WO2000003006A1

WO2000003006A1 - Sucrose biosynthesis genes and uses therefor

Info

Publication number: WO2000003006A1
Application number: PCT/AU1999/000557
Authority: WO
Inventors: Robert Furbank; John Lunn
Original assignee: Commonwealth Scientific Industrial And Research Organisation
Priority date: 1998-07-08
Filing date: 1999-07-08
Publication date: 2000-01-20
Also published as: AUPP457898A0

Abstract

The present invention provides genetic sequences which encode sucrose phosphate synthase-like (SPS-like) enzymes and homologues, derivatives and analogues thereof, in particular genetic sequences encoding the SPS-like enzyme of Synechocystis spp. (SynSPS), which enzymes utilise NDP-glucose in addition to UDP-glucose as a glucosyl donor substrate. The invention further provides SPS-like proteins from Synechocystis spp. and Anabaena spp. The invention further provides transformed bacterial cells and plants that stably express the SynSPS enzyme.

Description

SUCROSE BIOSYNTHESIS GENES AND USES THEREFOR

FIELD OF THE INVENTION

The present invention relates generally to novel genetic sequences which encode sucrose biosynthesis enzymes, peptides, oligopeptides and polypeptides and more particularly to novel genetic sequences which encode sucrose phosphate synthase-like (SPS-like) enzymes, peptides, oligopeptides and polypeptides which have broader substrate specificity than the SPS enzymes of higher plants and improved resistance to endogenous cellular processes which normally inactivate SPS activity in higher plants. In particular, the present invention provides genetic sequences which encode Synechocystis sp. sucrose phosphate synthase-like enzymes and homologues, derivatives and analogues thereof. The genetic sequences of the present invention provide the means by which sucrose metabolism and carbon partitioning may be altered or manipulated in higher plants which at least comprise the subject nucleic acid molecules in their genomes in the form of introduced transgenes. The invention extends to genetically modified plants which have been transformed with the subject genetic sequences and to methods of modifying sucrose metabolism and carbon partitioning.

GENERAL

Bibliographic details of the publications referred to in this specification are collected at the end of the description.

As used herein the term "derived from" shall be taken to indicate that a specified integer may be obtained from a particular specified source or species, albeit not necessarily directly from that specified source or species.

Throughout this specification and in the claims that follow, unless the context requires otherwise, the word "comprise", or variations such as "comprises" or "comprising", will be understood to imply the inclusion of a stated step or element or integer or group of steps or elements or integers but not the exclusion of any other step or element or integer or group of elements or integers.

SUBSΗTUTE SHEET (Rule 26) (RO/AU) Those skilled in the art will appreciate that the invention described herein is susceptible to variations and modifications other than those specifically described. It is to be understood that the invention includes all such variations and modifications. The invention also includes all of the steps, features, compositions and compounds referred to or indicated in this specification, individually or collectively, and any and all combinations or any two or more of said steps or features.

The present invention is not to be limited in scope by the specific embodiments described herein, which are intended for the purposes of exemplification only. Functionally-equivalent products, compositions and methods are clearly within the scope of the invention, as described herein.

Sequence identity numbers (SEQ ID NOS.) containing nucleotide and amino acid sequence information included in this specification are collected after the Abstract and have been prepared using the programme Patentln Version 2.0. Each nucleotide or amino acid sequence is identified in the sequence listing by the numeric indicator <210> followed by the sequence identifier (e.g. <210>1 , <210>2, etc). The length, type of sequence (DNA, protein (PRT), etc) and source organism for each nucleotide or amino acid sequence are indicated by information provided in the numeric indicator fields <211>, <212> and <213>, respectively. Nucleotide and amino acid sequences referred to in the specification are defined by the information provided in numeric indicator field <400> followed by the sequence identifier (eg. <400>1 , <400>2, etc).

The designation of nucleotide residues referred to herein are those recommended by the IUPAC-IUB Biochemical Nomenclature Commission, wherein A represents Adenine, C represents Cytosine, G represents Guanine, T represents thymine, Y represents a pyrimidine residue, R represents a purine residue, M represents Adenine or Cytosine, K represents Guanine or Thymine, S represents Guanine or Cytosine, W represents Adenine or Thymine, H represents a nucleotide other than Guanine, B represents a nucleotide other than Adenine, V represents a nucleotide other than Thymine, D represents a nucleotide other than Cytosine and N represents any nucleotide residue. The designation of amino acid residues referred to herein, as recommended by the IUPAC-IUB Biochemical Nomenclature Commission, are listed in Table 1.

TABLE 1

Amino Acid Three-letter One-letter

Abbreviation Symbol

Alanine Ala A Arginine Arg R

Asparagine Asn N

Aspartic acid Asp D

Cysteine Cys C

Glutamine Gin Q Glutamic acid Glu E

Glycine Gly G

Histidine His H

Isoleucine lie 1

Leucine Leu L Lysine Lys K

Methionine Met M

Phenylalanine Phe F

Proline Pro P

Serine Ser S Threonine Thr T

Tryptop an Trp w

Tyrosine Tyr Y

Valine Val V

Any amino acid as above Xaa X

BACKGROUND TO THE INVENTION

The rate of sucrose biosynthesis in the leaves of higher plants is a key determinant of growth rate, yield and duration of the life cycle. This is particularly relevant in the case of crop and pasture plants where high yields are desirable. Sucrose is produced in the cytoplasm of leaf cells from those phosphates exported from the chloroplast. Sucrose is the primary sugar transported around the plant and is used as an energy source in rapidly growing tissues and as a carbon source by storage organs such as seeds and tubers and for the biosynthesis of structural material (i.e. cell walls) in the form of cellulose and lignin. Sucrose also plays an important role in fruit development and the acclimation responses of plants to heat, cold, dehydration and anoxia.

A number of key enzymes have been identified which are capable of regulating the rate of sucrose biosynthesis in higher plants and under certain environmental conditions. Additionally, these enzymes play key roles in carbon partitioning, thereby determining whether carbon is mobilised into the sucrose biosynthetic pathway or deposited as starch or other carbohydrate (Stitt and Sonnewald, 1995).

One of the most important regulators of sucrose biosynthesis and carbon partitioning in higher plants is the sucrose phosphate synthase (SPS) enzyme. Transgenic tomato plants expressing maize SPS exhibit increased SPS activity compared to otherwise isogenic non-transformed plants and, as a consequence, have higher rates of sucrose biosynthesis at the expense of starch (Worrell et al, 1991) and increased net rates of photosynthesis, particularly at saturating light and high CO₂ concentrations (Galtier et al, 1993; Galtier et al, 1995), indicating that SPS is a major control point for regulating the rates of sucrose biosynthesis and of photosynthesis and the partitioning of carbon in higher plants.

Additionally, the export rate of photoassimilates produced by leaves has been correlated with SPS activity and sucrose formation in cotton (Hendrix and Huber, 1986) and soybean (Huber et al, 1984). The failure of tomato plants to increase fruit yield when grown in extended photoperiod conditions has also been attributed to low SPS activity (Dorais et al, 1996).

High SPS activity has also been suggested to be a requirement for the cold acclimation response of spinach plants (Martindale and Leegood, 1997). Reduced SPS activity has also been shown to correlate with reduced sucrose biosynthesis in sorghum, soybean and Craterostigma sp plants suffering from water deficit (Reddy, 1996; Bensari et al, 1990; Ingram et al, 1997) and in heat-stressed heat-intolerant lines of potato (Basu and Minhas, 1991), suggesting that the environmental regulation of carbon partitioning in particular plants may also be exerted via modified SPS activity and/or biosynthesis.

The properties of higher plant-derived forms of SPS have been studied in detail and are reviewed by Huber and Huber (1996). The enzyme utilises UDP-glucose as a glucosyl donor to fructose-6-phosphate in the following reaction:

UDP-glucose+fructose-6-phosphate → sucrose-6'-phosphate+UDP

The activities of many higher plant SPS enzymes are strongly inhibited by inorganic orthophosphate and activated by glucose-6-phosphate (Huber and Huber, 1992; Doehlert and Huber, 1984). SPS activity is generally inhibited in vivo under conditions wherein inorganic orthophosphate levels in the cell are high or phosphorylated intermediates are present at a low concentration. Light activation of higher plant SPS enzymes, brought about by dephosphorylation of the SPS protein, increases the affinity of the SPS enzyme for the substrates fructose-6-phosphate and UDP-glucose and decreases its inhibition by inorganic orthophosphate. If the ratio of hexose phosphate:orthophosphate in the cytoplasm remains constant then the dephosphorylation of higher plant SPS enzymes leads to increased enzyme activity and, as a consequence, increased synthesis of sucrose-6'-phosphate and sucrose.

An exception to this generality is the soybean SPS enzyme, which is not activated by glucose-6-phosphate and only weakly inhibited by inorganic orthophosphate. However, soybean SPS is subject to end-product inhibition by UDP (Nielsen and Huber, 1989).

The SPS activity isolatable from germinated pea seedlings is also activated by glucose-6-phosphate and fructose-1 ,6-bisphosphate and inhibited by UDP and inorganic orthophosphate (Lunn and Ap Rees, 1990). However, the inhibition of SPS by inorganic orthophosphate is highly variable (0-35% at 5mM or 10mM Pi concentration) in different plant species, including maize, tobacco, soybean, wheat and spinach (Crafts-Brandner and Salvucci 1989). In spinach leaves, the "dark" form of SPS is inhibited by greater than 90% at 5mM orthophosphate concentration (Stitt et al., 1988).

Photoaffinity labeling of spinach SPS with the substrate analogue(β-³²P)5'-N₃-UDP- glucose indicates that the holoenzyme may have a molecular mass of 253,000 and may comprise of two identical 120 kDa subunit polypeptides (Salvucci et al, 1990). However, the precise structure of the spinach SPS enzyme has not been determined because of the variation obtained using different methods of determination. In particular, it is not clear whether the enzyme comprises a dimeric 240-260 kDa protein or alternatively, a tetramer having a molecular mass of approximately 480 kDa and composed of 120 kDa subunits. The isoelectric point of the 120 kDa subunit is 5.2. Maize SPS is of similar subunit size and is functional when expressed in transformed Eschehchia coli cells (Worrell etal, 1991), indicating that the active form of maize SPS does not require additional polypeptides or any eukaryotic post-translational modification for activity.

The activation of spinach leaf SPS by glucose-6-phosphate is labile in the absence of dithiotheitol (DTT), the presence of hydrogen peroxide, N-ethylmaleimide or p- chloromercuribenzenesulfonic acid, without concomitant loss of catalytic activity being detected in the absence of activator. Additionally, inhibition of spinach SPS by inorganic orthophosphate is lost in the presence of N-ethylmaleimide or p- chloromercunbenzenesuifonic acid (Doehlert and Huber, 1985). These data suggest that glucose-6-phosphate and orthophosphate bind to a site in the spinach SPS holoenzyme at least, which is distinct from the catalytic site and that the activation/inhibition site may contain sulfhydryl groups which are accessible to DTT and essential for allosteric regulation of the SPS enzyme. Doehlert and Huber (1984) also showed that inorganic orthophosphate antagonizes the activation of spinach leaf SPS by glucose-6-phosphate activation, suggesting that there may be a single modifier site to which both activator or inhibitor bind. However, it is not clear whether identical amino acid residues in the SPS polypeptide would be involved in binding both activator and inhibitor molecules.

The UDP-glucose binding domain of SPS has been localised to a 26 kDa region of the 120 kDa polypeptide, by overexpressing part of the spinach SPS-encoding gene in Escherichia coli and photoaffinity-labeling the expressed protein with 5-azidouridine diphosphate-glucose (Salvucci and Klein, 1993). The UDP-glucose-binding domain is highly conserved between spinach and potato SPS enzymes.

The genes encoding SPS enzymes have been isolated from several plant species including maize, spinach, potato, rice, bean and sugar beet. Comparison of the derived amino acid sequences encoded by these genes indicates that there is considerable homology between the amino acid sequences of SPS enzymes in higher plants.

Levels of the SPS enzyme have been increased by over-expressing cDNA clones encoding SPS in transgenic tomato plants (Worrell et al, 1991 ; Galtier et al, 1993; Micallef et al, 1995; Laporte et al, 1997). These experiments showed some alteration in carbohydrate metabolism, by overexpressing SPS, in particular modifications in carbon partitioning, growth rate, yield and flowering time. However, high-level expression of SPS in transgenic plants has been difficult to obtain, partially due to inactivation of the SPS enzyme by endogenous cellular processes.

In work leading up to the present invention, the inventors sought to identify novel SPS enzyme activities having improved capacities to modify carbohydrate metabolisms in higher plants. The inventors discovered an isolated nucleotide sequence which encodes an SPS-like activity and which surprisingly exhibits very broad substrate specificity and/or which is less susceptible to the inactivation processes which have hitherto been reported to reduce SPS activity in the leaves of transgenic plants.

SUMMARY OF THE INVENTION One aspect of the invention provides an isolated nucleic acid molecule which encodes or is complementary to an isolated nucleic acid molecule which encodes a sucrose biosynthesis polypeptide and preferably, an SPS-like activity which does not utilise UDP-glucose as a sole glucosyl donor substrate.

A second aspect of the present invention provides an isolated nucleic acid molecule which is capable of encoding a polypeptide which possesses SPS-like activity and which does not contain a serine residue in the context of the amino acid sequence motif B-Hy-X-B-X-X-S wherein X is any amino acid residue, B is arginine or lysine and Hy is any hydrophobic amino acid residue.

A third aspect of the invention provides an isolated nucleic acid molecule which encodes or is complementary to an isolated nucleic acid molecule which encodes an SPS-like enzyme which comprises an amino acid sequence which at least includes 5- 10 contiguous amino acid residues derived from residues 465 to 720 of SEQ ID NO:2 or a homologue, analogue or derivative thereof.

A fourth aspect of the invention provides an isolated nucleic acid molecule which encodes or is complementary to an isolated nucleic acid molecule which encodes the amino acid sequence set forth in SEQ ID NO:2 or a homologue, analogue or derivative thereof which is at least about 50% identical to said amino acid sequence.

A fifth aspect of the invention provides an isolated nucleic acid molecule which comprises a sequence of nucleotides which is at least about 50% identical to SEQ ID NO:1 or a complementary sequence thereto.

A further aspect of the invention provides an isolated or synthetic nucleic acid molecule or amplification primer of at least about 10-15 nucleotides in length which is capable of hybridising under at least low stringency conditions to at least 20 contiguous nucleotides of SEQ ID NO:1 or a complementary sequence thereto.

A further aspect of the invention provides a genetic construct which comprises the isolated nucleic acid molecule according to any one or more embodiments described herein in operable connection with a promoter sequence.

A further aspect of the invention provides a method of altering the level of sucrose or sucrose phosphates in a plant cell, tissue or organ or a higher plant, said method comprising expressing the isolated nucleic acid molecule according to any one or more embodiments described herein in said cell, tissue or organ for a time and under conditions sufficient for sucrose-6'-phosphate synthesis to occur.

A still further aspect of the present invention contemplates a method of altering carbon partitioning and/or a developmental and/or growth response of a plant, said method comprising expressing the isolated nucleic acid molecule according to any one or more embodiments described herein in said plant for a time and under conditions sufficient for sucrose-6'-phosphate synthesis to occur.

A still further aspect of the invention provides a method of producing sucrose-6'- phosphate from a nucleoside diphosphate glucose other than UDP-glucose in a plant cell, said method comprising expressing the isolated nucleic acid molecule according to any one or more embodiments described herein in said cell for a time and under conditions sufficient for a catalytically-active SPS-like enzyme to be synthesized and incubating said enzyme in the presence of said nucleoside diphosphate glucose and fructose-6-phosphate.

A still further aspect of the invention provides a method of producing a recombinant enzymatically active SPS-like polypeptide in a transgenic plant comprising the steps of:

(i) producing a genetic construct which comprises the isolated nucleic acid molecule according to any one or more embodiments described herein placed operably under the control of a plant-expressible promoter and optionally an expression enhancer element, wherein said genetic sequences is also placed upstream of a transcription terminator sequence;

(ii) transforming said genetic construct into a cell or tissue of said plant; and (iii) selecting transformants which express an enzymatically-active SPS-like enzyme which comprises at least one polypeptide subunit encoded by said nucleic acid molecule.

A still further aspect of the invention provides a recombinant SPS-like polypeptide or functional SPS-like enzyme molecule, and preferably an isolated or recombinant polypeptide which comprises a sequence of amino acids set forth in SEQ ID NO: 2 or a homologue, analogue or derivative thereof which is at least about 50% identical thereto.

A further aspect of the invention contemplates an immunologically interactive molecule which binds to an isolated or recombinant SPS-like polypeptide as described herein or a homologue, analogue or derivative thereof.

A further aspect of the invention provides an isolated cell, tissue, organ or organism which comprises a non-endogenous nucleic acid molecule which includes the nucleotide sequence set forth in SEQ ID NO:1 or a homologue, analogue or derivative which is at least about 50% identical to 30 or more continuous nucleotides thereof or a complementary nucleotide sequence thereto. Preferably, the isolated cell, tissue, organ or organism is capable of expressing a recombinant SPS-like polypeptide encoded by the nucleic acid molecule which has been introduced thereto.

BRIEF DESCRIPTION OF THE DRAWINGS

Figure 1 is a schematic representation showing an alignment between the amino acid sequences of the Synechocystis sp. SPS-like enzyme and spinach SPS. Sequences were aligned using the Pileup function conservation. Regulatory phosphorylation sites in the spinach SPS enzyme as identified by Toroser and Huber (1997) are underlined. Figure 2 is a schematic representation showing a genetic construct suitable for expression of Synechocystis sp. SPS-like enzyme in Escherichia coli. The Synechocystis sp. SPS-like structural gene sequence (SEQ ID NO:1) was inserted between Bam\λ\ and EcoRI sites in the bacterial expression vector pTrcHisA (Invitrogen). The vector was then introduced into E. coli JM109 cells and the His₆- SPS-like fusion polypeptide expressed from the Trc promoter, following induction using IPTG.

Figure 3 is a copy of a photographic representation showing the molecular weight of the purified recombinant His₆-SPS-like enzyme as determined using SPS/PAGE. Recombinant Synechocystis sp. SPS-like enzyme was produced in E. coli as a fusion polypeptide with a histidine hexapeptide (i.e. His₆) and purified by affinity chromatography on Talon (immobilised Co²⁺). The crude cell lysate (20μg) (Lane 1) and affinity-purified fusion polypeptide (1μg) (Lane 2) were subjected to SDS/PAGE. Gels were stained with Coomassie Blue. M indicates molecule weight protein markers, the molecule weights of which are indicated to the left of the Figure.

Figure 4A is a copy of a graphical representation showing the ability of recombinant Synechocystis sp. SPS-like enzyme produced in E. coli to convert UDP-glucose to UDP in the absence (i) or presence (ii) of fructose-6-phosphate. UDP production was determined spectrophotometrically according to the method of Lunn and ApRees (1990).

Figure 4B is a copy of a graphical representation showing the ability of recombinant Synechocystis sp. SPS-like enzyme produced in E. coli to produce [U-¹⁴C]sucrose-6'- phosphate from UDP and [U-¹⁴C]fructose-6-phosphate. Reaction mixtures were incubated for zero time (Top panel) or for 10 mins (Lower panel). Alkaline phosphatase was then added and the sugars (sucrose, glucose and fructose) separated by paper chromatography. The percentage of [U-¹⁴C] label appearing in each fraction of the chromatogram is indicated for each sugar.

Figure 5 is a copy of a graphical representation showing the ability of recombinant Synechocystis SPS-like enzyme produced by E. coli cells to utilise UDP-glucose, ADP- glucose, GDP-glucose and CDP-glucose as a substrate. SPS-like enzyme activity (μmol/min/mg protein) was measured as the fructose 6-phosphate-dependent release of UDP, ADP, GDP or CDP from each NDP-glucose substrate (NDPGIc) tested.

Figure 6 is a copy of a graphical representation of a plot to determine kinetic characteristics of recombinant Synechocystis SPS-like enzyme produced by E. coli cells, using fructose 6-phosphate and UDP-glucose or ADP-glucose or GDP-glucose or CDP-glucose as a substrate for the reaction. Values for the K_M and V_max of the enzyme for each substrate tested were calculated by linear regression from a Hanes plot of these data.

Figure 7 is a copy of a graphical representation showing the activity of recombinant Synechocystis SPS-like enzyme produced by E. coli cells in the presence of 0-20mM inorganic orthophosphate. SPS-like enzyme activity (μmol/min/mg protein) was measured as the fructose 6-phosphate-dependent release of UDP from UDP-glucose. The reaction contained 10mM UDP-glucose +/- 5mM fructose-6-phosphate in the presence of 5mM MgCI₂ at pH8.0.

Figure 8 is a schematic representation of genetic constructs suitable for expression of recombinant Synechocystis sp. SPS-like enzyme in transgenic Nicotiana tabacum W38. The SPS-like gene (SyπSPS) was placed in operable connection with the CaMV 35S promoter in the plasmid vector pDH51 , which also utilises the CaMV 35S terminator sequence. The p35S-SynSPS-t35S construct was inserted into the EcoRI site of the binary vector pBS389 in both possible orientations. The binary plasmid pBS389 further comprises the kanamycin-resistance selectable marker gene (npt\\) placed operably under control of the sub-clover stunt virus (SCSV) gene 1 promoter (pSd) and upstream of the SCSV gene 3 terminator (tSc3). The pBS389 derivative vectors containing the Synechocystis sp. SPS-like gene under control of the CaMV promoter were then introduced into the Agrobacterium tumefaciens strain AGL1 and transformed into Nicotiana tabacum W38. Figure 9 is a copy of a photographic representation of a western blot showing the presence of recombinant Synechocystis sp. SPS-like enzyme in transgenic tobacco plants transformed with the binary vectors described in Figure 5. Lane 1 contained purified recombinant SPS-like enzyme (0.3 μg) produced in E. coli cells. Lane 2 contained a leaf extract (10μg) derived from untransformed tobacco cells. Lane 3 contained a leaf extract (10μg) derived from transformed tobacco comprising the binary vector described in the legend to Figure 5. Proteins were probed using a 1/10,000 dilution of antibody prepared against the purified Synechocystis sp. SPS-like enzyme. M indicates molecule weight marker proteins, the molecule weights of which are indicated to the left of the figure.

Figure 10 is a graphical representation of genetic constructs used to stably transform rice (Oryza sativa) plants (pUbi1-SynSPS-tm1 ; panel A) and tobacco (Nicotiana tabacum) plants or tobacco (N. plumbaginifolia) protoplasts (p35S-SyπSPS-t35S; panel B).

Figure 11 is a copy of a photographic representation showing the expression of recombinant Synechocystis SPS-like protein in protoplasts and transgenic plants as determined by western blotting using anti-SynSPS serum at a 1/10,000 dilution. Lanes A-C, 10 μg protein derived from Nicotiana plumbaginifolia protoplasts; Lanes D-E, 10 μg protein derived from rice leaf extracts; Lanes F-G, 10 μg protein derived from tobacco leaf extracts. Extracts shown in lanes A and G were derived from plants transformed with the genetic construct p35S-SyπSPS-t35S (Figure 10). Extract shown in lane E was derived from plants transformed with the genetic construct pUbil- SynSPS-tml (Figure 10). Extracts shown in lanes B, C, D and F were derived from untransformed plants.

Figure 12 is a copy of a photographic representation of a western blot showing the presence of SPS-like protein in Anabaena variabilis M3 cells. Protein extracts were prepared from Synechocystis sp. PCC6803 (panel A) and Anabaena vaήabilis M3

(panel B), electrophoresed, transferred to nylon membrane and probed with anti- SynSPS serum at a 1/10,000 dilution.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

One aspect of the invention provides an isolated nucleic acid molecule which encodes or is complementary to an isolated nucleic acid molecule which encodes a sucrose biosynthesis polypeptide having sucrose phosphate synthase-like (SPS-like) activity or a peptide, oligopeptide, polypeptide thereof.

As used herein, the term "sucrose biosynthesis polypeptide" shall be taken to refer to any polymer comprising amino acid residues which is capable of catalysing one or more enzymic reactions in the sucrose biosynthesis pathway or alternatively, which is capable of catalysing one or more enzymic reactions which is known by those skilled in the art to directly or indirectly produce sucrose from those.

As used herein, the term "sucrose phosphate synthase or "SPS" shall be taken to refer to any peptide, polypeptide, oligopeptide or enzyme which is capable of performing the reaction:

UDP-glucose+fructose-6-phosphate → sucrose-6'-phosphate+UDP

Those skilled in the art will be aware of the structural and physical characteristics of the SPS enzyme derived from higher plants and the present invention is not to be limited by such physical structural characteristics. Preferably, an active SPS enzyme as defined herein will comprise a dimer or tetramer of two identical or non-identical polypeptide subunits, each of which has a molecular weight in the range of 115-120 kDa as determined by SDP/PAGE or gel filtration and/or a pi value in the range 7.0- 7.5.

The term "sucrose phosphate synthase-like" or "SPS-like" shall be taken to refer to any peptide, oligopeptide, polypeptide or enzyme which is capable of catalysing the biosynthesis of sucrose-6 '-phosphate from any nucleoside diphosphate glucose (NDP- glucose) substrate molecule in the reaction: NDP-glucose+fructose-6-phosphate → sucrose-6'-phosphate+NDP;

wherein NDP is ADP or CDP or GDP or UDP, subject to the proviso that the NDP- glucose substrate is not exclusively UDP-glucose.

Accordingly, the sucrose phosphate synthase-like activity of the invention may be distinguished from a sucrose phosphate synthase activity by virtue of its broader substrate specificity and, whilst an SPS-like enzyme may utilise UDP-glucose as a glucosyl donor to fructose-6-phosphate, it does not exclusively utilise UDP-glucose as a glucosyl donor and as a consequence, may also utilise ADP-glucose or GDP-glucose or CDP-glucose as glucosyl donor. In this regard, the SPS-like enzyme exemplified herein, which is derived from the unicellular cyanobacterium Synechocystis sp. utilises UDP-glucose or ADP-glucose or CDP glucose or GPD glucose. Based upon K_m values, the preferred substrates of the Synechocystis SPS-like enzyme is GDP glucose (K_m=1.8mM), whilst the enzyme has a weaker affinity for CDP glucose (K =7.2 mM). Those skilled in the art will be aware that the preferred enzyme substrate of an SPS-like enzyme in vivo will vary depending upon local concentrations of nucleoside diphosphate glucose substrates in the cell.

The SPS-like enzyme exemplified herein has the added advantage of having a higher affinity for fructose-6-phosphate (K_m=0.22mM) compared to higher plant SPS enzymes (K_mfor fructose-6-phosphate in the range of about 0.7-5.5mM).

For the purposes of nomenclature, the term "SynSPS" shall be taken to refer to the SPS-like enzyme derived from Synechocystis sp., in particular, Synechocystis sp PCC6803.

The activity of an SPS-like enzyme may be determined by any means known to those skilled in the art for example, the SPS enzyme assay described by Salvucci and Crafts- Brandner (1991), which utilises a uridine 5'-diphosphate-[U- ¹⁴C]glucose substrate in combination with HPLC to separate the substrate from [U-¹ C]sucrose-6'-phosphate, may be adapted to assay SPS-like enzyme activity. Modifications of the HPLC conditions to separate adenosine 5'-diphosphate-[U-¹⁴C]glucose and/or cytosine 5'- diphosphate-[U-¹⁴C]glucose and/or guanosine 5'-diphosphate-[U-¹⁴C]glucose from [U- ¹⁴C]sucrose-6 '-phosphate, may be readily performed by those skilled in the art.

The SPS-like enzyme of the present invention will preferably exhibit distinct activation and inhibition kinetics compared to SPS enzymes which are derived from higher plants. In marked contrast to SPS enzymes, the SPS-like enzyme of the invention is preferably not allosterically-regulated by the concentration of glucose-6-phosphate and/or inorganic orthophosphate.

Alternatively or in addition, the SPS-like enzyme of the invention will comprise at least one polypeptide subunit having a molecular weight of less than about 110 kDa, more preferably less than about 100 kDa and even more preferably less than about 90 kDa. In a still more preferred embodiment, the SPS-like enzyme of the invention will comprise at least one polypeptide subunit having a molecular weight in the range of 80-90 kDa, as determined by SDS/PAGE or gel filtration or by the estimation of molecular weight from amino acid composition data or from amino acid sequence data.

Alternatively or in addition, the SPS-like enzyme of the invention will possess a pi value which is higher than the pi value of SPS enzymes derived from higher plants. For example, the polypeptide subunit of the Synechocystis sp. SPS-like enzyme includes a C-terminus which is not found in SPS enzymes and comprises greater than 10% glutamine plus asparagine plus histidine residues. Preferably, SPS-like enzymes will possess a pi value in the range of 7.6 to 9.0 and more preferably in the range of 8.0 to 8.5.

The present invention clearly extends to chimaeric enzymes which comprise at least one polypeptide subunit which is derived from an SPS-like enzyme, such as from Synechocystis sp. SPS-like enzyme exemplified herein, and one or more polypeptide subunits derived from another SPS-like enzyme or alternatively, derived from an SPS enzyme, subject to the proviso that such chimaeric enzymes possess at least partial SPS-like enzyme substrate specificity or catalytic activity, subject to the proviso that it does not utilise UDP-glucose as the sole glucosyl donor.

Accordingly, a chimaera between an SPS-like enzyme and an SPS enzyme may be capable of utilising ADP-glucose and/or CDP glucose and/or GDP glucose and/or UDP-glucose as a glucosyl donor to fructose-6-phosphate, in addition to retaining the characteristic activation and inhibition kinetics of higher plant SPS enzymes (i.e. activation by glucose-6-phosphate and/or inhibition by inorganic orthophosphate). Alternatively a chimaeric SPS-like/SPS enzyme may only be capable of utilising UDP- glucose as a glucosyl donor.

It is also within the scope of the present invention to produce chimaeric SPS-like/SPS multimeric or dimeric proteins which have reduced catalytic activity or no catalytic activity at all, for example wherein it is desirable to reduce the activity of a higher plant SPS enzyme in vivo. In one example, an SPS-like polypeptide subunit may be expressed in a higher plant cell, tissue or organ at a high level such that, in addition to functional SPS-like enzyme molecules being produced which preferably comprise monomers and/or homodimers and/or homotetramers, there are formed SPS-like/SPS chimaeric protein multimers which have no activity but serve to reduce the pool of endogenous SPS enzyme activity. Whilst not limiting the invention according to this embodiment to any theory or mode of action, the reduction in SPS activity in such circumstances may arise via a dominant negative mutation effect as a consequence of insufficient sequence identity between the SPS-like polypeptide subunits and SPS polypeptide subunits to produce enzyme activity, notwithstanding the fact that there may be sufficient similarity therebetween to facilitate their association into a dimeric or multimeric protein.

In a particularly preferred embodiment, an isolated nucleic acid molecule which encodes an SPS-like peptide, oligopeptide, polypeptide or enzyme, will further comprise a nucleotide sequence which is at least about 50% identical to the nucleotide sequence set forth in SEQ ID NO:1 or a homologue, analogue or derivative thereof or a complementary nucleotide sequence thereto. More preferably, the percentage identity to SEQ ID NO: 1 is at least about 60%, even more preferably at least about 70%, even more preferably at least about 80%, even more preferably at least about 90%. In a most particularly preferred embodiment, the nucleic acid molecule of the present invention will comprise a nucleotide sequence which is at least about 95% or alternatively, 100% identical to SEQ ID NO:1 or a homologue, analogue or derivative thereof or a complementary nucleotide sequence thereto.

For the purposes of nomenclature, the nucleotide sequence set forth in SEQ ID NO:1 comprises the Synechocystis sp. gene which encodes an SPS-like enzyme described supra. The amino acid sequence of the Synechocystis sp. SPS-like enzyme subunit polypeptide is set forth herein as SEQ ID NO:2.

The present invention clearly extends equally to the genes and cDNA equivalents of the nucleotide sequences exemplified herein.

Reference herein to a "gene" is to be taken in its broadest context and includes: (i) a classical genomic gene consisting of transcriptional and/or translational regulatory sequences and/or a coding region and/or non-translated sequences (i.e. introns, 5'- and 3'- untranslated sequences);or

(ii) mRNA or cDNA corresponding to the coding regions (i.e. exons) and 5'- and 3'- untranslated sequences of the gene; or (iii) amplified single-stranded or double-stranded DNA which is derived from sub- paragraph (i) or sub-paragraph (ii).

The term "gene" is also used to describe synthetic or fusion molecules encoding all or part of a functional product, which may be derived from a naturally-occurring sucrose biosynthesis gene by standard recombinant techniques.

Accordingly, the terms "SPS-like gene" or "gene encoding an SPS-like enzyme" or similar term refers to any gene which, in its native context at least, is capable of encoding a polypeptide subunit of an SPS-like enzyme or a peptide, oligopeptide, or fusion polypeptide derived therefrom.

For the present purpose, "homologues" of a nucleotide sequence shall be taken to refer to an isolated nucleic acid molecule which is substantially the same as the nucleic acid molecule of the present invention or its complementary nucleotide sequence, for example a nucleic acid molecule which is the catalytic and/or enzymatic and/or immunological equivalent of the nucleic acid molecule of the present invention, notwithstanding the occurrence within said sequence of one or more nucleotide substitutions, insertions, deletions, or rearrangements.

"Analogues" of a nucleotide sequence set forth herein shall be taken to refer to an isolated nucleic acid molecule which is substantially the same as a nucleic acid molecule of the present invention or its complementary nucleotide sequence, notwithstanding the occurrence of any non-nucleotide constituents not normally present in said isolated nucleic acid molecule, for example carbohydrates, radiochemicals including radionucleotides, reporter molecules such as, but not limited to DIG, alkaline phosphatase or horseradish peroxidase, amongst others.

"Derivatives" of a nucleotide sequence set forth herein shall be taken to refer to any isolated nucleic acid molecule which contains significant sequence similarity to said sequence or a part thereof.

Generally, homologues, analogues or derivatives of a nucleotide sequence of the invention are produced by synthetic means or alternatively, derived from naturally- occurring sources. For example, the nucleotide sequence of the present invention may be subjected to mutagenesis to produce single or multiple nucleotide substitutions, deletions and/or insertions.

In one embodiment of the invention, preferred homologues, analogues or derivatives of the nucleotide sequence set forth in SEQ ID NO:1 or complementary sequences thereto, encode immunologically-active or enzymatically-active polypeptides. As used herein, the terms "immunologically-active", "immunologically interactive" or similar term shall be taken to refer to the ability of a peptide, oligopeptide, polypeptide or enzyme molecule to elicit an immune response in a mammal, in particular an immune response sufficient to produce an antibody molecule such as, but not limited to, an IgM or IgG molecule or whole serum containing said antibody molecule. The term "immunologically-active" also extends to the ability of a polypeptide to elicit a sufficient immune response for the production of monoclonal antibodies, synthetic Fab fragments of an antibody molecule, single-chain antibody molecule or other immunointeractive molecule.

As used herein, the term "enzymatically-active" shall be taken to refer to the ability of a polypeptide molecule to catalyse an enzyme reaction. In the context of the present invention, an enzymatically-active homologue, analogue or derivative of SEQ ID NO:1 will comprise a nucleotide sequence which is capable of encoding an amino acid sequence which comprises sucrose biosynthesis activity, preferably SPS-like activity and in particular, encodes an amino acid sequence which comprises SPS-like activity as defined herein.

In an alternative embodiment, an isolated nucleic acid molecule which encodes a sucrose phosphate synthase-like peptide, oligopeptide, polypeptide or enzyme, will further encode an amino acid sequence which is at least about 50% similar or alternatively, at least about 50% identical, to the amino acid sequence set forth in SEQ ID NO:2 or a homologue, analogue or derivative thereof.

More preferably, the percentage similarity or identity to SEQ ID NO: 2 is at least about 60%, even more preferably at least about 70%, even more preferably at least about 80%, even more preferably at least about 90%. In a most particularly preferred embodiment, the nucleic acid molecule of the present invention will encode an amino acid sequence which is at least about 95% or alternatively, 100% similar or identical to SEQ ID NO:2 or a homologue, analogue or derivative thereof.

In the present context, "homologues" of an amino acid sequence refer to those amino acid sequences or peptide sequences which are derived from the peptide, polypeptide, enzyme or protein of the present invention or alternatively, correspond substantially to the amino acid sequence listed supra, notwithstanding any naturally-occurring amino acid substitutions, additions or deletions thereto.

For example, amino acids may be replaced by other amino acids having similar properties, for example hydrophobicity, hydrophilicity, hydrophobic moment, antigenicity, propensity to form or break α-helical structures or β-sheet structures, and so on.

Alternatively, or in addition, the amino acids of a homologous amino acid sequence may be replaced by other amino acids having similar properties, for example hydrophobicity, hydrophilicity, hydrophobic moment, charge or antigenicity, and so on.

Naturally-occurring amino acid residues contemplated herein are described in Table 1.

A homologue of an amino acid sequence may be a synthetic peptide produced by any method known to those skilled in the art, such as by using Fmoc chemistry.

Alternatively, a homologue of an amino acid sequence may be derived from a natural source, such as the same or another species as the polypeptides, enzymes or proteins of the present invention. Preferred sources of homologues of the amino acid sequences listed supra include any organism capable of producing sucrose from those, including any bacterium, cyanobacte um, algae or plant species.

In a particularly preferred embodiment, an analogue of the SyπSPS protein or enzyme exemplified herein or a homologue of the SyπSPS-encoding gene exemplified herein is derived from the distantly-related Anabaena sp., more particularly from A. vahabilis. As exemplified herein, the present inventors have shown clearly the presence of SPS- like protein in A. vahabilis M3 cells (Figure 12). "Analogues" of an amino acid sequence encompass those amino acid sequences which are substantially identical to the amino acid sequences listed supra notwithstanding the occurrence of any non-naturally occurring amino acid analogues therein.

Preferred non-naturally occurring amino acids contemplated herein are listed below in Table 2.

The term "derivative" in relation to an amino acid sequence shall be taken to refer hereinafter to mutants, parts, fragments or polypeptide fusions of the amino acid sequences listed supra. Derivatives include modified amino acid sequences or peptides in which ligands are attached to one or more of the amino acid residues contained therein, such as carbohydrates, enzymes, proteins, polypeptides or reporter molecules such as radionuclides or fluorescent compounds. Glycosylated, fluorescent, acylated or alkylated forms of the subject peptides are also contemplated by the present invention. Additionally, derivatives may comprise fragments or parts of an amino acid sequence disclosed herein and are within the scope of the invention, as are homopolymers or heteropolymers comprising two or more copies of the subject sequences.

Substitutions encompass amino acid alterations in which an amino acid is replaced with a different naturally-occurring or a non-conventional amino acid residue. Such substitutions may be classified as "conservative", in which case an amino acid residue is replaced with another naturally-occurring amino acid of similar character, for example Gly÷→-Ala, Vah->lle<- Leu, Asp<→Glu, Lys<->Arg, Asn<->Gln or Phe«→ rp<-Tyr. Substitutions encompassed by the present invention may also be "non-conservative", in which an amino acid residue which is present in a repressor polypeptide is substituted with an amino acid having different properties, such as a naturally- occurring amino acid from a different group (eg. substituted a charged or hydrophobic amino acid with alanine), or alternatively, in which a naturally-occurring amino acid is substituted with a non-conventional amino acid. Amino acid substitutions are typically of single residues, but may be of multiple residues, either clustered or dispersed. Procedures for derivatizing peptides are well- known in the art.

TABLE 2

Non-conventional Code Non-conventional Code amino acid amino acid

α-aminobutyric acid Abu L-N-met ylalanine Nmala α-amino-α-methylbutyrate Mgabu L-N-met ylarginine Nmarg aminocyclopropane- Cpro L-N-methylasparagine Nmasn carboxylate L-N-methylaspartic acid Nmasp aminoisobutyric acid Aib L-N-methylcysteine Nmcys aminonorbornyl- Norb L-N-met ylglutamine Nmgln carboxylate L-N-methylglutamic acid Nmglu cyclohexylalanine Chexa L-N-met ylhistidine Nmhis cyclopentylalanine Cpen L-N-met ylisolleucine Nmile

D-alanine Dal L-N-methylleucine Nmleu

D-arginine Darg L-N-methyllysine Nmlys

D-aspartic acid Dasp L-N-methylmethionine Nmmet

D-cysteine Dcys L-N-methylnorleucine Nmnle

D-glutamine Dgln L-N-methylnorvaline Nmnva

D-glutamic acid Dglu L-N-methylornithine Nmorn

D-histidine Dhis L-N-methylphenylalanine Nmphe

D-isoleucine Dile L-N-methylproiine Nmpro

D-leucine Dleu L-N-methyiserine Nmser

D-lysine Dlys L-N-methylthreonine Nmthr

D-methionine Dmet L-N-methyltryptophan Nmtrp

D-ornithine Dorn L-N-methyltyrosine Nmtyr

D-phenylalanine Dphe L-N-methylvaline Nmval

D-proline Dpro L-N-methylethylglycine Nmetg

D-serine Dser L-N-methyl-t-butylglycine Nmtbug

D-threonine Dthr L-norleucine Nle

D-tryptophan Dtrp L-norvaline Nva

D-tyrosine Dtyr α-methyl-aminoisobutyrate Maib

D-valine Dval α-methyl-γ-aminobutyrate Mgabu

D-α-methylalanine Dmala α-methylcyclohexylalanine Mchexa

D-α-methylarginine Dmarg α-methylcylcopentylalanine Mcpen D-α-met ylasparagine Dmasn α-methyl-α-napthylalanine Manap

D-α-methylaspartate Dmasp α-methylpenicillamine Mpen

D-α-methylcysteine Dmcys N-(4-aminobutyl)glycine Nglu

D-α-met ylglutamine Dmgln N-(2-aminoethyl)glycine Naeg

D-α-methylhistidine Dmhis N-(3-aminopropyl)glycine Norn

D-α-methylisoleucine Dmile N-amino-α-methylbutyrate Nmaabu

D-α-methylleucine Dmleu α-napthylalanine Anap

D-α-methyllysine Dmlys N-benzylglycine Nphe

D-α-met ylmethionine Dm met N-(2-carbamylethyl)glycine Ngln

D-α-methylornithine Dmorn N-(carbamylmethyl)glycine Nasn

D-α-methylphenylalanine Dmphe N-(2-carboxyethyl)g lycine Nglu

D-α-met ylproline Dmpro N-(carboxymethyl)glycine Nasp

D-α-met ylserine Dmser N-cyclobutylglycine Ncbut

D-α-methylt reonine Dmt r N-cycloheptylglycine Nchep

D-α-methyltryptophan Dmtrp N-cyclohexylg lycine Nchex

D-α-methyltyrosine Dmty N-cyclodecylg lycine Ncdec

D-α-met ylvaline Dmval N-cylcododecylglycine Ncdod

D-N-met ylalanine Dnmala N-cyclooctylglycine Ncoct

D-N-methylarginine Dnmarg N-cyclopropylglycine Ncpro

D-N-methylasparagine Dnmasn N-cycloundecylglycine Ncund

D-N-methylaspartate Dnmasp N-(2,2-diphenylethyl) glycine Nbhm

D-N-methylcysteine Dnmcys N-(3,3-diphenylpropyl) glycine Nbhe

D-N-methylglutamine Dnmgln N-(3-guanidinopropyl) glycine Narg

D-N-methylglutamate Dnmglu N-(1-hydroxyethyl)glycine Nthr

D-N-methylhistidine Dnmhis N-(hydroxyethyl))glycine Nser

D-N-methylisoleucine Dnmile N-(imidazolylethyl)) giycine Nhis

D-N-methylleucine Dnmleu N-(3-indolylyethyl) glycine Nhtrp

D-N-methyllysine Dnmlys N-methyl-γ-aminobutyrate Nmgabu

N-methylcyclohexylalanine Nmchexa D-N-methylmethionine Dnmmet

D-N-methylornithine Dnmorn N-methylcyclopentylalanine Nmcpen

N-methylglycine Nala D-N-methylphenylalanine Dnmphe

N-methylaminoisobutyrate Nmaib D-N-methylproline Dnmpro

N-(1-methylpropyl)glycine Nile D-N-methylserine Dnmser N-(2-methylpropyl)glycιne Nleu D-N-methylthreonme Dnmthr

D-N-methyltryptophan Dnmtrp N-(1-methylethyl)glycιne Nval

D-N-methyltyrosine Dnmtyr N-methyla-napthylalanine Nmanap

D-N-methylvahne Dnmval N-methylpenicillamme Nmpen

Y-aminobutyπc acid Gabu N-(p-hydroxyphenyl)glycιne Nhtyr

L-f-butylglycine Tbug N-(thιomethyl)glycιne Ncys

L-ethylglycme Etg penicillamme Pen

L-homophenylalanme Hphe L-α-methylalanine Mala

L-α-methylarginine Marg L- -methylasparagine Masn

L-α-methylaspartate Masp L-α-methyl-f-butylglycine Mtbug

L-α-methylcysteme Mcys L-methylethylglycine Metg

L-α-methylglutamine Mgln L-α-methylglutamate Mglu

L- -methylhistidine Mhis L-α-methylhomo phenylalanine Mhphe

L-α-methylisoleucme Mile N-(2-methylthιoethyl) glycine Nmet

L-α-methylleucine Mleu L-α-methyllysme Mlys

L-α-methylmethionine Mmet L-α-methylnorleucine Mnle

L-α-methylnorvahne Mnva L-α-methylornithine Morn

L-α-methylphenylalanme Mphe L-α-methylproline Mpro

L-α-methylseπne Mser L-α-methylthreonine Mthr

L-α-methyltryptophan Mtrp L-α-methyltyrosine Mtyr

L-α-methylva ne Mval L-N-methylhomo phenylalanine Nmhp e

N-(N-(2,2-dιphenylethyl) N-(N-(3,3-dιphenylpropyl) carbamylmethyl)glycιne Nnbhm carbamylmethyl)glycιne Nnbhe

1 -carboxy-1 -(2,2-dιphenyl- ethylamιno)cyclopropane Nmbc

Amino acid deletions will usually be of the order of about 1-10 amino acid residues, while insertions may be of any length. Deletions and insertions may be made to the N-terminus, the C-terminus or be internal deletions or insertions. Generally, insertions within the amino acid sequence will be smaller than amino-or carboxyl-terminal fusions and of the order of 1-4 amino acid residues

Reference herein to a percentage identity or percentage similarity between two or more nucleotide or amino acid sequences shall be taken to refer to the number of identical or similar residues in a nucleotide or amino acid sequence alignment, as determined using any standard algorithm known by those skilled in the art. In particular, nucleotide and/or amino acid sequence identities and similarities may be calculated using the GAP program, which utilises the algorithm of Needleman and Wunsch (1970) to maximise the number of residue matches and minimise the number of sequence gaps. The GAP program is part of the Sequence and Analysis Software Package of the Computer Genetics Group Inc., University Research Park, Madison, Wisconsin, United States of America (Devereux er a/., 1984). In nucleotide and amino acid sequence comparisons which contain no gaps, the percentage identity may be calculated from a direct comparison of the number of identical nucleotides or amino acids therebetween, as the case may be, expressed as a percentage of the total number of nucleotides or amino acids in the sequences.

The source from which the subject nucleic acid molecule may be derived is any organism which possesses the genetic capacity to synthesize sucrose and/or sucrose- 6'-phosphate from those via a nucleoside diphosphate glucose intermediate, subject to the proviso that the nucleoside diphosphate glucose substrate is not exclusively UDP-glucose. Preferably, the isolated nucleic acid molecule of the present invention is derived from a unicellular organism such as a cyanobacterium and in particular, from one or more Synechocystis sp. Additional sources of the subject nucleic acid molecule are not excluded.

In the exemplification of the invention, the inventors have demonstrated that the Synechocystis sp. gene sequence provided herein encodes an SPS-like polypeptide having SPS-like enzyme activity when expressed in isolated Escherichia coli cells. Additionally, the inventors have demonstrated that the SPS-like enzyme activity encoded by the isolated nucleic acid molecule of the invention possesses physical and kinetic characteristics which differ significantly from the physical and kinetic characteristics exhibited by SPS enzymes, such as those derived from higher plants (Table 3). As mentioned supra, the SPS-like activity of the invention possesses a broad substrate preference compared to SPS enzymes. Additionally the SPS-like activity described herein is not inhibited by inorganic orthophosphate or activated by glucose-6-phosphate, suggesting that it is of particular utility in shifting carbon from other metabolic pathways into sucrose.

TABLE 3

Properties of the SPS-like enzyme derived from Synechocystis sp. compared to the SPS enzymes derived from higher plants.

A further significant advantage of the isolated nucleic acid molecule of the invention, in addition to the fact that it encodes an SPS-like enzyme, is that it encodes an SPS- like enzyme which is less likely to be subjected to inactivation by higher plant cell processes such as phosphorylation, in the same manner as SPS enzymes derived from higher plants, by virtue of the significant dissimilarity between the amino acid sequences of SPS-like enzyme polypeptide subunits and SPS enzyme polypeptide subunits.

Accordingly, a second aspect of the present invention provides an isolated nucleic acid molecule which is capable of encoding a polypeptide which possesses SPS-like activity but which is not inactivated substantially by endogenous cellular processes of higher piants, in particular phosphorylation.

The inactivation of higher plant SPS enzymes as a consequence of their phosphorylation by protein kinases, appears to be mediated by the phosphorylation of specific serine residues. For example, spinach leaf SPS is phosphorylated at Ser 158 and Ser 424 (Toroser and Huber, 1997), a feature which is important in regulation of the activity of this enzyme in vivo, however poses particular problems to those skilled in the art who are attempting to increase sucrose biosynthesis in plant cells. The amino acid sequences between SPS enzymes derived from higher plants are highly- conserved, particularly in the region surrounding Ser 158 and Ser 424 of the spinach leaf enzyme (Tables 4 and 5), however these sequence motifs are not present in the SPS-like enzyme of the invention (see Figure 1).

TABLE 4 Amino acid sequences of SPS enzyme polypeptide subunits in the region of the phosphorylated serine residues Ser 158 of spinach SPS

TABLE 5

Amino acid sequences of SPS enzyme polypeptide subunits in the region of the phosphorylated serine residues Ser 454 of spinach SPS

Accordingly, in a preferred embodiment, the present invention provides an isolated nucleic acid molecule which encodes a polypeptide which possesses SPS-like activity however does not contain a serine residue in the context of the amino acid sequence motif:

BHyXBXXS wherein X is any amino acid residue, B is arginine or lysine and Hy is any hydrophobic amino acid residue.

In a most particularly preferred embodiment, the nucleic acid molecule of the invention will not encode an amino acid sequence which comprises a serine residue in the context of any one or more amino acid sequences set forth in Table 4 or Table 5 herein, wherein said serine residue is the equivalent of serine 158 or serine 424 of the spinach SPS enzyme, subject to the proviso that said SPS-like enzyme does not further comprise the Craterostigma2 amino acid sequence set forth in Table 5.

In this regard, the SPS-like enzyme of the invention does not comprise a serine residue in this context and preferably contains a basic amino acid residue, in particular asparagine, at the equivalent position to serine 424 of the spinach SPS enzyme, more preferably an asparagine residue in the context of the amino acid sequence:

BHyXBXAN, wherein X is any amino acid residue, B is arginine or lysine and Hy is any hydrophobic amino acid residue.

The present invention is not to be limited by the requirement for a basic amino acid residue in this position and other alternatives, such as the presence of an acidic amino acid residue or a non-polar amino acid residue or a hydrophobic amino acid residue at the equivalent position to serine 424 of the spinach SPS enzyme are contemplated, the only proviso being that siad residue is not a serine residue.

Preferably, the isolated nucleic acid molecule according to this aspect of the invention further encodes an SPS-like enzyme polypeptide subunit which comprises an amino acid sequence which is at least about 50% identical to SEQ ID NO: 2 or a homologue, analogue or derivative thereof.

Those skilled in the art will be aware that homologues, analogues or derivatives of a nucleotide sequence which encodes an SPS-like enzyme may be used as genetic probes in the isolation of related sequences falling within the scope of the presently- described invention, however which possess altered catalytic or kinetic characteristics from the subject matter exemplified herein. Preferred homologues, analogues or derivatives capable of such use comprise a sequence derived from SEQ ID NO:1 or "primer" molecules suitable for use in polymerise chain reaction assays or as hybridisation "probes".

Accordingly, a further aspect of the invention clearly extends to any isolated nucleic acid molecule which comprises a nucleotide sequence having at least about 50% sequence identity to SEQ ID NO:1 or a complementary nucleotide sequence thereto.

A further aspect of the invention contemplates an isolated or synthetic nucleic acid molecule or amplification primer of at least about 10-15 nucleotides in length which is capable of hybridising under at least low stringency conditions to at least 20 contiguous nucleotides derived from SEQ ID NO:1 or a complementary sequence thereto.

A further distinction between SPS-like polypeptides and SPS polypeptides is the significant divergence in amino acid sequence which occurs in the C-terminus of these two classes of sucrose biosynthesis enzymes. For example, the Synechocystis sp. SPS-like polypeptide set forth herein as SEQ ID NO:2 comprises an amino acid sequence having less than 50% sequence identity to a higher plant SPS enzyme polypeptide subunit in the region between residues 7 and 465 of SEQ ID NO:2, however residues from about position 465 to position 720 of SEQ ID NO:2 possess no significant similarity to known proteins.

Without being bound by any theory or mode of action, these amino acid sequence differences between SPS-like polypeptides and SPS polypeptides explain, at least in part, the altered catalytic activity and kinetic characteristics of SPS-like enzymes. Thus, the possibility exists that the C-terminal region of the Synechocystis sp. SPS- like polypeptide may comprise substrate and allosteric inhibitor binding domains not present in SPS enzymes derived from higher plants.

Additionally, it will be apparent to those skilled in the art of protein engineering that novel sucrose biosynthesis polypeptides may be produced by fusion of the C-terminal region of the Synechocystis sp. SPS-like polypeptide exemplified herein or a homologue, analogue or derivative thereof to the amino acid sequence of an SPS enzyme derived from a higher plant or a part thereof, to confer novel catalytic and kinetic characteristics thereon. Means for producing such fusion polypeptides will be known to those skilled in the art and will conveniently involve the splicing together of nucleotide sequences encoding both the SPS polypeptide and SPS-like polypeptide domains, amongst other methods. Such fusions may be assigned for catalytic activity by any means known to those skilled in the art, particularly in light of the instant disclosure.

Accordingly, a third aspect of the present invention provides an isolated nucleic acid molecule which is capable of encoding an SPS-like peptide, polypeptide, oligopeptide or enzyme which comprises an amino acid sequence which includes at least about 15- 20 contiguous amino acid residues derived from residues 465 to 720 of SEQ ID NO:2 or a homologue, analogue or derivative thereof.

Preferably, the length of the amino acid sequence derived from SEQ ID NO:2 comprises at least about 5-15 amino acids, more preferably at least about 5-10 contiguous amino acids and even more preferably about 5 contiguous amino acid residues derived from SEQ ID NO:2.

Standard methods may be used to define regions within the SPS-like enzyme subunits which are involved in substrate, inhibitor or activator binding. For example, photoaffinity-labelled substrate or inhibitor or activator analogues which are capable of competitively inhibiting the corresponding substrate, inhibitor or activator molecule(s) may be incubated with the active SPS-like enzyme and covalently linked thereto. Peptides comprising the covalently-bound labelled analogue compound are then isolated and the modified residues determined using standard amino acid sequence determination techniques.

Truncated peptides may also be expressed in E. coli or other cells and tested for their ability to bind substrate analogues and/or activator analogues and/or inhibitor analogues in isolated cells, cell extracts. The advantage with this approach is that the labelled peptides may be readily isolated and assayed for the presence of the covalently-bound label (eg ¹⁴C or ³² P), for example using any combination of ion exchange, gel filtration or reverse phase chromatographic procedures, such as on HPLC or FPLC. It is also possible to "map" putative substrate, inhibitor and activator domains of an SPS-like polypeptide by producing a library of expressible clones in E. coli wherein each clone of said library comprises a nucleotide sequence derived from the open reading frame of the complete SPS-like gene sequence such that the complete open reading frame is represented in said library. According to this embodiment, each clone of the library will express a short region of about 10 amino acids in length, derived from the full-length amino acid sequence of an SPS-like polypeptide and as a consequence, may be tested for its ability to bind the analogue compound. Clones which express peptides capable of binding analogue compounds are then isolated, their nucleotide sequences determined and the amino acid sequences encoded therefor are derived. The analogue-labelled peptides may also be isolated and determined directly as described supra.

The present inventors have shown that the Synechocystis SPS-like enzyme exemplified herein can utilise ADP-glucose, GDP-glucose, CDP-glucose or UDP- glucose as a substrate and there is limited homology between amino acids 63-74 of the Synechocystis SPS-like enzyme and the UDP-glucose binding site of higher plant SPS enzymes. Without being bound by any theory or mode of action, residues 63-74 of the Synechocystis SPS-like enzyme may comprise all or part of the NDP-glucose binding site, wherein the deviation in amino acid sequence from the higher plant UDP- glucose binding site accounts for the broader substrate specificity of the Synechocystis SPS-like enzyme.

Preferably, the percentage identity to SEQ ID NO:2 is at least about 70%, even more preferably at least about 80%, and even more preferably at least about 90%. In a most preferred embodiment, a homologue, analogue or derivative of SEQ ID NO:2 will comprise an amino acid sequence which is substantially identical or similar to SEQ ID NO:2, including at least about 95% identity or 98% identity or 99% identity thereto.

The invention is not to be limited in any way by the source of the isolated nucleic acid molecule or the source of the SPS enzyme or homologue, analogue or derivative encoded by said nucleic acid molecule.

Those skilled in the art will be aware that homologues, analogues or derivatives of a nucleotide sequence which encodes an SPS-like enzyme may be used as genetic probes in the isolation of related sequences falling within the scope of the presently- described invention, however which possess altered catalytic or kinetic characteristics from the subject matter exemplified herein. Preferred homologues, analogues or derivatives capable of such use comprise a sequence derived from SEQ ID NO:1 or "primer" molecules suitable for use in polymerase chain reaction assays or as hybridisation "probes".

A further aspect of the invention provides an isolated or synthetic nucleic acid molecule or amplification primer of at least about 10-15 nucleotides in length which is capable of hybridising under at least low stringency conditions to at least about 20 contiguous nucleotides derived from SEQ ID NO:1 or a complementary sequence thereto.

Preferably, the stringency of hybridization to SEQ ID NO:1 is at least moderate stringency, even more preferably at least high stringency.

For the purposes of defining the level of stringency, those skilled in the art will be aware that several different hybridisation conditions may be employed. For example, a low stringency may comprise a hybridisation and/or a wash carried out in 6xSSC buffer, 0.1% (w/v) SDS at 28°C or room temperature. A moderate stringency may comprise a hybridisation and/or wash carried out in 2xSSC buffer, 0.1% (w/v) SDS at a temperature in the range 45°C to 65°C. A high stringency may comprise a hybridisation and/or wash carried out in O.lxSSC buffer, 0.1% (w/v) SDS or Church Buffer at a temperature of at least 65°C. Variations of these conditions will be known to those skilled in the art.

Generally, the stringency is increased by reducing the concentration of SSC buffer, and/or increasing the concentration of SDS in the hybridisation buffer or wash buffer and/or increasing the temperature at which the hybridisation and/or wash are performed. Conditions for hybridisations and washes are well understood by one normally skilled in the art. For the purposes of clarification of parameters affecting hybridisation between nucleic acid molecules, reference can conveniently be made to pages 2.10.8 to 2.10.16. of Ausubel et al. (1987), which is herein incorporated by reference.

Alternatively, or in addition, the isolated nucleic acid molecule according to this aspect of the invention is capable of hybridizing to at least about 30 contiguous nucleotides, more preferably at least about 50 contiguous nucleotides, even more preferably at least about 100 contiguous nucleotides and still even more preferably at least about 500 contiguous nucleotides derived from SEQ ID NO:1 or a complementary sequence thereto.

A further aspect of the invention contemplates an isolated or synthetic nucleic acid molecule or amplification primer of at least about 10-15 nucleotides in length which is capable of hybridising under at least low stringency conditions to at least 20 continuous nucleotides derived from SEQ ID NO: 1 or a complementary sequence thereto.

Such nucleic acid molecules are particularly useful as reagents for isolating homologous nucleotide sequences and genes to those specifically exemplified herein or alternatively, as diagnostic reagents to distinguish between SPS-encoding and SPS- like encoding nucleotide sequences. Such information may be particularly useful in breeding programmes which aim to select for the presence of specific SPS-like variants and/or altered sucrose metabolism. The nucleic acid molecule of the invention according to any of the foregoing embodiments may be DNA, such as a gene, cDNA molecule, RNA molecule or a synthetic oligonucleotide molecule, whether single-stranded or double-stranded and irrespective of any secondary structure characteristics unless specifically stated.

The present invention clearly encompasses derivatives of the sucrose phosphate synthase-like (SPS-like) genes described herein which at least possess biological activity or alternatively, are at least useful as diagnostic reagents or molecular probes in the isolation of homologues, analogues or derivatives of the nucleotide sequences genes described herein.

The isolated nucleic acid molecules disclosed herein and described according to any one or more of the preceding embodiments may be used to isolate or identify related gene sequences, for example homologues, analogues or derivatives, from other cells, tissues, or organ types, or from the cells, tissues, or organs of another species using any one of a number of means known to those skilled in the art.

For example, genomic DNA, or mRNA, or cDNA may be contacted, under at least low stringency hybridisation conditions or equivalent, with a hybridisation-effective amount of an isolated nucleic acid molecule which comprises the nucleotide sequence set forth in SEQ ID NO:1 or a complementary sequence thereto, or a functional part thereof, and hybridisation detected using a detection means.

The detection means may be a reporter molecule capable of giving an identifiable signal (e.g. a radioisotope such as ³²P or³⁵S or a biotinylated molecule) covalently linked to the isolated nucleic acid molecule of the invention.

In an alternative method, the detection means is any known format of the polymerase chain reaction (PCR). According to this method, degenerate pools of nucleic acid "primer molecules" of about 15-50 nucleotides in length are designed based upon the nucleotide sequence disclosed in SEQ ID NO:1 or a complementary sequence thereto. In one approach the related sequences (i.e. the "template molecule") are hybridized to two of said primer molecules, such that a first primer hybridizes to a region on one strand of the double-stranded template molecule and a second primer hybridizes to the other strand of said template, wherein the first and second primers are not hybridized within the same or overlapping regions of the template molecule and wherein each primer is positioned in a 5'- to 3'- orientation relative to the position at which the other primer is hybridized on the opposite strand. Specific nucleic acid molecule copies of the template molecule are amplified enzymatically in a polymerase chain reaction, a technique that is well known to one skilled in the art. Several formats of the polymerase chain reaction are described by McPherson et a/ (1991).

The primer molecules may comprise any naturally-occurring nucleotide residue (i.e. adenine, cytidine, guanine, thymidine) and/or comprise inosine or functional analogues or derivatives thereof, capable of being incorporated into a polynucleotide molecule. The nucleic acid primer molecules may also be contained in an aqueous mixture of other nucleic acid primer molecules or be in a substantially pure form.

The detected sequence may be in a recombinant form, in a virus particle, bacteriophage particle, yeast cell, animal cell, or a plant cell. Preferably, the related genetic sequence originates from a plant species.

The present invention further extends to the subject isolated nucleic acid molecule when integrated into the genome of a cell as an addition to the endogenous cellular complement of sucrose biosynthesis genes. Alternatively, wherein the host cell does not normally encode enzymes required for sucrose biosynthesis, such as certain bacterial cells, insect cells, mammalian cells and yeast cells, amongst others, the present invention extends to the subject isolated nucleic acid molecule when integrated into the genome of said cell as an addition to the endogenous cellular genome.

The isolated nucleic acid molecule of the present invention is also useful for developing genetic constructs which comprise the subject nucleic acid molecules, preferably in a format designed for expression in cells into which they are subsequently introduced. Accordingly, a further aspect of the invention provides a genetic construct which comprises the isolated nucleic acid molecule according to any one or more of the embodiments described herein in operable connection with a promoter sequence.

Preferably, the genetic construct will comprise all or part of the nucleotide sequence set forth in SEQ ID NO:1 or a biologically-active homologue, analogue or derivative thereof or a fragment thereof which at least encodes a peptide of at least about 5 contiguous amino acids derived from SEQ ID NO:2. As stated supra, such fragments may be useful in determining substrate, activator and inhibitor binding regions of an SPS-like enzyme.

As used herein, the term "biologically active" means that the homologue, analogue or derivative of SEQ ID NO:1 is capable of encoding an SPS-like peptide, oligopeptide, polypeptide or enzyme, including one which possesses one or more of the catalytic activity or kinetic characteristics of an SPS-like enzyme.

The term "sense molecule" shall be taken to refer to an isolated nucleic acid molecule which encodes or is complementary to an isolated nucleic acid molecule of the invention, preferably one which is capable of encoding a functional SPS-like enzyme, wherein said nucleic acid molecule is provided in a format suitable for its expression to produce a recombinant polypeptide when introduced into a host cell by transfection or transformation procedures.

It is understood in the art that certain modifications, including nucleotide substitutions amongst others, may be made to a sense molecule of the present invention, without destroying the ability of said molecule to encode an SPS-like enzyme. It is therefore within the scope of the present invention to include any nucleotide sequence variants, homologues, analogues, or fragments of the said gene encoding same.

With particular regard to the genetic constructs of the invention comprising the sense molecule described herein, the expression of these sequences in a host cell will generally require the operable connection of said sense molecule with a promoter sequence. The choice of promoter for the present purpose may vary depending upon the level of expression required and/or the species from which the host cell is derived and/or the tissue-specificity or development-specificity of expression which is required.

Reference herein to a "promoter" is to be taken in its broadest context and includes the transcriptional regulatory sequences of a classical eukaryotic genomic gene, including the TATA box which is required for accurate transcription initiation, with or without a CCAAT box sequence and additional regulatory elements (i.e. upstream activating sequences, enhancers and silencers) which alter gene expression in response to developmental and/or external stimuli, or in a tissue-specific manner. In the context of the present invention, the term "promoter" also includes the transcriptional regulatory sequences of a classical prokaryotic gene, in which case it may include a -35 box sequence and/or a -10 box transcriptional regulatory sequences.

The term "promoter" is also used to describe a synthetic or fusion molecule, or derivative which confers, activates or enhances expression of a nucleic acid molecule to which it is operably connected in a cell. Preferred promoters may contain additional copies of one or more specific regulatory elements, to further enhance expression of the sense molecule and/or to alter the spatial expression and/or temporal expression of said sense molecule. For example, copper-responsive regulatory elements may be placed adjacent to a heterologous promoter sequence driving expression of a sense molecule to confer copper inducible expression thereon.

Placing a sense molecule under the regulatory control of a promoter sequence means positioning said molecule such that expression is controlled by the promoter sequence. A promoter is usually, but not necessarily, positioned upstream or 5' of a nucleic acid molecule which it regulates. Furthermore, the regulatory elements comprising a promoter are usually positioned within 2 kb of the start site of transcription of the sense molecule or a chimaeric gene comprising same. In the construction of heterologous promoter/sense molecule combinations, it is generally preferred to position the promoter at a distance from the gene transcription start site that is approximately the same as the distance between that promoter and the gene it controls in its natural setting, i.e., the gene from which the promoter is derived. As is known in the art, some variation in this distance can be accommodated without loss of promoter function. Similarly, the preferred positioning of a regulatory sequence element with respect to a heterologous gene to be placed under its control is defined by the positioning of the element in its natural setting, i.e., the genes from which it is derived. Again, as is known in the art, some variation in this distance can also occur.

Examples of promoters suitable for use in genetic constructs of the present invention include promoters derived from the genes of viruses, yeasts, moulds, bacteria, insects, birds, mammals and plants which are capable of functioning in isolated cells or whole organisms regenerated therefrom. The promoter may regulate the expression of the sense molecule constitutively, or differentially with respect to the tissue in which expression occurs or, with respect to the developmental stage at which expression occurs, or in response to external stimuli such as physiological stresses, pathogens, or metal ions, amongst others.

Preferred promoters according to this embodiment are those promoters which are capable of functioning in bacterial cells, yeasts, fungal cells and/or plant cells, tissues or organs.

In a more preferred embodiment, the promoter may be derived from a sucrose biosynthesis genomic gene, particularly a higher plant gene which encodes a sucrose phosphate synthase enzyme or a homologue, analogue or derivative thereof, such as one which is not subject to diurnal regulation or environmental down-regulation.

Particularly suitable promoters in the genetic constructs described herein are tissue- specific promoters which are operable in any tissue, organ or cell-type in which sucrose biosynthesis genes, in particular SPS genes, are expressed. Preferably, such promoters are at least operable in leaf tissues. According to this embodiment, the present invention clearly contemplates the modification of sucrose metabolism in higher plants by expressing the nucleic acid molecule of the invention in the sense orientation under the control of constitutive or tissue-specific or environmentally- regulated promoter sequences.

Examples of suitable promoters include the SCSV gene promoters, CaMV 35S promoter, ubiquitin (Ubi1) gene promoter, CaMV 19S promoter, NOS promoter, octopine synthase (OCS) promoter, Arabidopsis thaliana SSU gene promoter, napin seed-specific promoter, any plant Adh gene promoter, SPS gene promoter, sucrose synthase promoter, P₃₂ promoter, BK5-T imm promoter, Trc promoter, lac promoter, tac promoter, phage lambda λ_L or λ _R promoters, T7 promoter or lacUVδ promoter and the like. In addition to the specific promoters identified herein, plant-derived cellular promoters for so-called housekeeping genes are useful by virtue of their possible constitutive expression in plant cells.

The genetic construct of the invention may further comprise a terminator sequence and be introduced into a suitable host cell where it is capable of being expressed to produce a recombinant polypeptide gene product.

The term "terminator" refers to a DNA sequence at the end of a transcriptional unit which signals termination of transcription. Terminators are 3'-non-translated DNA sequences containing a polyadenylation signal, which facilitates the addition of polyadenylate sequences to the 3'-end of a primary transcript.

Terminators active in cells derived from viruses, yeasts, moulds, bacteria, insects, birds, mammals and plants are known and described in the literature. They may be isolated from bacteria, fungi, viruses, animals and/or plants.

Examples of terminators particularly suitable for use in the genetic constructs of the present invention are those which function in plant cells, including the nopaline synthase (NOS) gene terminator of Agrobacterium tumefaciens, the terminator of the Cauliflower mosaic virus (CaMV) 35S gene (i.e. t35S), the zein gene terminator from Zea mays, the Rubisco small subunit (SSU) gene terminator sequences, subclover stunt virus (SCSV) gene sequence terminators, the tm1 terminator, or any rho- independent E. coli terminator, amongst others. Those skilled in the art will be aware of additional promoter sequences and terminator sequences which may be suitable for use in performing the invention. Such sequences may readily be used without any undue experimentation.

The genetic constructs of the invention may further include an origin of replication sequence which is required for replication in a specific cell type, for example a bacterial cell, when said genetic construct is required to be maintained as an episomal genetic element (eg. plasmid or cosmid molecule in said cell.

Preferred origins of replication include, but are not limited to, the f -ori and co/E1 origins of replication.

The genetic construct may further comprise a selectable marker gene or genes that are functional in a cell into which said genetic construct is introduced.

As used herein, the term "selectable marker gene" includes any gene which confers a phenotype on a cell in which it is expressed to facilitate the identification and/or selection of cells which are transfected or transformed with a genetic construct of the invention or a derivative thereof.

Suitable selectable marker genes contemplated herein include the ampicillin resistance (Amp^r), tetracycline resistance gene (Tc^r), bacterial kanamycin resistance gene (Kan"), phosphinothricin resistance gene, neomycin phosphotransferase gene (npt\\), hygromycin resistance gene, β-glucuronidase (GUS) gene, chloramphenicol acetyltransferase (CAT) gene and luciferase gene, amongst others.

The present invention clearly extends to transfected or transformed cells, tissues, organs or whole organisms which contain and/or express a sense molecule which comprises or is derived from the isolated nucleic acid molecule described herein, in particular the Synechocystis sp. SPS-like gene, or a homologue, analogue or derivative thereof. Preferably, the isolated nucleic acid molecule is contained within a genetic construct as described herein. In a particularly preferred embodiment, the present invention provides transformed bacterial and plant cells capable of expressing SynSPS protein and enzyme activity. As exemplified herein, the inventors have transformed bacterial cells with the genetic construct designated pTrcHisA-SynSPS-Term (Figure 2) and expressed SynSPS activity therefrom. The inventors have also transformed plant cells with the genetic constructs designated p35S-SynSPS-t35S (Figure 8; Figure 10) and pUbM-SynSPS- tm1 (Figure 10) and expressed SynSPS activity therefrom.

Even more particularly, the present invention provides transformed Escherichia coli, Oryza sativa and Nicotiana tabacum cells capable of expressing SynSPS protein and enzyme activity. As shown in Figures 3 to 7, active SynSPS may be produced in E. coli cells. As shown in Figures 9 and 11 and Table 7, active SynSPS may be produced in tobacco and rice cells and whole plants transformed with the SynSPS-encoding gene. However, it is to be understood that the present invention is broadly applicable to species other than those exemplified herein and the invention is not to be limited to expression of SynSPS in E. coli, tobacco and rice.

To achieve such expression, a sense molecule which comprises the inventive nucleotide sequences or a genetic construct comprising same, may be introduced into a cell using any known method for the transfection or transformation of said cell. Wherein a cell is transformed by the genetic construct of the invention, a whole organism may be regenerated from a single transformed cell, using any method known to those skilled in the art.

By "transfect" is meant that the sense molecule or genetic construct comprising same is introduced into said cell without integration into the cell's genome.

By "transform" is meant that the sense molecule or genetic construct comprising same or a fragment said genetic construct comprising the SPS-like gene sequence is stably integrated into the genome of the cell.

Means for introducing recombinant DNA into bacterial cells or plant tissue or cells include, but are not limited to, transformation using CaCI₂ and variations thereof, in particular the method described by Hanahan (1983), direct DNA uptake into protoplasts (Krens et al, 1982; Paszkowski et al, 1984), PEG-mediated uptake into protoplasts (Armstrong et al, 1990) microparticle bombardment, electroporation (Fromm et al., 1985), microinjection of DNA (Crossway et al., 1986), microparticle bombardment of tissue explants or cells (Christou era/, 1988; Sanford, 1988), vacuum- infiltration of tissue with nucleic acid, or in the case of plants, T-DNA-mediated transfer from Agrobacterium to the plant tissue as described essentially by An et a/.(1985), Herrera-Estrella et al. (1983a, 1983b, 1985).

For microparticle bombardment of cells, a microparticle is propelled into a cell to produce a transformed cell. Any suitable ballistic cell transformation methodology and apparatus can be used in performing the present invention. Exemplary apparatus and procedures are disclosed by Stomp et al. (U.S. Patent No. 5,122,466) and Sanford and Wolf (U.S. Patent No. 4,945,050). When using ballistic transformation procedures, the genetic construct may incorporate a plasmid capable of replicating in the cell to be transformed.

Examples of microparticles suitable for use in such systems include 1 to 5 μm gold spheres. The DNA construct may be deposited on the microparticle by any suitable technique, such as by precipitation.

In the case of plant cells or tissues, where relevant technology is available, a whole organism may be regenerated from the transformed cell, in accordance with procedures well known in the art.

Plant tissue capable of subsequent clonal propagation, whether by organogenesis or embryogenesis, may be transformed with a genetic construct of the present invention and a whole plant regenerated therefrom. The particular tissue chosen will vary depending on the clonal propagation systems available for, and best suited to, the particular species being transformed. Exemplary tissue targets include leaf disks, pollen, embryos, cotyledons, hypocotyls, megagametophytes, callus tissue, existing meristematic tissue (e.g., apical meristem, axillary buds, and root meristems), and induced meristem tissue (e.g., cotyledon meristem and hypocotyl meristem).

The term "organogenesis", as used herein, means a process by which shoots and roots are developed sequentially from meristematic centres.

The term "embryogenesis", as used herein, means a process by which shoots and roots develop together in a concerted fashion (not sequentially), whether from somatic cells or gametes.

The regenerated transformed plants may be propagated by a variety of means, such as by clonal propagation or classical breeding techniques. For example, a first generation (or T1) transformed plant may be selfed to give homozygous second generation (or T2) transformant, and the T2 plants further propagated through classical breeding techniques.

The regenerated transformed organisms contemplated herein may take a variety of forms. For example, they may be chimeras of transformed cells and non-transformed cells; clonal transformants (e.g., all cells transformed contain the expression cassette); grafts of transformed and untransformed tissues (e.g., in plants, a transformed root stock grafted to an untransformed scion ).

Thus, the sucrose biosynthesis genes described herein may be used to develop single cells or whole organisms which synthesize sucrose from substrates which are not normally utilised by higher plant SPS enzymes. Accordingly, the present invention provides the means for mobilising carbon into sucrose from a wider range of biochemical pathways than would otherwise be possible in plants.

Accordingly, a further aspect of the present invention provides a method of altering the level of sucrose or sucrose phosphates in a plant cell, tissue or organ or in a higher plant, said method comprising expressing an isolated nucleic acid molecule which encodes an SPS-like enzyme in said cell, tissue, organ or higher plant for a time and under conditions sufficient for sucrose-6'-phosphate synthesis to occur.

In a preferred embodiment, the subject method comprises the additional first step of transforming the cell, tissue, organ or organism with a sense molecule which comprises the subject nucleic acid molecule. As discussed supra the isolated nucleic acid molecule may be contained within a genetic construct.

Those skilled in the art will be aware that, in higher plants, the partitioning of carbon between sucrose and starch and other carbohydrates is such that the mobilisation of carbon into sucrose may occur at the expense of starch accumulation. For example,

Dorais et al (1996) have suggested that low fructose-1 ,6-bisphosphatase activity and

SPS activity in tomato plants grown under extended photoperiod results in the accumulation of phosphorylated compounds which reduce inorganic orthophosphate levels in chloroplast stroma, thereby leading to significant increases in starch accumulation. Similarly, sucrose biosynthesis is normally greater during periods of reduced starch biosynthesis and transgenic plants expressing recombinant SPS enzymes exhibit higher levels of sucrose compared to starch. Accordingly, it will be apparent to the skilled artisan that transgenic plants expressing recombinant SPS-like enzymes will also exhibit altered carbon partitioning wherein sucrose levels are increased at the expense of starch.

In fact, this altered carbon partitioning may be more pronounced by virtue of the fact that SPS-like enzymes utilise ADP-glucose as a glucosyl donor, which is also the preferred glucosyl donor to starch. Accordingly, plants expressing the Synechocystis sp. SPS-like enzyme exemplified herein will, in a preferred embodiment preferentially divert ADP in the form of ADP-glucose into sucrose formation rather than starch biosynthesis. Such plants also have the potential to divert nucleoside 5'-diphosphates from nucleotide biosynthesis into increased sucrose biosynthesis. In light of the roles that carbon partitioning and nucleotide metabolism play in plant development, such effects will significantly alter plant cell growth and development. The isolated nucleic acid molecule of the invention has particular utility in altering one or more growth and developmental responses of a higher plant to produce novel phenotypes therein, for example by extending the period of sucrose biosynthesis throughout the light cycle, or increasing sucrose biosynthesis under low CO₂ concentrations or in low irradiance light or extended photoperiod in photoperiod- sensitive plants, or improving cold acclimation responses and/or dehydration responses and/or heat tolerance of plants or improving sucrose biosynthesis in unfavourable growth conditions, such as in limiting nitrogen availability, amongst others. All such effects may produce increases in yield and productivity of field and crop plants.

Accordingly, a still further aspect of the present invention contemplates a method of altering one or more developmental and/or growth responses of a plant, said method comprising expressing a sense molecule comprising an isolated nucleic acid molecule which encodes an SPS-like polypeptide or enzyme in said plant for a time and under conditions sufficient for the level of biosynthesis of sucrose or sucrose-6 '-phosphate to be increased therein.

In a preferred embodiment, the subject method comprises the additional first step of transforming the cell, tissue, organ or organism with the sense molecule.

As discussed supra the isolated nucleic acid molecule may be contained within a genetic construct.

It will be apparent to those skilled in the art that the isolated nucleic acid molecule of the invention and genetic constructs comprising same may also be useful in the production of recombinant polypeptides which possess sucrose biosynthesis activity. Such polypeptides may be useful, for example, in carrying out substrate conversions to produce intermediates of sucrose metabolism in isolated cells, cell cultures, whole organisms or in vitro.

Accordingly, a still further aspect of the invention provides a method of producing a recombinant enzymatically active SPS-like polypeptide in a cell, said method comprising the steps of:

(i) producing a genetic construct which comprises an isolated nucleic acid molecule as described herein which encodes said SPS-like polypeptide placed operably under the control of a promoter capable of conferring expression in said cell, and optionally an expression enhancer element;

(ii) transforming said genetic construct into said cell; and

(iii) selecting transformants which express a functional SPS-like polypeptide encoded by the genetic sequence at a high level.

(i) producing a genetic construct which comprises an isolated nucleic acid molecule which encodes said SPS-like polypeptide placed operably under the control of a plant-expressible promoter and optionally an expression enhancer element, wherein said nucleic acid molecule is also placed upstream of a transcription terminator sequence;

(ii) transforming said genetic construct into a cell or tissue of said plant; and (iii) selecting transformants which express a functional SPS-like polypeptide encoded by the genetic sequence at a high level.

A further aspect of the invention provides a recombinant SPS-like polypeptide or functional enzyme molecule.

The recombinant SPS-like enzymes and polypeptides described herein, in particular recombinant or isolated Synechocystis sp. oligopeptides, polypeptides and enzymes, or a homologue, analogue or derivative thereof, may also be immunologicaHy active molecules.

Accordingly, a further aspect of the present invention provides an immunologically- interactive molecule which is capable of binding to an isolated or recombinant sucrose biosynthesis polypeptide of the invention and in particular to an isolated or recombinant SPS-like peptide, oligopeptide, polypeptide or functional SPS-like enzyme molecule or a homologue, analogue or derivative thereof. Even more preferably, the invention provides an immunologically-interactive molecule which is capable of binding to the Synechocystis sp. SPS-like polypeptide exemplified herein or a part thereof.

In one embodiment, the immunologically interactive molecule is an antibody molecule. The antibody molecule may be monoclonal or polyclonal. Monoclonal or polyclonal antibodies may be selected from naturally occurring antibodies to an epitope, or peptide fragment, or synthetic peptide derived from a recombinant gene product or may be specifically raised against a recombinant product or a homologue, analogue or derivative thereof.

Both polyclonal and monoclonal antibodies are obtainable by immunisation with an appropriate gene product, or epitope, or peptide fragment of a gene product. Alternatively, fragments of antibodies may be used, such as Fab fragments. The present invention extends to recombinant and synthetic antibodies and to antibody hybrids. A "synthetic antibody" is considered herein to include fragments and hybrids of antibodies

The antibodies contemplated herein may be used for identifying genetic sequences which express related SPS-like polypeptides encompassed by the embodiments described herein. The only requirement for successful detection of a related SPS-like genetic sequence is that said genetic sequence is expressed to produce at least one epitope recognised by the antibody molecule. Preferably, for the purpose of obtaining expression to facilitate detection, the related genetic sequence is placed operably behind a promoter sequence, for example the bacterial lac promoter. According to this preferred embodiment, the antibodies are employed to detect the presence of a plasmid or bacteriophage which expresses the related polypeptide. Accordingly, the antibody molecules are also useful in purifying the plasmid or bacteriophage which expresses the related polypeptide. The subject antibody molecules may also be employed to purify the recombinant SPS- like polypeptide of the invention or a naturally-occurring equivalent or a homologue, analogue or derivative of same.

The present invention is further described by reference to the following non-limiting Examples.

EXAMPLE 1 Cloning of SynSPS gene and sequencing The Synechocystis sps gene was amplified by PCR from genomic DNA of Synechocystis sp. PCC 6803 using the following oligonucleotide primers:

5^' primer 5^'-CGCGCAGATCTATGAGCTATTCATCAAAATACATT-3^'

3^' primer 5^'-CGCGCGTCGACGAATTCGGCTGGTTAAACGGGGTCTAA-3^'

Primers were designed from the published Synechocystis sp. PCC 6803 genome sequence (Genbank Accession No. D64006). The amplified product was purified on a Wizard column (Promega, Madison, Wl, USA), digested with BglW and EcoRI and ligated into the Ba HI and EcoRI sites of pBluescript II SK. Both strands of the insert were sequenced by the chain termination method with thermal cycling using rhodamine dye chemistry (Applied Biosystems, Foster City, CA, USA).

EXAMPLE 2 Sequence alignment protocols Sequences were aligned with the GAP program, using the algorithm of Needleman and Wunsch (1970), or the PILEUP programme of the Wisconsin Sequence Analysis Package (Genetics Computer Group, Inc., Madison, Wl, USA).

Data presented in Figure 1 show the alignment of the spinach SPS polypeptide and Synechocystis sp. SPS-like polypeptide, using the PILEUP programme. As shown therein, there are significant differences between the spinach and Synechocystis sp. polypeptides, particularly with respect to the absence of the spinach Ser158 and Ser424 residues from the Synechocystis sp. protein.

EXAMPLE 3 Expression of SynSPS in E. Coli and purification of the SynSPS enzyme

The Synechocystis sps gene amplified by PCR as described above was purified on a Wizard column (Promega, Madison, Wl, USA), digested with BglW and EcoRI and ligated into the BamHI and EcoRI sites of the bacterial expression vector pTrcHisA (Invitrogen Corporation, San Diego, CA, USA). The pTrcHisA/SynSPS gene construct (Figure 2) was transformed into E. coli JM109 by electroporation.

A stationary phase culture of E Coli JM109 (pTrcHisA/SynSPS) grown in LB medium at 37°C was diluted 250-fold into 2 I of Terrific Broth containing 200 μg ml^"1 ampicillin divided equally between four 2-I flasks and incubated with shaking (180 rpm) at 37°C until cells reached an A₆₀₀ of 0.5. Hi^ -SynSPS protein expression was induced by addition of IPTG to a final concentration of 1 mM. After incubation for a further 20 h at 37°C, the cells were harvested by centrifugation at 2000 x g for 20 min (4°C).

For purification under native conditions the cells were resuspended in 500 mL of ice- cold lysis buffer (20 mM Tris-HCI, 100 mM NaCI, pH 8.0) containing 1 mM PMSF and lysed by sonication. The crude lysate was centrifuged at 10, 000 g for 10 min and the supernatant was filtered through one layer of Miracloth (Calbiochem-Novabiochem Pty. Ltd, Alexandria, NSW, Australia). The filtered extract was mixed with 1 ml of Talon immobilised Co²⁺ resin (Clontech, Palo Alto, CA, USA) by stirring for 1 h on ice. The resin was collected in a 10-ml column and washed successively with 10 ml of lysis buffer and 50 ml of lysis buffer containing 20 mM imidazole. The His₆-SPS fusion protein was eluted from the resin with lysis buffer containing 100 mM imidazole. All procedures were carried out at 4°C.

As shown in Figure 3, recombinant SynSPS protein can be purified using this procedure. EXAMPLE 4 Assay for SPS-like activity in Escherichia coli

SPS-like activity in extracts of Escherichia coli JM109 (pTrcHisA/SynSPS) and of purified SynSPS was measured as the Fru-6-P-dependent release of NDP from NDPGIc as described by Lunn and ap Rees (1990).

As shown in Figures 4A and 4B, recombinant SynSPS produced in E. coli cells converts UDP-glucose and fructose-6-phosphate to sucrose-6'-phosphate and UDP.

As shown in Figures 5 and 6, the recombinant SPS-like activity expressed in E.coli exhibits a broader substrate specificity than the higher plant SPS enzymes, in particular utilizing ADP-glucose or CDP-glucose or GDP-glucose or UDP-glucose as a glucosyl donor to fructose-6-phosphate.

From the data presented in Figure 6, the kinetic parameters of the recombinant Synechocystis SPS-like enzyme were determined and are presented in Table 6.

TABLE 6

Kinetic parameters of the Synechocystis SPS-like enzyme produced by recombinant means in E.coli cells

The ability of inorganic orthophosphate to inhibit the recombinantly-produced SPS-like enzyme was also investigated and it was shown that, at concentrations of orthophosphate up to 20mM, there is no significant inhibition of enzyme activity in the presence of UDP-glucose (Figure 7).

EXAMPLE 5 Construction of clones to express SynSPS in tobacco and rice The SynSPS gene was excised from pBluescript II SK/SynSPS by digestion with Not\ and EcoRI, and end-filled by incubation with the large fragment of DNA polymerase I (Klenow) in the presence of deoxyribonucleoside triphosphates. The SynSPS gene was isolated from the vector by gel electrophoresis in low melting point agarose and purified using a Bresa-Clean nucleic acid purification kit (Bresatec, Adelaide, SA, Australia).

To produce p35S-SynSPS-t35S (Figure 8; Figure 10), the SynSPS gene was ligated into the Smal site of pDH51 between the CaMV 35S promoter and CaMV 35S terminator. The promoter-gene-terminator construct was excised with EcoRI and isolated from the vector as described above and ligated in both orientations into the EcoRI site of the binary vector pBS389. The pBS389 vector containing the 35S- SynSPS gene construct was transferred from E. Coli DH5α into Agrobacterium tumefaciens AGLI by triparental mating with an E. Coli strain carrying the pRK2013 mobilisation plasmid.

To produce the construct pUbi1-SynSPS-tm1 (Figure 10) for use in rice, the SynSPS gene was ligated between the ubiquitin (Ubi1) gene promoter and tm1 gene terminator.

EXAMPLE 6

Transformation of tobacco

Tobacco (Nicotiana tabacum W38) leaf discs were cocultivated with Agrobacterium tumefaciens AGLI containing pBS389/35S-SynSPS-t35S in the dark at 26°C on MS9 medium. After 48 h the leaf disks were transferred to regeneration medium (MS9) containing 100 mg I^"1 kanamycin and 150 mg I^"1 timentin. After 10 d in the light at 26°C, the disks were transferred to fresh regeneration medium. After 7 d shoots were excised from the leaf disks and transferred onto rooting medium (MS0) containing 100 mg I^"1 kanamycin and 150 mg I^"1 timentin. The plants were grown in the light at 26 °C until roots were formed, then transferred into potting compost (25 cm diameter pots). Plants were grown in a naturally illuminated glasshouse maintained at 25°C during the day and 20°C at night.

Tobacco (N. plumbaginifolia) protoplasts were also transfected with the genetic construct p35S-SynSPS-t35S using standard procedures.

EXAMPLE 7

Expression of SynSPS in transgenic tobacco cells and whole tobacco plants

Leaf tissue (0.5 g) from wild type and transgenic tobacco plants was ground in an ice- cold mortar with 1 ml of extraction medium (50 mM Hepes-K\ pH 7.5, 1 mM EDTA, 5 mM DTT, 2% (w/v) polyvinylpolypyrrolidone, 1 mM PMSF, 1 mM benzamide, 1 mM benzamidine, 5 mM e-amino caproic acid, 2 μM leupeptin, 10 μM antipain).

Extracts were also produced from transiently transfected Nicotiana plumbaginifolia protoplasts.

The leaf and protoplast extracts were clarified by centrifugation at 13,000 g for 1 min. Proteins (20 μg) in the leaf extracts were resolved by SDS-PAGE on 9 % (w/v) polyacrylamide gels as described by Laemmli (1970) and transferred to a nitrocellulose membrane (Schleicher and Schϋell, Germany) using an electroblotting apparatus (BioRad, Hercules, CA, USA). The membrane was blocked by washing in three changes of blocking buffer (25 mM Tris-HCI, 150 mM NaCI, pH 7.5, containing 0.2 % dried milk powder and 0.2 % Tween) for 3 x 30 min and then incubated with anti- SynSPS antiserum (1 :10,000 dilution in blocking buffer) for 16 h. The membrane was washed with three changes of blocking buffer and then incubated with alkaline phosphatase-conjugated goat anti-rabbit IgG antibody (Promega, Madison, Wl, USA) for 2 h. The membrane was washed with three changes of blocking buffer for 3 x 10 min, then with 100 mM Tris-HCI, 4 mM MgCI₂, pH 9.8 for 5 min before developing with 130 μM nitroblue tetrazolium, 160 μM bromochloroindophenol in 100 mM Tris-HCI, 4 mM MgCI₂, pH 9.8. The reaction was stopped by washing with three changes of water.

Data present in Figure 9 (Lane 3) and Figure 11 (Lanes F-G) indicate that SynSPS protein accumulated in the leaves of stably-transformed tobacco plants. A small amount of protein was also detectable in the transiently-transfected N. plumbaginifolia protoplasts using this procedure (Figure 11 , Lane A).

EXAMPLE 8 Assay of SPS-like activity in tobacco Leaf tissue (0.5 g) from wild type and transgenic tobacco plants was ground in an ice- cold mortar with 1 ml of extraction medium (50 mM Tricine-Na⁺, pH 8.0, 1 mM EDTA, 10 mM MgCI₂, 5 mM DTT, 2% (w/v) PVPP, 1% (w/v) BSA, 0.1% (v/v) Triton X-100, 1 mM PMSF, 1 mM benzamide, 1 mM benzamidine, 5 mM e-amino caproic acid, 2 μM leupeptin, 10 μM antipain). The leaf extracts were clarified by centrifugation at 13,000 g for 1 min, and desalted by passage through a 3-ml column of Sephadex G-25M equilibrated with extraction buffer minus PVPP, BSA and Triton X-100.

SPS was assayed in tobacco leaf extracts by measuring the UDPGIc or GDPGIc- dependent synthesis of [¹⁴C]sucrose 6'-phosphate from [⁴ Cjfructose 6-phosphate. The reaction mixture (100 μ\) contained 50 mM Tricine-KOH, pH 8.0, 10 mM MgCI₂, 10 mM UDPGIc or GDPGIc, 2 mM [¹⁴C]fructose 6-phosphate and 7 mM [¹⁴C]glucose 6-phosphate and 50 μ\ of extract. The reaction was started by addition of extract. After incubation at 25°C for 30 min, the reaction was stopped by boiling for 2 min. To hydrolyse any remaining sucrose 6'-phosphate, 10 units of calf intestinal alkaline phosphatase was added to each reaction mixture. After incubation at 37°C for 16 h, 20-μl samples were transferred to a sheet of Whatman 3 MM filter paper and subjected to descending chromatography for 24 h with ethyl acetate: pyridine: water (8:2:1, by vol) as the developing solvent. The location of the sucrose was determined from marker strips run on the same sheet of chromatography paper and that section of the paper was excised, eluted with 2 ml of water and the ¹⁴C content determined by a standard scintillation counting procedure. EXAMPLE 9 Transformation of rice plants

Mature seeds of rice (Oryza sativa) cv. Taipei 309 were used for callus induction on NB solid medium and maintained in the dark at 27°C. Agrobacterium tumefaciens strain AGL1 carrying the binary vector pwbvecδ/SynSPS was grown for 2 days at 28°C on solid LB medium, supplemented with 25 mg 1^"1 rifampicin, 100 mg 1^"1 carbenicillin and 50 mg 1^"1 streptomycin. Freshly subcultured (5 days) embryogenic calli (2-4 mm in size) were soaked in a thick suspension of the Agrobacterium tumefaciens in NB liquid medium containing 100 μm acetosyringone (NB-AS) for 10 min. Calli were then placed on solid NB-AS medium and co-cultivated for 2 days in the dark at 25°C. Calli were then washed in sterile water supplemented with 150 mg 1^"1 Timentin (SmithKline Beecham), blotted dry on filter paper and placed on NB medium containing 150 mg 1^"1 Timentin and 30 mg 1^"1 hygromycin (NBTH30) and cultured for 3-4 weeks in the dark at 27°C. Hygromycin-resistant callus clusters were transferred onto NBTH50 (containing 50 mg 1^~1 hygromycin) and cultured for 2-3 weeks in the dark at 27°C. Resistant calli were then transferred to pre-regeneration medium (PRNTH50) in which 2,4-D in NB medium was replaced with benzylaminopurine (BAP, 2 mg 1^"1), 1- naphthalene acetic acid (NAA, 1 mg 1^"1) and abscisic acid (ABA, 5 mg 1^"1). Calli were kept in the dark for 5-10 days and later in diffused light for 5 days. The calli were then transferred to regeneration medium RNTH50 (containing 3 mg 1^"1 BAP and 0.5 mg 1 ^' and kept in the light, until green shoots formed. The shoots were transferred to half- strength MS medium and grown at 22-26°C with 16 h light (130 μE πr² s^"1). Well rooted plantlets were replanted in jiffy pots containing a mixture of soil, perlite, sand and peat moss (50:25:15:10 by volume) and kept in a mist chamber in a naturally illuminated glasshouse with 28°C day and 21 °C night temperatures for one week before transferring to 15 cm plastic pots containing the same potting mix and submerged in water.

Data presented in Table 7 indicate that low but detectable SynSPS activity is detectable in leaf extracts derived from transformed tobacco plants. Higher enzyme activity was observed in transgenic lines than for the untransformed control plants. EXAMPLE 10 Expression of SynSPS in transgenic rice plants

Leaf extracts of transformed and non-transformed rice plants were subjected to western blot analysis using anti-SynSPS serum at a 1/10,000 dilution according to standard procedures. Data present in Figure 11 (Lane E) indicate that SynSPS protein accumulated in the leaves of stably-transformed rice plants.

EXAMPLE 11 Assay of SPS-like activity in rice Rice leaves were frozen in liquid nitrogen and extracted by grinding in a mortar with 1-2 volumes of ice-cold extraction buffer [50 mM Tricine-KOH, 10 mM MgCI₂, 1 mM EDTA, 5 mM DTT, 1 mM PMSF, 1 mM benzamide, 1 mM benzamidine, 5 mM ε-aminocaproic acid, 2 μM leupeptin, 10 μM antipain, 0.5% (w/v) BSA, 0.1% (v/v) Triton X-100, 2% (w/v) Polyclar pH 8.0], containing about 0.1 g of quartz. The crude extract was centrifuged at 11 ,600xg for 1 min. A 500 μl aliquot of the supernatant was de-salted by passage through a column (bed volume 3 ml) of Sephadex G-25 (Pharmacia, Uppsala, Sweden), equilibrated with extraction buffer minus BSA, Triton X-100 and Polyclar. All procedures were carried out at 4°C.

SynSPS was assayed in rice leaf extracts by measuring the fructose 6-phosphate- dependent production of UDP or GDP from UDPGIc or GDPGIc, respectively, in a reaction mixture (100μl) containing 50 mM Tricine-KOH, pH 8.0, 10 mM MgCI₂, 10 mM UDPGIc or GDPGIc, 5 mM Fru-6-P, 17.5 mM Glc6P and 50 μl of extract. The reaction was started by addition of extract. After incubation at 25°C for 10 min, the reaction was stopped by boiling for 2 min. The reaction mixture was centrifuged at 11 600 g for 1 min. An aliquot (50 μl) of the supernatant was assayed for UDP or GDP by coupling to oxidation of NADH with pyruvate kinase (EC 2.7.1.40) and lactate dehydrogenase (EC 1.1.1.27). The reaction mixture (1 ml) contained 50 mM Hepes-KOH, pH 7.5, 200 mM KCI, 40 mM MgCI₂, 0.25 mM NADH, 2 mM phosphoeno/pyruvate, 2 units lactate dehydrogenase and 10 units pyruvate kinase.

Data shown in Table 7 indicate higher SynSPS activity in transformed rice leaf extracts than for the non-transformed controls, indicating that SynSPS can be expressed in higher plants to utilise UDP-glucose or GDP-glucose as a glucosyl donor.

TABLE 7 SynSPS activity in transformed tobacco and rice plants

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Claims

WE CLAIM:

1. An isolated nucleic acid molecule which encodes or is complementary to an isolated nucleic acid molecule which encodes a sucrose biosynthesis polypeptide possessing SPS-like activity wherein said polypeptide does not utilise UDP-glucose as a sole glucosyl donor substrate.

2. The isolated nucleic acid molecule according to claim 1 , wherein the sucrose biosynthesis polypeptide utilises UDP-glucose or ADP-glucose as a glucosyl donor substrate.

3. The isolated nucleic acid molecule according to claim 1 , wherein the sucrose biosynthesis polypeptide utilises UDP-glucose or GDP-glucose as a glucosyl donor substrate.

4. The isolated nucleic acid molecule according to claim 1 , wherein the sucrose biosynthesis polypeptide utilises UDP-glucose or CDP-glucose as a glucosyl donor substrate.

5. The isolated nucleic acid molecule according to any one of claims 1 to 4, wherein the sucrose biosynthesis polypeptide is not allosterically regulated by glucose- 6-phosphate and/or inorganic phosphate.

6. The isolated nucleic acid molecule according to any one of claims 1 to 5, wherein the sucrose biosynthesis polypeptide has a molecular weight in the range of

80-90 kDa as determined by SDS/PAGE or gel filtration and a pi value in the range 8.0 to 8.5.

7. The isolated nucleic acid molecule according to any one of claims 1 to 6, wherein the sucrose biosynthesis polypeptide comprises an amino acid sequence that is at least 50% identical to SEQ ID NO:2.

8. The isolated nucleic acid molecule according to any one of claims 1 to 7 derived from Synechocystis sp. or Anabaena sp.

9. An isolated nucleic acid molecule which encodes or is complementary to an 5 isolated nucleic acid molecule which encodes a sucrose biosynthesis polypeptide possessing SPS-like activity wherein said polypeptide comprises an amino acid sequence that is at least 50% identical to SEQ ID NO:2 and wherein said SPS-like activity does not utilise UDP-glucose as a sole glucosyl donor substrate.

10 10. The isolated nucleic acid molecule according to claim 9, wherein the percentage identity to SEQ ID NO:2 is 100%.

11. An isolated nucleic acid molecule which encodes a polypeptide which possesses SPS-like activity wherein said polypeptide does not contain a serine residue 15 in the context of the amino acid sequence motif B-Hy-X-B-X-X-S and wherein X is any amino acid residue, B is arginine or lysine and Hy is any hydrophobic amino acid residue, and wherein said SPS-like activity does not utilise UDP-glucose as a sole glucosyl donor substrate.

20 12. The isolated nucleic acid molecule according to claim 11, wherein said polypeptide further comprises an amino acid sequence that is at least 50% identical to SEQ ID NO:2.

13. An isolated nucleic acid molecule which encodes or is complementary to an 25 isolated nucleic acid molecule which encodes an SPS-like enzyme that does not utilise UDP-glucose as a sole glucosyl donor substrate, wherein said enzyme comprises an amino acid sequence which includes 5-10 contiguous amino acid residues derived from residues 465 to 720 of SEQ ID NO:2.

30 14. An isolated nucleic acid molecule which encodes or is complementary to an isolated nucleic acid molecule which encodes an SPS-like enzyme that does not utilise UDP-glucose as a sole glucosyl donor substrate, wherein said nucleic acid molecule comprises a sequence of nucleotides which is at least about 50% identical to SEQ ID NO:1 or a complementary sequence thereto.

15. The isolated nucleic acid molecule of claim 14, comprising a nucleotide 5 sequence that encodes the amino acid sequence set forth in SEQ ID NO:2.

16. An isolated or synthetic nucleic acid molecule probe or amplification primer of at least 10-15 nucleotides in length which is capable of hybridising under at least low stringency conditions to at least 20 contiguous nucleotides of SEQ ID NO:1 or a

10 complementary sequence thereto.

17. Use of the probe or amplification primer of claim 16 in the method of isolating or detecting a nucleic acid molecule which encodes an SPS-like activity that does not utilise UDP-glucose as a sole glucosyl donor substrate, said method comprising

15 contacting said nucleic acid molecule or amplification primer with DNA or RNA for a time and under conditions sufficient for hybridisation to occur and then detecting said hybridisation.

18. Use according to claim 17 wherein the hybridisation is detected by a polymerase 20 chain reaction.

19. Use according to claim 18 wherein the hybridisation is detected by means of a labelled reporter molecule bound to the probe.

25 20. A genetic construct comprising the isolated nucleic acid molecule according to any one of claims 1 to 15.

21. The genetic construct of claim 20, wherein the isolated nucleic acid molecule is placed in operable connection with a promoter sequence.

30

22. The genetic construct according to claim 21 , wherein the promoter sequence is selected from the group consisting of ubiquitin, CaMV 35S and SCSV promoter sequences.

23. The genetic construct according to any one of claims 21 or 22, further comprising a transcription termination sequence.

5

24. The genetic construct according to claim 23, wherein the transcription termination sequence is tm1 or the CaMV 35S transcription terminator.

25. A method of altering the level of sucrose or sucrose phosphates in a plant cell, 10 tissue or organ or a higher plant, said method comprising expressing the isolated nucleic acid molecule according to any one of claims 1 to 15 in said cell, tissue or organ for a time and under conditions sufficient for sucrose-6'-phosphate synthesis to occur.

15 26. The method according to claim 25 wherein the level of UDP-glucose-dependent sucrose-6'-phosphate synthesis in the cell, tissue or organ is increased.

27. The method according to claim 25 wherein the level of GDP-glucose-dependent sucrose-6'-phosphate synthesis in the cell, tissue or organ is increased.

20

28. The method according to claim 25 wherein the level of ADP-glucose-dependent sucrose-6'-phosphate synthesis in the cell, tissue or organ is increased.

29. The method according to claim 25 wherein the level of CDP-glucose-dependent 25 sucrose-╬┤'-phosphate synthesis in the cell, tissue or organ is increased.

30. The method according to any one of claims 25 to 29 wherein the plant is a tobacco plant.

30 31. The method according to any one of claims 25 to 29 wherein the plant is a rice plant.

32. The method according to any one of claims 25 to 31 , comprising the further first step of transforming a cell, tissue or organ with the nucleic acid molecule according to any one of claims 1 to 15 or a genetic construct comprising same.

5 33. A method of altering carbon partitioning and/or a developmental and/or growth response of a plant, said method comprising expressing the isolated nucleic acid molecule according to any one of claims 1 to 15 in said plant for a time and under conditions sufficient for sucrose-6'-phosphate synthesis to occur.

10 34. The method according to claim 32 wherein the level of UDP-glucose-dependent sucrose-6'-phosphate synthesis in the cell, tissue or organ is increased.

35. The method according to claim 32 wherein the level of GDP-glucose-dependent sucrose-6'-phosphate synthesis in the cell, tissue or organ is increased.

15

36. The method according to claim 32 wherein the level of ADP-glucose-dependent sucrose-6'-phosphate synthesis in the cell, tissue or organ is increased.

37. The method according to claim 32 wherein the level of CDP-glucose-dependent 20 sucrose-╬▓'-phosphate synthesis in the cell, tissue or organ is increased.

38. The method according to any one of claims 32 to 37 wherein the plant is a tobacco plant.

25 39. The method according to any one of claims 32 to 37 wherein the plant is a rice plant.

40. The method according to any one of claims 32 to 39, comprising the further first step of transforming a cell, tissue or organ with the nucleic acid molecule according to

30 any one of claims 1 to 15 or a genetic construct comprising same.

41. A method of producing sucrose-6'-phosphate from a nucleoside diphosphate glucose other than UDP-glucose in a plant cell, said method comprising expressing the isolated nucleic acid molecule according to any one of claims 1 to 15 in said cell for a time and under conditions sufficient for a catalytically-active SPS-like enzyme to be synthesized and incubating said enzyme in the presence of said nucleoside 5 diphosphate glucose and fructose-6-phosphate.

42. The method according to claim 41 wherein the nucleoside diphosphate glucose is GDP-glucose.

10 43. The method according to claim 41 wherein the nucleoside diphosphate glucose is ADP-glucose.

44. The method according to claim 41 wherein the nucleoside diphosphate glucose is CDP-glucose.

15

45. A method of producing a recombinant enzymatically active SPS-like polypeptide in a transgenic plant comprising the steps of:

(i) producing a genetic construct which comprises the isolated nucleic acid molecule according to any one of claims 1 to 15 operably under the control of 20 a plant-expressible promoter and optionally an expression enhancer element, wherein said genetic sequences is also placed upstream of a transcription terminator sequence;

(ii) transforming said genetic construct into a cell or tissue of said plant; and

(iii) selecting transformants which express an enzymatically-active SPS-like 25 enzyme which comprises at least one polypeptide subunit encoded by said nucleic acid molecule.

46. The method of claim 45 wherein the plant-expressible promoter is selected from the group consisting of ubiquitin, CaMV 35S and SCSV promoter sequences.

30

47. The method according to claim 45 or 46 wherein the terminator sequence is the CaMV 35S terminator or tm1 sequence.

48. The method according to any one of claims 45 to 47 wherein the plant is a tobacco plant.

49. The method according to any one of claims 45 to 47 wherein the plant is a rice 5 plant.

50. A recombinant SPS-like polypeptide or functional SPS-like enzyme molecule which comprises a sequence of amino acids that is at least 50% identical to the sequence set forth in SEQ ID NO: 2.

10 51. The recombinant SPS-like polypeptide or functional SPS-like enzyme molecule of claim 50 wherein said polypeptide or enzyme does not utilise UDP-glucose as a sole glucosyl donor substrate.

52. The recombinant SPS-like polypeptide or functional SPS-like enzyme molecule 15 of claim 50 or 51 comprising the amino acid sequence set forth in SEQ ID NO:2 or amino acid residues 465 to 720 thereof.

53. A polyclonal or monoclonal antibody that binds to the isolated or recombinant SPS-like polypeptide according to any one of claims 50 to 52.

20

54. An isolated cell, tissue, organ or organism which comprises a non-endogenous nucleic acid molecule that is capable of expressing a recombinant SPS-like polypeptide encoded by the nucleic acid molecule which has been introduced thereto, wherein said nucleic acid molecule is the nucleic acid molecule according to any one

25 of claims 1 to 15.

55. The cell, tissue, organ or organism of claim 54 of plant origin.

56. The cell, tissue, organ or organism of claim 55 wherein the plant is a rice plant. 30

57. The cell, tissue, organ or organism of claim 55 wherein the plant is a tobacco plant.