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  • C12N15/09—Recombinant DNA-technology
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  • C12P19/00—Preparation of compounds containing saccharide radicals
  • C12P19/26—Preparation of nitrogen-containing carbohydrates
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  • C12P19/00—Preparation of compounds containing saccharide radicals
  • C12P19/26—Preparation of nitrogen-containing carbohydrates
  • C12P19/28—N-glycosides
  • C12P19/38—Nucleosides
  • C12P19/385—Pyrimidine nucleosides

Definitions

• the present invention relates to the biological production of thymidine. More particularly it relates to a method utilizing genetically manipulated biological materials.
• the genetically manipulated biological materials may include a thymidine producing organism, such as a bacterium or one or more DNA constructs, for example plasmids, which in combination with a suitable host cell will, when grown in an appropriate culture media, give rise to the biological production of thymidine.
• the pyrimidine deoxyribonucleoside thymidine is useful as a pharmaceutical intermediate. It is particular important for the chemical synthesis of the AZT, also known as zidovidine or azidothymidine.
• This is an antiretro viral drug of the nucleoside reverse transcriptase inhibitor (NRTI) class, which, under the trademark Retrovir, was the first drug licensed to treat HIV infection.
• NRTI nucleoside reverse transcriptase inhibitor
• HIV/AIDS is usually treated with combinations of three or more antiretro viral drugs (De Clercq, Med. Chem. Res. 13: 439-478, 2004).
• AZT has an important and expanded role as a major component of combination therapies and is sold under a variety of trademarks including Retrovir, Zidovir, Viro-Z, Aviro-Z and Zido-H.
• AZT is particularly valuable when combined with the NRTI drug 3TC (lamivudine). These two drugs are available, formulated in a single pill, under the trademarks Combivir and Duovir. A triple NRTI combination of AZT, 3TC and abacavir is sold under the trademark
• Trizivir Trizivir.
• Other NRTI drugs used in conjunction with AZT include didanosine, emtricitabine and zalcitabine.
• AZT is also useful in combination with HIV protease inhibitor drugs including amprenavir, atazanavir, indinavir, ritonavir and saquinavir, and with non-nucleoside reverse transcriptase inhibitors (NNRTI) including delavirdine, efavirenz and nevirapine.
• NRTI non-nucleoside reverse transcriptase inhibitors
• AZT is the only anti-HIV drug approved for use during pregnancy (Lyall, et ah, HIV Med.
• Fermentation technology is an attractive alternative to chemical synthesis for commercial preparation of the large quantities of thymidine required for AZT manufacture. Fermentation processes are well established in industry as a means to produce biological molecules such as antibiotics, amino acids and vitamins at large scale and relatively low cost (Atkinson, & Mavituna, Biochemical Engineering and Biotechnology Handbook, 2nd
• thymidine is not usually found in its "free” form in nature, but produced as the monophosphate thymidylic acid and incorporated into DNA as the triphosphate. Biological systems do not naturally produce significant amounts of thymidine, hence a mutant or engineered organism is required.
• thymidine that involves the expression of deoxythymidylate phosphohydrolase (dTMPase) from the Bacillus subths bactenophage PBS l
• dTMPase deoxythymidylate phosphohydrolase
• PBS l This type of enzyme is found in nature expressed by bacteriophages that incorporate PdNs such as 2'- deoxyu ⁇ dine or 5-hydroxymethyl-2'-deoxyu ⁇ dme in their DNA in place of thymidine
• thymidine phosphorylase In the thymidine fermentation desc ⁇ bed in US 5,213,972 the enzymes that degrade thymidine, (thymidine phosphorylase and uridine phosphorylase), have been removed by mutation so that thymidine accumulates Thus, the use of the dTMPase enzyme helps create the pathway to allow thymidine synthesis An expression of dTMPase alone, however, may not assure a commercially viable level of thymidine production
• Levels of thymidine can be given as "specific productivity" figures where the measure is determined 4-6 hours after induction Levels of greater than 5mg TdR / 1/ hr/ g dry cell weight, more particularly greater than lOmg TdR / 1/ hr/ g dry cell weight and more preferably specific productivity figures of greater than 15 through 20 and 25 are preferred.
• organism includes an organism which can produce thymidine through expression of DNA encoded in its chromosome (as disclosed in, for example, EP 0344937) or DNA which it hosts (as disclosed in, for example, US 5,213,972 and WO 01/02580).
• This host DNA may be present as one or more DNA constructs, such as, for example, one or more plasmids.
• an organism capable of producing thymidine when grown in a culture medium, the organism comprising one or more genetic modification to its DNA, or some extra-chromosomal DNA necessary for its growth, characterized in that the organism comprises one or more genetic modifications which result in the thymidine biosynthetic pathway preferentially producing TdR at the expense of UdR as a consequence of one or more genetic modifications which increase the availability of a single carbon unit for conversion of dUMP to dTMP.
• the one or more modifications are preferably modifications which increase the amount of
• CH 2 -THF 10-methylenetetrahydro folate
• EC 2.1.1.45 and EC 2.1.1.148 The reactions catalysed by these two enzymes are compared in Figure 2.
• two hydrogen atoms are provided by the reduced form of a flavin nucleotide such as flavin adenine dinucleotide (FAD).
• FAD flavin adenine dinucleotide
• CH 2 - THF is converted to tetrahydro folate (THF) and FADH 2 to FAD in the thymidylate synthase reaction (Myllykallio, H. et al, Science 297: 105-107, 2002).
• CH 2 -THF is normally regenerated by transfer of a one-carbon unit to THF.
• thymidylate synthase there must be, as a first step, conversion of DHF to THF by the action of a dihydro folate reductase (EC 1.1.5.3), expressed for example by an E. coli folA gene or a T4 frd gene. With the EC 2.1.1.148 form of thymidylate synthase this step is not required.
• the primary metabolic route to generate CH 2 -THF from THF is a pathway through the amino acids serine and glycine, starting from 3-phosphoglycerate, which is an intermediate in carbohydrate metabolism.
• This serine-glycine-Ci pathway is shown in Figure 3.
• the first committed step is the generation of 3-phosphohydroxypyruvate by action of phosphoglycerate dehydrogenase encoded by, for example, the serA gene of E. coli.
• CH 2 -THF is generated at two steps in this pathway: the first is the conversion of L-serine to glycine by the action of serine hydroxymethyltransferase encoded by, for example, the E. coli glyA gene. The second is the conversion of glycine to carbon dioxide and ammonia by the glycine cleavage enzyme complex that is the product of at least four genes.
• CH 2 -THF can also be generated from THF in three steps using formate to provide the carbon atom, as shown in Figure 4.
• Formate can be derived from the central metabolic intermediate pyruvate.
• the three required enzyme activities may be present in a single trifunctional protein, as encoded for example by the ADE3 gene of the yeast Saccharomyces cerevisiae.
• the organism is a bacterium, more preferably E coli.
• Brevibacterium and Bacillus species as disclosed in the prior art documents identified previously, and other thymidine producing organisms e.g. the bacterium Corynebacterium or yeast could be selected and modified in accordance with the described invention. It will be appreciated that there are various modes by which the availability Of CH 2 -THF may be increased in a thymidine-producing organism, and that the most effective will in each case depend upon the genetic background of that organism.
• the organism will preferably also comprise a transcriptional unit (which may be chromosomal or extra-chromosomal) capable of over-expressing a thymidylate synthase encoded by, for example, a td or thyA gene, and one or more genes which cause thymidine to be produced from dTMP, such as, for example, a deoxythymidylate phosphohydrolase encoded by, for example, a dTMPase gene.
• a transcriptional unit which may be chromosomal or extra-chromosomal
• the organism includes a gene which, when expressed, facilitates the conversion of DHF to THF.
• This may be a dihydro folate reductase gene such as folA or frd.
• the preferred dihydrofolate reductase gene is a bacteriophage gene, more preferably a T even phage gene, and most preferably the T4 bacteriophage gene frd.
• the dihydrofolate reductase gene may be introduced directly into the organism's chromosomal DNA or incorporated into a DNA construct which may in turn be hosted by the organism.
• the DNA construct is a vector, such as a plasmid, virus, transposon, or mini-chromosome.
• DNA construct is used to introduce the DNA into a host cell it is preferable to ensure it is designed to be stably maintained in its intended host.
• the organism is E coli host strain CMG2576, derived from strain CMG2451 disclosed in WO 01/02580, harbouring plasmid pCG609, derived from pCG532 disclosed in WO 01/02580. Fuller details of their make up are given in Examples 3 and 4. This combination has been doubly stabilised to ensure that the plasmid is maintained within the host strain during growth under "non-inducing" conditions.
• the TMPase gene is inserted into the host chromosome, coupled with a bacteriophage lambda Pi promoter.
• the plasmid carries a bacteriophage lambda Cl 857 temperature sensitive repressor under control of its own promoter, relocated from the chromosome in the host strains disclosed in WO 01/02580. If the plasmid is lost there is no repressor made, and the TMPase gene is expressed so that TMP is converted to thymidine and the organism dies.
• this stabilization mechanism was insufficient for use in large scale cultures with the many generations of growth required, since plasmid loss accompanied by simultaneous inactivation of the chromosomally located TMPase gene is an event sufficiently frequent to allow the growth of plasmid-less derivatives.
• a second stabilization mechanism was added, where a gene essential for growth and survival was deleted from the chromosome and relocated to the plasmid. In the CMG2576 and pCG609 combination this gene is Thy A, cloned into the plasmid under control of a lac promoter.
• Thy A gene Without a functional Thy A gene, the cell cannot make TMP, hence cannot make TTP for synthesis of DNA and dies. This mechanism is particularly effective because genes can readily mutate to lose a function, but much less readily recreate one that is completely lost. Furthermore, the product of the missing enzyme is a phosphorylated compound that is unable to "cross-feed" by passing out of functional cells and into those having lost the plasmid-borne gene.
• shake flask test data often includes the introduction of a "test gene" in a second plasmid.
• test gene a "test gene" in a second plasmid.
• multi plasmid systems have not been subjected to scale testing and their genetic stability on large-scale culture has not been determined.
• a method of producing thymidine comprising growing an organism in a culture medium, the organism comprising one or more genetic modification to its DNA, or some extra-chromosomal DNA necessary for its growth, characterized in that the organism comprises one or more genetic modifications which result in the thymidine biosynthetic pathway preferentially producing TdR at the expense of UdR as a consequence of one or more genetic modifications which increase the availability of single carbon units for conversion of dUMP to dTMP.
• the UdR content is less than 5% of the thymidine content.
• the thymidine content is greater than 5mg TdR / 1/ hr/ g dry cell weight, more preferably greater than 7.5 through 10 in steps of 2.5 to 25mg TdR / 1/ hr/ g dry cell weight and higher.
• culture medium capable of supporting growth and thymidine production for an organism of the invention.
• zidovidine manufactured from thymidine produced according to the method of the invention.
• a gene is introduced into the organism indirectly as extra-chromosomal DNA, most preferably in the form of a plasmid (rather than directly into the chromosome).
• the preferred gene is one which expresses an enzyme which is able to ensure that a thymidylate synthase enzyme (e.g. EC 2.1.1.45, EC 2.1.1.148), which converts deoxyuridine monophosphate (dUMP) to thymidine monophosphate (dTMP), is provided with a ready supply of methyl groups such that the majority of dUMP is methylated to dTMP as opposed to being converted to 2 '-deoxyuridine (UdR).
• a thymidylate synthase enzyme e.g. EC 2.1.1.45, EC 2.1.1.148
• dUMP deoxyuridine monophosphate
• dTMP thymidine monophosphate
• the favoured thymidylate synthase enzyme is of the EC 2.1.1.45 type and the favoured gene is one which aids the conversion of dihydro folate (DHF) to tetrahydrofolate (THF).
• DHF dihydro folate
• THF tetrahydrofolate
• a suitable gene is a gene encoding dihydrofolate reductase. This may be a wild type or deregulated bacterial gene or a bacteriophage gene.
• a particular favoured gene, and one used to exemplify the invention, is the frd gene of bacteriophage T4 (T4frd).
• constructs of the present invention may be chromosomal or more preferably extra- chromosomal e.g. located on a vector.
• Vectors of the present invention include plasmid, virus (including phage), transposons, and mini-chromosomes, preferably plasmids.
• the vector may be introduced into a host cell according to any convenient method known to those skilled in the art, e.g. Pl transduction, electroporation or transformation.
• Suitable host cells useful in the present invention include eukaryotes (e.g. fungi) and prokaryotes (e.g. bacteria).
• Prokaryotes include E. coli, Salmonella, Pseudomonas, Brevibacterium, Bacillus, strains and mutants thereof.
• Vectors of the present invention preferably comprise a regulatory element (e.g. promoter such as lambda P L , operator, activator, repressor such as lambda repressor, particularly a temperature sensitive variant, and/or enhancer), appropriate termination sequences, initiation sequences and ribosome binding sites.
• a regulatory element e.g. promoter such as lambda P L , operator, activator, repressor such as lambda repressor, particularly a temperature sensitive variant, and/or enhancer
• the vector may further comprise a selectable marker.
• regulatory elements particularly lambda repressor may be located on the host cell chromosome.
• Host cells modified according to the present invention are particularly useful in the commercial production of thymidine.
• E. coli host cells comprising (harboring or incorporating) a plasmid modified according to the present invention (particularly in conjunction with the teachings of US 5,213,972 or WO 01/02580) may be used in the commercial production of thymidine.
• host cells modified according to the present invention may further comprise dTMPase derived from e.g. PBSl and the mutations taught in US 5,213,972, e.g. deoA, tdk- 1 and udp-1, along with the further improvements to the construct taught in WO 01/02580, e.g. T4 nrdCAB, T4 td, dcd, and udk.
• dTMPase derived from e.g. PBSl and the mutations taught in US 5,213,972, e.g. deoA, tdk- 1 and udp-1, along with the further improvements to the construct taught in WO 01/02580, e.g. T4 nrdCAB, T4 td, dcd, and udk.
• a fermentation method is employed as taught in WO 01/02580, which involves submerging the cells in a culture medium contained within a suitable vessel. Following culturing under appropriate conditions, produced thymidine is harvested and purified (enriched), if necessary, to pharmaceutical grade according to standard protocols. The purified thymidine may then be used in the production of medicaments, e.g. pharmaceutical compositions such as AZT.
• medicaments e.g. pharmaceutical compositions such as AZT.
• Fig. 1 illustrates a thymidine biosynthesis pathway
• Fig. 2 illustrates the thymidylate synthase reaction (converting dUMP to dTMP);
• Fig. 3 illustrates regeneration Of CH 2 -THF from L-serine and glycine
• Fig. 4 illustrates regeneration of CH 2 -THF from formate
• Fig. 5 summarises approaches to improve regeneration of CH 2 -THF
• Fig. 6 illustrates a map for plasmid pCG532 (the prior art teaching).
• Fig. 7 illustrates a map for plasmid pCG609
• Fig. 8 illustrates maps for plasmid pCG376
• Fig. 9 compares thymidine production in a number of hosts/ plasmids
• Figure 1 illustrates the complexity of the thymidine biosynthetic pathway and the associated genetics, using as an example E. coli Kl 2 with the addition of a TMPase gene.
• Thymidine is TdR and 2'-deoxyuridine is UdR, at the lower right hand side of the diagram.
• UdR differs chemically from TdR by the absence of a single methyl group.
• the structural similarity between the compounds makes purification difficult and costly.
• the invention concerns itself with making gene modifications which influence the preferential formation of TdR at the expense of the production of UdR.
• the applicant has determined that the levels of TdR relative to UdR can be effectively controlled by ensuring that the process step dUMP to dTMP, catalysed by expression of e.g. thymidylate synthase from a thymidylate synthase gene (e.g. td, thyA or thyX), is provided with an increased supply of one-carbon units as e.g. CH 2 -THF.
• thymidylate synthase from a thymidylate synthase gene (e.g. td, thyA or thyX)
• Figure 2 shows two alternative mechanisms for thymidylate synthase, each producing a different depleted one-carbon donor.
• thymidylate synthase is of the type EC 2.1.1.45, encoded for example by a thyA or td gene
• the depleted donor is DHF.
• the enzyme is of the EC 2.1.1.148 type, encoded for example by a thyX gene
• the depleted donor is THF.
• the direct precursor of CH 2 -THF is THF
• the first step must be conversion of DHF to THF.
• a dihydro folate reductase gene encoding an enzyme that catalyses this conversion, the applicant has been able to significantly reduce the levels of contaminating UdR and additionally increase thymidine productivity as is illustrated in Example 6.
• Example 8 this approach has been combined with further modifications to ensure that a plasmid construct carrying the dihydro folate reductase and other cloned genes required for TdR production is stably maintained in its appropriate E. coli host strain. This represents the current best mode improvement over the prior art process taught by WO 01/02580, but by no means reaches the limit of what may be expected from the present invention.
• Example 9 an enzyme is overexpressed, capable of converting THF to CH 2 -THF through conversion of L-serine to glycine.
• Example 10 another enzyme is overexpressed with the aim of improving the supply of one-carbon units for conversion of THF to CH 2 -THF by increasing the flow of metabolites into the serine/ glycine pathway shown in Figure 3.
• Examples 11 and 12 use different methods to introduce into E. coli a non-native pathway for conversion of THF to CH 2 -THF using formate as one-carbon donor.
• Fig. 6 illustrates the construct pCG532 and Table 1 gives the genotype of the E. coli host strain CMG2451. It will however be appreciated that not all the genes disclosed therein form an essential part of the present invention, although the presence of a thymidylate synthase gene (td, on the plasmid) and a TMPase (integrated into the chromosome) are important to the preferred embodiment thereof.
• td thymidylate synthase gene
• TMPase integrated into the chromosome
• Example 2 The construct and strain referred to in Example 1 were grown in culture medium (Table 2) according to the following methodology. Data were obtained for comparative purposes on both the rate of TdR production and the relative amount of UdR impurity (expressed as a percentage of TdR).
• Table 3 Average Specific Productivity (mg TdR/L/hr./g dry cell weight) for strains in enriched medium
• the host strain was modified through a series of steps as listed in Table 4 below. There were three significant changes overall, although it will be appreciated that none of these form an essential part of the present invention. In the Examples below, intermediate strains CMG2549 and CMG2560 as well as final strain CMG2576 are used to demonstrate aspects of the invention.
• Strain CMG2549 differs from CMG2451 in that the chromosomal ndk gene has been knocked out by replacement with an insertionally inactivated ndkr.kan. This change was made to improve the overall production of pyrimidine nucleosides according to the principles taught by US 5,213,972 and is important to the demonstration of the utility of the present invention detailed in Example 7.
• CMG2560 was constructed by eliminating a lambda prophage from the chromosome, at the same time repairing mutations causing requirements for biotin and nicotinic acid. This change was made in the presence of a plasmid carrying a lambda cl 857 temperature sensitive repressor, without which the dTMPase gene, inserted into the chromosome and expressed from a lambda P L promoter, would be active and the organism would degrade dTMP, be unable to make DNA and die.
• CMG2576 differs from CMG2560 in that the chromosomal thyA gene has been replaced with an insertionally inactivated thyA748::TnlO. This change was made in the presence of a plasmid carrying both a lambda cl 875 repressor and a cloned thyA. Without the plasmid-borne thyA gene the organism is unable to synthesize dTMP, thus unable to make DNA and dies.
• Plasmids pCG532 and pCG609 are illustrated by Figures 6 and 7 respectively, where the main features may be compared.
• T4 frd gene forms an essential part of one embodiment of the present invention.
• the effect of this modification is illustrated by Examples 6 and 7 below.
• the remaining changes were made to prevent loss of plasmid from the host strain, as detailed in Example 8 below.
• the effects of adding additional genes to established host strain/ plasmid combinations were tested by introducing these on a second plasmid.
• the second plasmid is designed to be compatible with pCG532 and its derivatives, so that both may be stably maintained within a single E. coli host.
• pCG532 is ultimately derived from the natural plasmid CoIEl : its detailed history is disclosed in WO 01/02580.
• the second plasmid is based on cloning vector pACYC177 (Chang & Cohen, J. Bacteriol. 134: 1141-1156, 1978), which is derived from natural plasmid pi 5 A and compatible with CoIEl.
• This system has the advantage that gene additions may be tested without the need to further modify the already complex constructs described in Example 4 above. However, it is recognized that this system may be unstable without selective antibiotics and therefore not suited to large scale use in an industrial fermentation process.
• pCG590 differs from the prior art construct pCG532 only in the addition of the frd gene from bacteriophage T4, encoding T4 dihydro folate reductase.
• the T4 enzyme is well characterised and readily expressed in E. coli (Purohit et al. J. Biol. Chem. 256: 9121-9125, 1981).
• This gene was added to the "operon" expressed from the lambda P L promoter and is therefore induced with dTMPase (in the chromosome) and the T4 genes nrdC, nrdA, nrdB and td (on the plasmid) when temperature is increased and the lambda clgsj repressor inactivated.
• T4 frd gene greatly reduced the level of UdR produced, thereby simplifying production of substantially pure TdR by fermentation. It is expected that improved conversion of dUMP to dTMP rather than to UdR should result in additional thymidine as well as reduced UdR, which appears to be true in this case.
• Host strain CMG2549 differs from the prior art host strain CMG2451 by a mutation in the ndk gene, encoding nucleoside diphosphate kinase. Mutation of this gene has been reported to increase intracellular pools of dCTP and dTTP in E. coli (Lu et al. J. MoI. Biol, 254: 337-341, 1995). According to the principles taught in US 5,213,972, the introduction of such a mutation in the pyrimidine deoxyribonucleoside biosynthetic pathway may be used to increase overall synthesis of TdR and UdR in an appropriately engineered organism.
• Example 2 shake flask data were obtained for the two host strains CMG2451 and CMG2549, each in combination with either pCG532 or pCG590. In this way the effects of the ndk mutation and the addition of T4 frd were compared both independently and in combination. The results are shown in Table 8 below.
• Table 8 Shake Flask Average Specific Productivity (mg TdR/L/hr/g dry cell weight) for strains in enriched medium with or without ndk mutation and with or without cloned T4frd.
• T4 frd becomes even more effective in reducing UdR levels when introduced into an E. coli host carrying an ndk mutation.
• ndk mutation clearly increases the deoxyribonucleoside synthetic capacity of an E. coli strain according to the prior art, UdR content is too high for such an improvement to be exploited in the cost-effective production of pure thymidine.
• additional dUMP produced by the ndk mutant can be directed towards dTMP and thereby converted into thymidine, rather than converted into UdR. The result is a very substantial improvement over the prior art in terms of both thymidine productivity and low UdR content.
• the cls 5 7 repressor acts to prevent the expression of the dTMPase gene integrated into the chromosome, which was cloned with a lambda PL promoter.
• the repressor is located on the plasmid, loss of that plasmid results in expression of dTMPase and consequent degradation of dTMP resulting in loss of viability for the plasmid-free cell.
• pCG596 was constructed as described in Example 4 and its host strain CMG2560 as in Example 3.
• the chromosomal thyA gene could then be deleted in a host strain harbouring this plasmid.
• Expression of thymidylate synthase from thyA is essential for growth and survival because there is no other means by which the cell can synthesize dTMP.
• dTMP itself cannot pass through cell walls and "cross-feed" from thy A + cells with plasmid to thyA cells having lost the plasmid.
• Table 9 Shake Flask Average Specific Productivity (mg TdR/L/hr/g dry cell weight) for strains in enriched medium with and without genetic stabilisation of the plasmid.
• This example illustrates the effect of adding to the prior art construct and host strain a cloned E. coli glyA gene encoding serine hydroxymethyltransferase.
• This enzyme catalyses the conversion of serine to glycine coupled to the regeneration Of CH 2 -THF from THF (see Figure 3). It will be recognized that the host strain already carries and expresses a glyA gene, and that the effect of this genetic modification is simply to boost the level of serine hydroxymethyltransferase when the plasmid-borne gene is expressed during temperature induction.
• the glyA gene was cloned into plasmid vector pCG376, compatible with pCG532, as discussed in Example 5 above.
• the resulting plasmid pCG601 carries the glyA gene with a lambda P L promoter.
• CMG2451 harbouring both pCG532 and pCG601 was compared with the prior art CMG2451 harbouring pCG532 alone
• Table 10 Shake Flask Average Specific Productivity (mg TdR/L/hr/g dry cell weight) for strains in enriched medium with and without the E. coli glyA gene overexpressed.
• the rate limiting step in CH 2 -THF synthesis may be elsewhere in E. coli strains with different or additional added genes and mutations and in other thymidine-producing organisms. Indeed if the thymidine-producing organism continues to be improved according to the principles taught in US 5,213,972 and WO 01/02580, then the rate limiting step may move to the serine hydroxymethyltransferase reaction and the addition of a cloned glyA gene show a more dramatic benefit in reducing the level of UdR in thymidine according to the principles of the present invention.
• Phosphoglycerate dehydrogenase is the enzyme catalyzing the first step in this pathway, which is the major source Of CH 2 -THF in many organisms including E. coli.
• CH 2 -THF is generated in two consecutive reactions: conversion of L-serine to glycine, catalysed by serine hydroxymethyltransferase, and conversion of glycine to carbon dioxide and water, catalysed by the glycine cleavage enzyme complex.
• E. coli serA gene was added to increase the level of the native 3-phosphoglycerate dehydrogenase, using the second plasmid test system as detailed in Example 5.
• the starting point for this new genetic modification was the best mode host strain and construct CMG2576/ pCG609 including the ndk mutation, T4 frd gene and genetically stabilised construct. It was reasoned that the benefit of increasing the capacity for synthesis of CH 2 -THF from THF would be best tested in this background, where capacity for conversion of DHF to THF had been improved.
• the serA gene was cloned into plasmid vector pCG376, as discussed in Example 5.
• the resulting plasmid pCG844 carries the serA gene with a lambda P L promoter.
• CMG2576 derivatives were compared, harbouring pCG609 combined with either pCG376 or pCG844. The results are given in Table 11.
• Table 11 Shake Flask Average Specific Productivity (mg TdR/L/hr/g dry cell weight) for strains in enriched medium with and without the E. coli serA gene overexpressed.
• This example illustrates the effect of adding to a thymidine-producing organism an additional pathway to provide regeneration of CH 2 -THF.
• regeneration Of CH 2 -THF using formate as the source of the required one-carbon unit is not a normal metabolic route.
• this mechanism exists in prokaryotes, including the yeast Saccharomyces cerevisiae, where a single gene may encode the trifunctional protein expressing the three enzyme activities required to convert THF to CH 2 -THF by this route (see Figure 4).
• formate is not the normal source of one-carbon units for CH 2 -THF in a prokaryotic organism such as E. coli, hence this aspect of the invention may be require further genetic manipulation in order to have its maximum effect.
• the pyruvate formate lyase enzyme that generates formate from pyruvate could be overexpressed to increase the availability of formate for methylation of THF (see Figure 5).
• Table 12 Shake Flask Average Specific Productivity (mg TdR/L/hr/g dry cell weight) for strains in enriched medium with and without the yeast ADE3 gene expressed.
• This example illustrates an alternative mechanism to introduce into E. coli a pathway to regenerate CH 2 -THF using formate.
• three enzymes are required to catalyse this reaction.
• E. coli is unable to use this pathway because it lacks the first of these enzymes, formyltetrahydrofolate synthetase (EC 6.3.4.3).
• the remaining two enzymes are present as a bifunctional protein encoded by the folD gene.
• Table 13 Shake Flask Average Specific Productivity (mg TdR/L/hr/g dry cell weight) for strains in enriched medium with and without the C. acidi-urici fhs gene expressed.

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