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  • C12N9/00—Enzymes; Proenzymes; Compositions thereof; Processes for preparing, activating, inhibiting, separating or purifying enzymes
  • C12N9/0004—Oxidoreductases (1.)
  • C12N9/0012—Oxidoreductases (1.) acting on nitrogen containing compounds as donors (1.4, 1.5, 1.6, 1.7)
  • C12N9/0026—Oxidoreductases (1.) acting on nitrogen containing compounds as donors (1.4, 1.5, 1.6, 1.7) acting on CH-NH groups of donors (1.5)
  • C12N9/0032—Oxidoreductases (1.) acting on nitrogen containing compounds as donors (1.4, 1.5, 1.6, 1.7) acting on CH-NH groups of donors (1.5) with oxygen as acceptor (1.5.3)
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  • C12N15/00—Mutation or genetic engineering; DNA or RNA concerning genetic engineering, vectors, e.g. plasmids, or their isolation, preparation or purification; Use of hosts therefor
  • C12N15/09—Recombinant DNA-technology
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Definitions

• a simple and efficient method for random in vitro mutagenesis and recombination of polynucleotide sequences based on walk-through with chain-elongating molecules and chain terminating molecules e.g. dNTPs/ddNTPs followed by reassembly and amplification is described.
• the utility of this method for improving protein structure and/or function is demonstrated by creating novel sarcosine oxidase variants and by recombining human placenta! alkaline phosphatase and calf intestinal alkaline phosphatase.
• Proteins are engineered with the goal of better understanding the molecular basis for their functions as well as much improving their performance for practical applications.
• the choice of approach for a given optimization problem will depend upon the degree of understanding of the relationships between sequence, structure and function.
• the rational redesign of an enzyme's catalytic site for example, often requires extensive knowledge of the enzyme structure, the structures of its complexes with various ligands and analogs of reaction intermediate and details of the catalytic mechanism. Such information is available only for a very few well-studied systems; little is known about the vast majority of potentially interesting enzymes.
• DNA- shuffling (US 5,834,252; US 5,605,793; US 5,830,721; US 5,837,458; US 5,811,238) DNA-shuffling comprises the following steps:
• the different double-stranded template polynucleotides may belong to the same family of nucleic acids or proteins (i.e. are related) but which differ in their sequence (i.e. are not identical) and hence in their biological activity.
• DNAse I is described for random fragmentation in the prior art.
• DNAse I cleaves the DNA non-randomly. During an early phase of reaction cleavage occurs preferably in the middle of a DNA sequence and in the later phase of fragmentation cleavage occurs preferably between purine - and pyrimidine analogues. This leads to a fragmentation procedure which can be hardly controlled. Furthermore the minimal length of the desired gene is limited because of the non-controllable digestion of the template polynucleotide. Moreover, DNAse has to be removed completely after the digestion step because it introduces disturbance into the further reassembly reaction.
• the aim of the present invention was, therefore, developing a novel technique for the recombination of DNA sequences.
• the subject of the present invention is a novel technique for in vitro walk-through recombination of DNA sequences.
• the technique involves walking through a template gene with a mixture of chain- elongating molecules and chain- terminating molecules, e.g. dNTPs/ddNTPs, to generate a pool of 3' end-randomly distributed DNA fragments with a low level of point mutations.
• a fragment ladder is generated similar to fragment ladders during sequencing reactions.
• these short DNA fragments can prime one another under appropriate reaction conditions based on homology and thus can be reassembled to form full-length genes by repeated thermocycling in the presence of thermostable DNA polymerase.
• genes can be further amplified by a conventional PCR and cloned into a proper vector for expression of the encoded proteins. Screening or selection of the expressed mutants leads to new variants with improved or even novel functions. These variants can be immediately used as partial solutions to a practical problem, or they can serve as new starting points for further cycles of walk-through mutagenesis and recombination.
• This technique has been proven by creating Bacillus subtilis BMTU3420 sarcosine oxidase variants and by recombining human placental alkaline phosphatase and calf intestinal alkaline phosphatase. It was found that the technique is both - simple and efficient.
• subject of the present invention is a method for forming a polynucleotide sequence comprising the steps of
• reassembly of the polynucleotide by hybridizing these fragments to one another or to template polynucleotides in presence of a thermostable enzyme having nucleic acid-synthesizing activity and conducting nucleic acid synthesis either i) in presence of a mixture of nucleic acid chain-terminating molecules and nucleic acid chain elongating molecules or ii) in presence of a reaction mixture of nucleic acid chain elongating molecules which does not contain chain-terminating molecules whereas in case of i) steps b) and c) will be repeated subsequently.
• the step of removing chain-terminators and the step of fragment reassembly step can also be done at the same time.
• steps a, b, c The subdivision of the process in steps a, b, c does not imply that three separate steps have to be performed. The three steps may also take place at the same time. It is also possible that the whole WalkThrough recombination maybe performed in more preferred forms, such as in a single container, vessel or tube.
• the template polynucleotide in step a) and/or c) is a mixture consisting of different template polynucleotides which may belong to the same family of nucleic acids or proteins (i.e. related) but which differ in their sequence (i.e. not identical) and hence differ in their biological activity. In this case gene shuffling may occur.
• This step is somehow similar to normal polynucleotide sequencing reaction, and, therefore, various conditions suitable for sequencing different targets may be applied to this step.
• the template can be single- or double-stranded polynucleotides in linear or closed circular form.
• the template may also be in the form of genomic DNA, that is, in native, intact, unpurified, or uncloned forms. Since, in most cases, the template genes are cloned in vectors into which no additional mutations should be introduced, they are usually first cleaved with restriction endonuclease(s) and purified from the vectors through agarose gel electrophoresis. For example linear DNA molecules are denatured, annealed to oligodeoxynucleotides for walkthrough reactions in the presence of an appropriate amount of chain-elongating/ chain- terminating molecules as e.g.
• dNTPs/ddNTPs prime the DNA of interest at different positions along the entire target region extend to generate DNA fragments complementary to each strand of the template DNA. Due to some synthesis errors, these DNA fragments also contain a low level of local mutations. After removing the chain-terminating ends of ddNMP's, these DNA fragments can prime one another under appropriate reaction conditions based on homology and be reassembled into full-length genes by repeated thermocycling in the presence of thermostable DNA polymerase. The resulting full-length genes will have diverse sequences, most of which, however, still resemble that of the original template DNA.
• the fragment synthesis can be carried out with either chain elongation under mild conditions using mesophilic DNA polymerases or thermocycle sequencing using thermostable polymerase.
• PCR to the WalkThrough synthesis provides a more convenient way for more nascent DNA fragments and makes our technique more robust.
• thermocycling is used for the fragment synthesis, the number of thermocycling can be from about 15 to about 55 cycles, depending on the amount of template and its purity.
• the concrete cycle number can be easily determined according to the quality and quantity of the newly synthesized fragments.
• the amount of initial DNA template(s) may vary according to the chain elongation conditions. Normally, 0.1-10 pmol of ds-DNA is sufficient for this kind of reaction.
• the length of the primers used may vary from 14-mer to 28-mer. The lengths of the selected primers should be long enough to prevent annealing to unspecific DNA binding positions and guarantee a good signal-to-noise ratio. We usually use primers of 18- to 21 -mer.
• the distances between the WalkThrough primers depend on the chain reaction modes and may vary from 100 to 800 nucleotides. The average distance between two primers in our cases is normally around 500 nucleotides.
• each primer can vary from 0.05 pmol to 2 pmol, depending on what kind of chain reaction is used. Normally, 0.1 pmol primer is used for the chain elongation of B. subtilis sarcosine oxidase gene, while 1.0 pmol each primer is used for the chain elongation of human placental alkaline phosphatase and calf intestinal alkaline phosphatase.
• thermostable DNA polymerases can be used for the WalkThrough synthesis. More preferably, other modified or mixture of DNA polymerases can be used which incorporate ddNTPs more efficiently than the wild type polymerases. For single-stranded RNA template, reverse transcriptase is preferred for the WalkThrough synthesis. Each of these enzymes is usually used under its optimized conditions to promote the chain elongation. Enzymes having nucleic acid-synthesizing activity can be DNA or RNA dependent polymerases.
• ddNTPs 2', 3'-dideoxynucleotide triphosphates
• each of the four elongation reactions contains a population of extended chains, all of which have a fixed 5' end determined by the annealed primers and a variable 3' end terminated at a specific dideoxynucleotide.
• the chain elongation and termination can also be done in one reaction in the presence of proper amount of ddNTPs/dNTPs.
• Chain terminating molecules means that these molecules terminate nucleic acid synthesis and, therefore, a fragment ladder is generated. After generation of the fragment ladder suitable chain-terminating molecules can be removed.
• suitable chain-terminating nucleotides are e.g. dideoxynucleotides as ddGTP, ddATP, ddCTP, ddTTP or derivatives thereof.
• Derivatives of dideoxynucleotides are defined as those dideoxynucleotides that are able to be incorporated by a thermostable DNA polymerase into growing DNA molecules that are synthesized in a thermocycling reaction. These dideoxynucleotides and derivatives are preferably used at a concentration of about 20 ⁇ M to 1.0 mM.
• Such derivatives can include thionucleotides, 7-deaza-2'-dGTP, 7-deaza-2'-dATP as well as deoxy- inosine triphosphate that can also be used as a substitute deoxynucleotide for dATP, dGTP, dTTP or dCTP, but are not limited to these. These deoxynucleotides and derivatives are preferably used at a concentration of about 4 ⁇ M to 400 ⁇ M.
• the concentrations and the ratios of the ddNTP/dNTP may vary from case to case in order to get shorter or longer fragments.
• a ratio of dNTP/ddNTP between 1/50 and 5/1 works well for the chain elongation, it has to be optimized for different individual template in order to get fragment pools with randomly distributed 3'-ends. This is very important for allowing every nucleotide of the template should be copied at a similar frequency into products, and therefore, providing possibility to recombine or dissect two or more mutations although they may be very close to each other.
• Preferred WalkThrough buffer systems normally include Tris-HCI at a concentration of about 50 to 500 mM, preferably of about 100 to 250 mM.
• the pH values of these buffer systems range from pH6.5 to pH 10.0, depending on polymerases used.
• MaCl 2 is generally included in these buffer systems at a concentration ranging from 1.0 to 5.0 mM.
• KCI may also be included at a concentration of 2-80 mM.
• Certain amount of mercaptoethanol (0.5-1.5%), Tween 20 (0.2-0.4%) and DMSO (1- 5%) may also exist in the buffers.
• Other agents, such as glycerol, betaine, etc., which can lower the melting point of the templates may also be included in the buffers to facilitate the WallcThrough reaction.
• the Walk-Through DNA synthesis is based on the chain elongation guided by template, and the nascent strand is synthesized from the 3'-OH termini at the primers using polymerase and the four deoxynucleoside triphosphates and stop after the ddNTP incorporation. Thus the reaction is independent of the length of the DNA template.
• This step is the second key step for successful WalkThrough recombination.
• the end nucleotide or incorporated terminator at each newly generated fragments must be removed so that each such fragment gains a free 3' -OH group necessary for further fragment reassembly.
• Chain- terminating molecules can be removed, for example, by DNA polymerases, many DNA polymerases have a 3' -5' exonuclease activity. This activity removes a single nucleotide at a time, releasing a nucleotide 5' monophosphate. In the absence of dNTPs, this activity will catalyze stepwise degradation from 3' end of both single- and double-stranded DNA.
• the polymerase which may serve this purpose include Klenow fragment of E. coli DNA polymerase I (Jacobsen, H., Klenow, H., and Over- gaard-Hansen, K. 1974.
• chain-terminating molecules can also be removed by exonucleases, such as Exonuclease III.
• This enzyme is a multifunctional enzyme that catalyzes hydrolysis of several types of phosphodiester bonds in double stranded DNA.
• the main application of Exo III is as a 3'-5' double- stranded specific exonuclease that catalyzes release of 3' nucleotides from the 3'- end of double stranded DNA (Roger, S.G. and Weiss, B. 1980.
• Preferred reaction buffers used for terminator-removing reactions normally include Tris-HCI at a concentration of about 40 to 200 mM, preferably of about 50 to 100 mM.
• the pH values of these buffer systems range from pH 6.5 to pH9.0, depending on enzymes with 3'-5' exonuclease activity used.
• MgCl 2 is generally mcluded in these buffer systems at a concentration ranging from 1.0 to 3.0 mM.
• KCI may also be included at a concentration of 5-50 mM. Certain amount of mer cap to ethanol (0.5-1.5%) may also exist in the buffers.
• the reaction normally is carried out at 37°C for 0.5-2.0 hours.
• thermostable DNA polymerases and thermostable enzymes with 3'->5' exonuclease activity in the proper buffer and under the optimized conditions the steps of terminator removal and reassembly can also be combined, so that no separate (B) and (C) steps are necessary.
• This method of the present invention is particularly preferred, as demonstrated in the example of recombining human placental alkaline phosphatase and calf intestinal alkaline phosphatase.
• the reassembling of the fragments is a PCR like reaction, comprising cycles of DNA-denaturation, annealing and DNA synthesis. During the annealing ssDNA- fragments hybridize in homologues areas. The overlapping ends of the ssDNA are extended by a polymerase. In case ssDNA random fragments from different polynucleotides hybridize, gene-shuffling occurs. Gene-shuffling means recombination between homologues but non-identical sequences. The term "identical" means that two nucleic acid sequences have the same sequence or a complementary sequence.
• areas of identity means that regions or areas of a nucleic acid fragment or polynucleotide are identical or complementary to another polynucleotide or nucleic acid fragment.
• homologues means that one single-stranded (ss) nucleic acid sequence may hybridize to a complementary ss nucleic acid sequence. The degree of hybridization may depend on a number of factors including the amount of identity between sequences and the hybridization conditions such as temperature and salt concentration as discussed later.
• the reassembly step is the most critical one in the whole WalkThrough process, as all synthesized fragments during Step (A) prime one another for elongation without any additional primers added.
• the concentration of the DNA is the most important variable, it is useful to set up three separate reactions with different concentrations (high, middle, low). According to our experience, the amount of DNA fragments between 0.1 ⁇ g to 2.0 ⁇ g usually gives satisfied reassembly results in a reaction volume of 20-50 ⁇ l.
• the deoxynucleotides during reassembly are preferably used at a concentration of about 100 ⁇ M to 400 ⁇ M, and the chosen concentration depends on the cycle number of the reaction.
• Preferred reassembly buffers normally include Tris-HCI at a concentration of about 5 to 50 mM, preferably of about 10 mM.
• the pH values of these buffer systems range from pH 7.5 to pH 10.0, depending on polymerases used.
• MgCl 2 is generally included in these buffer systems at a concentration ranging from 1.0 to 5.0 mM.
• KCI may also be included at a concentration of 20-80 mM.
• Certain amount of mercaptoethanol (0.5-1.5%), Tween 20 (0.2-0.4%) or Triton-X 100 (0.1-0.5%) may also exist in the buffers.
• a typical reassembly cycle consists of three steps: the first step is heat denaturation of the double- stranded target nucleic acid.
• the exact conditions required for denaturation of the sample nucleic acid depend on the length and composition of the sample nucleic acid. Typically, an incubation at 90°C-100°C for about 10 seconds up to 5 minutes is efficient to denature the sample nucleic acid.
• the annealing temperature used in reassembly reaction is about 40°C to 70°C, usually ranging from about 55°C to 65°C and lasting for a period of 15 second to 60 seconds.
• the elongation is usually done under conditions sufficient to provide for polymerization of nucleotides to the fragment ends.
• the temperature of the reaction mixture will typically be maintained at a temperature ranging from about 65°C to 75°C, more preferably at 68°C to 72°C for about 15 second to 2 minutes, preferably for 30 seconds to 1 minute depending on the length of the finally reassembled DNA.
• the number of thermocycling can be from about 15 to about 55 cycles, depending on the amount of template and the length of the finally reassembled DNA.
• the concrete cycle number can be easily determined according to the quality and quantity of the newly synthesized fragments.
• thermostable polymerases with different synthesis fidelity can be used for the reassembly, depending on what kind of the error rate the final reassembly product should have.
• Each of these enzymes is usually used under its optimized conditions to promote the reassembly. The goal is that the resulting full-length genes will have diverse sequences, most of which, however, still resemble that of the original template DNA.
• sequences obtained after the reassembling step can be further amplified by a conventional PCR and cloned into a vector for expression.
• Suitable vectors are known to a person skilled in the art and vectors cited in the following references are herein incorporated by reference Kingsman SM, Kingsman AJ. Philos Trans. R. Soc. Lond B. Biol. Sci. 1989, 324(1224):477-485; Bailey JE. Adv. Biochem. Engl. Biotechnol. 1993, 48:29-52.
• Suitable expression systems are known in the art as well and expression systems cited in the following references are herein incorporated by reference Shatzman AR, Rosenbrg M. Methods Enzymol. 1987; 152:661-73.
• Screening or selection of the expressed mutants should lead to variants with improved or even new specific functions. Suitable screening and selection systems are known in the art and screening and selection systems cited in the following references are herein incorporated by Kuchner O, Arnold FH. Trends Biotechno. 1997 Dec; 15(12):523-30; Patel PH, Loeb LA. Procc. Natl. Acad. Sci. USA, 2000 May 9; 97(10):5095-100. These variants can be immediately used as partial solutions to a practical problem, or they can serve as new starting points for further cycles of directed evolution.
• WalkThrough chains are a population of fragments that each stops in every position, they are uniform in their positional preference and lack a sequence bias.
• the sequence heterogeneity allows both, mutations and crossover may happen more randomly than, for example, with error-prone PCR or DNA 'shuffling'.
• (4) Normal error-prone PCR and DNA shuffling can not efficiently recombine or dissect two or more mutations if they are very close to each other. (Stemmer, W. P. C. 1994a. Rapid evolution of a protein in vitro by DNA shuffling. Nature, 370: 389-391).
• Walk Through approach allows recombination occuring at every position of templates and therefore, provides possibility to recombine or dissect two or more mutations although they may be very close to each other.
• the Walk-Through DNA synthesis is based on the chain elongation guided by template, and the nascent strands are synthesized from the 3'-OH termini at the primers using polymerase and the four deoxynucleoside triphosphates and stop after the ddNTP incorporation.
• the reaction is independent of the length of the DNA template. This is particularly useful for engineering small peptides or large enzymes or even enzyme pathways.
• DNase I is an endonuclease that hydrolyzes double-stranded DNA preferentially at sites adjacent to pyrimidine nucleotides
• its use in DNA shuffling (Stemmer, W. P. C. 1994a.. Nature, 370: 389-391; Stemmer, W. P. C. 1994b. Proc. Natl. Acad. Sci, USA, 91:10747-10751) may result in bias (particularly for genes with high G+C or high A+T content) at the step of template gene digestion. Effects of this potential bias on the overall mutation rate and recombination frequency have not yet been investigated, but they may be avoided by using the Walk- Through approach.
• thermostable DNA polymerases Since there are dozen of polymerases currently available, the synthesis of the nascent DNA fragments with randomly distributed 3' end can be achieved in more different fashions.
• bacteriophage T4 DNA polymerase Nossal, N.G. 1974. J. Biol. Chem. 249: 5668-5676
• T7 sequenase version 2.0 DNA polymerase Tabor, S. and Richardson, C. C. 1987. Proc. Natl. Acad. Sci., USA, 84:4767-4771, Tabor, S. and Richardson, C. C. 1989. J. Biol. Chem. 264:6447-6458
• thermostable DNA polymerases can be used for the WalkThrough synthesis.
• reverse trans- criptase is preferred for the WalkThrough synthesis. Since this enzyme lacks 3'- 5' exonuclease activity, it is therefore prone to error. In the presence of high concentrations of dNTPs and 2+
• One of the key steps in our technique is to control the 3' end of the nascent, single-strand DNA synthesized during the WalkThrough process. Under certain conditions, this step may be used for efficient terminal and/ or internal insertion/ deletion, resulting in molecules with different sizes. Efficient changing target molecule sizes is normally unachievable through error-prone PCR or DNA shuffling.
• PCR can be adjusted for the WalkThrough synthesis using thermostable polymerase for the short, nascent DNA fragments.
• thermostable polymerase for the short, nascent DNA fragments.
• inventive method of the present invention may, however, be combined with other known methods of the art if this seems to be advantageously.
• WalkThrough DNA synthesis was used to generate DNA fragments with randomly distributed 3' ends from denatured, linear, double-stranded DNA (e.g., restriction fragments purified by gel electrophoresis) to Bacillus subtilis BMTU3420 sarcosine oxidase gene.
• the purified DNA mixed with a molar excess of primers, was denatured, and synthesis was then carried out using the Stoffel fragment.
• This enzyme lacks 5'-»3' exonuclease activity, so that the WalkThrough product was synthesized exclusively by extension and was not degraded by exonuclease (see Example 1).
• the WalkThrough recombination technique is further demonstrated with another example of re- combining human placental alkaline phosphatase and calf intestinal alkaline phosphatase. (see Example 2).
• the step of removing chain-terminators and the step of fragment reassembly step can be done at the same time.
• the technique described here maybe used to explore the vast space of potentially useful catalysts for their optimal performance in a wide range of applications as well as to develop or evolve new enzymes for basic structure-function studies.
• Reverse transcriptases are derived from retrovirus, such as avian myelo- blastosis virus (AMV) or Moloney murine leukemia virus (MMLV), which use them to make DNA copies of their RNA genomes.
• AMV and MMLN reverse transcriptases (Nerma IM.
• the DNA of interest with appropriate restriction endonuclease(s) and purify the DNA fragment of interest was purified by gel electrophoresis using Roche High Pure PCR Prep Kit (Roche Diagnostics GmbH, Germany)).
• Roche High Pure PCR Prep Kit Roche High Pure PCR Prep Kit (Roche Diagnostics GmbH, Germany)
• Bacillus subtilis BMTU3420 sarcosine oxidase gene was cleaved as a 1.2 kb-long Pstl-Nsil fragment from the recombinant plasmid pBMTU5823.
• reaction products were subjected to Wizard DNA Clean up System (Promega, Wl, USA) to remove the enzymes, primers, reaction buffer components and dNTPs/ddNTPs.
• the purified products were incubated in 1 x Klenow buffer and 10 U of Klenow at 37°C for 40 minutes. Two ⁇ l of Dpnl (10 U/ ⁇ l) was added to the mixture, and the incubation was continued at 37°C for further 40 minutes.
• the digested WallcThrough products were purified with High Pure PCR Purification Kit (Roche Diagnostics GmbH, Germany) and was used for whole gene reassembly.
• thermocycles were performed, each with 1.0 min at 95°C, 1.0 min at 55°C and 0 + 5 sec/cycle at 72°C, with the extension step of the last cycle proceeding at 72°C for 10 min, in an Eppendorf Master cycler Gradient (Eppendorf, Germany).
• the correctly reassembled product of this first PCR was further amplified in a second PCR reaction which contained the PCR primers complementary to the ends of the template DNA.
• thermocycles were performed, each with 1.0 min at 95°C and 1.0 min at 72°C, with the extension step of the last cycle proceeding at 72°C for 10 min, in an Eppendorf Mastercycler Gradient (Eppendorf GmbH, Germany).
• E. coli XL1 F' cells were transformed with the above ligation mixture to form a mutant library.
• Walk through recombination' is a method used to recombine two or more DNA-sequences based on their homology. Furthermore point mutations can also be introduced during the recombination steps. This method consists mainly of five steps:
• Fragment synthesis results from a DNA synthesis reaction where extension is terminated by the incorporation of dideoxy nucleotides.
• Target DNA sequences selected for recombination serve as templates in this reaction.
• the number of primers should be chosen depending on the length of the target DNA sequences and the length of the resulting fragments.
• the fragment length may also be controlled by the reaction conditions.
• a ,Cycle Sequencing reaction' is favorable because of its higher product yield and easy use.
• U-DNA and Uracil-DNA Glycos lase can be used to remove uracil base at any site where a deoxyuridylate has been incorporated (U-DNA).
• U-DNA deoxyuridylate has been incorporated
• the resulting abasic site can subsequently be hydrolyzed by alkali- treatment, high temperatures or specific endonucleases. In our approach, a simple temperature treatment is sufficient.
• U-DNA can be prepared by a PCR reaction using dUTP's instead of dTTP's. After the U-DNA has served as the template for the fragment synthesis, the whole reaction is subjected to the uracil-DNA glycosylase treatment and temperature treatment. Another control PCR can be used to ensure the complete U-DNA removal.
• the fragments need to have 3'-OH end for the reassembly reaction. Since our fragments are terminated with dideoxynucleotides, they do not carry 3' -OH ends. These terminators can be removed by 3'-5'-exonuclease activity of several nucleases or DNA polymerases resulting in 3'-OH end. We combine terminator removal with the reassembly by employing a thermostable exonuclease III and Taq polymerase for this step.
• Exonuclease III cuts the terminator by it's 3'-5'-exonuclease activity (enzyme activity only at ds DNA, and therefor only after DNA annealing) and Taq polymerase extends the fragment by it's 5'-3'-polymerase activity.
• Reassembly is a PCR-like reaction without primers, there the fragments anneal to each other based on their homologies and extend. Through all cycles of denaturation, annealing and extension fragments growth or reassemble up to the length of the original DNA sequences.
• Amplifying the reassembled DNA takes place in a PCR with sequence flanking primers in order to provide enough material for subsequent cloning and analysing steps.
• the synthesized U-DNA is separated from the template DNA by preparative agarose gel electrophoresis (1% agarose/TAE) and gel extraction (, QIAquick Gel Extraction Kit' Qiagen # 28706).
• Cycle sequencing reactions (,DIG Taq DNA Sequencing Kit for Standard and Cycle Sequencing' Roche # 1449443) with some modifications. Twelve reaction mixtures are set up for a total of six primers and two U-DNA templates. A typical reaction mixture with cycling conditions used for recombining the two alkaline phosphatase genes are shown as following:
• Taq DNA polymerase (5U/ ⁇ l) 0,6 ⁇ l
• Termination mixture ddATP/dGTP 2,5 ⁇ l Termination mixture ddCTP/dGTP 2,5 ⁇ l
• the calf intestinal alkaline phosphatase gene (ciap) and the human placental alkaline phosphatase gene (hpap) show a sequence similarity of 81 % and have lengths of about 1530 bps.
• Six primers are used for fragment synthesis.
• Two external primers (APhpaF and APxbaR) share the same sequence.
• Figure 2 shows the primer arrangement along human placental alkaline phosphatase (hpap) and calf intestinal alkaline phosphatase (ciap) genes.
• This step is accomplished by addition of 1 ⁇ l Uracil-DNA Glycosylase ( 1 U/ ⁇ l; Roche , #1775375) to each fragment synthesis mixture and by incubation at 37 °C for 4 h to cleavage the U-bases followed by incubation at 95 °C for 2 min to hydrolyse the abasic sites and to inactivate the enzyme.
• Uracil-DNA Glycosylase 1 U/ ⁇ l; Roche , #1775375
• DNA fragments are purified from the cleavage products by using Microcon (Microcon 50; Millipore # 42415) and are retained in H 2 O.
• Microcon Microcon 50; Millipore # 42415.
• a typical mixture and cycling conditions used for recombining the two alkaline phosphatase genes are shown below:
• the amplification PCR is a standard PCR with the two sequence flanking primers (AphpaF and ApxbaR) .
• the following is a typical mixture with cycling conditions used for recombining the two alkaline phosphatase genes:
• Taq DNA polymerase (Roche # 1 418 432) 2,5 U dATP, dCTP, dGTP, dTTP (Roche # 1 969064) 0,2 mM each
• the amplification product is purified by preparative gel electrophoresis (1% agarose/TAE) and gel extraction (,QIAquick Gel Extraction Kit' Quiagen # 28706), cloned into vector pCR®-XL-TOPO® according to the suppliers manual (Invitrogen # K 475020) and expressed in E. coli TOP 10 cells delivered with the vector.
• Figure 3 shows the N-terminal sequence ahgnment of parental ciap and hpap genes and their recombination variants (APOl, AP03, AP05, AP06, AP11 and AP15)
• Figure 4 shows the C-terminal sequence alignment of parental parental ciap and hpap genes and their recombination variants (AP2, AP9 and AP13)
• Figure 5 shows sequences of hpap, ciap, APOl, AP03, AP05, AP06, APll, AP15, AP2, AP9 and AP13.

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• Plant Pathology (AREA)
• Enzymes And Modification Thereof (AREA)
• Measuring Or Testing Involving Enzymes Or Micro-Organisms (AREA)
• Preparation Of Compounds By Using Micro-Organisms (AREA)
• Peptides Or Proteins (AREA)

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• 2002
  • 2002-06-26 EP EP02747439A patent/EP1404825A2/en not_active Withdrawn
  • 2002-06-26 JP JP2003509098A patent/JP3967319B2/ja not_active Expired - Fee Related
  • 2002-06-26 WO PCT/EP2002/007060 patent/WO2003002736A2/en active Search and Examination
  • 2002-06-26 CA CA002453864A patent/CA2453864A1/en not_active Abandoned

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