Method for cloning and producing the AatII restriction endonuclease and methylase

Abstract: The present invention provides a novel approach to the production of restriction endonucleases. More specifically, there is provided a novel method for the overexpression of these enzymes, which comprises transforming a host cell with an expression vector containing DNA coding for the restriction endonuclease of interest and a vector for the expression of a methylase which is capable of protecting the host cell DNA against the restriction endonuclease of interest. The methylase vector is compatible with the restriction endonuclease expression vector. Preferably, the expression vector is a T7 expression vector and the cell is transformed with a third compatible vector which regulates the expression vector. The cloning and overexpression of the Aat II restriction endonuclease and methylase is also described in detail.

Description: The present invention relates to recombinant DNA which encodes the Aat II restriction endonuclease and modification methylase, and the production of these enzymes from the recombinant DNA. Type II restriction endonucleases are a class of enzymes that occur naturally in bacteria. When they are purified away from other contaminating bacterial components, restriction endonucleases can be used in the laboratory to cleave DNA molecules into precise fragments for molecular cloning and gene characterization. Restriction endonucleases act by recognizing and binding to particular sequences of nucleotides (the 'recognition sequence') along the DNA molecule. Once bound, they cleave the molecule within, or to one side of, the recognition sequence. Different restriction endonucleases have affinity for different recognition sequences. Over one hundred and fifty restriction endonucleases with unique specificities have been identified among the many hundreds of bacterial species that have been examined to date. Bacteria tend to possess at most, only a small number of restriction endonucleases per species. The endonucleases typically are named according to the bacteria from which they are derived. Thus, the species Deinococcus radiophilus for example, synthesizes three different restriction endonucleases, named Dra I, Dra II and Dra III. These enzymes recognize and cleave the sequences TTTAAA, PuGGNCCPy and CACNNNGTG respectively. Escherichia coli RY13, on the other hand, synthesizes only one enzyme, Eco RI, which recognizes the sequence GAATTC. While not wishing to be bound by theory, it is thought that in nature, restriction endonucleases play a protective role in the welfare of the bacterial cell. They enable bacteria to resist infection by foreign DNA molecules like viruses and plasmids that would otherwise destroy or parasitize them. They impart resistance by cleaving invading foreign DNA molecule each time that the recognition sequence occurs. The cleavage that takes place disables many of the infecting genes and renders the DNA susceptible to further degradation by non-specific endonucleases. A second component of bacterial protective systems are the modification methylases. These enzymes are complementary to restriction endonucleases and they provide the means by which bacteria are able to protect their own DNA and distinguish it from foreign, infecting DNA. Modification methylases recognize and bind to the same nucleotide recognition sequence as the corresponding restriction endonuclease, but instead of cleaving the DNA, they chemically modify one or other of the nucleotides within the sequence by the addition of a methyl group. Following methylation, the recognition sequence is no longer bound or cleaved by the restriction endonuclease. The DNA of a bacterial cell is always fully modified by virtue of the activity of its modification methylase. It is therefore completely insensitive to the presence of the endogenous restriction endonuclease. It is only unmodified, and therefore identifiably foreign DNA, that is sensitive to restriction endonuclease recognition and cleavage. With the advent of genetic engineering technology, it is now possible to clone genes and to produce the proteins and enzymes that they encode in greater quantities than are obtainable by conventional purification techniques. The key to isolating clones of restriction endonuclease genes is to develop a simple and reliable method to identify such clones within complex 'libraries', i.e. populations of clones derived by 'shotgun' procedures, when they occur at frequencies as low as 10 -3 to 10 -4 . Preferably, the method should be selective, such that the unwanted majority of clones are destroyed while the desirable rare clones survive. Type II restriction-modification systems are being cloned with increasing frequency. The first cloned systems used bacteriophage infection as a means of identifying or selecting restriction endonuclease clones ( Eco RII: Kosykh et al., Molec. gen. Genet 178: 717-719, (1980); Hha II: Mann et al., Gene 3: 97-112, (1978); Pst I: Walder et al., Proc. Nat. Acad. Sci. 78 1503-1507, (1981)). Since the presence of restriction-modification systems in bacteria enable them to resist infection by bacteriophages, cells that carry cloned restriction-modification genes can, in principle, be selectively isolated as survivors from libraries that have been exposed to phage. This method has been found, however, to have only limited value. Specifically, it has been found that cloned restriction-modification genes do not always manifest sufficient phage resistance to confer selective survival. Another cloning approach involves transferring systems initially characterized as plasmid-borne into E. coli cloning plasmids ( Eco RV: Bougueleret et al., Nucl. Acid. Res. 12: 3659-3676, (1984); Pae R7: Gingeras and Brooks, Proc. Natl. Acad. Sci. USA 80: 402-406, (1983); Theriault and Roy, Gene 19: 355-359 (1982); Pvu II: Blumenthal et al., J. Bacteriol. 164: 501-509, (1985)). A third approach, and one that is being used to clone a growing number of systems are now being cloned by selection for an active methylase gene (refer to our EPO No.: 192,413 published September 3, 1986 and Bsu RI: Kiss et al., Nucl. Acid. Res. 13: 6403-6421, (1985)). Since restriction and modification genes are often closely linked, both genes can often be cloned simultaneously. This selection does not always yield a complete restriction system however, but instead yields only the methylase gene ( Bsp RI: Szomolanyi et al., Gene 10: 219-225, (1980); Bcn I: Janulaitis et al, Gene 20: 197-204 (1982); Bsu RI: Kiss and Baldauf, Gene 21: 111-119, (1983); and Msp I: Walder et al., J. Biol. Chem. 258: 1235-1241, (1983)). In some systems the cloning problem may lie in trying to introduce the endonuclease gene into a host not already protected by modification. If the methylase gene and endonuclease gene are introduced on a common DNA fragment, the methylase gene must modify or protect the host before the endonuclease gene cleaves the host's genome. Another obstacle to cloning these systems in E. coli was discovered in the process of cloning diverse methylases. Many E. coli strains (including those normally used in cloning) have systems that resist the introduction of DNA containing cytosine methylation. (Raleigh and Wilson, Proc. Natl. Acad. Sci., USA 83: 9070-9074, (1986)). Therefore, it is also necessary to carefully consider which E. coli strain(s) to use for cloning. When foreign restriction modification systems are cloned and introduced into E. coli , sometimes the endonuclease yield is very low compared to the native endonuclease-producing strain probably due to inefficient transcription or translation of the gene in E. coil . In some cases, E. coli cells carrying cloned restriction modification system grow poorly which is probably caused by insufficient methylation protection and unregulated constitutive expression of the restriction endonuclease gene. Therefore, a tightly regulated expression system would be desirable in order to express in E. coli toxic genes such as restriction endonuclease genes. It would be desirable to have an expression system which will produce a minimal level of endonuclease under noninduced conditions and produce large amounts of the desired endonuclease upon induction. In this way foreign restriction modification systems can be stably maintained in E. coli host. Because purified restriction endonucleases, and to a lesser extent, modification methylases, are useful tools for characterizing genes in the laboratory, there is a commercial incentive to obtain bacterial strains through recombinant DNA techniques that synthesize these enzymes in abundance. Such strains would be useful because they would simplify the task of purification as well as providing the means for production in commercially useful amounts. SUMMARY OF THE INVENTION The present invention relates to recombinant DNA encoding the genes for the Aat II restriction endonuclease and modification methylase obtainable from Acetobacter aceti as well as related methods for the production of Aat II restriction enzyme from the recombinant DNA. This invention also relates to a transformed host which expresses the restriction endonuclease Aat II, an enzyme which recognizes the DNA sequence 5'-GACGTC-3' and cleaves between T and C on both DNA strands to leave a 4 base 3' overhang (Sugisaki, H. et al. Nucl. Acids Res. 10: 5747-5752). Aat II endonuclease produced in accordance with the present invention is substantially pure and free of the contaminants normally found in restriction endonuclease preparations made by conventional techniques as described in Example 1. One preferred method for cloning the Aat II restriction-modification system comprises: selecting an appropriate vector, forming several libraries containing DNA from Acetobacter aceti , isolating those clones which contain DNA coding for the Aat II modification methylase, cloning additional chromosomal DNA adjacent to the methylase gene using a vector, screening for those clones that produce Aat II endonuclease, mapping the aatIIR and aatIIM genes on the clone ( aatIIR and aatIIM , genes coding for Aat II endonuclease and Aat II methylase, respectively), determining the DNA sequences of aatIIR and aatIIM genes, and overexpressing aatIIR and aatIIM genes in E. coli . BRIEF DESCRIPTION OF THE FIGURES Figure 1 is a scheme for cloning and producing the Aat II restriction endonuclease. Figure 2 is the DNA sequence of aatIIM gene (SEQ ID:11) and its encoded protein sequence (SEQ ID:12). Figure 3 is the DNA sequence of aatIIR gene (SEQ ID:13) and its encoded protein sequence (SEQ ID: 14). Figure 4 shows IPTG-induced and uninduced cell extracts and purified Aat II endonuclease protein. Arrow A and B indicate Aat II endonuclease protein and bovine serum albumin (BSA), respectively. DETAILED DESCRIPTION OF THE INVENTION This invention relates to recombinant DNA which encodes the Aat II restriction endonuclease and modification methylase, as well as to the method of producing Aat II restriction endonuclease from such a recombinant DNA. The method described herein by which the Aat II restriction gene and methylase gene are preferably cloned and expressed is illustrated in Figures 1 and includes the following steps: 1. The genomic DNA of Acetobacter aceti is purified. 2. The DNA is digested partially with a restriction endonuclease such as Hind III, or any of its isoschizomers, that generates a DNA fragment(s) containing the entire Aat II methylase gene. Alternatively, one could make a library which contains the entire restriction modification system such as Nde I genomic library (skip to step 7). The fragment(s) should also be of cloneable size, that is, between 1.5-14kb. 3. The Hind III-digested genomic DNA is ligated into pUC19 cloning vector. The resulting mixtures are used to transform an appropriate host, i.e. a hsdR - , mcrBC - , mrr - strain, such as E. coli strain RR1. The DNA/cell mixtures are plated on ampicillin selective media for transformed cells. After incubation, the transformed cell colonies are harvested together to form the primary cell libraries. As described above, a total of 10 such primary cell libraries were constructed using complete or partial digestion of the Acetobacter aceti DNA by the respective cloning endonuclease, and host strains. 4. The recombinant plasmids are purified in toto from the primary cell libraries to make primary plasmid libraries. The purified plasmid libraries are then digested to completion in vitro with Aat II endonuclease which is prepared from Acetobacter aceti cells, or any Aat II isoschizomer. Aat II endonuclease digestion causes the selective destruction of unmodified, non-methylase-containing clones, resulting in an increase in the relative frequency of Aat II methylase-carrying clones. 5. Identification of Aat II methylase clones: The digested plasmid library DNA is transformed back into a host such as E. coli strain RR1 and transformed colonies are again obtained by plating on ampicillin plates. The colonies are picked and their DNA is analyzed for the presence of the Aat II methylase gene by incubating purified plasmid DNA in vitro with Aat II endonuclease to determine whether it is resistant to Aat II digestion. 6. Once it has been established that the methylase gene has been cloned, the clone is assayed for Aat II endonuclease activity. If activity is detected, then the Aat II restriction gene is linked to the methylase gene and is present in the clone. In such a case one could then skip to step 10 below. However, in accordance with the present invention, no restriction activity was detected. The lack of restriction activity indicates that either the restriction gene is not linked to the methylase gene, or it is linked but not cloned intact with the methylase gene, or it is cloned intact but not expressed or the entire system has been cloned but expression of the methylase gene is insufficient to fully protect against the restriction endonuclease and the only clones that survive the selection have undergone spontaneous mutations or deletions in the endonuclease gene.. In order to determine which of the above possibilities is the situation, the cloned fragment is restriction-mapped and deletions are made to determine where the relative position of the methylase gene within the cloned fragment. The information is then used to determine if there is enough DNA on either side of the methylase gene to encode a restriction gene, if they are linked. If there is enough length of DNA, the restriction gene is assumed not to be linked, or to be present in the clone but not expressed (skip to step 10). If there is not enough DNA on both sides of the methylase gene in the cloned DNA to encode a linked restriction gene, as was found for the Hind III clone, p(UC) A atIIM18, of the present invention, a portion of the methylase gene is used to probe digests of the Acetobacter aceti chromosome to generate by Southern hybridization a genomic map of the region extending beyond the boundaries of the existing cloned DNA. This data helps identify certain endonucleases such as Nde I that cleave the restriction-modification region into individual fragments that carry the methylase gene as well as larger amounts of adjacent DNA. The exact sizes of the fragments generated by such endonucleases are calculated from the data as well. Presumably, if the restriction and modification genes are found to be linked, such fragments would also encode the restriction gene. 7. Enriched libraries are constructed by digestion of Acetobacter aceti DNA with Nde I and ligating the generated fragments into an appropriate vector such as pUC19. Clones carrying DNA the methylase gene as well as the adjoinig regions can be isolated by methylase selection (EPO No.: 193,413 published September 3, 1986 and Bsu RI: Kiss et al., Nucl. Acid. Res. 13: 6403-6421, (1985)). 8. Identification of restriction gene clones: After methylase selection and transformation, ampicillin resistant transformants are screened for DNA insert that extend beyond the boundaries of the existing methylase clone. Once such clones are identified, crude cell extracts are prepared from these clones, cell extracts diluted by serial dilution, and assayed for Aat II endonuclease activity. Clones carrying both aatIIM and aatIIR genes are identified by Aat II endonuclease activity assay of their cell extracts and the ability to confer vector backbone resistance to Aat II endonuclease cleavage. 9. The region containing aatIIR and aatIIM genes were mapped by deletion mapping and subcloning and DNA sequence determined by dideoxynucleotide termination method (Sanger,F., et. al., Proc. Natl. Acad. Sci., 1977, 74:5463-5467). 10. Overexpression: One prefered approach for overexpression of Aat II restriction endonuclease and/or other restriction endonucleases comprises 1) constructing an increased copy number mutant plasmid containing, for example, the pSC101 replication origin and cloning the aatIIM gene or other methylase gene into the above made medium-copy-number plasmid to premodify and protect an E. coli host. The aatIIM gene ( or other methylase genes) is expressed from lac promoter. 2) for aatIIR , the restriction endonuclease gene can be inserted downstream of a T7lac promoter on an expression vector, such as pSYX22 (the construction of pSYX22 will be described in detail in example 1). This may be accomplished by incorporating restriction sites in primers to amplify the endonuclease gene by polymerase chain reaction (Saiki, R. K. et al. Science 230: 1350-1354) or introducing restriction sites in the beginning and end of the endonuclease gene by site-directed mutagenesis (Kunkel, T. A. Proc. Natl. Acad. Sci. USA, 82: 488-492. (1985)). In addition, a strong ribosome binding site (Shine & Dalgarno, Proc. Natl. Acad. Sci. USA 71, 1342-1346 (1974)) can be placed in front of the gene to increase translation efficiency. Other promoters which can be used for endonuclease expression are P lac , P lacUV5 , P tac , P R , T3 and λP L on pUC19 and pBR322 derivatives. But these promotors are not tightly repressed under noninduced condition and therefore they give a low induction ratio. The DNA sequence of the aatIIR and aatIIM genes can be synthesized by conventional phosphoramidite or phosphotriester chemistry using codons that are more efficiently utilized in E. coli . (Applied Biosystems user bulletins #13, 1988; Usman, N. et al., J. Am. Chem. Soc. 109: 7845-7854, 1987). 11. Aat II production and purification: The Aat II methylase or endonuclease may be produced from clones carrying the Aat II methylase gene (or a heterologous methylase) and the overexpressed restriction endonuclease gene by propagation in a fermenter in a rich medium containing antibiotics, for example ampicillin, chloramphenicol, and kanamycin. The cells are thereafter harvested by centrifugation and disrupted by sonication to produce a crude cell extract containing Aat II methylase and restriction endonuclease activity. The crude cell extract containing the Aat II methylase and endonuclease is purified by standard protein purification techniques such as affinity-chromatography, gel filtration, or ion-exchange chromatography. The following example is given to illustrate embodiments of the present invention as it is presently preferred to practice. It will be understood that this example is illustrative, and that the invention is not to be considered as restricted thereto except as indicated in the appended claims. EXAMPLE 1 1. Cloning of Aat II restriction and modification system: The aatIIM gene was first cloned in a Hind III fragment using methylase seletion by challenging a plasmid library with Aat II endonuclease and looking for survivors after transformation. As discovered later, only part of the aatIIR gene was present in this Hind III insert. Restriction mapping of the Acetobacter aceti genomic DNA indicated that an Nde I fragment should extend one end of the DNA adjacent to the aatIIM gene. The Aat II restriction-modification system, aatIIR and aatIIM genes, was cloned in an Nde I restriction fragment in pUC19 vector by the methylase selection method. This clone was named p(UC)AatIIR + M + 18 which makes 2000 units of Aat II endonuclease per gram of wet E. coli cells. p(UC)AatIIR + M + 21 contains the same Nde I insert but in the opposite orietation. 2. Restriction mapping of aatIIM and aatIIR genes: The aatIIM and aatIIR genes were mapped as follows: There are two XbaI sites (about 300 bp apart) in the 5500-bp Nde I fragment insert and one Xba I site in the multiple cloning site of pUC19. An Xba I fragment deletion subclone (a deletion of about 1000 bp) was constructed which deleted out the two Xba I restriction fragments. Two µg of p(UC)AatIIR + M + 18 was digested with 20 units of Xba I in a 1X restriction buffer (50 mM NaCl, 10 mM Tris-HCl, 10 mM MgCl 2 , 1 mM dithiothreitol) in 50 µl at 37°C for 1 hour. The digested DNA was extracted with equal volume of phenol-CHCl 3 and CHCl 3 and precipitated with ethanol and dried. 0.2 µg of the resulting DNA was ligated with T4 DNA ligase a 50 µl in a 1x ligation buffer (50 mM Tris-HCl, 10 mM MgCl 2 , 10 mM dithiothreitol, 1 mM ATP 25 µg /ml of bovine serum albumin) at 16°C overnight. The ligated DNA was diluted by addition of equal volume of sterile-distilled water. 10 µl of the diluted DNA was mixed with 0.1 ml of ER1821 Δ( hsd-mrr-mcrBC ) competent cells and incubated at 4°C for 30 minutes, 42°C for 3 minutes. After addition of 0.1 ml of LB medium and incubation at 37°C for 1 hour the cell/DNA mixture was plated on LB agar plate plus 50 µg/ml of ampicillin (Ap) and incubated at 37°C overnight. 12 individual ampicillin resistant AP r ) transformants were picked and inoculated into 2 ml of LB plus Ap and shaken at 37°C overnight. 1.5 ml cells each were centrifuged in 1.5 ml Eppendorf tube to make plasmid DNA by a mini-purification procedure adapted from the method of Birnboim and Doly (Nucleic Acids Res. 7: 1513-1523 , (1979)). Plasmid mini-preparation procedure: 1.5 ml of culture grown overnight was pelleted at 6,000xg for 3 minutes. The supernatant was poured off and the cell pellet was resuspended in 0.2 ml of 25 mM Tris-HCl, pH 8.0, 10 mM EDTA, 50 mM glucose, 20 µg/ml RNaseA. After five minutes at room temperature, 0.2 ml of 0.2 M NaOH, 1% SDS was added and the tube was inverted to lyse the cells. After five minutes at room temperature, 0.15 ml of 3 M sodium acetate, pH 4.8, was added and inverted to mix well. The precipitate that formed was spun down at 12,000xg, 4°C for 10 minutes. The supernantant was removed and extracted with an equal volume of phenol/chloroform (1:1). The layers were separated by centrifugation at 12,000xg for five minutes. The upper phase was taken into a new centrifuge tube and extracted with equal volume of chloroform. The DNA was mixed with 0.9 ml of 95% ethanol. The tube was spun at 12,000xg for 10 minutes to pellet the precipitated nucleic acids. The supernatant was discarded and the pellet was washed again with 1 ml of 70% ethanol, repelleted and dried for 15 minutes under vacuum. Once dry, the pellet was resuspended in 100 µl of TE buffer (10 mM Tris-HCl, 1 mM EDTA, pH 8.0).Twelve plasmid DNA were analyzed by Xba I endonuclease digestion and agarose gel electrophoresis to confirm the Xba I deletion. All of them contain the correct size deletion. The Xba I deletion clone, p(UC)AatIIM + (XbaIΔ), still contains the intact aatIIM gene because when challenged with AatII endonuclease the plasmid DNA is resistant to Aat II cleavage. To further narrow down the size of aatIIM gene an Eco O109I fragment was deleted between the Eco O109I site on the vector and an Eco O109I site on the remaining insert. This deletion was constructed in the same way as the Xba I deletion except the Eco O109I ends were first filled-in with Klenow fragment of E.coli DNA polymerase I and then self-ligated. The resulting Eco O109I deletion clone was partially resistant to Aat II endonuclease digestion, indicating that the aatIIM gene is still contained in the clone. The XbaI deletion clone and the Eco O109I deletion clone defined the aatIIM within a DNA fragment of approximately 1800 bp. The XbaI deletion clone was also assayed for Aat II endonuclease activity to see if it contains the intact aatIIR gene. Crude cell extract was prepared as follows: 200 ml LB plus Ap were inoculated with 2 ml of cells grown overnight and shaken at 37°C overnight. The cells were centrifuged and cell pellet resuspended in 20 ml sonication buffer (10 mM Tris-HCl, 10 mM β-mercaptoethanol), sonicated for ten times at 30 seconds burst, centrifuged at 15000xg for 30 minutes to remove cell debris. 1 µl, 2 µl, 4 µl, and 8 µl of cell extract was incubated with 1 µg of λ DNA at 37°C for one hour in 30 µl in a 1x Aat II restriction buffer (50 mM potassium acetate, 20 mM Tris actate, 10 mM magnesium acetate, 1 mM dithiothreitol). Three µl of stop sulution was added and DNA was analyzed by agarose gel electrophoresis. No Aa tII endonuclease activity was found in the XbaI deletion clone, indicating part of the endonuclease gene has been deleted.In order to map the boundaries of aatIIR gene, the aatIIM gene was first cloned into a vector pR976, a pACYC184 derivative containing P tac and multiple cloning sites (constructed by Paul Riggs, New England Biolabs, Beverly, MA). A 3000 bp Pst I fragment containing the aatIIM gene was inserted into the Pst I site of pR976. The resulting plasmid, pR976AatIIM + , was transformed into E. coli strain ER1821 Δ( hsd-mrr-mcrBC ). Deletion subclones were transformed into premodified ER1821 [p(R976)AatIIM + ] to see whether they contain intact aatIIR gene.There are two Hinc II sites in p(UC)AatIIR + M + 18, one in the multiple cloning sites of the vector and the other in the insert. A Hinc II deletion, a deletion of about 4500 bp, was constructed in the same way as the Xba I deletion clone. When this plasmid was transformed into ER1821 [p(R976)AatIIM + ] cells, and-cell extracts were prepared from the transformants, no Aat II endonuclease activity was detected. Another deletion clone contains an Eco RV/ Sma I fragment deletion which deleted about 4200 bp DNA. This deletion clone shows Aat II endonuclease activity in crude cell extract. These results showed that the aatIIR gene is within a DNA fragment of appoximately 1300 bp. 3. DNA sequence analysis of aatIIM and aatIIR genes: In order to sequence the DNA region containing both aatIIM and aatIIR genes, four more deletion subclones ( Bam HI fragment deletion , Eco RI fragment deletion, Hind III fragment deletion, and Pst I fragment deletion) were constructed in the same way as was done for the Xba I fragment deletion by the respective endonuclease cleavage and self-ligation. A total of 3365 bp DNA was sequenced on both stands by Sanger dideoxy-termination sequencing method (Sanger, F. et. al. Proc. Natl. Acad. Sci. U.S.A. 74: 5463-5467). Two large open reading frames, 996 bp and 1038 bp, respectively, were found in the region in which the aatIIM and aatIIR genes were mapped. Subcloning of the 996 bp open reading frame in a plasmid showed Aat II site modification and thus this open reading frame was assigned to aatIIM gene (Figure 2 and SEQ ID:11). The predicted amino acid sequence from the aatIIM gene (SEQ ID:12) showed that it contains conserved motifs for the N-6 adenine methylases (motifs DPPY and DPF-GSG-T). Subcloning of the 1038 bp open reading frame in a plasmid and transforming into Aat II methylase-modified cells showed Aat II endonuclease activity in crude cell extract. Thus this open reading frame was assigned to aatIIR gene (Figure 3 and SEQ ID:13, encoding amino acid sequence SEQ ID:14). 4. Overexpression of aatIIR gene in E. coli : A. Expression of aatIIM and aatIIR gene under λP L promoter: A Bsp EI/ Nde I DNA fragment containing both aatIIM and aatIIR genes was cloned into expression vector pDNEV5, a derivative of pUC19 that contains the λ P L promoter and λ temperature-sensitive repressor gene cI 857 (pDNEV5 constructed by Donald Nwankwo, New England Biolabs, Beverly, MA). The resulting plasmid was named p(DNEV5)AatIIR + M + . ER1821 [p(DNEV5)AatIIR + M + ] cells make 40,000 units of Aat II endonuclease per gram of wet cells after induction at 43°C overnight. The yield is 20 times higher than the native Acetobacter aceti strain. This overproducing strain, however, is not stable. It grew poorly in a 100 liter fermentor and sob cells were lysed in the culture. Therefore, construction of more stable clones were attempted. B. Expression of aatIIR gene under P tac promoter: In order to stabilize the Aat II endonuclease producing clone, the aatIIM gene contained within a Pst I/ Nde I fragment was subcloned into pUC19. The four Aat II sites in the resulting plasmid, p(UC)AatIIM + (PstI/NdeI) were completely modified by the Aat II methylase. An Eco RI restriction fragment carrying the aatIIR gene was cloned into a compatible expression plasmid pR976 under P tac promoter and Lac repressor control. When E. coli cells ER2267 [p(UC)AatIIM + ( Pst I/ Nde I), p(R976)-AatIIR + ( Eco RI/ Eco RI)] were induced by addition of 0.5 mM IPTG in a 1 liter cell culture, Aat II endonuclease yield was 10,000 units per gram of wet cells, a 5-fold overexpression over the native Acetobacter aceti strain. However, when the culture was scaled up to 100 liter fermenter, the yield dropped to 2000 units per grams of wet cells. The reason for the failure to scale up is unknown. C. Expression of aatIIR gene under P lacUV5 promoter on expression vector pRRS: 1) Cloning of aatIIM gene into pWSK129 . Plasmid pWSR129 is a low-copy-number plasmid that contains kanamycin resistance (Kan r ) gene and pSC101 replication origin which is compatible with ColEI origin carried on pUC19 and pBR322 derived vectors (Wang R.F. and Kushner, S.R. Gene, 100: 195-199, 1991). Plasmid pWSK129 also contains multiple cloning sites flanked by T7 and T3 RNA polymerase promotors. The aatIIM gene was amplified by PCR from p(UC)AatIIR + M + 21 using two primers (Primer 1 has a Bam HI site near the 5' end: 5' CG G GAT CC G GAG GAA TAA AAT GAC CGC TCG TCC 3',SEQ ID:1; Primer 2 contains a Pst I site near the 5' end: 5' CCT TAA CTG CAG TCA TTT GTT GAT ATC CAG AG 3' ,SEQ ID:2). The PCR products were purified and cleaved with Bam HI and Pst I and ligated into Bam HI/ Pst I digested pWSK129. The ligated DNA was used to transform E. coli ER2206 cells and transformants were selected for kanamycin resistance. Eighteen individual transformants were screened for aatIIM insert in mini-preparation plasmid DNA. Eleven transformants contained the correct size insert and the resulting plasmid was named p(WSK129) Aat IIM + . The plasmid is resistant to Aat II digestion, indicating full modification by Aat II methylase. 2) Cloning of aatIIR gene into expression vector pRRS2 : Expression vector pRRS is a pUC19-derived vector that contains a lacUV5 promoter, a multiple cloning sequence, and a positive retroregulator stem-loop element downstream of the multiple cloning site (Skoglund C. M. et al. Gene, 88: 1-5, (1990)). A pRRS derivative, pRRS2, was constructed by filling-in the Eco RI site distal to the lacUV5 promoter and inserting a new Eco RI linker into the Hind III site proximal to the promoter. Two primers were used to amplify the aatIIR gene from p(UC)AatIIR + M + 21. Primer 1 contains an Eco RI site and ribosome binding site (5' CT G AAT TC G GAG GTT TAA AAT ATG AAC CCA GAC GAA GTA TTT TCA 3',SEQ ID:3). A Sal I site was engineered in primer 2 (5' TCG AGG GTC GAC TTT AGG ATT CTG ATT GTG GGA 3',SEQ ID: 4). The aatIIR gene was amplified by PCR reaction with Vent® DNA polymerase (95°C for 1 minute, 50°C for 2 mintues, 72°C for 1 minute, 20 cycles). The DNA products were extracted twice with phenol-CHCl 3 and twice with CHCl 3 and ethanol-precipitated. After restriction digestion with Eco RI and Sal I for 1 hour at 37°C, The DNA was extracted again with phenol-CHCl 3 and CHCl 3 , ethanol-precipitated, dried, and resuspended in 100 µl of TE buffer. The aatIIR gene was ligated into Eco RI/ Sal I cleaved pRRS2 and transformed into ER2206 [p(WSK129)AatIIM + ]/F' lacI q , Tn10 (Tc r ). Two kinds of colony morphology were observed among the transformants, one of which is normal colonies, and the other is flat, translucent colonies. The latter contain aatIIR insert in pRRS2 and produced Aat II endonuclease in 10 ml small culture. It failed to make Aat II endonuclease in 200 ml subculture, indicating the clone is not stable. The instability is probably caused by under methylation because the copy number of pUC19-derived vector can be as high as 400 per cell, but the copy number of the methylase containing plasmid is only about 6-8 copies per cell. The other reason may be that there is not sufficient Lac repressor produced in the cell to repress aatIIR gene expression under non-induced condition since the lacI q gene is carried on the F' low-copy-number plasmid. A tightly regulated expression system is needed for Aat II expression. D. Construction of a medium-copy-number vector for expression of aatIIM gene in E. coli : In order to stably overexpress Aat II endonuclease, an increased copy-number mutant plasmid was isolated from pWSK129 as follows: XL1-Blue cells carrying pWSK129 was grown to 2 x 10 8 cells /ml in LB plus 50 µg/ml of kanamycin (Kan) and concentrated 10-fold by spining down the cells and resuspending in one tenth volume of 10 mM Tris-HCl, 10 mM MgSO 4 , pH 7.8. The cells were mutagenized with 15 seconds UV treatment (6 J per second). The mutageneized cells were plated on 2 mg /ml Kan plate which is 40 times more Kan than the regular Kan plate and incubated at 37°C overnight. Thirty-five Kan r colonies were found and pooled together and innoculated into 10 ml of LB plus 2 mg/ml Kan. To eliminate those clones that are capable of growing in 2 mg/Kan due to host mutations, plasmid DNA was prepared from 1.5 ml of culture grown overnight and retransformed into a new host ER1821 and plated on 2 mg Kan/ml plate. To estimate the plasmid copy number, Kan r transformants were grown in LB plus kan and plasmid DNA prepared from equal number of cells by a mini-preparation method in parallel with pWSK129 and pBR322 plasmids. Five ml of the pWSK129, the copy number mutant, and pBR322 DNA were analyzed by agarose gel electrophoresis. A picture was taken from the ethidium bromide stained DNA in the gel. The negative of the picture was scanned by densitometry to estimate the relative amount of DNA in each lane. It was found that one mutant (#3) increased plasmid copy number 3-fold (18-24 copies per cell). This plasmid was named pSYX19. To clone the aatIIM gene into pSYX19 two primers were designed to amplify aatIIM gene in such a way that the first primer annealed to the beginning of aatIIM gene on the top strand (Watson strand) and the second primer annealed to the end of the aatIIM gene on the bottom strand (Crick strand). Primer 1 (5' CG G GAT C CG GAG GAA TAA AAT GAC CGC TCG TCC 3',SEQ ID:1) contains a Bam HI site and an efficient ribosome binding site 6 bases upstream of the ATG start codon. Primer 2 (5' CCT TAA CTG CAG TCA TTT GTT GAT ATC CAG AG 3',SEQ ID:2) contains a Pst I site near the 5' end. The aatIIM gene was amplified with Vent® DNA polymerase (New England Biolabs, Inc.) by polymerase chain reaction (95°C for 1 minute, 50°C for 2 mintues, 72°C for 1 minute, 20 cycles). The DNA products were extracted twice with phenol-CHCl 3 and twice with CHCl 3 and ethanol-precipitated. After restriction digestion with Bam HI and Pst I for 1 hour at 37°C. The DNA was extracted again with phenol-CHCl 3 and CHCl 3 , ethanol-precipitated, dried, and resuspended in 100 µl of TE buffer. Five µg of pSYX19 vector DNA was also cleaved with Bam HI and Pst I, extracted with phenol-CHCl 3 and CHCl 3 , and ethanol-precipitated, and resuspended in 100 µl of TE buffer. Ten µl of aatIIM DNA and ten µl of the pSYX19 vector DNA was ligated overnight at 16°C in a 1X ligation buffer in 30 µl total volume. The ligated DNA was diluted by addition of 70 µl of sterile-distilled water and 10 µl DNA was used to transform ER1821 cells and plated on Kan (50 µg/ml) plate. Eighteen Kan r transformants were grown overnight in 1.5 ml LB plus kanamycin and plasmid DNA was prepared from these cultures grown overnight. Eleven out of eighteen contain the correct size insert by Bam HI and Pst I restriction digests.When five of them were digested with Aat II endonuclease, all five plasmids are resistant to Aat II digestion. One plasmid was named p(SYX19)AatIIM + and used for co-expression of Aat II endonuclease. Expression of aatIIR gene in a medium-copy-number plasmid under T7lac promoter: pET-11a is a T7 expression vector derived from pBR322 that contains a lac operator 4 bp down stream of the T7 promoter (This T7 promoter plus lac operator is called T7lac promoter), the lacI q gene, and two restriction sites, Nde I and Bam HI, for cloning (Dubendorff, J. W. and Studier, F.W., J. Mol. Biol. 219: 45-59, 1991). A pET-11a derivative, pAII17, was constructed that contains four copies of rrnB transcription terminator (pAII17 constructed by William Jack at New England Biolabs, Beverly, MA) upstream of the T7 promoter. The transcription terminator upstream of T7 promoter futher decreases basal level of expression of the target gene. Since a Bam HI site occurs in the aatIIR gene , the Bam HI site on the vector was filled-in with klenow fragment of E. coli DNA polymerase I and a Sal I linker was inserted to replace the Bam HI site. This plasmid was named pSYX22. Two primers were designed in such a way that the first primer contains an Nde I site which annealed to the start of aatIIR gene and a Sal I site was engineered into the second primer which is complementary to the end of the aatIIR gene (primer 1: 5' CAA CAT ATG AAT CCA GAC GAA GTA TTT TCA3',SEQ ID:5; primer 2: 5' TCG AGG GTC GAC TTT AGG ATT CTG ATT GTG GGA 3',SEQ ID:6). The aatIIR gene was amplified by PCR with Vent® DNA polymerase (95°C for 1 minute, 50°C for 2 mintues, 72°C for 1 minute, 20 cycles). PCR products were purified by phenol-CHCl 3 and CHCl 3 extractions and ethanol-precipitation, and cleaved with Nde I and Sal I. The DNA was again purified by phenol-CHCl 3 , and CHCl 3 extraction and ethanol-precipitation and resuspended in 100 µl of TE buffer. 10 µl of the aatIIR DNA was ligated into 0.5 µg of Nde I/ Sal I treated pSYX22 at 16°C overnight. The ligated DNA was diluted with the addition of 70 µl of sterile-distilled water and 10 µl of which was used to transform BL21 (λDE3) Δ( hsd-mrr-mcrBC ) [p(SYX19)AatIIM + , Kan r ; pLysS, Cm r ] and plated on LB agar plus Ap (50 µg/ml), Cm (30 µg/ml), Kan (50 µg/ml). Plasmid pLysS encodes phage T7 lysozyme which inhibits T7 RNA polymerase to reduce the basal level target gene expression from the T7lac promoter (Studier, F.W., J. Mol. Biol. 219: 37-44, 1991). Plasmid DNA was prepared from 1.5 ml culture of 12 individual transformants and analyzed for aatIIR DNA insert. Four plasmids contain the correct size insert. One clone, p(SYX22)AatIIR + was tested for AatII endonuclease activity as follows: 200 ml rich broth medium plus Ap (50 µg/ml), Cm (30 µg/ml), Kan (50 µg/ml) were inoculated with 2 ml of cells grown overnight and shaken at 37°C to cell density of Klett 100 (midlog phase). IPTG inducer was added to the culture to 0.5 mM final concentration. The cells were induced for two hours and harvested at Klett 175. Cells were pelleted by centrifugation at 4°C, 5000xg for 15 minutes, resuspended in 20 ml sonication buffer (10 mM Tris-HCl, pH 7.8, 10 mM β-mercaptoethanol), and sonicated on ice ten times at 30 seconds burst. Cell debris was removed by centrifugation at 4°C 15000xg for 30 minutes. The supernatant was transferred to a new tube and diluted 10 1 -, 10 2 -, 10 3 -, and 10 4 -fold in sonication buffer and 2 µl of the diluted extract was used to cleave 1 µg of Hind III-linearized pUC19 in 1x Aat II restriction buffer in a total volume of 30 µl. There is a single Aat II site in pUC19. When the Hind III-linearized pUC19 was cleaved with Aat II endonuclease, two fragments, 516 bp and 2170 bp in size, were generated. After one hour at 37 °C the reaction was stopped by addition of 3 µl of stop solution and DNA analyzed by agarose gel electrophoresis. It was found that a complete digestion was achieved with as little as a 100 fold diluted cell extract. It was estimated that this clone makes 10 6 units of Aat II endonuclease per gram of wet cells. One unit of Aat II is that amount of enzyme that will completely digest one µg of λ DNA in one hour at 37° C in a 50µl reaction.Stability test: BL21 (λDE3) Δ( hsd-mrr-mcrBC ) [p(SYX22)AatIIR + , p(SYX19)AatIIM + , pLysS] cells were frozen in 25% glycerol at -70°C overnight. Cells were streaked out on LB agar plus Ap, Cm, and Kan from the frozen stock to obtain a single colony. 500 ml of rich broth plus antibiotics was inoculated with a single colony and shaken overnight. Colony-forming units were determined from the culture grown overnight on rich agar plate or rich agar plate supplemented with Ap, Cm, and Kan. It was found that the overproducing strain has the same plating efficiency on both plates (1.5 x 10 9 colony forming units/ml). Five ml of the culture grown overnight was inoculated into 500 ml of fresh rich broth plus antibiotics and grown to Klett units 100. The culture was induced by addition IPTG to 0.5 mM and incubation continued at 37° C. Cells were recovered by centrifugation. Cell extract was prepared and Aat II endonuclease activity determined on linear pUC19 DNA. At least 10 6 units Aat II endonuclease/gram of wet cells were detected. 6. Purification of Aa tII restriction endonuclease from Aat II overproducer strain (NEB #725) a deposit of which has been deposited at the American Type Culture Collection on July 7, 1992 and has received accession # 69028 : Two liters of rich broth medium plus 50 µg/ml Ap, 30 µg Cm/ml, 50 µg/ml Kan were inoculated with 20 ml of BL21 cells grown overnight carrying the AatII overproducing plasmid p(SYX22)AatIIR + , p(SYX19)AatIIM + , and pLysS. Cells were grown to mid-log phase (Klett units 100) at 37°C. IPTG inducer was added to the culture to a final concentration of 0.5 mM and incubation continued for 2 hours. Cells were centrifuged at 5000xg at 4°C for 20 minutes and cell pellet resuspended in 80 ml of sonication buffer (10 mM Tris-HCl, pH 7.8, 10 mM β-mercaptoethanol) and sonicated. The following purification procedure was done at 4°C. The extract was centrifuged at 15000xg for 30 minutes and supernatant applied onto a DEAE sepharose column (1.5 x 15 cm). A salt gradient was applied from zero to 1 M NaCl and fractions collected and assayed for Aat II endonuclease activity. The fractions containing Aat II endonuclease were pooled and dialyzed against a buffer containing 75 mM NaCl, 10 mM KPO4, pH 7, 1 mM DTT, 1 mM EDTA. The enzyme mixture was applied onto a heparin sepharose column (1.5 x 15 cm) and proteins eluted with a gradient of zero to 1 M NaCl. The fractions containing Aat II endonuclease were pooled as before and dialyzed against a 0.15 M NaCl buffer. The solution was applied to a 1.5 x 15 cm phophocellulose column and a gradient from 0.15 M to 1 M NaCl applied. The fractions with Aat II endonuclease were pooled and dialyzed against a storage buffer (50% glycerol, 50 mM KCl, 10 mM Tris-HCl, pH 7.4, 0.1 mM EDTA, 1 mM DTT, and 200 µg/ml BSA). The final yield is 10 6 units of Aat II endonuclease from 9 grams of cells (240,000 units Aat II/ml). The enzyme was diluted 10-fold to 24,000 units/ml and 2 µl to 10 µl of the diluted enzyme was analyzed by SDS-polyacrylamide gel electrophoresis. IPTG-induced and uninduced cell extracts were also loaded onto the same gel. The result shown in Figure 4 indicated that the purified Aat II endonuclease protein is at least 90% pure. (Note: the 68,000 dalton bovine serum albumin was not a contaminating protein. It was added into the Aat II enzyme preparation to stablize the Aat II endonuclease). The Aat II restriction endonuclease obtained from this purification was substantially pure and free of non-specific endonuclease and exonuclease. The purity of the Aat II restriction endonuclease preparation was checked by looking at the following criteria: 1) Ligation : After a 10-fold overdigestion of λ DNA, greater than 90% of the DNA fragments produced were ligated with T4 DNA Ligase. 95% of the ligated fragements could be recut with Aat II. 2) Prolonged digestion : After incubating a 50 µl reaction containing 1 µg of λ DNA and 72 units of enzyme for 16 hours, the same pattern of DNA bands was produced as a reaction performed in one hour with one unit of enzyme. 3) Exonuclease Activity : After incubation of 72 units of enzyme for 4 hours at 37°C in a 50 µl reaction containing 1 µg sonicated 3 H DNA (10 5 cpm/µg) less than 0.01% radioactivity was released. All tests were performed in the following reaction buffer: 50 mM potassium acetate, 20 mM Tris actate, 10 mM magnesium acetate, 1 mM dithiothreitol. EXAMPLE 2 Overexpression of Nla III endonuclease in E. coli : The Nla III methylase gene ( nlaIIIM ) and Nla III endonuclease gene ( nlaIIIR ) were cloned (see EPO publication #477,532, published April 1, 1992) and sequenced. Patent application relating to the methods for Nla III cloning and production of Nla III endonuclease from the clone has been filed (patent application number 07/575,285). Here we describe a method for overexpression of Nla III endonuclease in E . coli using the expression system of the present invention. In order to construct a constitutive expression vector, a Bss HII restriction fragment containing the lacZa gene was deleted from pSYX19 and an Ssp I /Pvu II restriction fragment carrying the tetracycline resistance (Tc r ) gene was ligated with the remaining DNA. The resulting plasmid pSYX20 contains both Kan r gene and Tc r gene, a deposit of which has been deposited at the American Type Culture Collection on July 7, 1992 and has received accession # 75260 . Cloning DNA into the Tc r gene inactivates it, which provides an easy screening method for detection of insert. The unique Eco RV, Bam HI, Sal I and Sph I restriction sites are convenient sites for cloning into the Tc r gene. A foreign gene inserted can be expressed constitutively from the Tc promoter. 1. Cloning of nlaIIIM gene into pSYX20 vector: Plasmid pSYX20 is a medium-copy-number plasmid that carries Kan r and Tc r genes and a pSC101 replication origin. The nlaIIIM gene inserted into the Tc r gene can be expressed constitutively from the Tc promoter. Two primers were used to amplify the nlaIIIM gene from p(UC)NlaIIIR + M + . Primer 1 contains a Bam HI site near 5' end and an efficient ribosome binding site 7 bp upstream of ATG start codon (5' CG G GAT CC G GAG GTT AAT TAA ATG AAC TAC ATC GGC TCC AAA CTA 3',SEQ ID:7). Primer 2 has a Sal I site near the 5' end (5' CAA GTC GAC TTA AAA GGT CTT TTC TAA AAT ATG 3',SEQ ID:8). The nlaIIIM gene was amplified with Vent® DNA polymerase by polymerase chain reaction (95°C for 1 minute, 50°C for 2 mintues, 72°C for 1 minute, 20 cycles). The PCR products were extracted twice with phenol-CHCl 3 , twice with CHCl 3 , ethanol-precipitated, and resuspended in 100 µl of TE buffer. After restriction digestion with Bam HI and Sal I for 1 hour at 37°C, The DNA was extracted again with phenol-CHCl 3 and CHCl 3 , ethanol-precipitated, dried, and resuspended in 100 µl of TE buffer. Five µg of pSYX20 vector DNA was also cleaved with Bam HI and Sal I, extracted with phenol-CHCl 3 and CHCl 3 , and ethanol-precipitated, and resuspended in 100 µl of TE buffer. Ten µl of nlaIIIM DNA and ten µl of the pSYX20 vector DNA was ligated overnight at 16°C in a 1X ligation buffer in 30 µl total volume. The ligated DNA was diluted by addition of 70 µl of sterile-distilled water and 10 µl DNA was used to transform ER1821 cells and plated on Kan (50 µg/ml) plate. Eighteen Kan r transformants were grown overnight in 1.5 ml LB plus Kan and plasmid DNA were prepared and analyzed by Bam HI and Sal I restriction digestion. Four clones contain the correct size insert. When they were challenged with Nla III endonuclease, all four plasmids were resistant to Nla III digestion, indicating full modification by Nla III methylase. One plasmid was named p(SXY20)NlaIIIM + and used to transform BL21 (λDE3) Δ( hsd-mrr-mcrBC ) [pLysS] cells and plated on LB agar plate plus 30 µg/ml of Cm, 50 µg/ml of Kan. Cm r and Kan r transformants were obtained and used later as the host for expression of Nla III endonuclease. 2. Cloning of nlaIIIR gene into expression vector pSYX22. Expression vector pSYX22 contains the T7lac promoter, the lacI q gene, two restriction sites, Nde I and Sa lI, for translation fusion cloning. The construction of pSYX22 has been described in Example I. Two primers were designed in such a way that the first primer contains an Nde I site which annealed to the start of nlaIIIR gene and a Sal I site was engineered into the second primer which is complementary to the end of the nlaIIIR gene (primer 1: 5' TTG CAT ATG AAA ATC ACA AAA ACA GAA CTA 3',SEQ ID: 9; primer 2: 5' CGA GTC GAC TCA TCC GTT ATC TTC TTC ATA TAA 3',SEQ ID:10). The nlaIIIR gene was amplified from p(UC)NlaIIIR + M + by PCR with Vent® DNA polymerase (95°C for 1 minute, 50°C for 2 mintues, 72°C for 1 minute, 20 cycles). PCR products were purified by phenol-CHCl 3 and CHCl 3 extractions and ethanol-precipitation, and cleaved with Nde I and Sal I. The DNA was again purified by phenol-CHCl 3 and CHCl 3 extraction and ethanol-precipitation and resuspended in 100 µl of TE buffer. 10 µl of the nlaIIIR DNA was ligated into 0.5 µg of Nde I/ Sal I treated pSYX22 at 16°C overnight. The ligated DNA was diluted with the addition of 70 l of sterile-distilled water and 10 µl of which was used to transform E. coli strain ER2267 recA1 Δ( hsd-mrr-mcrBC ) and plated on Ap plates. Forty eight individual transformants were inoculated into 2 ml of LB plus Ap and shaken overnight at 37°C. Plasmid DNA was prepared and analyzed by Nde I and Sal I restriction digestion. Ten plasmids contain the correct size insert. One clone p(SYX22)NlaIIIR + (#4) was used to transform BL21 (λDE3) Δ( hsd-mrr-mcrBC ) [p(SYX20)NlaIIIM + , pLysS] and selection for transformants was on LB agar plus Ap (50 µg/ml), Cm (30 µg/ml), Kan (50 µg/ml). Plasmid pLysS encodes phage T7 lysozyme which inhibits T7 RNA polymerase to reduce the basal level target gene expression from the T7lac promoter (Studier, F.W., J. Mol. Biol. 219: 37-44, 1991). E . coli strain BL21 (λDE3) Δ( hsd-mrr-mcrBC ) [p(SYX22)NlaIIIR + , p(SYX20)NlaIIIm + , pLysS] was tested for Nla III endonuclease production as follows: 200 ml of rich broth medium plus Ap (50 µg/ml), Cm (30 µg/ml), Kan (50 µg/ml) were inoculated with 2 ml of cells grown overnight and shaken at 37°C to cell density of Klett 75. IPTG inducer was added to the culture to 0.5 mM final concentration. The cells were induced for two hours and harvested at Klett 175. Cells were pelleted by centrifugation at 4°C, 5000xg for 15 minutes, resuspended in 20 ml of sonication buffer (10 mM Tris-HCl, pH 7.8, 10 mM β-mercaptoethanol), and sonicated on ice ten times with 30 second bursts. Cell debris was removed by centrifugation at 4°C 15000xg for 30 minutes. The supernatant was transferred to a new tube and diluted 10 -1 , 10 -2 , 10 -3 , and 10 -4 -fold in the sonication buffer and 5 µl of the diluted extract was used to cleave 1 µg of λ DNA in 1x Nla III restriction buffer in a total volume of 30 µl. After one hour digestion at 37 °C the reaction was stopped by addition of 3 µl of stop solution and DNA analyzed by agarose gel electrophoresis. It was found that a complete digestion was achieved at 10 -3 dilution of cell extract. It was estimated that this clone makes 2 x 10 6 units of Nla III endonuclease per gram of wet cells. when the Nla III overproducing strain was assayed for Nla III endonuclease production when grown in noninducing conditions, the cell extract contained only 1000 units per gram of wet uninduced cells. Thus, the induction ratio is 2000-fold. The expression system of the present invention with characteristics of high level of induction and low basal level expression is a very useful expression system for expression of toxic genes such as restriction endonuclease genes in E . coli . 3. Purification of Nla III endonuclease: Approximately 5 grams of cells from a liter culture were resuspended in 75 ml of sonication buffer (10 mM Tris-HCl, pH 7.8, 10 mM β-mercaptoethanol) and sonicated. The cell debris was removed by centrifugation at 15000xg for 30 minutes and 10 % glycerol was added and supernatant frozen at -70°C. Upon thawing, KCl was added to a final concentration of 0.2 M and the cell extract was applied to a phosphocellulose column (2.5 x 7 cm). A salt gradient was then applied from 0.2 M to 2 M KCl and fractions collected and analyzed for Nla III endonuclease activity. Those fractions with Nla III activity were pooled. After addition of 200 µg/ml of BSA they were loaded onto a hydroxylapatite column equilibrated with a buffer (10 mM KPO4, pH 7, 10 mM β-mercaptoethanol, 0.1 mM EDTA, 0.3 M NaCl, 10% glycerol). The enzyme was eluted with a phosphate gradient from 10 mM to 0.7 M KPO4. Fractions containing Nla III endonuclease were pooled and dialyzed against buffer containing 10 mM Tris-HCl, pH 7.6 , 10 mM β-mercaptoethanol, 0.1 mM EDTA, 50 mM NaCl and 10% glycerol. 200 µg/ml of BSA was again added to the enzyme pool which was applied to a DEAE sepharose column( 2.5 x 4.5 cm). The enzyme flowed through and was quickly applied to another hydroxylapatite column. The enzyme was eluted in a phosphate gradient of 10 mM to 0.7 M KPO4. The fractions with Nla III endonuclease were pooled and dialyzed against a storage buffer ( 10 mM Tris-HCl, pH 7.5, 0.1 mM EDTA, 1 mM dithiothretol, 0.2 M KCl, 50% glycerol) and stored at -70°C. A total yield of 400,000 units Nla III was obtained with the above purification procedure. The purified Nla III endonuclease was analyzed by SDS-polyacrylamide gel electrophoresis. It was found that the Nla III endonuclease protein with an apparent molecular weight of 27000 daltons is at least 90% pure (Figure 5. Note: the 68000 dalton BSA protein is not a contaminating protein. It was added into the Nla III preparation to stablize the endonuclease). The Nla III restriction endonuclease obtained from this purification was substantially pure and free of non-specific endonuclease and exonuclease. The purity of the Nla III restriction endonuclease preparation was checked by looking at the following criteria: 1) Ligation : After a 20-fold overdigestion of x DNA with the purified enzyme, greater than 95% of the DNA fragments produced were ligated with T4 DNA ligase. Of these ligated fragments, 95% could be recut with Nla III. 2) Prolonged digestion : After incubating a 50 ul reaction containing 1 µg of x174 DNA and 200 units of enzyme for 16 hours, the same pattern of DNA bands was produced as a reaction performed in one hour with one unit of enzyme. 3) Exonuclease Activity : After incubation of 200 units of enzyme for 4 hours at 37°C in a 50 µl reaction containing 1 µg sonicated 3 H DNA (10 5 cpm/µg) 0.21% radioactivity was released. All tests were performed in the following reaction buffer: 50 mM potassium acetate, 20 mM Tris actate, 10 mm magnesium acetate, 1 mM dithiothreitol. EXAMPLE 3 1. Cloning of a methylase gene into pSYX20 vector: Plasmid pSYX20 is a medium-copy-number plasmid that carries Kan r and Tc r genes and a pSC101 replication origin. A methylase gene inserted into the Tc r gene can be expressed constitutively from the Tc promoter. The construction of pSYX20 has been described in detail in Example 2. Alternatively, any plasmid vector, preferably a medium-copy number, which is compatible with ColEI and p15A can be used for cloning of the methylase gene; Bacteriophage vectors can also be used to clone the methylase gene and can be integrated into E. coli host chromosome for host DNA modification. Two primers with desired restriction sites can be used to amplify the methylase gene from a plasmid or from genomic DNA by the polymerase chain reaction. An efficient ribosome binding site, such as, GGAGGT can be engineered into the primer 7-10 bp upstream of ATG start codon. The polymerase chain reaction conditions are typically 95°C for 1 minute, 50°C for 2 mintues, 72°C for 1-2 minutes, 20-30 cycles. The PCR products can be extracted twice with phenol-CHCl 3 , twice with CHCl 3 , ethanol-precipitated, and resuspended in TE buffer. After restriction digestion the DNA can be extracted again with phenol-CHCl 3 and CHCl 3 , ethanol-precipitated, dried, and resuspended in TE buffer. The vector DNA pSYX20 is then cleaved with the same restriction endonuclease, extracted with phenol-CHCl 3 and CHCl 3 , and ethanol-precipitated, and resuspended in TE buffer. The DNA containing the methylase gene and the pSYX20 vector DNA can be ligated overnight at 16°C in a 1X ligation buffer. The ligated DNA can be diluted if necessary by addition of sterile-distilled water and a fraction of DNA can be used to transform E . coli cells ( mcrA - , mcrBC - , hsd - , mrr - ) and plated on Kan (50 µg/ml) plate. Kan r transformants can be screened for the correct size insert by mini-preparation of plasmid DNA and restriction digestion. The plasmid with the correct size insert can be challenged with the cognate endonuclease to confirm full modification. The methylase gene containing plasmid can be used to transform for example BL21 (λDE3) Δ( hsd-mrr-mcrBC ) [pLysS] cells and plated on LB agar plate plus 30 µg/ml of Cm, 50 µg/ml of Kan. Cm r and Kan r transformants can be obtained and used later as the host for expression of the corresponding endonuclease. Alternatively, a restriction fragment containing the intact methylase gene can be directly cloned into pSYX20 using restriction sites Aat II, Eco RI, Cla I/ Bsp DI, Eco RV, Bam HI, Sph I, Sal I, Psh AI, Eag I, Nru I, Bsp MI, Sma I, or Xho I. 2. Cloning of restriction endonuclease gene into expression vector pSYX22. Expression vector pSYX22 contains the T7lac promoter, rrmB transcription terminators upstream of T7lac promoter the lacI q gene, two restriction sites, Nde I and Sal I, for translational fusion cloning. The construction of pSYX22 has been described in Example I. Alternatively, derivative of pSYX22 can be constructed to contain Nco I, Sph I, Bsp HI, or Afl III restriction sites for insert cloning. Two primers can be designed in such a way that the first primer contains an Nde I site which annealed to the start of the endonuclease gene and a Sal I site engineered into the second primer which is complementary to the end of the endonuclease gene. The endonuclease gene can be amplified from plasmid clone or genomic DNA by PCR with heat stable DNA polymerase, for example; Vent®, (95°C for 1 minute, 50°C for 2 mintues, 72°C for 1-2 minutes, 20-30 cycles). PCR products can be purified by phenol-CHCl 3 and CHCl 3 extractions and ethanol-precipitation, and cleaved with Nde I and Sal I. The DNA can be again purified by phenol-CHCl 3 and CHCl 3 extraction and ethanol-precipitation and resuspended in TE buffer. DNA containing the endonuclease gene can he ligated into Nde I/ Sal I treated pSYX22 at 16°C overnight. The ligated DNA can be diluted with the addition of sterile-distilled water and a portion of which can be used to transform E . coli cells and plated on Ap plates. Individual transformants can be inoculated into 2 ml of LB plus Ap and shaken overnight at 37°C. Plasmid DNA can be prepared and analyzed by Nde I and Sal I restriction digestion. Alternatively, a restriction fragment containing the desired endonuclease gene can be directly cloned downstream of T7lac promoter in the pSYX22 expression vector. Plasmids containing the correct size insert can be used to transform for example BL21 (λDE3) Δ( hsd-mrr-mcrBC ) [p(SYX20)Methylase + , pLysS] and plated on LB agar plus Ap (50 µg/ml), Cm (30 µg/ml), Kan (50 µg/ml). E . coli strain BL21 (λDE3) Δ( hsd-mrr-mcrBC ) [p(SYX22)R + , p(SYX20)M + pLysS] can be tested for endonuclease production as follows: 200-1000 ml of rich broth medium plus Ap (50 µg/ml), Cm (30 µg/ml), Kan (50 µg/ml) can he inoculated with 2 ml of cells grown overnight and shaken at 37°C to cell density of Klett 75-100. IPTG inducer can he added to the culture to 0.5 mM final concentration. The cells can be induced for 2-14 hours and harvested. Cells can be pelleted by centrifugation at 4°C, 5000xg for 15 minutes, resuspended in sonication buffer (10 mM Tris-HCl, pH 7.8, 10 mM β-mercaptoethanol), and sonicated on ice till complete cell lysis. Cell debris can be removed by centrifugation at 4°C 15000xg for 30 minutes. The supernatant can be transferred to a new tube and diluted in the sonication buffer and 1-10 µl of the diluted extract can he used to cleave 1 µg of λ DNA or any substrate DNA in 1x restriction buffer. After one hour digestion at 37 °C the reaction can be stopped by addition of stop solution and DNA analyzed by agarose gel electrophoresis. Endonuclease yield can be estimated per gram of wet induced cells. The desired restriction endonuclease can be purified by ion exchange columns, gel filtration, or affinity columns. The purified restriction endonuclease can be analyzed by SDS-polyacrylamide gel electrophoresis.

Claims: Isolated DNA segment coding for Aat II restriction endonuclease wherein the isolated DNA segment is obtainable from E. Coli BL21 host cells NEB #725: ATCC Deposit Number 69028. A recombinant vector containing an isolated DNA segment coding for Aat II restriction endonuclease, as defined in claim 1. Isolated DNA coding for Aat II restriction endonuclease and methylase, wherein the isolated DNA segment is obtainable from E. Coli BL21 host cells NEB #725: ATCC Deposit Number 69028. A recombinant vector containing an isolated DNA segment coding for Aat II endonuclease and methylase, as defined in claim 3. A host cell transformed with a recombinant vector of claim 2 or 4. A method of producing Aat II restriction endonuclease comprising culturing a host cell transformed with the vector of claim 2 or 4 under conditions for expression of Aat II endonuclease and recovering Aat II endonuclease. An isolated DNA segment according to claim 1 or 3 which consists of, includes or comprises the nucleic acid sequence of the Aat II R gene SEQ ID 13, or the corresponding deduced amino acid sequence of SEQ ID 14.

Inventors: NWANKWO DONALD O, XU SHUANG-YONG, NWANKWO, DONALD O., XU, SHUANG-YONG
Applicants: NEW ENGLAND BIOLABS INC, NEW ENGLAND BIOLABS, INC.
Application year: 1993
Main classification: C12N15
IPC codes: C12N15, C12N9, C12N1, C12R1