Cloning of Sac II restriction endonuclease and methylase

Abstract: The present invention is directed to a method for cloning and producing the Sac II restriction endonuclease by 1) introducing the restriction endonuclease gene from Streptomyces achromogenes into a host whereby the restriction gene is expressed; 2) fermenting the host which contains the plasmid encoding and expressing the Sac II restriction endonuclease activity, and 3) purifying the Sac II restriction endonuclease from the

Description: The present invention relates to : Isolated DNA segment coding for the SacII restriction endonuclease, wherein the isolated DNA segment is obtainable from S. achromogenes ATCC No. 12767. At the onset of the cloning project, it was not known which endonucleases or conditions would be successful in cloning the Sac II restriction-modification system, nor where the restriction and modification genes were located within such clones. The cloning results and subsequent mapping, and characterization of the clones described in Figure 1A and Example I reveal the previously unknown direct pathway for cloning and expressing the Sac II restriction-modification system. More specifically, the cloning of the Sac II restriction-modification genes from Streptomyces achromogenes into E . coli proved to be complicated by the discovery that unlike many other Type II restriction-modification systems, Sac II genes do not express well in E . coli . Since methylase selection (the identification of methylase clones by their ability to resist and survive Sac II digestion) relies on methylase expression, selection for the Sac II methylase is not always successful. Expression was also found to be a problem for the Sac II restriction endonuclease gene. Methylase clones from many other Type II restriction-modification systems can be screened for restriction endonuclease activity with in - vitro assays. However, none of the Sac II restriction-modification clones expressed endonuclease activity detectable by in vitro assays, such as the assay described in the above-referenced EPO Publication 0193413, even after concentrating crude cell extracts over phosphocellulose columns. In order to determine whether the R gene was present in the M clones, numerous additional steps were required. The steps included: a) cloning S . achromogenes chromosomal DNA on both sides of the M gene ( Eco RV and Pst I clones), b) subcloning these methylase clones onto a Streptomyces cloning vector and transforming these plasmids into Streptomyces lividans . In this manner, the Eco RV methylase clones were found to carry the R gene, whereas the other methylase clone was found not to carry the R gene. In order to obtain overexpression of the Sac II restriction endonuclease, smaller subclones were generated in S . lividans . The method described herein by which the Sac II restriction gene and methylase gene are preferably cloned and expressed is illustrated in Figures 1A and 1B and includes the following steps: 1. Streptomyces achromogenes (ATCC No. 12767) is grown in nutrient broth with 68 g/l sucrose, 0.2 g/l MgCl₂, 5 g/l glucose, pH adjusted to 7.2 with NaOH and grown with aeration and agitation. The cells are lysed and the genomic DNA purified by the techniques described by Caserta, M. et al. J. Biol. Chem ., 262:4770-4777 (1987) and is described in detail in the example. 2. The S . achromogenes chromosomal DNA is digested completely and partially with a restriction endonuclease such as Pst I. Other restriction enzymes such as Sca I and Eco RV can also be used. 3. The digested DNA's are each ligated to a cloning vector, such as pJRD184 (Davidson, et al., Gene 39:299-304 (1984)), which contains at least one Sac II site. Other cloning vectors such as pACYC184 and pACYC177 (Chang, ACY, J. Bact ., 134:1141-1156 (1978)) can also be used. The resulting mixtures are used to transform an appropriate host such as E . coli strain RR1 or K802 cells (ATCC No. 31343 and ATCC No. 33526, respectively). RR1 which are mrr⁻ is the preferred host cell. 4. The DNA/cell mixtures are preferably plated on antibiotic media selective for transformed cells, such as ampicillin or chloramphenicol. After incubation, the transformed cell colonies are collected together to form the primary cell libraries. 5. The recombinant plasmids are purified in toto from the primary cell libraries to make primary plasmid libraries. 6. The plasmid libraries are then digested to completion in vitro with the Sac II restriction endonuclease, which is prepared from Streptomyces achromogenes cells using standard purification technique. Sac II restriction endonuclease digestion causes the selective destruction of unmodified, non-methylase-containing clones, resulting in an increase in the relative frequency of Sac II methylase-carrying clones. Exonuclease and/or phosphatase may also be added to the digestion to enhance the destruction of non-methylase clones. 7. Identification of Sac II methylase clones: The digested plasmid library DNA's are transformed back into a convenient host such as E . coli strain RR1 or K802, and transformed colonies are again obtained by plating on antibiotic plates. DNA from individual colonies is analyzed for the presence of the Sac II modification gene in the following manner: The plasmid DNA that they carry is purified and incubated in vitro with Sac II restriction endonuclease to determine whether it is resistant to digestion by Sac II. The plasmid DNA should be completely or substantially resistant to digestion. The total cellular DNA (chromosomal and plasmid) of the clone is also purified and incubated with Sac II restriction endonuclease. Further proof that the methylase gene has been cloned involves deleting the insert and checking the remaining vector for presence of intact Sac II sites. 8. Once it has been established that the methylase gene has been cloned, the clone is assayed for Sac II restriction endonuclease activity. If activity is detected, then the Sac 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 15 below. However, in accordance with the present invention, it has been found that even if present, the restriction gene is not expressed without further genetic manipulation as discussed below. 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. In order to determine which of the above three possibilities is the true situation, the cloned fragment is restriction-mapped and deletions are made to determine where the methylase gene lies 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 it were linked. If there is enough room, the restriction gene is assumed to be not linked, or to be present in the clone but not expressed (and one could skip to step 10). If there is not enough room on both sides of the methylase gene in the cloned DNA to encode a linked restriction gene, as was found for the Pst I clone of the present invention, a portion of the methylase gene is used to probe digests of the Sac II chromosome to generate a genomic map of the region extending beyond the boundaries of the existing cloned DNA. This data helps identify certain endonucleases 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 known 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. 9. Libraries are constructed by digesting genomic DNA from S . achromogenes with restiction endonucleases identified by Southern blot analysis to generate fragments which might encode the intact Sac II restriction modification system. These fragments are ligated into an appropriate vector such as pUC19 (ATCC No. 37017). This is used to transform an appropriate host such as E . coli strain RR1. Clones carrying the methylase gene and adjacent DNA are identified by colony hybridization using a portion of the methylase clone as a probe. Once isolated, these clones are analyzed for their ability to produce Sac II methylase and restriction endonuclease. 10. Identification of restriction gene clones: In accordance with the present invention, it has been found that clones carrying the Sac II restriction endonuclease gene cannot be identified by the usual crude cell extract assay because of the low-level expression of the gene in E . coli . Therefore, subclones are constructed which can be used to transform a host more closely related to S . achromogenes than E . coli . The DNA fragments containing the insert DNA are isolated away from the E . coli vector and ligated into a Streptomyces cloning vector such as pIJ486 (Ward, J.M. et al., Mol. Gen. Genet. , 203:468-478). The ligation mixture can then be used to transform protoplasts of an appropriate host such as S . lividans TK24, in accordance with the procedure of Hopwood, D.A., et al., Genetic Manipulation of Streptomyces, a Laboratory Manual , the disclosure of which is herein incorporated by reference. 11. The DNA/cell mixtures are preferably plated on regeneration media. Once regenerated the transformants are selected with an antibiotic, such as thiostrepton. After incubation, the transformed cell colonies can be picked and streaked for isolated colonies. 12. The recombinant plasmids are purified from several of the transformants. 13. The plasmids are then digested with Sac II and examined on an agarose gel to determine if the clones have any methylase activity. They are also digested with other restriction endonucleases to insure that the correct fragment has been cloned. 14. If the clones have methylase activity, several of these clones are individually grown in rich liquid media. Crude extracts of the methylase clones are prepared and assayed for Sac II restriction endonuclease activity. 15. A restriction map is made and different subclones constructed in E . coli and in S . lividans to determine the location of the intact Sac II methylase and restriction endonuclease genes. Each subclone is tested for overexpression of the Sac II restriction endonuclease gene. 16. Production: In one preferred embodiment, the Sac II methylase or endonuclease may be produced from transformed host cells transformed with one or more plasmid clones carrying the Sac II modification gene and the overexpressed restriction gene by propagation in a fermenter in a rich medium containing thiostrepton. The cells are thereafter harvested by centrifugation and disrupted in a French press to produce a crude cell extract containing Sac II methylase and restriction endonuclease activity. In another preferred embodiment, the host cell can be pre-protected by transformation with plasmids carrying the methylase gene, followed by introduction of plasmids carrying the endonuclease gene. 17. Purification: The crude cell extract containing the Sac II methylase and endonuclease is purified by standard product purification techniques such as affinity-chromatography, or ion-exchange chromatography. 18. The endonuclease so purified is found to be free of other contaminating enzymes including Sac I and Sac III, and substantially free of non-specific nucleases. Although the above-outlined steps represent the preferred mode for practicing the present invention, it will be apparent to those skilled in the art that the above described approach can vary in accordance with techniques known in the art. 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 I Cloning of SacII Modification Methylase and Restriction Endonuclease Genes 1. S . achromogenes DNA purification: 10 g of frozen Streptomyces achromogenes cells (ATCC #12767) were thawed on ice for 1 h, then resuspended in 20 ml of 25% sucrose, 50 mM Tris pH 8.0. 10 ml of 0.25 M EDTA pH 8.0, and 6 ml of 10 mg/ml lysozyme in 0.25 M Tris pH 8.0 were added. The suspension was kept on ice for 2 h, then lysed by the addition of 24 ml of 1% Triton X-100, 50 mM Tris pH 8.0, 67 mM EDTA and 5 ml of 10% SDS. The solution was extracted with 70 ml of phenol, (previously equilibrated with 0.5 M Tris pH 8.0), and 70 ml of chloroform. The emulsion was centrifuged at 10K rpm for 30 min and the viscous upper layer was withdrawn. The layer was reextracted with phenol/chloroform, and the emulsion was again centrifuged to separate the phases. The upper layer was withdrawn and dialyzed against four changes of 10 mM Tris pH 8.0, 1 mM EDTA. The dialyzed solution was then digested with RNase at a final concentration of 100 ug/ml for 1 h at 37 o C. The DNA was then precipitated by adding NaCl to a final concentration of 0.4 M, overlaying with 0.55 vol of isopropyl alcohol, and spooling the DNA onto a glass rod by mixing the phases together. The DNA was resuspended in DNA buffer (10 mM Tris pH 8.0, 1 mM EDTA) and stored at 4 o C. 2. Partial digestion of S . achromogenes DNA: 200 ug of S . achromogenes DNA was diluted into 2 ml of restriction endonuclease digestion buffer (10 mM Tris pH 7.5, 10 mM MgCl₂, 10 mM 2-mercaptoethanol, 100 mM NaCl). The solution was dispensed into 5 tubes, 300 ul into the first tube and 150 ul into each of the remaining tubes. 210 U of Pst I (10 ul) was mixed into the first tube to achieve 7 U enzyme/ug of DNA; 150 ul was withdrawn and transferred to the second tube (3.5 U/ug). 150 ul was then withdrawn from the second tube and transferred to the third tube, and so on, each transfer effecting a 2-fold serial dilution of Pst I. The tubes were incubated for 1 h at 37 o C, then heated for 15 min at 72 o C to stop the reactions. 5 ul from each tube was analyzed by agarose gel electrophoresis. Tubes in which moderate, but incomplete, digestion had occurred were combined. Two additional digestion series were performed in a similar manner using Sau 3A and Eco RI. 3. Ligation and transformation: 6 ug (60 ul) of Pst I-digested S . achromogenes DNA was mixed with 3 ug (6 ul) of Pst I-cleaved and dephosphorylated pJRD184 (obtained from Labofina s.a., of Feluy, Belgium. pJRD184 is described in the publication, Davidson et al., Gene 39:299-304, (1984)). 20 ul of 500 mM Tris pH 7.5, 100 mM MgCl₂, 100 mM DTT, 5 mM ATP, and 106.5 ul of sterile distilled water were added to bring the volume to 192.5 ul. 7.5 ul of T4 DNA ligase was added and the solution was incubated at 17 o C overnight. The solution was sterilized by extraction with 10 ul of chloroform, then clarified by microcentifugation for 15 s. 70 ul of the ligation solution was mixed with 1.0 ml of 50 mM NaCl, 5 mM Na₃Citrate, 67 mM CaCl₂ and 2.0 ml of ice-cold, competent E . coli K802 (ATCC No. 33526) cells were added. The solution was incubated at 44 o C for 5 min, then 12.5 ml of Luria-broth (L-broth) was added and incubation was continued at 37 o C for 3 h. Similar ligations were also set up between the Sau3A digest of S . achromogenes DNA and Bam HI-digested pJRD184, and between the Eco RI digest of S . achromogenes DNA and Eco RI-digested pJRD184. The additional ligations were also transformed into E . coli K802. 4. Primary cell libraries: The transformed cultures were gently centrifuged, the supernatants were discarded, and the cells from each culture were resuspended in 1.0 ml of Luria-broth. 200 ul portions of the resuspended cells were plated onto Luria-agar (L-agar) plates containing 100 ug/ml ampicillin. The plates were incubated overnight at 37 o C. The colonies that grew up were collected into three pools, one for each ligation, by flooding each plate with 1.5 ml of 10 mM Tris pH 7.5, 10 mM MgCl₂, and scraping the colonies together. 5. Primary plasmid libraries: 2.0 ml of each cell library was inoculated into 500 ml of L-broth containing 100 ug/ml ampicillin. The cultures were shaken overnight at 37 o C then centrifuged at 4K rpm for 5 min. The supernatants were discarded and the cell pellets were resuspended in 10 ml of 25% sucrose, 50 mM Tris pH 8.0, at room temperature. 5 ml of 0.25 M EDTA, pH 8.0, and 3 ml of 10 mg/ml lysozyme in 0.25 M Tris pH 8.0 was added to each. The solutions were kept on ice for 1 h, then 12 ml of 1% Triton X-100, 50 mM Tris pH 8.0, 67 mM EDTA was added to each, and the suspensions were gently swirled to induce cell lysis. The lysed mixtures were transferred to a 50 ml tubes and centrifuged for 45 min. at 17K rpm, 4 o C. The supernatants were removed with a pipette. 20.0 gm of solid CsCl was weighed into three 50 ml plastic screw-cap tubes and 22.0 gm of each supernatant was pipetted into each tube and mixed. 1.0 ml of 5 mg/ml ethidium bromide in 10 mM Tris pH 8.0, 100 mM NaCl, 1 mM EDTA was added to each tube. Each of the solutions was transferred to two 5/8 in. x 3 in. centrifuge tubes and spun in a Beckman Ti70 rotor for 42 h at 44K rpm, 17 o C. To collect the plasmid DNA in each tube, they were illuminated with ultraviolet light and the lower of the two fluorescent bands was collected by syringe. The ethidium bromide was removed from each of the collected bands by extracting four times with an equal volume of CsCl-saturated, isopropanol. The extracted solutions were dialyzed against 4 changes of DNA buffer, then the nucleic acids were precipitated overnight at -20 o C by the addition of 2 vol of isopropanol and NaCl to a final concentration of 0.4 M. The solutions were centrifuged for 15 min at 15K rpm, 0 o C, the supernatants were discarded, the pellets were air-dried for 15 min and then each was dissolved in 500 ul of 10 mM Tris pH 7.5, 1 mM EDTA and stored at -20 o C. The plasmid DNA concentrations were approximately 100 ug/ml. 6. Digestion of plasmid pool: 1 ug quantities of the plasmid libraries in 100 ul of 10 mM Tris pH 7.5, 10 mM MgCl2, 10 mM 2-mercaptoethanol, 10 mM NaCl, were digested with 0, 20, and 40 U of Sac II restriction endonuclease at 37 o C for 2 h. Following the digestions, 1.4 U (1.4 ul) of CIP (calf intestine phosphatase) were added to each reaction and incubated for 15 min. at 37 o C and then for 15 min. at 56 o C. An additional 1.4 U were added to each reaction and the temperature steps repeated. After completion of the dephosphorylating reactions, 20 ul (0.2 ug) of each library was transformed into E . coli K802. The mixtures were plated onto Luria-agar plates containing 100 ug/ml ampicillin, and incubated overnight at 37 o C. Digestion with Sac II restriction endonuclease reduced the number of transformants approximately 10³-fold. 7. Analysis of survivors: Between ten and thirty-five colonies were picked from among the survivors of each library. Each colony was inoculated into 10 ml of Luria-broth containing 100 ug/ml ampicillin, and grown overnight at 37 o C. Each of the plasmids present in the seventy-seven isolates was prepared by the following miniprep purification procedure, adapted from Birnboim and Doly, Nucleic Acids Res. 7:1513 (1979): Miniprep purification procedure : Each culture was centrifuged at 8 Krpm for 5 min; the supernatant was discarded and the cell pellet was resuspended in 1.0 ml of 25 mM Tris, 10 mM EDTA, 50 mM glucose, pH 8.0, containing 1 mg/ml lysozyme. After 10 min at room temperature, 2.0 ml of 0.2 M NaOH, 1% SDS was added to each tube; the tubes were shaken to lyse the cells, then they were placed on ice. Once the solutions had cleared, 1.5 ml of 3 M sodium acetate, pH 4.8, was added to each and shaken. The precipitates that formed were spun down at 15K rpm, 4 o C for 10 min. Each supernatant was poured into a centrifuge tube containing 3 ml of isopropanol and mixed. After 10 min at room temperature, the tubes were spun at 15K rpm for 10 min to pellet the precipitated nucleic acids. The supernatants were discarded and the pellets were air-dried at room temperature for 30 min. Once dry, the pellets were resuspended in 850 ul of 10 mM Tris, 1 mM EDTA, pH 8.0. 75 ul of 5 M NaCl was added to each and the solutions were transferred to Eppendorf tubes containing 575 ul of isopropanol, and again precipitated for 10 min at room temperature. The tubes were then spun for 45 s in a microfuge, the supernatants were discarded and the pellets were air-dried. The pellets were then dissolved in 500 ul of 10 mM Tris, 1 mM EDTA, pH 8.0, containing 100 ug/ml RNase and incubated for 1 h at 37 o C to digest the RNA. The DNA was precipitated once more by the addition of 50 ul of 5 M NaCl followed by 350 ul of isopropanol. After 10 min at room temperature, the DNA was spun down by centrifugation for 45 s, the supernatants were discarded and the pellets were redissolved in 150 ul of 10 mM Tris 1 mM EDTA, pH 8.0. The plasmid minipreps were subsequently analyzed by digestions with Sac II, Pst I, Eco RI and Sau 3A. 8. Identification of a Sac II methylase clone: Seventy-six of the seventy-seven plasmids that were analyzed were found to be sensitive to Sac II-digestion and to carry diverse fragments of S . achromogenes DNA. These plasmids were spurious and they were discarded. The remaining plasmid was found to be resistant to Sac II-digestion and to carry a single 5.9 kb Pst I fragment. However, the clone failed to synthesize the Sac II endonuclease. The 5.9 kb fragment was excised out of the pJRD184 plasmid and ligated into pUC19 for more convenient manipulations. This plasmid was designated pMMsacIIM 11-4. 9. Location of the methylase gene within the 5.9 kb Pst I fragment: We assumed that the Sac II endonuclease gene was located next to the Sac II methylase gene in the chromosome of S . achromogenes . Extensive mapping and subcloning DNA fragments of the original clone with restriction enzymes revealed that the methylase was located at one extreme end of the original 5.9 kb fragment. This information suggested that possibly the DNA encoding for the Sac II endonuclease gene was not present on the 5.9 kb fragment. At this point, the distance between the two genes, the exact size of the genes, and whether or not they were linked was not known. The lack of Sac II endonuclease activity in the clone indicated that the restriction gene was either not present in the clones, or was present but not expressed. In the event that the entire restriction gene was not present, the cloning of larger regions of DNA adjacent to the methylase gene was achieved as follows in steps 10-13. In the event that the larger clones still did not express the Sac II restriction endonuclease, these clones would be subcloned into Streptomyces to determine if a more closely related host would express the restriction endonuclease gene (steps 14-17). 10. A genomic map of the adjacent regions was determined using the Southern blot technique (Southern, E., J. Mol. Bio. , 98:503 (1975)). The pMMsacIIM11-4 plasmid DNA was purified and nick-translated to prepare a hybridization probe for Southern blots. 0.5 ug (5 ul) of plasmid DNA was mixed with 1.5 ul of 10 X nick-translation buffer (0.5 M Tris.HCl, pH 7.5, 50 mM MgCl₂, 10 mM 2-mercaptoethanol); 1 ul of dATP, dCTP, dGTP, dTTP mix (500 pMole of each nucleotide in dH₂O); 5 ul of [³²P]-dATP (Amersham; 800 Ci/mMol, 20 mCi/ml); 2 ul (20 U) of E . coli DNA polymerase I (New England Biolabs); and 1 ul of DNase I (1.0 ug/ml). The mixture was incubated at 16 o C for 2 h, then the reaction was stopped by the addition of 100 ul of 10 mM EDTA, pH 8.0. The Southern blot was prepared as follows: S . achromogenes DNA was digested separately with the restriction endonuclease Fsp I, Sca I, Kpn I, Pvu II, Bal I, Eco RV, Xmn I, Stu I, Dra I and Ssp I. The digests were electrophoresed on a 1.0% agarose gel. The gel was soaked in 0.25 M HCl for 15 min; 0.5 M NaOH, 1 M NaCl for 30 min; and then in 1 M Tris.HCl pH 7.5, 3 M NaCl for 30 min. A nitrocellulose sheet was soaked in water for 1 h, then briefly immersed in 5 X SSC (0.75 M NaCl, 75 mM Na₃Citrate). The sheet was applied to the surface of the gel and backed with chromatography paper (Whatman) to act as a wick. The sandwich was weighted down and transfer of the gel contents to the nitrocellulose sheet was allowed to proceed at room temperature for 4 h. The sheet was then baked in a vacuum oven at 80 o C for 1 h. to fix the transferred DNA fragments to the nitrocellulose support. The sheet was transferred to a plastic bag containing 15 ml of a solution composed of 3 ml of 10 gm/L Ficoll, 10 gm/L polyvinylpyrrolidone, 10 gm/L bovine serum albumin; 4.5 ml of 3 M NaCl, 0.3 M Na₃Citrate; 1.5 ml 10% SDS; 3 ml 10% dextran sulfate; 3 ml water, and prehybridized by incubating at 63 o C for 1 h. The entire radioactive probe was added to the bag, and incubation was continued at 65 o C overnight. The nitrocellulose sheet was then washed at room temperature three times for 5 minutes in 2 X SSC, 0.5% SDS. This step was followed by three 20 minute washes at 65 o C in 2 X SSC, air-dried then autoradiographed overnight. The probe hybridized to a single, 14 kb band in the Fsp I-digest, to a 9.4 kb band in the Sca I-digest, to a 8.9 kb band in the Eco RV-digest and to a 14 kb band in the Stu I-digest. These bands were judged to be of suitable size for cloning, and to be likely to contain the Sac II endonuclease gene as well as the Sac II methylase gene. 11. Preparation of S . achromogenes libraries: Four new libraries consisting of S . achromogenes DNA inserted into the plasmid pUC19, were prepared in E . coli RR1. In separate reactions, S . achromogenes DNA was digested to completion with Fsp I, Sca I, Eco RV, and Stu I. The reactions were stopped by heating to 72 o C for 15 min. 1 ug of each of the digested DNA's was combined with 1 ug of Hinc II-cleaved and dephosphorylated pUC19 DNA (ATCC No. 37017). 3 ul of 500 mM Tris pH 7.5, 100 mM MgCl2, 100 mM DTT, 5 mM ATP was added, and the vol of each reaction was brought to 30 ul with water. 3 ul of T4 DNA ligase was added to each mixture, and ligation was carried out at 16 o C overnight. The ligations were terminated by extraction with 10 ul of chloroform, and then the mixtures were transformed separately into competent E . coli RR1. The transformation mixtures were plated onto Luria-agar plates containing 100 ug/ml ampicillin. The plates were incubated overnight at 37 o C. 12. Isolation of overlapping Sac II methylase gene fragments: Colonies were screened for hybridization to the 5.9 kb PstI fragment containing the Sac II methylase gene. The colonies representing the new libraries prepared as described in Section 13, above, were transferred to nitrocellulose filters by contact-lifts. The filters were immersed in 0.5 M NaOH, 2 M NaCl for 30 s; 0.5 M Tris.HCl, pH 7.5, 3 M NaCl for 1 min; 0.3 M NaCl, 0.03 M Na₃Citrate, 0.1% SDS for 5 s; 0.3 M NaCl, 0.03 M Na₃Citrate for 10 s. The filters were air-dried, and then they were baked in a vacuum-oven at 80 o C for 30 min. The filters were prehybridized, and then hybridized (using the procedure described in Section 10) with the 2.0 kb Sac II methylase gene probe. Preparation of Sac II methylase gene probe: A 2.0 kb Bgl II to Eco RV subfragment of the 5.9 kb Pst I insert carried in pMMsacIIM11-4 was gel-purified and nick-translated. 10 ug of the plasmid DNA was incubated at 37 o C for 1 h in 10 mM Tris pH 7.5, 10 mM MgCl₂, 150 mM NaCl, 10 mM 2-mercaptoethanol with 120 U each of Bgl II and Eco RV. The digest was fractionated by gel electrophoresis on a 1.0% Tris-acetate agarose gel. The gel was run for 2.5 h, at 100 mA, then illuminated with long-wave UV light and the 2.0 kb band was cut out of the gel and transferred to a syringe. The gel slice was extruded through an 18 gauge needle into a 5 ml centrifuge tube. The tube was centrifuged at 43K rpm for 45 min at 25 o C in a Beckman SW 50.1 rotor. The supernatant was collected and the DNA was precipitated at -70 o C for 1 h by the addition of NaCl to 0.5 M, and 2 vol of isopropanol. The precipitated DNA was resuspended in 500 ul of 10 mM Tris HCl pH 8.0, 1 mM EDTA. The DNA was nicked translated using the procedure outlined in Section 10. The filters were air-dried, and then they were autoradiographed overnight. 13. Identification of new Sac II methylase gene clones: Approximately 4,000 colonies were screened from the Eco RV-library; of these, six colonies hybridized strongly to the probe. The six clones were analyzed; two were found to carry the sought-after, 8.9 kb Eco RV-fragment, and the other four were found to be spurious. Approximately 8,000 colonies were screened from the Sca I-library and four of fifteen strongly-hybridizing colonies from this library contained the 9.4 kb Sca I fragment. There were no strongly-hybridizing colonies found from the screening of the Fsp I and Stu I-libraries (∼5,000 colonies screened/library). One positive clone from each of the Eco RV and Sca I-libraries was retained and analyzed. The clone from the Eco RV-library, designated pMMsacIIRM3, was found to have 6.4 kb flanking the side of the Sac II methylase gene which had the least DNA in the original Sac II methylase clone (recalling the Sac II methylase gene was at one extreme end in the original clone.). The analysis of the clone from the Sca I-library, designated pMMsacIIRM8, determined that there is 1.0 kb flanking one side of the Sac II methylase and 3.0 kb flanking the other side. Extracts of the clones were assayed for restriction endonuclease activity and no activity was detected from either clone. 14. Subcloning the Sac II methylase clones into S . lividans : With the recovery of the new clones, there was now enough DNA cloned on both sides of the methylase gene to encode a restriction endonuclease gene, if it were linked, regardless of which side encoded the linked gene. However, none of the clones expressed any restriction endonuclease activity. With still no proof that the two Sac II restriction-modification genes were linked, the clones were subcloned into S . lividans , a species more closely related to S . achromogenes than E . coli . 10 ul (1.5 ug) of pMMsacIIRM3 (the Eco RV methylase clone in pUC19, Fig. 2) was digested in 50 ul of 10 mM Tris pH 7.5, 10 mM MgCl₂, 100 ug/ml bovine serum albumin, 100 mM NaCl containing 20 U of Eco RI and 20 U of Hind III at 37 o C for 1 h. The entire volume was electrophoresed in a 0.7% agarose gel for 2 h. The 7.5 kb Eco RI- Hind III restriction fragment was electrophoresed into DEAE anion exhange paper for two h. The paper was washed two times in 150 ul of a buffer containing 0.1 M NaCl, 10 mM Tris pH 8.0, and 1 mM EDTA. Subsequently, the DNA was eluted from the paper by washing the paper four times with 75 ul of a buffer containing 1.0 M NaCl, 10 mM Tris pH 8.0 and 1 mM EDTA. The resulting solution containing the DNA fragment was extracted with 300 ul phenol/chloroform followed by extraction with 300 ul chloroform and precipitated with 1 ml absolute ethanol by placing in a dry ice/ethanol bath for 15 min. The DNA was pelleted at 14k rpm for 5 min. The pellet was rinsed with 70% ethanol, air dried and resuspended in a final volume of 10 ul 10 mM Tris pH 8, and 1 mM EDTA. 9 ul (0.5 ug) of the Eco RI- Hind III purified DNA fragment were ligated to 1 ul (0.2 ug) of Eco RI- Hind III - cleaved and dephosphorylated pIJ486 (obtained from Hopwood, D.A. of Norwich, England. pIJ486 is described in the publication, Ward, J.M. et al., Mol. Gen. Genet . 203:468-478.) in 50 ul 1 X ligation buffer containing 1 ul T4 DNA ligase (400 U) at 12 o C overnight. 10 ul of the ligation mix was added to approximately 4 x 10⁹ S . lividans TK24 (obtained from Hopwood, D.A.. TK24 is described in Hopwood, D.A., et al., Genetic Manipulation of Streptomyces, a Laboratory Manual ) protoplasts, prepared as described in Hopwood D.A., et al., ibid , in P Buffer [103 g Sucrose, 0.25 g K₂SO₄, 2.02 g MgCl₂.6H₂O, 2 ml Trace elements solution, and distilled water to 800 ml. 80 ml aliquots are dispensed and autoclaved. Before use add to each 80 ml: 1 ml 0.5% KH₂PO₄, 10 ml 3.68% CaCl₂.2H₂O and 10 ml 5.73% TES buffer pH7.2. Trace elements solution per liter: 40 mg ZnCl₂, 200 mg FeCl₃.6H₂O, 10 mg CuCl₂.4H₂O, 10 mg MnCl₂.4H₂O, 10 mg Na₂B₄O₇.10H₂O and 10 mg (NH₄)6Mo₇O₂₄.4H₂O] 0.5 ml of 25% polyethylene glycol 1000 was added to the protoplast/DNA mixture. This was drawn up and down 3 times in a 1 ml pipette. 0.1 ml of the transformation mix was plated on each of six R2YE plates [103 g Sucrose, 0.25 g K2SO4, 10.12 g MgCl2.6H2O, 10 g Glucose, 0.1 g Difco Casaminoacids and 800 ml H2O. 80 ml of this solution are mixed with 2.2 g Difco agar and autoclaved. To prepare the plates the base agar solution is melted and the following sterile solutions are added: 1 ml 0.5% KH₂PO₄, 8 ml 3.68% CaCl₂.2H₂O, 1.5 ml 20% L-proline 10 ml 5.73% TES buffer pH7.2, 0.2 ml Trace elements solution, and 0.5ml 1N NaOH. The plates are poured and dried in a laminar flow hood for at least 1 h.]. The plates were overlayed after incubating overnight at 30 o C with 1.0 ml of an aqueous solution of thiostrepton (0.5 mg/ml). The plates were returned to 30 o C for 3 to 4 days until the colonies have grown. 15. Analysis of transformants: 6 of the surviving colonies obtained from selection 15 were streaked on R2YE plates containing 5 ug/ml thiostrepton for isolated colonies. Once grown, these were used to inoculate 5 ml of TSB with 5 ug/ml thiostrepton. These cultures were incubated at 30 o C with aeration for 24 h. Minipreps were done on 0.5 ml of the cultures. This procedure is identical to the procedure described by Birnboim and Doly ( Nucleic Acids Research , 7:1513 (1979)) with the exception that a 30 minute incubation in 4 mg/ml of lysozyme, 50 mM Glucose, 25 mM Tris pH8.0, and 10 mM EDTA at 37 o C is necessary before adding the NaOH-SDS solution. 10 ul of the miniprep DNA was analysed by running on an 0.7% agarose gel. 3 of the 6 clones appeared to be larger than pIJ486. Spores from these three isolates were harvested and used to inoculate 500 ml TSB + Thiostrepton. CsCl plasmid prep was prepared on 450 ml of the culture following a scaled up (20 x) version of Procedure 3 P. 93 in Hopwood et al. ibid . The resulting pellet was resuspended in 17 ml 10 mM Tris pH 8.0, 1mM EDTA, 18.7 g CsCl and 0.44 ml ethidium bromide (5 mg/ml). The solution was transferred to two 5/8 in. x 3 in. polyallomer centrifuge tubes and sealed. These tubes were centrifuged in a Beckman Ti70 rotor for 44k rpm for 48 h, 17 o C. To collect the plasmids, the tops of the tubes were pierced with a scalpel and the lower of the two fluorescent DNA bands was collected by syringe under ultraviolet light. The lower band from both tubes was combined into a 15 ml Corex tube and the ethidium bromide was removed by adding an equal volume of water and three volumes of ethanol. After 2 h at -20 o C the DNA was pelleted by spinning at 12k rpm for 20 min.. The pellet was resuspended in 2 ml 10 mM Tris pH 8.0, 1 mM EDTA . 50 ul of 8 M LiCl was added and the DNA was extracted with phenol/chloroform followed by a chloroform extraction. The DNA was precipitated by adding 3 volumes ethanol to the aqueous solution as described above. The pellet was resuspended in 500 ul 10 mM Tris pH 8.0, 1 mM EDTA. The purified plasmid was digested with Eco RI and Hind III to confirm the presence of the insert as well as with Sac II to determine if the subclone in S . lividans had any Sac II methylase activity. All three subclones were apparently identical having the correct construction as well as having methylase activity i.e., were uanable to be digested with the Sac II endonuclease. 50 ml of the culture used to make the plasmid prep was washed with 10.3% sucrose the pellet was frozen at -70 o C. Upon thawing the pellet was resuspended in 3 ml/g of wet cell weight with a solution of 50 mM Tris pH 8, 10 mM 2-mercaptoethanol and 1 mM PMSF. After sonication on ice the debris were removed by centrifugation at 16k rpm for 45 min. The supernatant was assayed for Sac II restriction endonuclease activity. These subclones are denoted pEGsacIIRM1-32,1-34 and 1-36 (Fig. 3) in S . lividans had 2 x 10⁵ U/g Sac II endonuclease activity, approximately the same level as that observed in S . achromogenes . 16. Determining the location of the Sac II restriction endonuclease gene: Further subcloning of the Eco RV fragment was done in S . lividans to determine the approximate location of the Sac II restriction endonuclease gene. An Mlu I deletion was constructed denoted pEGsacIIRM4-2 (Fig. 3) which had both endonuclease at a level of 8.5 x 10⁵ U/g and methylase activity. Two Pst I fragments were cloned individually into pIJ486. Only the leftward Pst I subclone (pEGsacIIM6-1) displayed Sac II methylase activity. Neither subclone gave Sac II endonuclease activity, indicating that the Sac II endonuclease gene spans that Pst I site (Fig. 3). 17. Overexpression of the Sac II restriction endonuclease. Another subclone that was constructed was with the Sca I- Hind III fragment (Fig. 3). The DNA fragment first had to be ligated into the Sma I- Hind III sites of pUC19 and transformed into E . coli ED8767 to obtain useful restriction sites for cloning into pIJ486. This subclone, pEGsacIIMC4A, in E . coli showed methylase activity but no endonuclease activity was detected. The relevant fragment was cut out of the pUC19 subclone with Eco RI and Hind III and ligated into Eco RI- Hind III digested pIJ486. S . lividans TK24 transformants which carried this recombinant plasmid, pEGsacIIRMC5, showed Sac II methylase activity. When this strain, denoted ATCC 68391, was assayed for Sac II restriction endonuclease activity, it showed activity at a level of 4 x 10⁷ U/g. In Figure 4, 500 ul of XbaI -digested ØC31 DNA was prepared in 1x NEB Buffer 4 (20 mM Tris, 10 mM Mg acetate, 50 mM K acetate, and 1 mM DTT) 50 ul was placed in two tubes and 25 ul was placed in each of 7 tubes. Additionally, 50 ul was placed in one tube and 25 ul in each of 5 tubes to be used as controls. 1 ul of crude cell extract (from cells containing the clone) was added to the first 50 ul tube and mixed. 1 ul was transferred to the second 50 ul tube and mixed, then 25 ul from that tube was transferred to a third tube (containing 25 ul) and mixed, and so on, until six such 1:2 dilutions had been performed using the first seven tubes. For the control, 1 ul (20 units) Sac II endonuclease was added to the first control tube, and five 1:2 dilutions (transfer of 25 ul) were carried out amoung the five control tubes. All tubes were incubated at 37 o C for one hour. 25 ul from each tube was analyzed by gel electrophoresis. Comparing the control with the crude cell extract of the clone, there are an estimated 40,000,000 units of Sac II activity per gram of clone cells. A sample of NEB No. 673 has been deposited at the American Type Culture Collection on September 4, 1990 and bears ATCC designation No. 68391. 18. Sac II modification methylase and endonuclease were produced from clones ATCC 68391, carrying the Sac II modification gene and the overexpressed restriction gene by propagation in a fermenter in a rich medium containing thiostrepton. The cells were thereafter harvested by centrifugation and 337 g were diluted 1:3 in 0.3 M NaCl SB pH 7.7 (10 mM KPO₄, 0.1 mM EDTA, 10 mM 2-mercaptoethanol) and disrupted in a French press to produce a crude cell extract containing Sac II methylase and restriction endonuclease activity. 19. The Sac II restriction endonuclease was prepared from the crude cell extract containing the Sac II methylase and endonuclease by loading on a DEAE-Sepharose column (5 cm x 17 cm with a bed volume of 333 ml) which had been equilibrated with 0.3 M NaCl SB pH 7.7. This was washed with 500 ml 0.2 M NaCl SB pH 7.0. The flow-through and the washes were diluted up to 3 l with this same buffer. This was loaded on a Heparin-Sepharose column (4 cm x 14 cm with a bed volume of 176 ml). After loading the sample the column was washed with 250 ml 0.2 M NaCl SB pH 7.0. To elute the protein, a gradient was run from 0.2 M NaCl to 1.2 M NaCl in SB pH 7.0. The pooled fractions (300 ml) containing endonuclease activity were dialyzed against 0.05 M NaCl SB pH 7.8. This was loaded on to a DEAE-Sepharose column (2.5 cm x 19 cm with a bed volume of 93 ml) which had been equilibrated with 50 mM NaCl SB pH7.8. Once loaded the column was washed with the same buffer. The endonuclease was eluted from the column with a gradient run from 50 mM NaCl to 0.8 M NaCl in SB pH 7.8. The fractions with the peak of activity were collected and dialyzed against storage buffer (50 mM KCl, 10 mM Tris-HCl pH 7.4, 0.1 mMEDTA, 1 mM DTT, 200 ug/ml BSA and 50% glycerol and stored at -20 o C. The total yield once purified was 3 x 10⁷U. The Sac II endonuclease obtained from this purification was substantially pure and free of non-specific endonuclease and exonucleases, and was entirely free from contamination with Sac I and Sac III endonuclease. The purity of the Sac II restriction endonuclease preparation was checked by looking at the following criteria: 1) Ligation : 95% of the DNA fragments produced by an 10-fold overdigestion were ligated with T4 DNA Ligase (at a 5' termini concentration of 1-2 uM at 16 o C). Of these ligated fragments, 95% were able to be recut. 2) Prolonged digestion : A 50 ul reaction containing 1 ug of DNA and 80 U of enzyme incubated for 16 hours resulted in the same pattern of DNA bands as a reaction produced in 1 h with 1 U of enzyme. 3) Exonuclease activity : Incubation of 100 U for 4 hours at 37 o C in 50 ul assay buffer (50 mM K acetate, 20 mM Tris acetate, 10 mM mg acetate, 1 mM DTT pH 7.9) with 1 ug sonicated ³H DNA (10⁵ cpm/ug) released 0.15% radioactivity. 4) Endonuclease contamination : Incubation of 80 U with 1 ug pBR322 DNA which has no Sac II sites (4 hours at 37 o C in 50 ul) gave 20% concentration to RFII.

Claims: Isolated DNA segment coding for the SacII restriction endonuclease, wherein the isolated DNA segment is obtainable from S . achromogenes ATCC No. 12767. A recombinant DNA vector comprising a vector into which a DNA segment coding for the SacII endonuclease produced by S . achromogenes ATCC No. 12767 has been inserted. Isolated DNA segment coding for the SacII restriction endonuclease and methylase, wherein the isolated DNA segment is obtainable from S . achromogenes ATCC No. 12767. A cloning vector which comprises the isolated DNA segment according to claim 1. A cloning vector which comprises the isolated DNA segment according to claim 3. A Streptomyces lividans host cell transformed by the vector according to claim 4 or 5. A method of producing SacII restriction endonuclease comprising culturing the transformed host cell of claim 6 under conditions for expression of said endonuclease.

Inventors: GUTHRIE ELLEN, MEDA MARTA, GUTHRIE, ELLEN, MEDA, MARTA
Applicants: NEW ENGLAND BIOLABS INC, NEW ENGLAND BIOLABS, INC.
Application year: 1991
Main classification: C12N15
IPC codes: C12N9, C12N15, C12N1, C12R1