Abstract:
A method for obtaining E. coli cell lines which carry the deoR mutation is described, as well as the cell lines themselves. These cell lines are useful in cell transfection and transformation, as they transfect transform at much higher frequencies than the previously available cell lines.

Description:
FIELD OF THE INVENTION 
     This invention relates to novel cell lines (or strains) of Escherichia coli with enhanced properties as a transformation host for DNA, as well as methods for producing said cell lines. 
     BACKGROUND AND PRIOR ART 
     The ability to transfer plasmids into E. coli has come to be an integral part of the repertoire of tools used in molecular biology. Plasmid transformation of E. coli was first observed by Cohen, et al, Proc. Natl. Acad. Sci. 69: 2110 (1972), by applying the observation of Mandel, et al, J. Mol. Biol. 53: 159 (1970) that E. coli and bacteriophage λ, when combined in CaCl 2  containing solutions at 0° C. caused transfection of the E. coli with the phage. 
     Much work has been undertaken since then, with the goals of improving transformation frequency, and characterizing the parameters involved. Examples of this work include Hanahan, J. Mol. Biol. 166: 557-580 (1983), which is incorporated by reference herein; Bergmans, et al, J. Bacteriol 146: 564 (1981); Jones, et al, J. Bacteriol 146: 841 (1981); Norgard, et al, Gene 3: 279 (1978). 
     What the art as a whole teaches, in summary, is that E. coli and DNA interact at low temperatures in an environment containing divalent cations. Many factors improve transformation frequency, including heat shock, inclusion of monovalent cations in the transforming buffer, the addition of hexamine cobalt chloride, treatment with solvents and sulfhydryl reagents, and growth in media containing elevated magnesium levels. These treatments have improved transformation from 1 plasmid in 10 5  cells, to up to 1 in 10 2  cells. 
     One cell line which has been of particular use in this regard is cell line DH-1, which is described in Hanahan, J. Mol. Biol. 166: 557-580 (1983). This cell line is an E. coli mutant (recAI), and has served as a host for, e.g., cDNA cloning, plasmid rescue, and cloning of large DNA fragments. 
     Recently, it has been learned that a mutation, identified as deoR, can be induced in E. coli cells. When present, the cells which carry the mutant serve as unexpectedly better hosts for all of the techniques described supra, as compared to DH-1. The DH-1 cell line itself has been mutated to contain the deoR mutation, and the resulting cell line is known as DH-5. 
     Cell line DH-5 has been deposited with the American Type Culture Collection, 12301 Parklawn Drive, Rockville Maryland and bears accession number 53868. This deposit affords permanency of cell line and provides ready access to the public. 
     A method for obtaining deoR mutated E. coli cell lines, the mutated cell lines themselves, and methods of using these for, e.g., cell transformation, are the subject of this invention. 
     While the deoR mutations might occur in nature, substantially pure cultures of these have never before been available. The inventions described herein provides a method to one skilled in the art whereby a regular source of the mutant line now becomes available. 
    
    
     DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS 
     General Protocol for Obtaining deoR Mutations 
     The deoR mutation involves a gene which encodes a repressor protein. This repressor suppresses the expression of a set of genes involved in nucleotide utilization. In the absence of this protein, the regulated genes are expressed constitutively. When the repressor is absent, as it is in deoR mutations, colonies of the mutants grow more rapidly than non-mutants on inosine containing medium. Inosine is a nucleotide which does not induce derepression of the coordinated genes in question, as do other nucleotides. Munch-Peterson, et al, Eur. J. Biochem 27: 208-215 (1972). 
     To select for the deoR mutant, whether arising naturally or by, e.g., chemical radiation, or genetically induced mutation, the subject cell sample is inoculated (10 4   to 10 5  cells) into SynIn medum. SynIn medium consists of 10 mM NaCl, 10 mM NH 4  Cl, 1 mM MgSO 4 . 7H 2  O, 2.5 mM K 2  HPO 4 , 0.1 mg/ml thiamine, and 1.5 mg/ml Inosine. When used in Agar plates, 1.5% Bacto-Agar is added. For Syn-Adenosine Medium or plates, adenosine replaces inosine. 
     5-50 ml of cell sample is used, depending upon the mutagenization protocol. When naturally occurring mutants are to be selected, larger amounts of sample should be used, while smaller amounts (5-10 ml) are appropriate for induced mutations. 
     Cells are cultured at 37° C. with agitation until confluency is reached (10 9  cells/ml), and then 50 μl of cells are taken into 5 mls of fresh SynIn medium, and cultured at 37° C. until confluency is reached. 
     The cycle described supra is repeated 2-5 times, with confluency requiring less and less time, because normal cells will be overgrown by deoR mutations. 
     Serial dilutions of the cells are then plated into SynIn agar plates, and these are incubated at 37° C. to produce colonies. deoR colonies are noticeably larger. 
     Candidate deoR cells are then streaked on fresh SynIn plates, together with parenteral deoR +   cells. Incubation is at 37° C. deoR colonies appear much faster. 
     The foregoing protocol was used on E. coli cell line DH1, which is described in Hanahan, J. Mol. Biol 166: 557-580 (1983) but can be used on any E. coli cell line. 
     In the examples that follow, deoR mutant cell line DH-5 was used, although one skilled in the art will recognize the applicability of the protocols and experiments set forth herein to any E. coli cell line. 
     EXAMPLE I 
     Preparation And Isolation of Cell Line DH 5 
     A sample of E. coli cell line DH 1 was mutagenized with nitrosoguanidine, following the protocol set forth in Miller, Experiments in Molecular Genetics (1972). These mutagenized cells were grown in a minimal essential medium which contained either uridine or inosine. These nucleosides were used as carbon sources because of the observation that cytR and deoR mutant cell lines could be discerned in mixed cell cultures due to the higher growth rate of the mutants on these media (cytR on uridine, deoR on inosine). 
     After the cells were grown for a relatively long period of time, samples were either streaked on plates, or studied in liquid medium to determine their rate of growth on either uridine or inosine containing media. The results are set forth in following Table 1: 
     
                       TABLE 1______________________________________cytR and deoR mutantsSelection of Mutants Liquid Medium                    Assay of cloned cells Serial   Plate            by growth on plateSample passages)          Medium    Uridine                           containing Inosine______________________________________1     Ur       Ur        f      f2     Ur       In        f      f3     Ur       In        mf     mf4     In       Ur        f      m5     In       Ur        m      m6     In       Ur        m      m7     In       In        m      f8     In       In        m      m9     In       In        m      m10    In       In        m      m11    In       In        s      vf12    In       In        m      fDH 1                     vs     vs______________________________________ f = fast s = slow vf = very fast vs = very slow m = medium 
    
     Cell sample 11, it will be seen, grows very fast on inosine containing medium, and slowly on uridine containing medium. This is indicative of the presence of a deoR mutation, and the absence of cytR. This colony was selected as cell line DH 5. Addtional cell lines, which are referred to infra, are DH-4 (cytR, but not deoR), and DH 6 (both cytR and deoR). 
     EXAMPLE II 
     Transfection With Plasmids 
     Additional experiments were performed to determine the rate of transfection with various plasmids. In these experiments, plasmids pBR322, pXAD, and p66 which is a 66 kilobase plasmid composed of human B globin gene sequences inserted into pBR322, were used. These are derivative plasmids of pBR322, which are known to the skilled artisan. See, e.g., Hanahan, supra which also sets forth the protocols used for the transformation experiments. The results are set forth in terms of colony forming units, for two separate runs on each cell line. 
     
         ______________________________________            PlatePlasmid          [%]    Cell line                            CFU  (Avg)______________________________________10 pg pBR322         1%     DH 1   31,41                                   36                       DH 4   50,34                                   42                       DH 5   80,90                                   85                       DH 6   70,50                                   60200 pg pXAD           2%     DH 1   24,26                                   25                       DH 4   40,34                                   37                       DH 5   58,68                                   63                       DH 6   58,70                                   64500 pg p66            15%    DH 1   2,0  1                       DH 4   0,0  0                       DH 5   3,3  3                       DH 6   5,4  410 pg pBR322 + DMSO         DH 5   85 pBR322 (no DMSO)      DH 5   31,37                                   34 pBR322 + DMSO         DH 6   60 pBR322 (no DMSO)      DH 6   22,28                                   25______________________________________ 
    
     Clearly, DH 5, an example of a deoR mutant, is showing transfection far in excess of other mutants, and especially in excess of the preferred cell line for transformation, DH-1. DH6 transforms similarly, as it also carries the deoR mutation. 
     EXAMPLE III 
     Comparison of DH1 and deoR mutant (DH5) 
     The cell cultures of DH1 and DH5, described supra, were used in comparative tests. pBR322 and p66 were used as well as a preparation of cDNA ligated into the vector pUC8 (in 20:1 mass ratio of cDNA to vector), at 5ng, 2.5ng, 1.25ng, and 0.5ng concentrations, as well as pUC9 +cDNA at the same amounts. pUC8 + pUC 9 are described in the art. 
     The cDNA used was RI-SalI DNA fragments from rat muscle. Additionally, as controls, 10pg pBR322 and 1ng p66 were used. The results of these experiments are set forth as follows: 
     
         ______________________________________       DH1         DH5       1%   10%        1%     10%______________________________________10 pg pBR322 20,20   --         50,60                                --1 ng p66     --      2,4        --   30,40Frequency of pBR322  2 × 10.sup.8                                5.5 × 10.sup.8transformation (ascolonies formed permicrogram of DNA)0.5 ng pUC8  7       51         17   1601.25 ng pUC8 16      70         32   2072.5 ng pUC8  7       82         32   2845.0 ng pUC8  18      145        64   5500.5 ng pUC9  2       21         5    921.25 ng pUC9 3       40         23   1202.5 ng pUC9  2       48         24   1805.0 ng pUC9  10      90         35   340______________________________________ 
    
     Yet again, it can be seen that DH5 is tranformed by all of the plasmids used at rates 3 and 4 times greater than those for DH1. As the only difference in the experimental parameters was the use of DH5 as compared to DH1, clearly the improvement resides in the deoR mutation. 
     EXAMPLE IV 
     Another set of experiments were performed using 30 mls of cultured cells, as described supra, in which the various plasmids described, supra, were used. These results were as follows: 
     
         ______________________________________Plasmid              CFUs    AVG.______________________________________      DH1 (1%)      10,10   1010 pg pBR322      DH5 (1%)      48,78   63200 pg pXAD      DHl (10%)     11,13   12      DH5 (10%)     205,185 1951 ng p66   DH1 (10%)     1,1     1      DH5 (10%)     34,26   301 ng pUC9  DHl (10%)     11,15   13+ cDNA     DH5 (10%)     115,125 1201 ng pUC8  DH1 (10%)     24,24   24+ cDNA     DH5 (10%)     240,260 250______________________________________ 
    
     EXAMPLE V 
     Tests were than performed to determine if the cells retained their enhanced tranformation properties after cold storage. 
     5×2.5ml colonies of the cells (DH1 or DH5), were mixed with 40 mls of SOB medium and grown to OD 550  of 0.6 DH1), and 0.48 (DH5). 
     3×10 ml samples were then treated by centrifugation for 30 minutes on ice to obtain cell pellets. These pellets were then resuspended in one of the following media: 
     (A) SF: 10% glycerol, 10 μM KCH 3  COO, 50 μM CaCl 2 .2H 2  O, 100 μM KCl at pH 6.1. 
     (B) RFT: 15% glycerol; 30 μM, KCH 3  COO; 100 μM RbCl; 10 μM CaCl2.2H 2  O; 50 μM MnCl2.4H 2  O (pH 5.8). 
     (C) Frozen Storage Buffer (FSB) 
     
         ______________________________________KCl          7.4 g           100 mMMnCl.sub.2.4H.sub.2 O        8.9 g           45 mMCaCl.sub.2.2H.sub.2 O        1.5 g           10 mMHACoCl.sub.3 0.8 g           3 mMPotassium acetate        10 ml of a 1 M stock                        10 mM        (pH 7.5)Redistilled glycerol        100 g           10% (w/v)(final pH 6.20 ± 0.10)______________________________________ 
    
     These were kept on ice for 50 minutes, and then resuspended in the same medium. DMSO was added to FSB, and each sample was aliquoted into 5 nunc tubes, and flash frozen at -70° C. 
     Twenty four hours later, samples were thawed until just liquid, and then were mixed with samples of pBR322 (100pg), and incubated, followed by plating. The following results were obtained. 
     
         ______________________________________        CFUs  AVG.______________________________________DH1SB             10,8    9RB             6,10    8FSB            8,2     5DH5SB             55,57   56RB             52,48   47FSB            34,40   37______________________________________ 
    
     Not only does DH5 transform at much higher frequencies than DH1, it also appears to remain more stable following storage than the cell line of choice, DH1. 
     As will be understood from the foregoing, especially Example I, the transformant used was an E. coli cell line which had been mutagenized by treatment with nitrosoguanadine and which contained the deoR mutant. These mutation appears to be key to the enhanced transformation frequency as DH1 and DH5 do not differ in any respect other than this. Additionally, comparison to other E. coli mutants, including ctyR and cytR +deoR shows that this ctyR mutation does not effect the transformation frequency of the cell line. 
     EXAMPLE VI 
     One skilled in the art will note immediately that while a specific chemical agent was used to obtain the deoR mutant used herein, additional chemical mutagens may be used to create the deoR mutation. Presence of the mutation can be determined in a given cell sample following the method set forth in Example I, supra, wherein growth on an inosine containing medium is seen to indicate its presence. 
     Additional methods of mutagenizing the E. coli cell line to obtain the desired deoR mutant cell line will be apparent to one skilled in the art. For example, both ultraviolet and X-irradiation may be used to produce mutations which may then be selected using the inosine containing medium described supra. Irradiation protocols may be found in, e.g., Miller, Experiments in Molecular Genetics (1972), (Cold Spring Harbor Laboratory, New York). Transposon insertional or deletional mutagenesis may be used as well. In this form of mutagenesis, transposons known to the art such as Tn5 and Tn10 can be inserted into the E. coli DNA, frequently with associated DNA deletions. Mutagenesis protocols may be found in, e.g., Davis et al., Advanced Bacterial Genetics (1980; Cold Spring harbor Laboratory, New York); Gilhavy et al., Experiments With Gene Fusions (1984; Cold Spring Harbor Laboratory, New York). 
     Spontaneous mutations to deoR can be selected and isolated by use of the inosine synthetic medium. While a low rate of mutation is to be expected (˜10 -7  cell/generation), this can be increased by inoculating about 50 ml of a cell sample to the SynIn medium described supra. 
     The terms and expressions which have been employed are used as terms of description and not of limitation, and there is no intention in the use of such terms and expressions of excluding any equivalents of the features shown and described or portions thereof, it being recognized that various modifications are possible within the scope of the invention.