Abstract:
Methods and compositions are provided that relate to obtaining a recombinant DNA and RNA cleaving nuclease. This involves the over-expression of a fusion protein between maltose-binding protein and a truncated nuclease in a soluble form in the cytoplasm of a host cell from which it can be readily extracted.

Description:
CROSS REFERENCE 
     This application is a continuation of U.S. patent application Ser. No. 11/660,110 filed Feb. 12, 2007, which is a §371 Application of PCT Application No. PCT/US05/28739 filed Aug. 12, 2005, which claims priority from U.S. Provisional Application No. 60/601,309 filed Aug. 13, 2004, herein incorporated by reference. 
    
    
     BACKGROUND 
     Nucleases are useful reagents for removing nucleic acids from protein preparations. All cells contain nucleases. Some nucleases can degrade both DNA as well as RNA in their double- and single-stranded forms. Some of nucleases degrade either DNA or RNA. Some of nucleases have more specific substrates, for example, single-stranded DNA. Reagent nucleases have been isolated either from the native host or as recombinant proteins. A problem encountered with cloning nucleases, which cleave both DNA and RNA, is that these proteins become toxic to host cells such as  E. coli  if overexpression is attempted using  E. coli  or other hosts. The toxicity effect can be ameliorated to some extent by causing the host cell either to efficiently secret the nuclease or to sequester the nuclease in an inclusion body in the host cell. Both methods have some disadvantages. Obtaining purified nuclease from media requires handling large volumes, which can be laborious. Release of nucleases from inclusion bodies in lysed cells requires denaturing quantities of urea (for example 6M urea) after which the nuclease has to be renatured to restore its enzymatic activity. It would therefore be desirable to have methods of obtaining nucleases in a simple way. 
     Descriptions of certain nucleases and their preparation by cloning are provided by Eaves et al. ( J. Bacteriol.  85:273-8 (1963)), Filimonova et al. ( Biochem. Mol. Biol. Int.  33(6):1229-36 (1994)), Ball et. al. ( Gene  57(2-3):183-92 (1987), Molin et. al. (U.S. Pat. No. 5,173,418), and Friedhoff et al. ( Protein Expr. Purif.  5(1):37-43 (1994)). 
     SUMMARY 
     In an embodiment of the invention, a modified nuclease in the form of a fusion protein is provided. This fusion protein contains a maltose-binding protein (MBP) fused to the nuclease such that the fusion protein is capable of being over-expressed in a host cell without causing lethality. A feature of the modified nuclease is that more than 80% of the expressed nuclease fusion protein is dispersed in the cytoplasm of the host cell. Examples of modified nuclease are provided from  Serratia marcescens  ( S. marcescens ) and  Staphylococcus aureus  ( S. aureus ). 
     The modified nuclease may be further characterized as truncated wherein the truncation corresponds to the removal of substantially all of the signal sequence. 
     In another embodiment of the invention, a DNA segment is provided for encoding the MBP-nuclease fusion protein. 
     In a further embodiment of the invention, a host cell is provided that is capable of expressing the nuclease-MBP fusion protein. 
     In a further embodiment of the invention, a method is provided for obtaining a nuclease for cleaving DNA and RNA, that includes the steps of (a) obtaining a DNA segment encoding a nuclease substantially lacking a signal sequence and a MBP; (b) expressing the DNA segment within a host cell to produce a significant concentration of nuclease fusion protein in the cytoplasm; (c) lysing the host cell; and (d) obtaining the isolated DNA and RNA nuclease. 
     In an example of the method, the nucleases are encoded by the DNAs derived from  S. marcescens  or  S. aureus.    
     In a further embodiment of the invention, a method for preparing a non-viscous extract of a prokaryotic cell is provided having the steps of: adding MBP-nuclease fusion protein, decanoyl-N-methylglucamide and a lysozyme to a cell pellet or cell suspension; and preparing the non-viscous extract from the prokaryotic cells. 
    
    
     
       LIST OF FIGURES 
         FIG. 1  shows an assay for nuclease activity using lambda-DNA substrate. A 2-fold serial dilution of the  S. marcescens  nuclease fused to MBP is used in each lane to digest 1 μg of λ genomic DNA. * symbol in Lane 4 is the amount of enzyme defined as one unit of nuclease activity. 
         FIG. 2  shows that expressed  S. marcescens  nuclease fused to MBP remains in the  E. coli  cytoplasm and is not secreted. Lanes are as follows: 
       Lane M is the DNA marker; 
       Lanes 1, 2 and 3 show the DNA nuclease activity from sonicated cells with 1×, 10× and 100× dilution; 
       Lanes 4, 5 and 6 show the enzymes from cell-free culture with 1×, 10× and 100× dilution. 
         FIG. 3  shows that the amount of expressed  S. marcescens  nuclease fused to MBP (MBP-nuclease(S)) in the crude extracts of  E. coli  are similar. Lanes are as follows: 
       Lane 1 is protein marker (New England Biolabs, Inc., Ipswich, Mass.—NEB #P7702); 
       Lane 2 is a crude extract from ER2683; 
       Lane 3 is a crude extract from ER1992; 
       Lane 4 shows the unbound fraction of a crude extract after fractionation on an amylose column; 
       Lanes 5 and 6 show the eluted  S. marcescens  nuclease fused to MBP after addition of 10 mM maltose to the column. 
         FIG. 4  shows (a) the DNA sequence (SEQ ID NO:1) and (b) the amino acid sequence (SEQ ID NO:2) of a non-secreted form of  S. marcescens  nuclease fused to MBP. 
         FIG. 5  shows the use of  S. marcescens  nuclease fused to MBP for the detergent-based cell-lysis reaction. Two  E. coli  strains were lysed by three different methods: sonication, CD-lysis reagent and PR-lysis reagent. Each method released similar total amount of proteins in the cell lysate as evidenced by the Bradford protein assay. 
         FIG. 6  shows that the expressed  S. aureus  nuclease fused to MBP (MBP-nucleases(S7)) is substantially non-secreted. The  E. coli  crude extracts, which were prepared from the harvested cells followed by sonication and centrifugation, were diluted 3-fold in serial reactions and showed to contain the nuclease activities in lanes 1 to 5. Lanes 6-10 showed the nuclease activity of purified fusion enzymes from an amylose column with 3-fold serial dilution from lanes 1-5 to lanes 6-10. 
         FIG. 7  shows that substantially all  S. aureus  nuclease fused to MBP is obtained in the crude extracts of  E. coli  (ER1992). Lanes are as follows: 
       Lane 1 is a protein marker (New England Biolabs, Inc., Ipswich, Mass., #P7702); 
       Lane 2 shows the crude extracts from ER1992; 
       Lane 3 shows the purified  S. aureus  nuclease fused to MBP eluted by addition of 10 mM maltose to the column. 
         FIG. 8  shows (a) the DNA sequence (SEQ ID NO:3) and (b) the amino acid sequence (SEQ ID NO:4) of a non-secreted form of  S. aureus  nuclease fused to MBP. 
     
    
    
     DESCRIPTION 
     An improved method for obtaining a nuclease capable of cleaving both DNA and RNA has been developed. The method involves intracellular over-production of the nuclease in the form of a fusion protein having substantially reduced toxicity. In one embodiment, reduced toxicity is achieved by fusion of the nuclease to MBP. The fusion may be made by removing a substantial portion of the signal peptide portion of the nuclease and adding MBP. For example, the signal sequence can be substituted for the MBP. 
     The portion of the signal sequence from the nuclease protein that should be removed is not required to be a precise number of amino acids. For example, whereas the length of the signal sequence may be 21 amino acids (von Heijne, G.  Nucleic Acids. Res.  14(11):4683-90 (1986)), as few as 19 amino acids in the signal peptide can be removed. Cell survival is improved by substituting substantially all the signal sequence with MBP. 
     The addition of the MBP moiety in the nuclease fusion protein results in a nuclease that can be trapped in the cytoplasm of the host cell in the soluble form and at high expression levels. The fusion protein can then be purified from lysed cells by means of amylose affinity chromatography in a simple purification step (pMal protein fusion and purification system described in New England Biolabs, Inc., Ipswich, Mass. catalog). Using this approach, over-expression of soluble, non-secreted nucleases in the cytoplasm of  E. coli  region yielded &gt;25 mg/L of nuclease. 
     The formation of an MBP-nuclease fusion protein can be achieved by ligating the DNA coding for the nuclease absent substantially all of the coding region for the signal sequence and the DNA coding for MBP. The DNA segment can be inserted into any suitable commercially available cloning vector for growing in an appropriate host cell. There is no known limitation on the type of prokaryotic or eukaryotic host cells used for this purpose. In an embodiment of the invention, the nuclease is over-expressed which here refers to being capable of being visualized as a distinct band on a 10%-20% polyacrylamide gel. 
     The present method is suitable for any DNA and RNA cleaving nuclease encoded by prokaryotic, eukaryotic or archeal cells or functional variants or derivatives of these nucleases. The method is exemplified here by two examples:  S. marcescans  nuclease and  S. aureus  nuclease. 
     One use for nucleases as reagents is in breaking up prokaryotic cells that contain proteins of interest. Cells can be broken up by non-mechanical means such as with detergents. For prokaryotic cells that lack a nuclear membrane, lysis of cells results in production of a high viscosity mixture because of released nucleic acid. The addition of a nuclease that digests DNA and RNA results in a clarified protein extract permitting further purification. In a preferred embodiment, a method is provided here for using a specific detergent together with a nuclease and lysozyme. Any type of lysozyme can be used such as chicken, T7 or T4 lysozyme. 
     The references cited above and below as well as U.S. Provisional Application Ser. No. 60/601,309 filed Aug. 13, 2004 are herein incorporated by reference. 
     EXAMPLES 
     Example 1 
     Over-Expression of  S. marcescens  Nuclease Fused to MBP 
     Bacterial Strain and Growth 
     The  E. coli  strains ER2683 and ER1992 were used to propagate plasmids and express fusion proteins. These strains are characterized by the following genotypes: ER2683 (MM294 background) F′ proA+B+ lacIq DlacZM15 miniTn10 (KanR) fhuA2 D(lacI-lacA)200 glnV44 e14− rfbD1? relA1? endA1 spoT1? thi-1 D(mcrC-nmr)114::IS10 ER1992 (MM294 background) D(argF-lac)U169 glnV44 mcr-67 rfbD1? relA1? endA1 dinD2::MudI1734 (KanR, lacZ+) spoT1? thi-1 D(mcrC-nmr)114::IS10. 
     Plasmid pMAL-c2X (New England Biolabs, Inc., Ipswich, Mass.) was used to clone the nuclease DNA (SEQ ID NO:1) with expression controlled by P tac  promotor and the lad repressor. Normal growth medium was LB supplemented with 100 μg/ml Amp and 0.2% (w/v) of glucose.  E. coli  was grown at 37° C. 
     A single bacterial colony with a red color was isolated from Japanese beetles by crushing beetles and spreading a sample on an LB plate at 30° C. The strain of  S. marcescens  was identified according to the sequences of 16S ribosomal DNA (bp27-1492). This isolate was used to obtain the nuclease gene as described below. 
     Preparation of Genomic DNA 
     A one ml preparation of an overnight culture at 30° C. of  S. marcescens  cells was centrifuged and resuspended in 100 μl of H 2 O containing 0.2% NP-40 and 10 mM EDTA. The solution was incubated at 100° C. for 10 min to release the genomic DNA of  S. marcescens  and to serve as a template for PCR. 
     Cloning of Nucleases from  S. marcescens    
     In order to amplify the nuclease by Vent DNA polymerase (New England Biolabs, Inc., Ipswich, Mass., NEB #M0254S), two primers were synthesized: 5′-GCCGACACGCTCGAATCCATCGACAAC-3′ (SEQ ID NO:5) and 5′-AGTCGGATCCTCAGTTTTTGCAGCCCATCAACTCCGG-3′ (SEQ ID NO:6). The PCR product was 741 bp and was cut with BamHI. Gel-purified BamHI-cut PCR products were kinased and cloned into the XmnI/BamHI region of pMAL-c2X vector, followed by transforming into ER1992. Two out of 18 colonies were found to carry the nuclease activity according to the assay below. The isolated plasmids containing the nuclease gene were sequenced using the ABI Bigdye terminator V3.1 sequencing kit (Applied Biosystems, Foster City, Calif.). DNA sequence confirmed that one of the clones carries the gene of  S. marcescens  nuclease with DNA nuclease activity. This plasmid was named pMAL-c2X-nuclease(S) (S for  S. marcescens ). 
     New transformants (ER1992 or ER2683 as hosts) carrying pMAL-c2X-nuclease(S) plasmid were grown at 37° C. on an LB plate with Amp. A single colony was inoculated into 2 ml LB+ glucose and grown overnight. Two hundred microliters of the overnight culture were added to fresh 10 ml of LB+Amp+glucose medium and grown to a density of OD 600nm =1.0, followed by addition of 1 mM of IPTG. After two hours induction, cells were harvested by centrifugation. 
     Assay for Nuclease Activity 
     The nuclease activity assay buffer used here contains 50 mM Tris-HCl (pH 8.0), 10 mM of MgCl 2  and 1 μg of λ-DNA. One unit of nuclease equals the amount of nuclease in the assay buffer at 37° C. for 20 min that causes the disappearance of high molecular weight of λ-DNA and accumulation of 100-400 bp (around or just below the migration of the bromophenol blue dye) of low molecular weight DNA fragments on a 1% agarose gel stained with ethidium bromide. The lane with a * symbol (lane 4,  FIG. 1 ) indicates the amount of enzyme which is defined as one unit of nuclease activity. 
     Cellular Fractions of  E. coli  Carrying pMAL-c2X-nucS 
     Cellular fractionations from 10 ml of induced cell culture (ER1992 as a host) were prepared as described using the spheroplast protocol (Randall and Hardy.  Cell  46(6):921-8 (1986)). Two well-known enzymatic activities are used to indicate the compartments of  E. coli : the presence of the β-galactosidase activity indicates the cytoplasmic fraction whereas the alkaline phosphatase activity indicates the periplasmic fraction. 
     Assay for β-Galactosidase Activity 
     β-galactosidase activity was assayed in a reaction mixture containing 180 μl of Z-buffer (0.06 M Na 2 HPO 4 /0.04M NaH 2 PO 4 , 0.1 M KCl, 0.001 M MgSO 4 . 0.05 M β-mercaptoethanol) and 40 μl of O-nitrophenyl-β-galactoside (10 mg/ml) at 37° C. Color was monitored at 420 nm. 
     Assay for Alkaline Phosphatase Activity 
     Alkaline phosphatase activity was assayed in a reaction mixture containing 180 μl of 1 M Tris-HCl (pH 8.0) and 20 μl of 10 mM p-nitrophenylphosphate at 37° C. Color was monitored at 420 nm. 
     Expression of Intracellular MBP-Nuclease Fusion Protein from  E. coli    
     The nuclease activity was predominantly present in the cell pellet and less than 0.1% of total activity was present in the medium ( FIG. 2 ). 
     To further narrow down the location of  S. marcescens  nuclease fused to MBP, the  E. coli  compartments were fractionated as described previously. As shown in Table 1, cellular fractionation confirmed that most of the nuclease activity was present in the cytoplasm where more than 98% of the β-galatosidase activity is present. 
                                                                 TABLE 1                   Cellular fractions of  E. coil  carrying pMAL-c2X-nucS gene                Cell culture   Periplasm   cytoplasm   Total activity                        Alkaline   N.A.   80%   20%   100%       Phosphatase       β-galactosidase   N.A.   &lt;2%   98%   100%       Nuclease   &lt;0.1%   &lt;5%   95%   100%                    
Purification of MBP-Nuclease
 
     MBP-nuclease fusion proteins were expressed in  E. coli  strains ER2683 and ER1992. 100 ml of cell culture were grown and cells were harvested by centrifugation. Cells were sonicated followed by centrifugation to remove cell debris as well as inclusion bodies. The supernatant fraction was called crude extracts thereafter. The amount of MBP-nuclease fusion in the crude extracts of these two strains did not show any difference as shown in  FIG. 3 , lane 2 (from ER2683) and lane 3 (from ER1992). Lane 1 is a protein marker (New England Biolabs, Inc., Ipswich, Mass., #P7702). Crude extracts were then loaded onto an amylose column; lane 4 shows the unbound fraction from ER1992 host strain; lanes 5 and 6 show the  S. marcescens  nuclease fused to MBP eluted by the addition of 10 mM maltose to the column. About 3.2 mg nuclease/cells were obtained from 100 ml LB broth. 
     Use of MBP-Nuclease Fusion for a Non-Mechanical Lysis of Bacterial Cells 
     MBP-nuclease fusion protein was used in conjunction with a detergent decanoyl-N-methylglucamide (MEGA-10) and chicken egg-white lysozyme. To a cell culture, CD-lysis reagent, containing 500 mM Tris-HCl (pH 7.9), 2 mM EDTA, 3% MEGA-10, 500 mM NaCl and 5% glycerol, was added directly to the cell culture to lyse cell. 
     For example, 100 μl of CD-lysis buffer, lysozyme (final concentration 15 μg/ml) and  S. marcescens  nuclease fused to MBP (1 unit) was added to 1 ml of cell culture to lyse cells prior to centrifugation. 
     Alternatively, 1 ml of cell culture was spun down and the pellets were resuspended in 100 μl of PR-lysis with additional of lysozyme (final concentration 150 μg/ml) and 1 unit of  S. marcescens  nuclease fused to MBP. The PR-lysis buffer containing 50 mM Tris-HCl (pH 7.9), 0.2 mM EDTA, 0.5% MEGA-10, 50 mM NaCl and 5% glycerol. 
     As shown in  FIG. 5 , similar amount of protein was released to extracellular environment of  E. coli  as determined by a Bradford protein assay, using non-mechanical methods (i.e., CD-lysis or PR-lysis reagents) or mechanical method (i.e., sonication). Two different strains of  E. coli , BL21 and BL21(DE3), were tested and both cells were lysed in a similar manner in terms of the amount of released proteins from cells. 
     Example II 
     Over-Expression of  S. aureus  Nuclease Fused to MBP 
     Bacterial Strain and Growth 
     The  E. coli  strain ER1992 was used to propagate plasmids and express fusion proteins. Plasmid pMAL-c2X (New England Biolabs, Inc., Ipswich, Mass.) was used to clone the nuclease DNA ( FIG. 8   a , SEQ ID NO:3) with expression controlled by P tac  promotor and the lad repressor. Normal growth medium was LB supplemented with 100 μg/ml Amp and 0.2% (w/v) of glucose.  E. coli  was grown at 37° C. 
     Preparation of Genomic DNA 
     An  S. aureus  V8 strain was purchased from ATCC (#49775). A one ml preparation of  S. aureus  cells grown overnight was centrifuged and resuspended in 100 μl of H 2 O containing 0.2% NP-40 and 10 mM EDTA. The solution was incubated at 100° C. for 10 min to release the genomic DNA of  S. aureus  and to serve as a template for PCR. 
     Cloning of Nucleases from  S. aureus    
     In order to amplify the nuclease by Vent DNA polymerase (New England Biolabs, Inc., Ipswich, Mass., NEB #M0254S), two primers were synthesized: 5′-GCAACTTCAACTAAAAAATTACATAAAGAACC-3′ (SEQ ID NO:7) and 5′-TTAAGGATCCTTATTGACCTGAATCAGCGTTGTCTTC-3′ (SEQ ID NO:8). The PCR product was 450 bp and was cut with BamHI. Gel-purified BamHI-cut PCR products were kinased and cloned into the XmnI/BamHI region of pMAL-c2X vector, followed by transforming into ER1992. Six out of 40 colonies were found to carry the nuclease activity according to the assay below. The isolated plasmids containing the nuclease gene were sequenced using the ABI Bigdye terminator V3.1 sequencing kit (Applied Biosystems, Foster City, Calif.). DNA sequence confirmed that six of the clones carry the gene of  S. aureus  nuclease. This plasmid was named pMAL-c2X-nuclease(S7) (S7 for  S. aureus ). 
     New transformants (ER1992) carrying pMAL-c2X-nuclease(S7) plasmid were grown at 37° C. on an LB plate with Amp. A single colony was inoculated into 2 ml LB+glucose and grown overnight. Two hundred microliters of overnight cells were added to fresh 10 ml of LB+Amp+glucose medium and were grown until OD 600nm  reached 1.0, followed by addition of 1 mM of IPTG. After two hours induction, cells were harvested by centrifugation. 
     Assay for Nuclease Activity 
     The nuclease activity assay buffer used here contains 50 mM Tris-HCl (pH 8.0), 10 mM of CaCl 2  and 1 μg of λ-DNA. DNA nuclease activity can be visualized by the disappearance of high molecular weight of λ-DNA and accumulation of 100-400 bp (around or just below the migration of the bromophenol blue dye) of low molecular weight DNA fragments on a 1% agarose gel stained with ethidium bromide ( FIG. 6 ). 
     Expression of Intracellular  S. aureus  Nuclease Fused to MBP and Purification from  E. coli    
       S. aureus  nuclease fused to MBPs were expressed in ER1992 hosts. 10 ml of cell culture were grown in LB medium and IPTG-induced cells were harvested by centrifugation. Cell pellets were resuspended in 50 mM Tris-HCl (pH 7.5)/50 mM NaCl buffer followed by sonication. Cell debris was spun down and the crude extracts, where the DNA nuclease activity was found ( FIG. 6 , lane 1-5), were then loaded onto an amylose column. As shown on  FIG. 7 , lane 1 is the protein marker (New England Biolabs, Inc., Ipswich, Mass., #P7702), lane 2 shows the crude extracts, lane 3 shows the  S. aureus  nuclease fused to MBP eluted by the addition of 10 mM maltose to the column. Purified  S. aureus  nuclease fused to MBP DNase activity was shown on  FIG. 6  (lane 6-10). Hence, it was concluded that the  S. aureus  nuclease fused to MBP was expressed as a soluble form in the cell.