Site-specific methylases induce the SOS DNA repair response in Escherichia coli

Isolation of temperature-sensitive McrA and McrB mutations and complementation analysis of the McrBC region of Escherichia coli K-12

We isolated temperature-sensitive mcrA and mcrBC mutants of Escherichia coli. At 42 degrees C, they were unable to restrict the T-even bacteriophages T6gt and T4gt or plasmids encoding cloned DNA methylase genes whose specificities confer sensitivity to the McrA and McrBC nucleases. Complementation analysis of the McrBC region (mcrB251) with the complete cloned McrBC system or a derivative with mcrB alone indicated that the mutation shows an absolute defect for the restriction of DNA containing hydroxymethylcytosine and a thermosensitive defect for the restriction of DNA containing methylcytosine. The properties of the McrA temperature-sensitive mutants suggest that some of these mutations can also influence the restriction of DNA containing hydroxymethylcytosine or methylcytosine residues.

Genetic and sequence organization of the mcrBC locus of Escherichia coli K-12

The mcrB (rglB) locus of Escherichia coli K-12 mediates sequence-specific restriction of cytosine-modified DNA. Genetic and sequence analysis shows that the locus actually comprises two genes, mcrB and mcrC. We show here that in vivo, McrC modifies the specificity of McrB restriction by expanding the range of modified sequences restricted. That is, the sequences sensitive to McrB(+)-dependent restriction can be divided into two sets: some modified sequences containing 5-methylcytosine are restricted by McrB+ cells even when McrC-, but most such sequences are restricted in vivo only by McrB+ McrC+ cells. The sequences restricted only by McrB+C+ include T-even bacteriophage containing 5-hydroxymethylcytosine (restriction of this phage is the RglB+ phenotype), some sequences containing N4-methylcytosine, and some sequences containing 5-methylcytosine. The sequence codes for two polypeptides of 54 (McrB) and 42 (McrC) kilodaltons, whereas in vitro translation yields four products, of approximately 29 and approximately 49 (McrB) and of approximately 38 and approximately 40 (McrC) kilodaltons. The McrB polypeptide sequence contains a potential GTP-binding motif, so this protein presumably binds the nucleotide cofactor. The deduced McrC polypeptide is somewhat basic and may bind to DNA, consistent with its genetic activity as a modulator of the specificity of McrB. At the nucleotide sequence level, the G+C content of mcrBC is very low for E. coli, suggesting that the genes may have been acquired recently during the evolution of the species.

Specificity of restriction endonucleases and DNA modification methyltransferases a review (Edition 3)

The properties and sources of all known class-I, class-II and class-III restriction endonucleases (ENases) and DNA modification methyltransferases (MTases) are listed and newly subclassified according to their sequence specificity. In addition, the enzymes are distinguished in a novel manner according to sequence specificity, cleavage position and methylation sensitivity. Furthermore, new nomenclature rules are proposed for unambiguously defined enzyme names. In the various Tables, the enzymes are cross-indexed alphabetically according to their names (Table I), classified according to their recognition sequence homologies (Table II), and characterized within Table II by the cleavage and methylation positions, the number of recognition sites on the DNA of the bacteriophages lambda, phi X174, and M13mp7, the viruses Ad2 and SV40, the plasmids pBR322 and pBR328, and the microorganisms from which they originate. Other tabulated properties of the ENases include relaxed specificities (integrated within Table II), the structure of the generated fragment ends (Table III), interconversion of restriction sites (Table IV) and the sensitivity to different kinds of DNA methylation (Table V). Table VI shows the influence of class-II MTases on the activity of class-II ENases with at least partially overlapping recognition sequences. Table VII lists all class-II restriction endonucleases and MTases which are commercially available. The information given in Table V focuses on the influence of methylation of the recognition sequences on the activity of ENases. This information might be useful for the design of cloning experiments especially in Escherichia coli containing M.EcodamI and M.EcodcmI [H16, M21, U3] or for studying the level and distribution of site-specific methylation in cellular DNA, e.g., 5'- (M)CpG-3' in mammals, 5'-(M)CpNpG-3' in plants or 5'-GpA(M)pTpC-3' in enterobacteria [B29, E4, M30, V4, V13, W24]. In Table IV a cross index for the interconversion of two- and four-nt 5'-protruding ends into new recognition sequences is complied. This was obtained by the fill-in reaction with the Klenow (large) fragment of the E. coli DNA polymerase I (PolIk), or additional nuclease S1 treatment followed by ligation of the modified fragment termini [P3]. Interconversion of restriction sites generates novel cloning sites without the need of linkers. This should improve the flexibility of genetic engineering experiments [K56, P3].(ABSTRACT TRUNCATED AT 400 WORDS)

Cloning, nucleotide sequence, and expression of the HincII restriction-modification system

Two genes, coding for the HincII from Haemophilus influenzae Rc restriction-modification system, were cloned and expressed in Escherichia coli RR1. Their DNA sequences were determined. The HincII methylase (M.HincII) gene was 1,506 base pairs (bp) long, corresponding to a protein of 502 amino acid residues (Mr = 55,330). The HincII endonuclease (R.HincII) gene was 774 bp long, corresponding to a protein of 258 amino acid residues (Mr = 28,490). The amino acid residues predicted from the R.HincII and the N-terminal amino acid sequence of the enzyme found by analysis were identical. These methylase and endonuclease genes overlapped by 1 bp on the H. influenzae Rc chromosomal DNA. The clone, named E. coli RR1-Hinc, overproduced R.HincII. The R.HincII activity of this clone was 1,000-fold that from H. influenzae Rc. The amino acid sequence of M.HincII was compared with the sequences of four other adenine-specific type II methylases. Important homology was found between tne M.HincII and these other methylases.

Differential plasmid rescue from transgenic mouse DNAs into Escherichia coli methylation-restriction mutants

Plasmids comprising transgene insertions in four lines of transgenic mice have been retrieved by plasmid rescue into a set of Escherichia coli strains with mutations in different members of the methylation-dependent restriction system (MDRS). Statistical analysis of plasmid rescue frequencies has revealed that the MDRS loci detect differential modifications of the transgene insertions among mouse lines that show distinctive patterns of transgene expression. Plasmids in mice that express hybrid insulin transgenes during development can be readily cloned into E. coli strains carrying mutations in two of the MDRS loci, mcrA and mcrB. In mice in which transgene expression is inappropriately delayed into adulthood, plasmids can only be cloned into E. coli that carry mutations in all known MDRS activities. Differential cloning frequencies in the presence or absence of the various methylation-dependent restriction genes represent a further way to distinguish regions of mammalian chromosomes. These multiply deficient E. coli strains will also facilitate the molecular cloning of modified chromosomal DNA.

Expression of cloned restriction and modification genes, hjaIRM from Hyphomonas jannaschiana in Escherichia coli

A type-II RM system, HjaI, was identified in the marine bacterium, Hyphomonas jannaschiana. The ENase recognizes GATATC, and DNA fragments generated after cleavage with this enzyme contain blunt ends. A DNA fragment encoding these enzymes was cloned and expressed in Escherichia coli, although the level of expression of the cloned genes was low. DNA methylated by M.HjaI was not restricted by the Mcr or Mrr restriction systems of E. coli. Although H. jannaschiana is a marine bacterium isolated near the thermal vents on the floor of the Pacific Ocean, the biochemical properties of the ENase were similar to those of EcoRV, an isoschizomer isolated from E. coli.

Characterization of the genetic determinants of SsoII-restriction endonuclease and modification methyltransferase

The genes encoding SsoI and SsoII restriction endonuclease (ENase) and methyltransferase (MTase) are located on the small plasmids P6 and P4, respectively, of Shigella sonnei strain 47. Functions provided by plasmids P5, P7 and P9, which include colicinogenicity and immunity to colicin E1, resistance to streptomycin (Sm), and conjugative DNA transfer, respectively, have also been identified. The genes of the SsoII restriction-modification (R-M) system have been cloned into Escherichia coli expressing the 35-kDa (ENase) and 43-kDa (MTase) products. A restriction map of the P4 plasmid DNA was determined, and the approximate location of the genes encoding SsoII ENase and MTase (ssoIIR and ssoIIM) on that have been established. SsoI is an isoschisomer of EcoRI and SsoII cleaves the 5'-/CCNGG/recognition sequence producing 5'-protruding 5-nt long cohesive ends.

Substrate recognition by the EcoRI endonuclease

The EcoRI restriction endonuclease is one of the most widely used tools for recombinant DNA manipulations. Because the EcoRI enzyme has been extremely well characterized biochemically and its structure is known at 3 A resolution as an enzyme-DNA complex, EcoRI also serves as a paradigm for other restriction enzymes and as an important model of DNA-protein interactions. To facilitate a genetic analysis of the EcoRI enzyme, we devised an in vivo DNA scission assay based on our finding that DNA double-strand breaks induce the Escherichia coli SOS response and thereby increase beta-galactosidase expression from SOS::lacZ gene fusions. By site-directed mutagenesis, 50 of 60 possible point mutations were generated at three amino acids (E144, R145, and R200) implicated in substrate recognition by the crystal structure. Although several of these mutant enzymes retain partial endonuclease activity, none are altered in substrate specificity in vivo or in vitro. These findings argue that, in addition to the hydrogen bond interactions revealed by the crystal structure, the EcoRI enzyme must make additional contacts to recognize its substrate.

Phage Trojan horses: a conditional expression system for lethal genes

The EcoRI restriction enzyme (ENase) cleaves DNA molecules within the sequence GAATTC. Cells expressing this lethal activity normally make a second enzyme, the M.EcoRI methyltransferase (MTase), which protects their chromosomal DNA by modifying the EcoRI recognition sites. To isolate mutants of the EcoRI ENase, its gene was cloned into a filamentous phage vector (M13mp18) under control of the lac promoter. Normally, filamentous phages (M13, f1 and their derivatives) form turbid plaques by impairing the growth of their host cell without killing it. In contrast, phages expressing the EcoRI ENase kill the host cell, but survive long enough to produce plaques which are very clear. Expression of the M.EcoRI MTase rescues the host and restores turbid plaque formation. EcoRI ENase mutants were isolated by screening for mutants that make turbid, instead of clear, plaques on an M- host. This conditional expression system may be useful for cloning and mutating genes for other toxic proteins.

Genetic and physical mapping of the mcrA (rglA) and mcrB (rglB) loci of Escherichia coli K-12

We have genetically analyzed, cloned and physically mapped the modified cytosine-specific restriction determinants mcrA (rglA) and mcrB (rglB) of Escherichia coli K-12. The independently discovered Rgl and Mcr restriction systems are shown to be identical by three criteria: 1) mutants with the RglA- or RglB- phenotypes display the corresponding McrA- or McrB- phenotypes, and vice versa; 2) the gene(s) for RglA and McrA reside together at one locus, while gene(s) for RglB and McrB are coincident at a different locus; and 3) RglA+ and RglB+ recombinant clones complement for the corresponding Mcr-deficient lesions. The mcrA (rglA) gene(s) is on the excisable element e14, just clockwise of purB at 25 min. The mcrB (rglB) gene(s), at 99 min, is in a cluster of restriction functions that includes hsd and mrr, determinants of host-specific restriction (EcoK) and methyladenine-specific restriction respectively. Gene order is mcrB-hsdS-hsdM-hsdR-mrr-serB. Possible models for the acqusition of these restriction determinants by enteric bacteria are discussed.

Quantitative evaluation of Escherichia coli host strains for tolerance to cytosine methylation in plasmid and phage recombinants

Many strains of E. coli K12 restrict DNA containing cytosine methylation such as that present in plant and animal genomes. Such restriction can severely inhibit the efficiency of cloning genomic DNAs. We have quantitatively evaluated a total of 39 E. coli strains for their tolerance to cytosine methylation in phage and plasmid cloning systems. Quantitative estimations of relative tolerance to methylation for these strains are presented, together with the evaluation of the most promising strains in practical recombinant cloning situations. Host strains are recommended for different recombinant cloning requirements. These data also provide a rational basis for future construction of 'ideal' hosts combining optimal methylation tolerance with additional advantageous mutations.

Characterization and cloning of MwoI (GCN7GC), a new type-II restriction-modification system from Methanobacterium wolfei

R.MwoI, a type-II restriction enzyme with the new specificity 5'-GCN7GC-3', was found in extracts of the thermophilic archaebacterium, Methanobacterium wolfei. R.MwoI cleaves duplex DNA producing fragments with 3-nt, 3'-terminal extensions, thus: GCN5/N2GC. The genes coding for the MwoI restriction and modification enzymes were cloned into Escherichia coli on the plasmid vector pBR322. The clones synthesize a low level of R.MwoI endonuclease. The plasmids display incomplete MwoI-specific modification, suggesting that the clones synthesize a low level of the M.MwoI methyltransferase, too.

Identification of a second polypeptide required for McrB restriction of 5-methylcytosine-containing DNA in Escherichia coli K12

The McrB restriction system in Escherichia coli K12 causes sequence-specific recognition and inactivation of DNA containing 5-methylcytosine residues. We have previously located the mcrB gene near hsdS at 99 min on the E. coli chromosome and demonstrated that it encodes a 51 kDa polypeptide required for restriction of M.AluI methylated (A-G-5mC-T) DNA. We show here, by analysis of maxicell protein synthesis of various cloned fragments from the mcrB region, that a second protein of approximately 39 kDa is also required for McrB-directed restriction. The new gene, designated mcrC, is adjacent to mcrB and located distally to hsdS. The McrB phenotype has been correlated previously with restriction of 5-hydroxy-methyl-cytosine (HMC)-containing T-even phage DNA that lacks the normal glucose modification of HMC, formally designated RglB (for restriction of glucoseless phage). This report reveals a difference between the previously correlated McrB and RglB restriction systems: while both require the mcrB gene product only the McrB system requires the newly identified mcrC-encoded 39-kDa polypeptide.

Nucleotide sequence of the McrB region of Escherichia coli K-12 and evidence for two independent translational initiation sites at the mcrB locus

The McrB restriction system of Escherichia coli K-12 is responsible for the biological inactivation of foreign DNA that contains 5-methylcytosine residues (E. A. Raleigh and G. Wilson, Proc. Natl. Acad. Sci. USA 83:9070-9074, 1986). Within the McrB region of the chromosome is the mcrB gene, which encodes a protein of 51 kilodaltons (kDa) (T. K. Ross, E. C. Achberger, and H. D. Braymer, Gene 61:277-289, 1987), and the mcrC gene, the product of which is 39 kDa (T. K. Ross, E. C. Achberger, and H. D. Braymer, Mol. Gen. Genet., in press). The nucleotide sequence of a 2,695-base-pair segment encompassing the McrB region was determined. The deduced amino acid sequence was used to identify two open reading frames specifying peptides of 455 and 348 amino acids, corresponding to the products of the mcrB and mcrC genes, respectively. A single-nucleotide overlap was found to exist between the termination codon of the mcrB gene and the proposed initiation codon of the mcrC gene. The presence of an additional peptide of 33 kDa in strains containing various recombinant plasmids with portions of the McrB region has been reported by Ross et al. (Gene 61:277-289, 1987). The analysis of frameshift and deletion mutants of one such hybrid plasmid, pRAB-13, provided evidence for a second translational initiation site within the McrB open reading frame. The proposed start codon for translation of the 33-kDa peptide lies 481 nucleotides downstream from the initiation codon for the 51-kDa mcrB gene product. The 33-kDa peptide may play a regulatory role in the McrB restriction of DNA containing 5-methylcytosine.

Repair of the Escherichia coli chromosome after in vivo scission by the EcoRI endonuclease

We prepared a set of temperature-sensitive mutants of the EcoRI endonuclease. Under semipermissive conditions, Escherichia coli strains bearing these alleles form poorly growing colonies in which intracellular substrates are cleaved at EcoRI sites and the SOS DNA repair response is induced. Strains defective in SOS induction (lexA3 mutant) or SOS induction and recombination (recA56 and recB21 mutants) are not more sensitive to this in vivo DNA scission, whereas strains deficient in DNA ligase (lig4 and lig ts7 mutants) are extremely sensitive. We conclude that although DNA scission induces the SOS response, neither this induction nor recombination are required for repair. DNA ligase is necessary and may be sufficient to repair EcoRI-mediated DNA breaks in the E. coli chromosome.

Cloning the BamHI restriction modification system

BamHI, a Type II restriction modification system from Bacillus amyloliquefaciensH recognizes the sequence GGATCC. The methylase and endonuclease genes have been cloned into E. coli in separate steps; the clone is able to restrict unmodified phage. Although within the clone the methylase and endonuclease genes are present on the same pACYC184 vector, the system can be maintained in E. coli only with an additional copy of the methylase gene present on a separate vector. The initial selection for BamHI methylase activity also yielded a second BamHI methylase gene which is not homologous in DNA sequence and hybridizes to different genomic restriction fragments than does the endonuclease-linked methylase gene. Finally, the interaction of the BamHI system with the E. coli Dam and the Mcr A and B functions, have been studied and are reported here.

Cloning type-II restriction and modification genes

We have cloned into Escherichia coli the genes for 38 type-II bacterial modification methyltransferases. The clones were isolated by selecting in vitro for protectively modified recombinants. Most of the clones modify their DNA fully but a substantial number modify only partially. In approximately one-half of the clones, the genes for the corresponding endonucleases are also present. Some of these clones restrict infecting phages and others do not. Clones carrying endonuclease genes but lacking methyltransferase genes have been found, in several instances, to be viable.

Molecular cloning, sequencing, and mapping of the bacteriophage T2 dam gene

Bacteriophage T2 codes for a DNA-(adenine-N6)methyltransferase (Dam), which is able to methylate both cytosine- and hydroxymethylcytosine-containing DNAs to a greater extent than the corresponding methyltransferase encoded by bacteriophage T4. We have cloned and sequenced the T2 dam gene and compared it with the T4 dam gene. In the Dam coding region, there are 22 nucleotide differences, 4 of which result in three coding differences (2 are in the same codon). Two of the amino acid alterations are located in a region of homology that is shared by T2 and T4 Dam, Escherichia coli Dam, and the modification enzyme of Streptococcus pneumoniae, all of which methylate the sequence 5' GATC 3'. The T2 dam and T4 dam promoters are not identical and appear to have slightly different efficiencies; when fused to the E. coli lacZ gene, the T4 promoter produces about twofold more beta-galactosidase activity than does the T2 promoter. In our first attempt to isolate T2 dam, a truncated gene was cloned on a 1.67-kilobase XbaI fragment. This construct produces a chimeric protein composed of the first 163 amino acids of T2 Dam followed by 83 amino acids coded by the pUC18 vector. Surprisingly, the chimera has Dam activity, but only on cytosine-containing DNA. Genetic and physical analyses place the T2 dam gene at the same respective map location as the T4 dam gene. However, relative to T4, T2 contains an insertion of 536 base pairs 5' to the dam gene. Southern blot hybridization and computer analysis failed to reveal any homology between this insert and either T4 or E. coli DNA.

Highly efficient positive selection of recombinant plasmids using a novel rglB-based Escherichia coli K-12 vector system

We have developed pBR328-derived vectors which allow highly efficient positive selection of recombinant plasmids. The system is based on the rglB-coded restriction activity of Escherichia coli K-12 directed against 5-methylcytosine (5mC)-containing DNA. The vectors code for cytosine-specific, temperature-sensitive DNA methyltransferases (ts-Mtases), whose specificity elicits RglB restriction. 5mC-free vector DNA - a prerequisite to allow establishment of such plasmids in cells expressing the RglB nuclease activity - can be prepared from cultures grown at 42 degrees C. At 30 degrees C the vector plasmids are vulnerable to RglB restriction due to the expression of suicidal Mtase activity. Cloning a DNA fragment into the ts-Mtase-coding gene disrupts the lethal methylation and thus permits selection of such recombinant plasmids at 30 degrees C. The standard vector used, pBN73, contains unique recognition sites for nine restriction enzymes within the ts-Mtase-coding gene, which can be used independently or in combination for the construction of recombinant plasmids selectable by the rglB-coded activity. Plasmid pBN74, which carries the determinants for both the ts-Mtase and the RglB nuclease, contains seven unique sites within the ts-Mtase-coding gene. While selection of recombinant plasmids derived from pBN73 obligatorily requires the employment of rglB+ strains, selection of pBN74 derivatives can be performed independent of the E. coli-host genotype. It remains to be elucidated whether positive selection of pBN74-derived recombinant plasmids can also be achieved in hosts other than E. coli. Plasmids pBN73, pBN74 and the recombinants are structurally stable. Generally applicable procedures, as developed during the establishment of this vector system, are described; they allow the isolation of ts-Mtases and facilitate the cloning of genes coding for nucleases directed against 5mC-containing DNA.

Overproduction, purification and crystallization of TaqI restriction endonuclease

Under phoA promoter control, TaqI endonuclease was overproduced to 5% of Escherichia coli cellular proteins. This was achieved by fusing the endonuclease gene to the first four codons of the alkaline phosphatase signal sequence. For maximal overproduction (30% of cellular proteins), a putative 14-bp hairpin within the endonuclease coding sequence was replaced with degenerate codons. In addition, TaqI methylase was required to protect host DNA. The endonuclease was purified in sufficient amounts for crystallization.

Survival and mutagenic effects of 5-azacytidine in Escherichia coli

Survival and mutagenesis caused by 5-azacytidine was studied in Escherichia coli. Survival was partially lexA- and recA-dependent and was decreased by the presence of a DNA (cytosine-5)methyltransferase. The dcm, MspI, and EcoRII methyltransferase genes all decreased survival. There was no direct relationship between amount of methylase enzyme present and cell survival, but only plasmids containing a methylase gene sensitized cells to 5-azacytidine. Survival was not affected by uvrA, uvrB or umuCD mutations. Induction of sulA::lacZ fusions by 5-azacytidine was inhibited in strains containing elevated levels of DNA methylase. Cells resistant to 5-azacytidine when they contained a plasmid specifying the EcoRII methylase were sensitive if the plasmid specified the complete EcoRII restriction-modification system. The mechanism of cell death in these situations is therefore different. Mutation of the rpoB gene by 5-azacytidine was studied. The mutation rate was decreased by the presence of recA and lexA mutations. Mutation in umuCD had little effect on the mutation rate. The recA430 mutation, which does not support SOS-dependent mutagenesis induced by UV light, does support 5-azacytidine induced mutagenesis. The presence of DNA (cytosine-5)methyltransferase had no effect on the mutation rate caused by 5-azacytidine treatment. The mutagenic and lethal lesions caused by 5-azacytidine in the absence of methylase therefore differ from the lethal lesions that occur in the presence of methylase. The former could be due to the opening of the 5-azacytosine ring in DNA. Cell death in the presence of methylase could be due to tight binding of methylase to azacytosine containing DNA as well as inhibition of induction of the SOS response.

McrA and McrB restriction phenotypes of some E. coli strains and implications for gene cloning

The McrA and McrB (modified cytosine restriction) systems of E. coli interfere with incoming DNA containing methylcytosine. DNA from many organisms, including all mammalian and plant DNA, is expected to be sensitive, and this could interfere with cloning experiments. The McrA and B phenotypes of a few strains have been reported previously (1-4). The Mcr phenotypes of 94 strains, primarily derived from E. coli K12, are tabulated here. We briefly review some evidence suggesting that McrB restriction of mouse-modified DNA does occur in vivo and does in fact interfere with cloning of specific mouse sequences.

Nucleotide sequence of the DdeI restriction-modification system and characterization of the methylase protein

The DdeI restriction-modification system was previously cloned and has been maintained in E. coli on two separate and compatible plasmids (1). The nucleotide sequence of the endonuclease and methylase genes has now been determined; it predicts proteins of 240 amino acids, Mr = 27,808, and 415 amino acids, Mr = 47,081, respectively. Inspection of the DNA sequence shows that the 3' end of the methylase gene had been deleted during cloning. The clone containing the complete methylase gene was made and compared to that containing the truncated gene; only clones containing the truncated form support the endonuclease gene in E. coli. Bal-31 deletion studies show that methylase expression in the Dde clones is also dependent upon orientation of the gene with respect to pBR322. The truncated and complete forms of the methylase protein were purified and compared; the truncated form appears to be more stable and active in vitro. Finally, comparison of the deduced amino acid sequence of M. DdeI with that of other known cytosine methylases shows significant regions of homology.

A genetic system for isolation and characterization of TaqI restriction endonuclease mutants

The gene encoding TaqI restriction endonuclease has been subcloned downstream from an inducible phoA promoter. Certain strains of Escherichia coli remain viable when endonuclease is expressed, even in the absence of (protective) methylation. Infecting lambda phage DNA is not restricted in vivo. One E. coli strain, MM294, exhibited a temperature-sensitive phenotype when TaqI endonuclease was induced. This allowed for design of an in vivo plate assay for identification of specially constructed two-codon insertion mutants in the endonuclease gene. These mutants exhibited a wide range of in vitro activities, including wild-type activity, greater activity in low-salt buffer, and sequence-specific nicking activity.