Organization of restriction-modification systems

The 'endo-blue method' for direct cloning of restriction endonuclease genes in E. coli

A new E. coli strain has been constructed that contains the dinD1::LacZ+ fusion and is deficient in methylation-dependent restriction systems (McrA-, McrBC-, Mrr-). This strain has been used to clone restriction endonuclease genes directly into E. coli. When E. coli cells are not fully protected by the cognate methylase, the restriction enzyme damages the DNA in vivo and induces the SOS response. The SOS-induced cells form blue colonies on indicator plates containing X-gal. Using this method the genes coding for the thermostable restriction enzymes Taql (5'TCGA3') and Tth111l (5'GACNNNGTC3') have been successfully cloned in E. coli. The new strain will be useful to clone other genes involved in DNA metabolism.

CAATTG-specific restriction-modification munI genes from Mycoplasma: sequence similarities between R.MunI and R.EcoRI

The genes coding for the MunI restriction-modification (R-M) system, which recognize the sequence 5'-CAATTG, have been cloned and expressed in Escherichia coli, and their nucleotide sequences have been determined. The restriction endonuclease (ENase; R.MunI) is encoded by an open reading frame (ORF) of 606 bp, and a 699-bp ORF codes for the methyltransferase (MTase). The two genes are transcribed divergently from a 355-bp region. The gene encoding the ENase is preceded by a short co-linear ORF of 222 bp. The deduced amino acid (aa) sequence of this short ORF (SORF) closely resembles the sequences of a family of regulatory proteins that are associated with other type-II R-M systems. Comparative analysis of the deduced aa sequence of R.MunI revealed several regions of similarity to the EcoRI and RsrI ENases that recognize the GAATTC sequence. The similar mode of interaction of MunI, EcoRI and RsrI with the tetranucleotide AATT, common to the recognition sequences of these ENases, was suggested.

Cloning and analysis of translational control for genes encoding the Cfr9I restriction-modification system

The complete type-II Cfr9I restriction-modification (R-M) system of Citrobacter freundii strain RFL9, recognizing the DNA sequence CCCGGG, has been cloned and expressed, and functionally active enzymes have been produced in Escherichia coli. Both the methyltransferase (MTase; M.Cfr9I) and restriction endonuclease (ENase; R.Cfr9I) were found to be encoded on a 2.3-kb cloned fragment in the same transcriptional orientation, but differing in translational phases. The last codon (underlined) (ATGA) of the MTase-encoding gene (Cfr9IM) overlaps with the start codon for the ENase-encoding gene (overlined) (cfr9IR). A nucleotide sequence complementary to a predicted Shine-Dalgarno sequence preceding cfr9IR is within this gene. Predicted free energy (delta G) for formation of the mRNA secondary structure involving these complementary sequences was found to be -16.1 kcal/mol. Amino-acid sequence homology of 80% was found between R.Cfr9I and R.XcyI.

The DNA (cytosine-5) methyltransferases

The m5C-MTases form a closely-knit family of enzymes in which common amino acid sequence motifs almost certainly translate into common structural and functional elements. These common elements are located predominantly in a single structural domain that performs the chemistry of the reaction. Sequence-specific DNA recognition is accomplished by a separate domain that contains recognition elements not seen in other structures. This, combined with the novel and unexpected mechanistic feature of trapping a base out of the DNA helix, makes the m5C-MTases an intriguing class of enzymes for further study. The reaction pathway has suddenly become more complicated because of the base-flipping and much remains to be learned about the DNA recognition elements in the family members for which structural information is not yet available.

Cloning, purification and characterization of the BseCI DNA methyltransferase from Bacillus stearothermophilus

The gene (bseCIM) encoding the BseCI DNA methyltransferase (MTase; M.BseCI) from a Bacillus stearothermophilus species was cloned and expressed in Escherichia coli using plasmid vector pBR322. Selection of transformants carrying bseCIM was based on the resistance of the modified plasmid to cleavage by BseCI. The MTase was purified to homogeneity and further characterized. Its size as determined by sodium dodecyl sulfate-polyacrylamide-gel electrophoresis and size exclusion chromatography was 68 kDa, suggesting that the MTase exists as a monomer. When phage lambda DNA was used as a substrate, the optimum temperature for MTase activity was determined to be 50-55 degrees C and optimum pH approx. 7.4. M.BseCI is inhibited by concentrations of NaCl and KCl greater than 50 mM, and it does not require Mg2+ for activity. Finally, M.BseCI methylates the 3' adenine residue in the sequence, 5'-ATCGAT-3', similarly to its isoschizomer M.ClaI.

Conserved sequence motif DPPY in region IV of the phage T4 Dam DNA-[N6-adenine]-methyltransferase is important for S-adenosyl-L-methionine binding

Comparison of the deduced amino acid sequences of DNA-[N6-adenine]-methyltransferases has revealed several conserved regions. All of these enzymes contain a DPPY [or closely related] motif. By site-directed mutagenesis of a cloned T4 dam gene, we have altered the first proline residue in this motif [located in conserved region IV of the T4 Dam-MTase] to alanine or threonine. The mutant enzymic forms, P172A and P172T, were overproduced and purified. Kinetic studies showed that compared to the wild-type [wt] the two mutant enzymic forms had: (i) an increased [5 and 20-fold, respectively] Km for substrate, S-adenosyl-methionine [AdoMet]; (ii) a slightly reduced [2 and 4-fold lower] kcat; (iii) a strongly reduced kcat/KmAdoMet [10 and 100-fold]; and (iv) almost the same Km for substrate DNA. Equilibrium dialysis studies showed that the mutant enzymes had a reduced [4 and 9-fold lower] Ka for AdoMet. Taken together these data indicate that the P172A and P172T alterations resulted primarily in a reduced affinity for AdoMet. This suggests that the DPPY-motif is important for AdoMet-binding, and that region IV contains or is part of an AdoMet-binding site.

Characterization of the bstVIRM genes encoding the Bacillus stearothermophilus V restriction-modification system

The nucleotide (nt) sequence of a 2.7-kb HindIII-EcoRI DNA fragment encoding the bstVIR and bstVIM genes has been determined. The sequence predicts a restriction endonuclease of 224 amino acids (aa), M(r) 25,104, and a methyl-transferase of 561 aa, M(r0 65,702. Both genes are aligned in the same orientation and are separated by a 102-nt intergenic region. No homology was found between R.BstVI and M.BstVI when their deduced aa sequences were compared. Significant similarity at the aa level was found, however, when both enzymes were compared to their equivalents in the paeR7IRM system of Pseudomonas aeruginosa PAO303.

Biology of DNA restriction

Our understanding of the evolution of DNA restriction and modification systems, the control of the expression of the structural genes for the enzymes, and the importance of DNA restriction in the cellular economy has advanced by leaps and bounds in recent years. This review documents these advances for the three major classes of classical restriction and modification systems, describes the discovery of a new class of restriction systems that specifically cut DNA carrying the modification signature of foreign cells, and deals with the mechanisms developed by phages to avoid the restriction systems of their hosts.

Gene structure and expression of the MboI restriction--modification system

The genes from Moraxella bovis encoding the MboI restriction--modification system were cloned and expressed in Escherichia coli. Three open reading frames were found in the sequence containing the genes. These genes, which we named mboA, mboB, and mboC, had the same orientation in the genome. Genes mboA and mboC encoded MboI methyltransferases (named M.MboA and M.MboC) with 294 and 273 amino acid residues, respectively. The mboB gene coded for MboI restriction endonuclease (R.MboI) with 280 amino acid residues. Recombinant E.coli-MBOI, which contained the whole MboI system, overproduced R.MboI. R.MboI activity from E.coli-MBOI was 480-fold that of M.bovis. The amino acid sequences deduced from these genes were compared with those of other restriction--modification systems. The protein sequences of the MboI system had 38-49% homology with those of the DpnII system.

Determination of the order of substrate addition to MspI DNA methyltransferase using a novel mechanism-based inhibitor

The cloning and overexpression of the MspI DNA methyltransferase as a functional fusion with glutathione S-transferase is described. The fusion enzyme retains full biological activity and has been used to investigate the interaction of substrates and inhibitors with MspI DNA methyltransferase. The fusion enzyme has been purified to homogeneity in a single step on GSH-agarose and is free from contaminating exonuclease activity. The enzyme can be photolabelled with S-adenosyl-L-methionine and the level of incorporation of label is enhanced by the presence of a nonspecific DNA duplex. In the presence of a cognate oligodeoxynucleotide, no photolabelling was observed since methyl transfer occurs instead. The inclusion of a mechanism-based inhibitor of C-5 deoxycytidine DNA methylation (an oligodeoxynucleotide containing the base 2-pyrimidinone-1-beta-D-2'-deoxyribofuranoside in the position of the deoxycytidine to which methyl addition occurs), which is thought to form a covalent interaction with the reactive cysteine of such enzymes, led to an enhancement of S-adenosyl-L-methionine photolabelling which suggests that, in contrast with results obtained with EcoRII DNA methyltransferase [Som and Friedman (1991) J. Biol. Chem. 266, 2937-2945], methylcysteine is not the photolabelled product. The implications of the results obtained with this mechanism-based inhibitor are discussed with respect to other C-5-specific DNA methyltransferases. Gel-retardation assays in the presence of cognate oligodeoxynucleotides that contain the reactive pyrimidinone base in place of the deoxycytidine target base are described. These demonstrate that most probably a stable covalent bond is formed between the methyltransferase and this oligodeoxynucleotide. However, the alternative of extremely tight non-covalent binding cannot be rigorously excluded. Furthermore, the results from these experiments indicate that the reaction mechanism proceeds in a manner similar to that of HhaI DNA methyltransferase with sequence-specific DNA binding being followed by addition of S-adenosyl-L-methionine and concomitant isomerization of the ternary complex leading to methyl transfer. S-Adenosyl-L-homocysteine appears to inhibit the reaction pathway as a result of either competition with the methyl donor and potentiation of a high-affinity interaction between the enzyme and DNA in an abortive ternary complex or through an allosteric interaction.

In vivo methylation in Escherichia coli by the Bacillus subtilis phage phi 3T I methyltransferase to protect plasmids from restriction upon transformation of Clostridium acetobutylicum ATCC 824

The restriction endonuclease Cac824I has been shown to be a major barrier to electrotransformation of Clostridium acetobutylicum ATCC 824 (L. D. Mermelstein, N. E. Welker, G. N. Bennett, and E. T. Papoutsakis, Bio/Technology 10:190-195, 1992). Methylation by the phi 3T I methyltransferase encoded by Bacillus subtilis phage phi 3T was shown to protect plasmid DNA from restriction by Cac824I. Expression in Escherichia coli of the phi 3tI gene (which encodes the phi 3T I methyltransferase) from pAN1, which replicates via the p15A origin of replication, was sufficient to completely methylate coresident E. coli-C. acetobutylicum shuttle vectors with ColE1 origins of replication. Three shuttle vectors (pIMP1, pSYL2, and pSYL7) methylated in this manner were used to efficiently electrotransform strain ATCC 824. These vectors could not be introduced into strain ATCC 824 when unmethylated because the E. coli portions of the plasmids contain a large number of Cac824I sites. This method obviates the need to use B. subtilis-C. acetobutylicum shuttle vectors with few Cac824I sites to introduce DNA into C. acetobutylicum ATCC 824.

ScrFI restriction-modification system of Lactococcus lactis subsp. cremoris UC503: cloning and characterization of two ScrFI methylase genes

Two genes from the total genomic DNA of dairy starter culture Lactococcus lactis subsp. cremoris UC503, encoding ScrFI modification enzymes, have been cloned and expressed in Escherichia coli. No homology between the two methylase genes was detected, and inverse polymerase chain reaction of flanking chromosomal DNA indicated that both were linked on the Lactococcus genome. Neither clone encoded the cognate endonuclease. The DNA sequence of one of the methylase genes (encoded by pCI931M) was determined and consisted of an open reading frame 1,170 bp long, which could encode a protein of 389 amino acids (M(r), 44.5). The amino acid sequence contained the highly characteristic motifs of an m5C methylase. Extensive regions of homology were observed with the methylases of NlaX, EcoRII, and Dcm.

Analysis of the nucleotide and derived amino acid sequences of the SsoII restriction endonuclease and methyltransferase

A 2648-bp fragment from the P4 plasmid of Shigella sonnei strain 47 coding for the SsoII restriction endonuclease (ENase) and methyltransferase (MTase) (recognition sequence 5'-CCNGG) was sequenced. Two divergently arranged open reading frames of 905 bp for the SsoII ENase (R.SsoII) and 1137 bp for the MTase (M.SsoII) were identified. The coding regions are separated by 110 bp. The calculated M(r) of R.SsoII (35937) and M.SsoII (42887) are in good agreement with values previously obtained by in vitro transcription-translation experiments, i.e., 35 and 43 kDa for the ENase and MTase, respectively. The M.SsoII amino acid (aa) sequence revealed a considerable similarity to m5C-MTases recognizing the related sequences--M.EcoRII, M.dcm, M.MspI, M.BsuFI, M.HpaII, and M.HhaI. Surprisingly, the greatest degree of homology has been observed between the aa sequences of M.SsoII and M.NlaX, with an unidentified recognition sequence. The multiple alignment of aa sequences helps to identify the blocks of conserved aa in variable regions of MTases. These conserved aa can play a key role in target recognition. Some aspects of evolution of m5C-MTases are discussed.

Purification and characterisation of Bst LVI restriction endonuclease, a thermostable isoschizomer of ClaI from Bacillus stearothermophilus LV

This work describes the purification and biochemical characterization of BstLVI restriction endonuclease, a thermostable isoschizomer of ClaI, from Bacillus stearothermophilus LV. The enzyme was purified by successive DEAE-cellulose, Affi-Gel Blue and Heparin-Sepharose CL-6B column chromatography. A molecular weight of 37,000 was determined for Bst LVI by gel filtration. As expected from thermophilic proteins, the enzyme showed a high stability towards heat and also to other known protein-denaturing agents.

BsuBI--an isospecific restriction and modification system of PstI: characterization of the BsuBI genes and enzymes

The enzymes of the Bacillus subtilis BsuBI restriction/modification (R/M) system recognize the target sequence 5'CTGCAG. The genes of the BsuBI R/M system have been cloned and sequenced and their products have been characterized following overexpression and purification. The gene of the BsuBI DNA methyltransferase (M.BsuBI) consists of 1503 bp, encoding a protein of 501 amino acids with a calculated M(r) of 57.2 kD. The gene of the restriction endonuclease (R.BsuBI), comprising 948 bp, codes for a protein of 316 amino acids with a predicted M(r) of 36.2 kD. M.BsuBI modifies the adenine (A) residue of the BsuBI target site, thus representing the first A-N6-DNA methyltransferase identified in B. subtilis. Like R.PstI, R.BsuBI cleaves between the A residue and the 3' terminal G of the target site. Both enzymes of the BsuBI R/M system are, therefore, functionally identical with those of the PstI R/M system, encoded by the Gram negative species Providencia stuartii. This functional equivalence coincides with a pronounced similarity of the BsuBI/PstI DNA methyltransferases (41% amino acid identity) and restriction endonucleases (46% amino acid identity). Since the genes are also very similar (58% nucleotide identity), the BsuBI and PstI R/M systems apparently have a common evolutionary origin. In spite of the sequence conservation the gene organization is strikingly different in the two R/M systems. While the genes of the PstI R/M system are separated and transcribed divergently, the genes of the BsuBI R/M system are transcribed in the same direction, with the 3' end of the M gene overlapping the 5' end of the R gene by 17 bp.

In vivo specificity of EcoRI DNA methyltransferase

The EcoRI adenine DNA methyltransferase forms part of a bacterial restriction/modification system; the methyltransferase modifies the second adenine within the canonical site GAATTC, thereby preventing the EcoRI endonuclease from cleaving this site. We show that five noncanonical EcoRI sites (TAATTC, CAATTC, GTATTC, GGATTC and GAGTTC) are not methylated in vivo under conditions when the canonical site is methylated. Only when the methyltransferase is overexpressed is partial in vivo methylation of the five sites detected. Our results suggest that the methyltransferase does not protect host DNA against potential endonuclease-mediated cleavage at noncanonical sites. Our related in vitro analysis of the methyltransferase reveals a low level of sequence-discrimination. We propose that the high in vivo specificity may be due to the active removal of methylated sequences by DNA repair enzymes (J. Bacteriology (1987), 169 3243-3250).

Cloning and characterization of genes for the PvuI restriction and modification system

The genes encoding the endonuclease and the methylase of the PvuI restriction and modification system were cloned in E.coli and characterized. The genes were adjacent in tandem orientation spanning a distance of 2200 bases. The PvuI endonuclease was a single polypeptide with a calculated molecular weight of 27,950 daltons. The endonuclease was easily detectable when the gene was expressed from its endogenous promotor and present on a low copy plasmid, but expression was considerably enhanced when the endonuclease gene was placed under the control of a strong promoter on a high copy plasmid. The methylase did not completely protect plasmid DNA from R.PvuI digestion until the methylase gene was placed under lac promotor control in a multicopy plasmid. In the absence of the M.PvuI methylase, expression of the R.PvuI endonuclease from the lac promotor on a multicopy plasmid was not lethal to wild type E.coli, but was lethal in a temperature-sensitive ligase mutant at the non-permissive temperature. Moreover, induction of the R.PvuI endonuclease under lambda pL promotor control resulted in complete digestion of the E.coli chromosome by R.PvuI.

Regulation of the BamHI restriction-modification system by a small intergenic open reading frame, bamHIC, in both Escherichia coli and Bacillus subtilis

BamHI, from Bacillus amyloliquefaciens H, is a type II restriction-modification system recognizing and cleaving the sequence G--GATCC. The BamHI restriction-modification system contains divergently transcribed endonuclease and methylase genes along with a small open reading frame oriented in the direction of the endonuclease gene. The small open reading frame has been designated bamHIC (for BamHI controlling element). It acts as both a positive activator of endonuclease expression and a negative repressor of methylase expression of BamHI clones in Escherichia coli. Methylase activity increased 15-fold and endonuclease activity decreased 100-fold when bamHIC was inactivated. The normal levels of activity for both methylase and endonuclease were restored by supplying bamHIC in trans. The BamHI restriction-modification system was transferred into Bacillus subtilis, where bamHIC also regulated endonuclease expression when present on multicopy plasmid vectors or integrated into the chromosome. In B. subtilis, disruption of bamHIC caused at least a 1,000-fold decrease in endonuclease activity; activity was partially restored by supplying bamHIC in trans.

Alw26I, Eco31I and Esp3I--type IIs methyltransferases modifying cytosine and adenine in complementary strands of the target DNA

The specificity of three DNA methyltransferases M.Alw26I, M.Eco31I and M.Esp3I, isolated from Acinetobacter Iwoffi RFL26, Escherichia coli RFL31 and Hafnia alvei RFL3+, respectively, was determined. All the enzymes methylate both strands of asymmetric recognition sites yielding m5C in the top-strand and m6A in the bottom-strand, as below: 5'-GTm5CTC 5'-GGTm5CTC 5'-CGTm5CTC 3'-Cm6AGAG 3'-CCm6AGAG 3'-GCm6AGAG (M.Alw26I) (M.Eco31I) (M.Esp3I) They are the first members of type IIs methyltransferases that modify different types of nucleotides in the recognition sequence.

Identification of the CTAG-recognizing restriction-modification systems MthZI and MthFI from Methanobacterium thermoformicicum and characterization of the plasmid-encoded mthZIM gene

Two CTAG-recognizing restriction and modification (R/M) systems, designated MthZI and MthFI, were identified in the thermophilic archaeon Methanobacterium thermoformicicum strains Z-245 and FTF, respectively. Further analysis revealed that the methyltransferase (MTase) genes are plasmid-located in both strains. The plasmid pFZ1-encoded mthZIM gene of strain Z-245 was further characterized by subcloning and expression studies in Escherichia coli followed by nucleotide sequence analysis. The mthZIM gene is 1065 bp in size and may code for a protein of 355 amino acids (M(r) 42,476 Da). The deduced amino acid sequence of the M.MthZI enzyme shares substantial similarity with four distinct regions from several m4C- and m6A-MTases, and contains the TSPPY motif that is so far only found in m4C-MTases. Partially overlapping with the mthZIM gene and in reverse orientation, an additional ORF was identified with a size of 606 bp potentially coding for a protein of 202 amino acids (M(r) 23.710 Da). This ORF is suggested to encode the corresponding endonuclease R.MthZI.

Purification and biochemical characterization of the EcaI DNA methyltransferase

The EcaI GGTNACC-specific DNA-adenine modification methyltransferase has been purified to apparent homogeneity. The active form of the DNA methyltransferase is a single polypeptide. The enzyme has a pH optimum at pH 8.0 and a temperature optimum at 25 degrees C. EcaI DNA methyltransferase transfers one methyl group to the adenine of the recognition site in a single binding event. The Km was 170 nM for DNA and 1.8 microM for the methyl donor S-adenosylmethionine. Methylated DNA is a competitive inhibitor with respect to DNA (Ki = 3.5 nM). The other product of the DNA-methylation reaction, S-adenosylhomocysteine was found to be a competitive inhibitor with respect to S-adenosylmethionine (Ki = 2.7 microM). The S-adenosylmethionine analog sinefungin was shown to be a very strong inhibitor (Ki = 3.5 nM) of the DNA methyltransferase reaction.

Biological function of DNA methylation

Structural and functional properties of prokaryotic DNA methyltransferases are summarized. The different aspects of the role of DNA methylation which influences DNA-protein interaction in restriction and modification of DNA and in mismatch repair, DNA replication and gene expression are discussed.

Cloning and linkage analysis of Neisseria gonorrhoeae DNA methyltransferases

We have cloned DNA methyltransferases (MTases) from various strains of Neisseria gonorrhoeae. Each of these clones represents a single specificity, indicating that the multiple gonococcal MTase specificities are encoded by monospecific MTases. The DNAs of five strains (FA5100, F62, MS11, Pgh3-2, and WR302) were digested with NheI, SpeI, or NheI plus SpeI and subjected to pulsed-field gel electrophoresis. The DNA MTase clones were used to probe Southern blots of these pulsed-field gels to determine whether the MTase genes are linked and whether there are strain-to-strain differences. The results indicate that none of these genes are closely linked, but variable hybridization patterns indicate that there exist restriction fragment length polymorphisms between the strains tested. Most of the chromosomal regions containing these restriction fragment length polymorphisms are clustered in regions containing gonococcal genes known or suspected to antigenically vary via genetic recombination.

Characterization of the archaeal, plasmid-encoded type II restriction-modification system MthTI from Methanobacterium thermoformicicum THF: homology to the bacterial NgoPII system from Neisseria gonorrhoeae

A restriction-modification system, designated MthTI, was localized on plasmid pFV1 from the thermophilic archaeon Methanobacterium thermoformicicum THF. The MthTI system is a new member of the family of GGCC-recognizing restriction-modification systems. Functional expression of the archaeal MthTI genes was obtained in Escherichia coli. The mthTIR and mthTIM genes are 843 and 990 bp in size and code for proteins of 281 (32,102 Da) and 330 (37,360 Da) amino acids, respectively. The deduced amino acid sequence of M.MthTI showed high similarity with that of the isospecific methyltransferases M.NgoPII and M.HaeIII. In addition, extensive sequence similarity on the amino acid level was observed for the endonucleases R.MthTI and R.NgoPII. Moreover, the endonuclease and methyltransferase genes of the thermophilic MthTI system and those of the Neisseria gonorrhoeae NgoPII system show identical organizations and high (54.5%) nucleotide identity. This finding suggests horizontal transfer of restriction-modification systems between members of the domains Bacteria and Archaea.

Stepwise cloning and genetic organization of the seemingly unclonable HgiCII restriction-modification system from Herpetosiphon giganteus strain Hpg9, using PCR technique

The genes, hgiCIIR and hgiCIIM, that encode the HgiCII restriction and modification (R-M) system from Herpetosiphon giganteus strain Hpg9, an AvaII isoschizomer recognizing the sequence, GGATCC, were cloned in Escherichia coli. Cloning the respective hgiCIIM gene was achieved via in vitro selection both from a Sau3AI- and an NheI-generated plasmid gene library using AvaII, a commercially available isoschizomer of HgiCII. However, all attempts to clone the closely linked hgiCIIR and M genes in a single step resulted in deletions spanning parts of the coding region of hgiCIIR. Therefore, cloning of the missing 3'-terminal part of this gene was achieved by applying the inverse polymerase-chain-reaction technique. All attempts to construct an enzymatically active R.HgiCII failed; only the inactivated hgiCIIR gene could be cloned. Sequencing of the hgiCIIRM region (carrying predesigned small mutations in the R gene) disclosed three open reading frames (ORFs): one small ORF preceding the methltransferase (MTase)-encoding gene, plus those encoding M.HgiCII (49,620 Da) and R.HgiCII (30,891 Da). M.HgiCII exhibits the common motif of ten conserved amino-acid blocks typically found within the group of m5C-MTases. The R-M system of HgiCII reveals strong homologies to the isoschizomeric R-M system of HgiBI from H. giganteus strain Hpg5, which, in contrast, could be cloned in one step.

IncN plasmid pKM101 and IncI1 plasmid ColIb-P9 encode homologous antirestriction proteins in their leading regions

The IncN plasmid pKM101 (a derivative of R46), like the IncI1 plasmid ColIb-P9, carries a gene (ardA, for alleviation of restriction of DNA) encoding an antirestriction function. ardA was located about 4 kb from the origin of transfer, in the region transferred early during bacterial conjugation. The nucleotide sequence of ardA was determined, and an appropriate polypeptide with the predicted molecular weight of about 19,500 was identified in maxicells of Escherichia coli. Comparison of the deduced amino acid sequences of the antirestriction proteins of the unrelated plasmids pKM101 and ColIb (ArdA and Ard, respectively) revealed that these proteins have about 60% identity. Like ColIb Ard, pKM101 ArdA specifically inhibits both the restriction and modification activities of five type I systems of E. coli tested and does not influence type III (EcoP1) restriction or the 5-methylcytosine-specific restriction systems McrA and McrB. However, in contrast to ColIb Ard, pKM101 ArdA is effective against the type II enzyme EcoRI. The Ard proteins are believed to overcome the host restriction barrier during bacterial conjugation. We have also identified two other genes of pKM101, ardR and ardK, which seem to control ardA activity and ardA-mediated lethality, respectively. Our findings suggest that ardR may serve as a genetic switch that determines whether the ardA-encoded antirestriction function is induced during mating.

Isolation and characterization of the cDNA encoding human DNA methyltransferase

We have cloned a series of overlapping cDNA clones encoding a 5194 bp transcript for human DNA methyltransferase (DNA MTase). This sequence potentially codes for a protein of 1495 amino acids with a predicted molecular weight of 169 kDa. The human DNA MTase cDNA has eighty percent homology at the nucleotide level, and the predicted protein has seventy-four percent identity at the amino acid level, to the DNA MTase cDNA cloned from mouse cells. Like the murine DNA MTase, the amino terminal two-thirds of the human protein contains a cysteine-rich region suggestive of a metal-binding domain. The carboxy terminal one-third of the protein shows considerable similarity to prokaryotic (cytosine-5)-methyltransferases. The arrangement of multiple motifs conserved in the prokaryotic genes is preserved in the human DNA MTase, including the relative position of a proline-cysteine dipeptide thought to be an essential catalytic site in all (cytosine-5)-methyltransferases. A single 5.2 kb transcript was detected in all human tissues tested, with the highest levels of expression observed in RNA from placenta, brain, heart and lung. DNA MTase cDNA clones were used to screen a chromosome 19 genomic cosmid library. The DNA MTase-positive cosmids which are estimated to span a genomic distance of 93 kb have been localized to 19p13.2-p13.3 by fluorescence in situ hybridization. Isolation of the cDNA for human DNA MTase will allow further study of the regulation of DNA MTase expression, and of the role of this enzyme in establishing DNA methylation patterns in both normal and neoplastic cells.

Sequence and characterization of pvuIIR, the PvuII endonuclease gene, and of pvuIIC, its regulatory gene

An open reading frame partially overlaps pvuIIR, and genetic evidence implies that this open reading frame, named pvuIIC, specifies a positive regulator of pvuIIR (T. Tao, J. C. Bourne, and R. M. Blumenthal, J. Bacteriol. 173:1367-1375, 1991). Inducible constructs of pvuIIC produced a protein of the expected size. The site of C.PvuII action appears to lie within pvuIIC itself; thus, pvuIIC may be a self-contained regulatory cassette.

The corrected nucleotide sequences of the TaqI restriction and modification enzymes reveal a thirteen-codon overlap

The nucleotide sequence of the genes encoding methyltransferase TaqI (M.TaqI) and restriction endonuclease TaqI (R.TaqI) with the recognition sequence, TCGA, were analyzed in clones isolated from independent libraries. The genes, originally reported as 363 and 236 codons long [Slatko et al., Nucleic Acids Res. 15 (1987) 9781-9796] were redetermined as 421 and 263 codons long, respectively. The C terminus of the taqIM gene overlaps the N terminus of the taqIR gene by 13 codons, as observed with the isoschizomeric TthHB8I restriction-modification system [Barany et al., Gene 112 (1992) 13-20]. Removal of the overlapping codons did not interfere with in vivo M.TaqI activity. We postulate the overlap plays a role in regulating taqIR expression.

Cloning and sequencing of genes encoding the TthHB8I restriction and modification enzymes: comparison with the isoschizomeric TaqI enzymes

Genes encoding the TthHB8I restriction and modification (R-M) system from Thermus thermophilus HB8 (recognition sequence T decreases CGA) were cloned in Escherichia coli. The genes have the same transcriptional orientation, with the last 13 codons of the methyltransferase (MTase) overlapping the first 13 codons of the endonuclease (ENase). Nucleotide sequence analysis of the TthHB8I ENase revealed a single chain of 263 amino acid (aa) residues that share a 77% identity with the corrected isoschizomeric TaqI ENase. Likewise, the Tth MTase (428 aa) shares a 79% identity with the corrected sequence of the TaqI MTase. This high degree of aa conservation suggests a common origin between the Taq and Tth R-M systems. However, codon usage and G+C content for the R-M genes differed markedly from that of other cloned Thermus genes. This suggests that these R-M genes were only recently introduced into the genus Thermus.

Correlation between insertion mutant activities and amino acid sequence identities of the TaqI and TthHB8 restriction endonucleases

A two-codon insertion mutagenesis method has been generalized. Over two dozen insertion mutants throughout the gene encoding TaqI restriction endonuclease were constructed and activity was characterized. All mutants with activity either cleaved or nicked the canonical T decreases CGA recognition sequence. Some insertion mutants created duplication of gene regions, termed Gemini proteins, which still retained activity. The correlation between mutants with poor activity and the regions of shared amino acid identity between the isoschizomeric TaqI and TthHB8I suggests these regions are involved in DNA recognition and/or catalysis.

Cloning and expression of the HpaI restriction-modification genes

The genes from Haemophilus parainfluenzae encoding the HpaI restriction-modification system were cloned and expressed in Escherichia coli. From the DNA sequence, we predicted the HpaI endonuclease (R.HpaI) to have 254 amino acid residues (Mr 29,630) and the HpaI methyltransferase (M.HpaI) to have 314 amino acid residues (37,390). The R.HpaI and M.HpaI genes overlapped by 16 base pairs on the chromosomal DNA. The genes had the same orientation. The clone, named E. coli HB101-HPA2, overproduced R.HpaI. R.HpaI activity from the clone was 100-fold that from H. parainfluenzae. The amino acid sequence of M.HpaI was compared with those of other type II methyltransferases.

Cloning and molecular characterization of the HgiCI restriction/modification system from Herpetosiphon giganteus Hpg9 reveals high similarity to BanI

The genes coding for the GGYRCC specific restriction/modification system HgiCI from Herpetosiphon giganteus Hpg9 have been cloned in Escherichia coli in three steps. As an initial step, the methyltransferase gene could be obtained after heterologous in vitro selection of a plasmid gene bank by cleavage with the isoschizomeric restriction endonuclease BanI. The adjacent endonuclease gene was cloned following Southern blot analysis of flanking genomic regions. The two genes code for polypeptides of 420 amino acids (M.HgiCI) and 345 amino acids (R.HgiCI). Establishing a functional endonuclease gene could only be achieved using a tightly regulated expression system or by methylation of the genomic DNA prior to transformation of the endonuclease gene. The methyltransferase M.HgiCI shows significant similarities to the family of 5-methylcytidine methyltransferases. Striking similarities could be found with both the isoschizomeric endonuclease and methyltransferase of the BanI restriction/modification system from Bacillus aneurinolyticus.

Cloning, characterization and evolution of the BsuFI restriction endonuclease gene of Bacillus subtilis and purification of the enzyme

The restriction endonuclease (R.BsuFI) of Bacillus subtilis recognizes the target DNA sequence 5' CCGG. The R.BsuFI gene was found in close proximity to the cognate M.BsuFI gene, which had previously been characterized (1). Cloning of the R.BsuFI gene in E.coli was only possible with the M.BsuFI Mtase gene present on a compatible plasmid. The cloned R.BsuFI gene was expressed in E. coli and restriction activity was observed in vivo and in vitro. The R.BsuFI gene consists of 1185 bp, coding for a protein of 395 amino acids with a calculated molecular weight of 45.6 kD. The R.BsuFI enzyme was purified to homogeneity following overexpression. It presumably works as a dimer and cleaves the 5' CCGG target sequence between the two cytosines to produce sticky ends with 5' CG overhangs, like the isoschizomers R.MspI and R.HpaII. The relatedness between R.BsuFI and R.MspI is reflected by significant similarities of the amino acid sequences of both enzymes. This is the first case where such similarities have been observed between isoschizomeric restriction endonucleases which belong to 5mC specific R/M systems. This observation suggests that R.BsuFI and R.MspI genes derive from a common ancestor. In spite of such functional and evolutionary relatedness, the R/M systems differ in the arrangement of their R and M genes. In the BsuFI system transcription of the two genes is convergent, whereas divergent transcription occurs in the MspI system.

The sequence specificity domain of cytosine-C5 methylases

Prokaryotic DNA[cytosine-C5]methyltransferases (m5C-methylases) share a common architectural arrangement of ten conserved sequence motifs. A series of eleven hybrids have been constructed between the HpaII (recognition sequence: Cm5CGG) and HhaI (recognition sequence: Gm5CGC) DNA-methylases. The hybrids were over-expressed in E.coli and their in vivo methylation phenotypes investigated. Six were inactive by our assay while five of them retained partial methylation activity and full specificity. In all five cases the specificity matched that of the parent methylase which contributed the so-called variable region, located between conserved motifs VIII and IX. This was the only sequence held in common between the active hybrids and for the first time provides unequivocal evidence that the specificity determinants of the mono-specific m5C-methylases are located within the variable region. Correlation of the hybrid methylase structure with the efficiency of methylation suggests that conserved motif IX may interact with the variable region whereas motif X most probably interacts with the N-terminal half of the molecule.

Cloning, sequence and characterization of m5C-methyltransferase-encoding gene, hgiDIIM (GTCGAC), from Herpetosiphon giganteus strain Hpa2

We have cloned the gene (hgiDIIM) encoding the methyltransferase (MTase) of the SalI isoschizomeric restriction-modification (R-M) system, HgiDII (GTCGAC), into Escherichia coli. The hgiDIIM gene has been isolated from the same plasmid library of Herpetosiphon giganteus strain Hpa2, as was the previously cloned R-M system, HgiDI [AcyI/GRCGYC; Düsterhöft et al., Nucleic Acids Res. 19 (1991) 1049-1056]. Sequencing and functional localization of hgiDIIM revealed an open reading frame (ORF) of 354 codons (39786 Da) with significant homologies to the group of m5C-, rather than the m4C-/m6A-, MTases. Subsequent cloning and analysis of adjacent chromosomal segments led to the identification of two additional ORFs upstream (ORF15, 139 codons) and downstream (ORF68, 611 codons) from hgiDIIM with the same transcriptional orientation as the hgiDIIM gene. However, the expected restriction enzyme function was not found in either of these ORFs.

Complementation by detached parts of GGCC-specific DNA methyltransferases

Individually inactive N- and C-terminal fragments of the m5C-methyltransferase M.BspRI can complement each other resulting in specific, in vivo methylation of the DNA. This was shown by cloning the coding regions for N- and C-terminal parts of the enzyme in compatible plasmids and co-transforming them into E.coli cells. The enzyme could be detached at several different sites, producing either non-overlapping or partially overlapping fragments capable of complementation. Reconstitution of the active methyltransferase from inactive fragments was demonstrated in vitro, as well. Another GGCC-specific methyltransferase, M.BsuRI, showed a similar complementation phenomenon. Moreover, interspecies complementation was observed between appropriate fragments of the two closely related enzymes M.BspRI and M.BsuRI. Fragments of structurally and functionally more different methyltransferases were unable to complement each other.