Agmenellum quadruplicatum M.AquI, a novel modification methylase.

The complete type II modification methylase of Agmenellum quadruplicatum was cloned in Escherichia coli as an R.Sau3A fragment of approximately 4.5 kilobases. The coding sequence was contained in a stretch of 1,156 base pairs which was organized into two parallel, partly overlapping open reading frames of 248 and 139 codons. In vivo complementation experiments showed that the synthesis of both predicted peptides was required for full methylase activity. The amino acid sequences were considerably similar to regions of other deoxycytidylate methylases.     

The cloning, purification and characterization of the Eco RV modification methylase.

The gene for the Eco RV methylase has been cloned into a plasmid under control of the strong lambda PL promoter and overexpressed in E. coli. This plasmid, pVIC1, gives reliable overexpression of the methylase at levels of about 20% of total protein. Maximum yields of soluble protein are achieved after about 6 hours of induction. If the cells are harvested later than this much of the enzyme is found in the pellet fraction following centrifugation. A two column purification scheme using phosphocellulose and Blue-Sepharose chromatography has been developed. This yielded pure methylase in amounts of 5mg per gram E. coli cell paste. The enzyme is monomeric and methylates the first deoxyadenosine residue in its recognition sequence GATATC.     

Cloning and structure of the BepI modification methylase.

The gene coding for a CGCG specific DNA methylase has been cloned in E. coli from Brevibacterium epidermidis. The enzyme, named BepI methylase, is probably the cognate methylase of the FnuDII isoschizomer BepI endonuclease isolated from this strain. The expression of BepI methylase in E. coli is dependent on the orientation of the cloned fragment suggesting that the gene is transcribed from a promoter on the plasmid vector. No BepI endonuclease could be detected in the clones producing BepI methylase. The nucleotide sequence of the BepI methylase gene has been determined, it predicts a protein of 403 amino acids (MR: 45,447). Analysis of the amino acid sequence deduced from the nucleotide sequence revealed similarities between the BepI methylase and other cytosine methylases. M. BepI methylates the external cytosine in its recognition sequence.     

Substrate recognition and selectivity in the type IC DNA modification methylase M.EcoR124I.

The type I DNA modification methylase M.EcoR124I binds sequence specifically to DNA and protects a 25bp fragment containing its cognate recognition sequence from digestion by exonuclease III. Using modified synthetic oligonucleotide duplexes we have investigated the catalytic properties of the methylase, and have established that a specific adenine on each strand of DNA is the site of methylation. We show that the rate of methylation of each adenine is increased at least 100 fold by prior methylation at the other site. However, this is accompanied by a significant decrease in the affinity of the methylase for these substrates according to competitive gel retardation assays. In contrast, methylation of an adenine in the recognition site which is not a target for the enzyme results in only a small decrease in both DNA binding affinity and rate of methylation by the enzyme.     

Sequence specificity of the P1 modification methylase (M.Eco P1) and the DNA methylase (M.Eco dam) controlled by the Escherichia coli dam gene.

Labeled oligonucleotides have been fractionated from pancreatic DNase digests of DNA that had been methylated in vitro with the P1 modification enzyme (M·Eco P1) or with the DNA-adenine methylase (M·Eco dam) controlled by the Escherichia coli dam gene. The sequences of methylated oligonucleotides were established for M·Eco dam modification of calf thymus DNA. The results show that M·Eco dam inethylates adenine residues contained in the twofold symmetrical sequence, 5′ … G-A-T-C … 3′. The sequence for the site methylated by M·Eco P1 has also been deduced; we propose that M·Eco P1 modification produces the following methylated pentameric sequence: 5′ … A-G-A¹-C-Py … 3′ (where $\text{A}{}^1\text{= N}{}^6$ methyladenine and Py is C or T).     

Isolation and characterization of two proteins possessing Hpa II methylase activity

Two proteins exhibiting Hpa II methylase activity have been purified to homogeneity from Haemophilus parainfluenzae and their physical and catalytic properties have been studied. Separation of the two Hpa II methylase activities was achieved by DEAE-Sephadex A-50 chromatography. In subsequent steps, each methylase was purified separately by chromatography on Sephacryl S-200, phosphocellulose, and hydroxylapatite. The proteins have molecular weights of 38,500 +/- 1,000 (Hpa II) and 41,500 +/- 1,000 (Hpa II') as judged by polyacrylamide gel electrophoresis in the presence of sodium dodecyl sulfate. Sedimentation equilibrium analyses of the native proteins yield molecular weights of 38,800 +/- 3,000 and 42,200 +/- 3,000 for Hpa II and Hpa II', respectively, indicating that both enzymes are composed of a single subunit. Furthermore, both methylases exhibit identical specificity in the methylation of the nucleotide sequence dC-C-G-G in simian virus 40 (SV40) DNA and in a short synthetic oligonucleotide duplex. Although pH, temperature, and salt optima are the same for both enzymes, homogeneous Hpa II' methylase is more stable than Hpa II methylase. Preliminary peptide mapping indicates that the two enzymes are structurally related, suggesting the possibility that Hpa II' methylase may represent a precursor form of Hpa II methylase.     

Cloning and characterization of the HpaII methylase gene.

The HpaII restriction-modification system from Haemophilus parainfluenzae recognizes the DNA sequence CCGG. The gene for the HpaII methylase has been cloned into E. coli and its nucleotide sequence has been determined. The DNA of the clones is fully protected against cleavage by the HpaII restriction enzyme in vitro, indicating that the methylase gene is active in E. coli. The clones were isolated in an McrA-strain of E. coli; attempts to isolate them in an McrA+ strain were unsuccessful. The clones do not express detectable HpaII restriction endonuclease activity, suggesting that either the endonuclease gene is not expressed well in E. coli, or that it is not present in its entirety in any of the clones that we have isolated. The derived amino acid sequence of the HpaII methylase shows overall similarity to other cytosine methylases. It bears a particularly close resemblance to the sequences of the HhaI, BsuFI and MspI methylases. When compared with three other methylases that recognize CCGG, the variable region of the HpaII methylase, which is believed to be responsible for sequence specific recognition, shows some similarity to the corresponding regions of the BsuFI and MspI methylases, but is rather dissimilar to that of the SPR methylase.     

Sequence-specific BamHI methylase. Purification and characterization

BamHI methylase has been purified to apparent homogeneity. The isolated form of the enzyme is a single polypeptide with a molecular weight of 56,000 as determined by sodium dodecyl sulfate-polyacrylamide electrophoresis. Unlike BamHI endonuclease, which is isolated as a dimer and higher aggregates, the methylase has no apparent higher form. The methylase requires S-adenosyl-L-methionine as the methyl-group donor and is inhibited by Mg2+. The enzyme is also inhibited by 2,3-butanedione and reagents specific for sulfhydryl groups, such as N-ethylmaleimide, which suggests a role for arginine and cysteine residues, respectively. DNA efficiently protects the enzyme against the butanedione modification while S-adenosylmethionine has no effect. In contrast, S-adenosylmethionine protects against cysteine modification while DNA produces only small amounts of protection. Studies on the mechanism of methylation indicate that both strands of the recognition sequence are modified in a single binding event. The sequence specificity of the methylase is relaxed upon the addition of glycerol in the reaction mixture. In the presence of 30% glycerol the enzyme methylates sequences that are also recognized by BamHI endonuclease when acting under conditions of relaxed specificity.     

Isolation and characterization of canine secretory immunoglobulin M.

Canine secretory immunoglobulin M, isolated from both colostrum and bronchial secretions, contained the unique glycoprotein bound secretory component. The presence of this extra subunit accounted for the differences in size, quaternary structure, and antigenicity observed upon comparison of secretory immunoglobulin M with its serum counterpart. Approximately 90% of the isolated secretory immunoglobulin M contained covalently bound secretory component while, in the remainder of the population, secretory component was loosely attached and easily dissociated from the immunoglobulin. Following peptide bond cleavage with cyanogen bromide, the release of bound secretory component and J chain from secretory immunoglobulin M was not detected. Because cyanogen bromide cleavage of secretory immunoglobulin A results in the release of these subunits, differences in the primary structure of secretory immunoglobulin M and secretory immunoglobulin A must exist around the binding sites for secretory component and J chain.     

Isolation and characterization of the precursor of type M collagen.

A 225000-Mr peptide has been purified from rat chondrosarcoma which is immunologically and biochemically related to type M collagen. Rotary shadowing shows this molecule to be twice the length of the type M molecule and has a prominent kink close to one end. We believe this molecule represents parent type M, the form of the molecule in vivo.     

Purification and biochemical characterisation of the EcoR124 type I modification methylase.

Large scale purification of the type I modification methylase EcoR124 has been achieved from an over-expressing strain by a two step procedure using ion-exchange and heparin chromatography. Pure methylase is obtained at a yield of 30 mg per gm of cell paste. Measurements of the molecular weight and subunit stoichiometry show that the enzyme is a trimeric complex of 162 kDa consisting of two subunits of HsdM (58 kDa) and one subunit of HsdS (46 kDa). The purified enzyme can methylate a DNA fragment bearing its cognate recognition sequence. Binding of the methylase to synthetic DNA fragments containing either the EcoR124 recognition sequence GAAN6RTCG, or the recognition sequence GAAN7RTCG of the related enzyme EcoR124/3, was followed by fluorescence competition assays and by gel retardation analysis. The results show that the methylase binds to its correct sequence with an affinity of the order 10(8) M-1 forming a 1:1 complex with the DNA. The affinity for the incorrect sequence, differing by an additional base pair in the non-specific spacer, is almost two orders of magnitude lower.     

Mode of action of the Spiroplasma CpG methylase M.SssI.

The cytosine DNA methylase from the wall-less prokaryote, Spiroplasma strain MQ1 (M.SssI) methylates completely and exclusively CpG-containing sequences, thus showing sequence specificity which is similar to that of mammalian DNA methylases. M.SssI is shown here to methylate duplex DNA processively as judged by kinetic analysis of methylated intermediates. The cytosine DNA methylases, M.HpaII and M.HhaI, from other prokaryotic organisms, appear to methylate in a non-processive manner or with a very low degree of processivity. The Spiroplasma enzyme interacts with duplex DNA irrespective to the presence of CpG sequences in the substrate DNA. The enzyme proceeds along a CpG-containing DNA substrate molecule methylating one strand of DNA at a time.     

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.     

Purification and characterization of a tRNA methylase from Salmonella typhimurium.

A tRNA methylase, in which supK strains of Salmonella typhimurium are deficient, was purified from strain LT2 and characterized. Column chromatography of protein extracts from wild-type cells on phosphocellulose, diethylaminoethyl-Sephadex A-50, and hydroxlapatite resulted in an enzyme that was estimated to be about 50% pure. tRNA from S. typhimurium which had been incubated at pH 9.0 served as a substrate for this methylase. The enzyme has a molecular weight of about 50,000 as estimated by gel chromatography and by electrophoresis on sodium dodecyl sulfate-polyacrylamide gels. The optimal assay conditions, as well as the kinetics and stability of the enzyme, were studied. As with other tRNA-methylating enzymes, S-adenosylhomocysteine is a potent inhibitor.     

Isolation and characterization of the cell-associated region of group A streptococcal M6 protein.

DNA sequence analysis of the complete M6 protein gene revealed 19 hydrophobic amino acids at the C terminus which could act as a membrane anchor and an adjacent proline- and glycine-rich region likely to be located in the cell wall. To define this region within the cell wall and its role in attaching the molecule to the cell, we isolated the cell-associated fragment of the M protein. Assuming that the cell-associated region of the M protein would be embedded within the wall and thus protected from trypsin digestion, cells were digested with this enzyme, and the wall-associated M protein fragment was released by phage lysin digestion of the peptidoglycan. With antibody probes prepared to synthetic peptides of C-terminal sequences, a cell wall-associated M protein fragment (molecular weight, 16,000) was identified and purified. Amino acid sequence analysis placed the N terminus of the 16,000-molecular-weight fragment at residue 298 within the M sequence. Amino acid composition of this peptide was consistent with a C-terminal sequence lacking the membrane anchor. Antibody studies of nitrous acid-extracted whole bacteria suggested that, in addition to the peptidoglycan-associated region, a 65-residue helical segment of the C-terminal domain of the M protein is embedded within the carbohydrate moiety of the cell wall. Since no detectable amino sugars were associated with the wall-associated fragment, the C-terminal region of the M6 molecule is likely to be intercalated within the cross-linked peptidoglycan and not covalently linked to it. Because the C-terminal region of the M molecule is highly homologous to the C-terminal end of protein A from staphylococci and protein G from streptococci, it is likely that the mechanism of attachment of these proteins to the cell wall is conserved.     

Isolation and chemical characterization of lipopolysaccharides from four Mycoplana species (M. bullata,M. segnis,M. ramosa and M. dimorpha)

Lipopolysaccharides (LPS), isolated from four Mycoplana species, i.e. the type strains of M. bullata, M. segnis, M. ramosa and M. dimorpha, were characterized onto their chemical composition and their respective lipid A-types. Those of M. bullata and M. segnis showed on DOC-PAGE an R-type character and had lipid A's of the Lipid A_DAG-type which exclusively contained 2,3-diamino-2,3-dideoxy-d-glucose as lipid A sugar. LPS's of M. ramosa and M. dimorpha showed, although only weakly expressed, ladder-like patterns on DOC-PAGE indicating some S-type LPS's and lipid A of the d-glucosamine type (Lipid A_GlcN). M. bullata LPS contained mannose and glucose in major amounts and additionally l-glycero-d-mannoheptose, whereas M. segnis LPS was composed of rhamnose, mannose and glucose together with both, d-glycero-d-manno- and l-glycero-d-manno-heptoses in a molar ratio of 1:2. All LPS's contained 2-keto-3-deoxy-octonic acid (Kdo), phosphate and an unidentified acidic component X. In addition to X, M. segnis LPS contained glucuronic and galacturonic acids, whereas M. ramosa LPS contained only galacturonic acid. Acetic acid hydrolysis of the LPS resulted in splitting off lipid A moieties, very rich in 3-hydroxy fatty acids, in particular in 3-OH-12:0 (in Lipid A_DAG), or in 3-OH-14:0 (in Lipid A_GlcN). Analysis of the 3-acyloxyacyl residues revealed major amounts of amide-linked 3-OH(3-OH-13:0)12:0 in lipid A of M. bullata and 3-OH(12:0)12:0 in lipid A of M. segnis. The rare 4-oxo-myristic acid (4-oxo-14:0) was observed only in M. bullata LPS, where it is ester-linked. Amide linked diesters could not be traced in M. ramosa and M. dimorpha. All four lipid A's lacked erster-bound acyloxyacyl residues.Non-standard abbreviations DAG 2,3-diamino-2,3-dideoxy-d-glucose - Kdo 2-keto-3-deoxy-octonate - LPS lipopolysaccharide - PITC phenyl isothiocyanate - NANA N-acetyl neuraminic acid