title: Conservation of motifs within the unusually variable polypeptide sequences of type I restriction and modification enzymes.
abstract: Type I restriction enzymes comprise three subunits encoded by genes designated hsdR, hsdM, and hsdS; S confers sequence specificity. Three families of enzymes are known and within families, but not between, hsdM and hsdR are conserved. Consequently, interfamily comparisons of M and R sequences focus on regions of putative functional significance, while both inter- and intrafamily comparisons address the origin, nature and role of diversity of type I restriction systems. We have determined the sequence of the hsdR gene for EcoA, thus making available sequences of all three hsd genes of one representative from each family. The predicted R polypeptide sequences share conserved regions with one superfamily of putative helicases, so-called 'DEAD box' proteins; these conserved sequences may be associated with the ATP-dependent translocation of DNA that precedes restriction. We also present hsdM and hsdR sequences for EcoE, a member of the same family as EcoA. The sequences of the M and R genes of EcoA and EcoE are at least as divergent as typical genes from Escherichia coli and Salmonella, perhaps as the result of selection favouring diversity of restriction specificities combined with lateral transfer among different species.
authors.affiliation: Institute of Cell and Molecular Biology, University of Edinburgh, UK.
authors.forename: N E
authors.initials: NE
authors.lastname: Murray
authors.forename: A S
authors.initials: AS
authors.lastname: Daniel
authors.forename: G M
authors.initials: GM
authors.lastname: Cowan
authors.forename: P M
authors.initials: PM
authors.lastname: Sharp
mesh_terms: D000595:Amino Acid Sequence; D001665:Binding Sites; D004265:DNA Helicases / Q000737:chemistry / Q000235:genetics; D015253:Deoxyribonucleases, Type I Site-Specific / Q000737:chemistry* / Q000235:genetics; D004926:Escherichia coli / Q000201:enzymology / Q000235:genetics; D005798:Genes, Bacterial; D008969:Molecular Sequence Data; D010802:Phylogeny; D012486:Salmonella typhimurium / Q000201:enzymology / Q000235:genetics; D016415:Sequence Alignment; D017386:Sequence Homology, Amino Acid
chemical_list: C040446:endodeoxyribonuclease EcoAI; C047525:endodeoxyribonuclease EcoE; C029602:endodeoxyribonuclease EcoK; C048242:endodeoxyribonuclease EcoR124I; D015253:Deoxyribonucleases, Type I Site-Specific; D004265:DNA Helicases