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Conservation of motifs within the unusually variable polypeptide sequences of type I restriction and modification enzymes
Author(s) -
Murray Noreen E.,
Daniel Anne S.,
Cowan Gill M.,
Sharp Paul M.
Publication year - 1993
Publication title -
molecular microbiology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 1.857
H-Index - 247
eISSN - 1365-2958
pISSN - 0950-382X
DOI - 10.1111/j.1365-2958.1993.tb01675.x
Subject(s) - biology , restriction enzyme , type (biology) , enzyme , genetics , computational biology , biochemistry , gene , ecology
Summary 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 Eco A and Eco E 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.