title: DNA restriction-modification systems mediate plasmid maintenance.
abstract: Two plasmid-carried restriction-modification (R-M) systems, EcoRI (from pMB1 of Escherichia coli) and Bsp6I (from pXH13 of Bacillus sp. strain RFL6), enhance plasmid segregational stability in E. coli and Bacillus subtilis, respectively. Inactivation of the endonuclease or the presence of the methylase in trans abolish the stabilizing activity of the R-M systems. We propose that R-M systems mediate plasmid segregational stability by postsegregational killing of plasmid-free cells. Plasmid-encoded methyltransferase modifies host DNA and thus prevents its digestion by the restriction endonuclease. Plasmid loss entails degradation and/or dilution of the methylase during cell growth and appearance of unmethylated sites in the chromosome. Double-strand breaks, introduced at these sites by the endonuclease, eventually cause the death of the plasmid-free cells. Contribution to plasmid stability is a previously unrecognized biological role of the R-M systems.
authors.affiliation: Institut National de la Recherche Agronomique, Jouy-en-Josas, France.
authors.forename: S
authors.initials: S
authors.lastname: Kulakauskas
authors.forename: A
authors.initials: A
authors.lastname: Lubys
authors.forename: S D
authors.initials: SD
authors.lastname: Ehrlich
mesh_terms: D001412:Bacillus subtilis / Q000201:enzymology; D001419:Bacteria / Q000201:enzymology* / Q000235:genetics; D015254:DNA Modification Methylases / Q000378:metabolism; D004262:DNA Restriction Enzymes / Q000378:metabolism; D015280:DNA Restriction-Modification Enzymes / Q000502:physiology*; D004926:Escherichia coli / Q000201:enzymology; D010957:Plasmids / Q000378:metabolism*
chemical_list: D015280:DNA Restriction-Modification Enzymes; D015254:DNA Modification Methylases; D004262:DNA Restriction Enzymes