DNA RESTRICTION-MODIFICATION SYSTEMS MEDIATE PLASMID MAINTENANCE

Two plasmid-carried restriction-modification (R-M) systems, EcoRI (fro m pMB1 of Escherichia coli) and Bsp6I (from pXH13 of Bacillus sp. stra in RFL6), enhance plasmid segregational stability in E. cell and Bacil lus subtilis, respectively. Inactivation of the endonuclease or the pr esence of the methylase in trans abolish the stabilizing activity of t he R-M systems. We propose that R-M systems mediate plasmid segregatio nal stability by postsegregational killing of plasmid-free cells. Plas mid-encoded methyltransferase modifies host DNA and thus prevents its digestion by the restriction endonuclease. Plasmid loss entails degrad ation and/or dilution of the methylase during cell growth and appearan ce of unmethylated sites in the chromosome. Double-strand breaks, intr oduced at these sites by the endonuclease, eventually cause the death of the plasmid-free cells. Contribution to plasmid stability is a prev iously unrecognized biological role of the R-M systems.