Cellular responses to postsegregational killing by restriction-modification genes

Plasmids that carry one of several type II restriction modification gene co mplexes are known to show increased stability. The underlying mechanism was proposed to be the lethal attack by restriction enzyme at chromosomal reco gnition sites in cells that had lost the restriction modification gene comp lex. In order to examine bacterial responses to this postsegregational cell killing, we analyzed the cellular processes following loss of the EcoRI re striction modification gene complex carried by a temperature-sensitive plas mid in an Escherichia coli strain that is mild type with respect to DNA rep air. A shift to the nonpermissive temperature blocked plasmid replication, reduced the increase in viable cell counts and resulted in loss of cell via bility. Many cells formed long filaments, some of which were multinucleated and others anucleated. In a mutant defective in RecBCD exonuclease/recombi nase, these cell death symptoms were more severe and cleaved chromosomes ac cumulated. Grow th inhibition was also more severe in recA, ruvB, ruvC, rec G, and recN mutants. The cells induced the SOS response in a RecBC-dependen t manner. These observations strongly suggest that bacterial cells die as a result of chromosome cleavage after loss of a restriction modification gen e complex and that the bacterial RecBCD/RecA machinery helps the cells to s urvive, at least to some extent, by repairing the cleaved chromosomes. Thes e and previous results have led us to hypothesize that the RecBCD/Chi/RecA system serves to destroy restricted "nonself" DNA and repair restricted "se lf" DNA.