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Journal of Bacteriology, April 2000, p. 2218-2229, Vol. 182, No. 8
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.

Cellular Responses to Postsegregational Killing by Restriction-Modification Genesdagger

Naofumi Handa, Asao Ichige, Kohji Kusano,Dagger and Ichizo Kobayashi*

Department of Molecular Biology, Institute of Medical Science, University of Tokyo, Shirokanedai, Tokyo 108-8639, Japan

Received 16 August 1999/Accepted 13 January 2000

Plasmids that carry one of several type II restriction modification gene complexes are known to show increased stability. The underlying mechanism was proposed to be the lethal attack by restriction enzyme at chromosomal recognition sites in cells that had lost the restriction modification gene complex. In order to examine bacterial responses to this postsegregational cell killing, we analyzed the cellular processes following loss of the EcoRI restriction modification gene complex carried by a temperature-sensitive plasmid in an Escherichia coli strain that is wild type with respect to DNA repair. A shift to the nonpermissive temperature blocked plasmid replication, reduced the increase in viable cell counts and resulted in loss of cell viability. Many cells formed long filaments, some of which were multinucleated and others anucleated. In a mutant defective in RecBCD exonuclease/recombinase, these cell death symptoms were more severe and cleaved chromosomes accumulated. Growth inhibition was also more severe in recA, ruvAB, ruvC, recG, and recN mutants. The cells induced the SOS response in a RecBC-dependent manner. These observations strongly suggest that bacterial cells die as a result of chromosome cleavage after loss of a restriction modification gene complex and that the bacterial RecBCD/RecA machinery helps the cells to survive, at least to some extent, by repairing the cleaved chromosomes. These and previous results have led us to hypothesize that the RecBCD/Chi/RecA system serves to destroy restricted "nonself" DNA and repair restricted "self" DNA.

* Corresponding author. Mailing address: Department of Molecular Biology, Institute of Medical Science, University of Tokyo, Shirokanedai, Tokyo 108-8639, Japan. Phone: (81) 3-5449-5326. Fax: (81) 3-5449-5422 and -5645. E-mail: ikobaya{at}ims.u-tokyo.ac.jp.

dagger Dedicated to Tokio Kogoma.

Dagger Present address: Laboratory of Genetics, University of Wisconsin, Madison, WI 53706.

Journal of Bacteriology, April 2000, p. 2218-2229, Vol. 182, No. 8
0021-9193/00/$04.00+0
Copyright © 2000, American Society for Microbiology. All rights reserved.


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