Contribution of RecFOR machinery of homologous recombination to cell survival after loss of a restriction–modification gene complex

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Abstract
Loss of a type II restriction–modification (RM) gene complex, such as EcoRI, from a bacterial cell leads to death of its descendent cells through attack by residual restriction enzymes on undermethylated target sites of newly synthesized chromosomes. Through such post-segregational host killing, these gene complexes impose their maintenance on their host cells. This finding led to the rediscovery of type II RM systems as selfish mobile elements. The host prokaryote cells were found to cope with such attacks through a variety of means. The RecBCD pathway of homologous recombination inEscherichia colirepairs the lethal lesions on the chromosome, whilst it destroys restricted non-self DNA.recBCDhomologues, however, appear very limited in distribution among bacterial genomes, whereas homologues of the RecFOR proteins, responsible for another pathway, are widespread in eubacteria, just like the RM systems. In the present work, therefore, we examined the possible contribution of the RecFOR pathway to cell survival after loss of an RM gene complex. ArecFmutation reduced survival in an otherwiserec-positive background and, more severely, in arecBC sbcBCbackground. We also found that its effect is prominent in the presence of specific non-null mutant forms of the RecBCD enzyme: the resistance to killing seen withrecC1002,recC1004,recC2145andrecB2154is severely reduced to the level of a nullrecBCallele when combined with arecF,recOorrecRmutant allele. Such resistance was also dependent on RecJ and RecQ functions. UV resistance of these non-nullrecBCDmutants is also reduced byrecF,recJorrecQmutation. These results demonstrate that the RecFOR pathway of recombination can contribute greatly to resistance to RM-mediated host killing, depending on the genetic background.