Anti-SOS effects induced in Bacillus subtilis by a phi 105 mutant prophage.

The presence of the mutant prophage phi 105cts23 in Bacillus subtilis strains strongly affected several biological parameters including the viability of protoplasts and the establishment of plasmid pC194. A defective inducibility of the prophage after treatments that de-repress the SOS-like response were also observed. Although these alterations suggested a Rec-deficient phenotype, homologous recombination was not impaired in these lysogenic derivatives. In fact, chromosomal DNA transformation in these competent cells was more efficient than in cells carrying the wild type prophage: cell death due to prophage induction upon competence development was lower than expected. Alterations in the response to SOS-inducing agents and to osmotic stress correlated with the presence of this particular mutant prophage or the cloned thermosensitive repressor at the permissive temperature. The induction of an anti-SOS effect is discussed.