Superoxide stress decreases expression of srfA through inhibition of transcription of the comQXP quorum-sensing locus in Bacillus subtilis

During the course of screening for competence-deficient mutants in the mutant collection constructed by the Japan Consortium of Bacillus Functional Genomics, a disruption mutant of sodA encoding superoxide dismutase was identified as a mutant with decreased transformation efficiency. In fact, in the sodA mutant we observed a severe decrease in the expression of srfA required for the development of genetic competence. Northern and primer extension analyses revealed inhibition of the transcription of the comQXP quorum-sensing locus in the sodA mutant, thereby preventing srfA expression. Furthermore, an excess amount of superoxide anion induced by the addition of paraquat also resulted in a decrease in comQXP transcription. Thus, it was concluded that high levels of superoxide are able to inhibit specifically the transcription of the comQXP operon. In support of this conclusion, the effect of added paraquat was significantly alleviated in a comX-independent srfA expression system.