Repressor of PBSX. Binds to four sites close to its own gene. Necessary for the maintenance of the lysogenic state. The defective prophage of Bacillus subtilis 168, PBSX, is a chromosomally based element which encodes a non-infectious phage-like particle with bactericidal activity. PBSX is induced by agents which elicit the SOS response (PubMed:2125016).