An unprecedented DNA recognition-mimicry switch governs induction in arbitrium phages

Temperate phages integrate multiple information sources to regulate lysis-lysogeny transitions. SPBeta-like phages use arbitrium signalling and DNA damage to control repressor activity during lytic induction, but how the repressor functions and is inactivated by the SOS response remains unclear. Here, we show that SroF, the SPBeta-like phage repressor, binds DNA via a novel mechanism involving its integrase-like fold, enabling stable prophage repression. Upon DNA damage, the host SOS response triggers derepression of a newly identified antirepressor, Sar. Sar binds SroF by mimicking the DNA structure recognised by the repressor, inactivating its function and inducing phage. This mechanism is conserved across SP{beta}-like phages, which encode multiple, specific SroF-Sar pairs. Surprisingly, repressor inactivation alone is insufficient for induction when arbitrium levels are high. Our results uncover the mechanism underlying the double layer of control that ensures phage induction occurs only under SOS conditions and in the absence of neighbouring prophages.