Parallel pathways of repression and antirepression governing the transition to stationary phase in Bacillus subtilis

The AbrB protein of the spore-forming bacterium Bacillus subtilis is a repressor of numerous genes that are switched on during the transition from the exponential to the stationary phase of growth. The gene for AbrB is under the negative control of the master regulator for entry into sporulation, Spo0A-P. It has generally been assumed that derepression of genes under the negative control of AbrB is achieved by Spo0A-P-mediated repression of abrB followed by rapid degradation of the AbrB protein. Here, we report that AbrB levels do decrease during the transition to stationary phase, but that this decrease is not the entire basis by which AbrB-controlled genes are derepressed. Instead, AbrB is inactivated by the product of a uncharacterized gene, abbA (formerly ykzF), whose transcription is switched on by Spo0A-P. The abbA gene encodes an antirepressor that binds to AbrB and prevents it from binding to DNA. Combining our results with previous findings, we conclude that Spo0A-P sets in motion two parallel pathways of repression and antirepression to trigger the expression of diverse categories of genes during the transition to stationary phase.