Role of adenosine nucleotides in the regulation of a stress-response transcription factor in Bacillus subtilis

Abstract

The RNA polymerase sigma factor sigma B is a stress-response regulatory protein in Bacillus subtilis. The activity of sigma B is controlled in part by RsbW, a protein that inhibits sigma B, and RsbV, a protein that counteracts this inhibition. We now demonstrate that purified RsbW is capable of forming alternative complexes with either sigma B or RsbV. Sigma B in the RsbW. sigma B complex was transcriptionally inactive. RsbV reversed this inhibition by sequestering RsbW in a RsbW-RsbV complex, thereby allowing sigma B to remain free and active. In contrast to interactions among the components of the homologous regulatory system for the sporulation transcription factor sigma F, the binding of RsbW to RsbV and sigma B did not require adenosine nucleotides. Experiments involving the exchange of proteins between the two regulatory systems demonstrated that RsbW and its homolog in the sigma F system, SpoIIAB, exhibit strong preference in binding to RsbV and sigma B, and SpoIIAA and sigma F, respectively, and that the difference in nucleotide-dependence of binding between these two systems is attributable to a difference between RsbW and SpoIIAB. In confirmation and extension of previous results, we show that RsbW is also a protein kinase that uses ATP to phosphorylate RsbV, thereby blocking the capacity of RsbV to bind to RsbW and activate transcription. A close correlation was observed between the concentration of ATP required for efficient RsbW-mediated phosphorylation of RsbV, inhibition of RsbW.RsbV comlex formation, and inhibition of sigma B-directed transcription. These results are consistent with the hypothesis that activation of sigma B under certain stress condition is due to a decrease in cellular ATP levels.