Structural insights into the regulation of SigB activity by RsbV and RsbW

Bacillus subtilis SigB is an alternative sigma factor that initiates the transcription of stress-responsive genes. The anti-sigma factor RsbW tightly binds SigB to suppress its activity under normal growth conditions and releases it when nonphosphorylated RsbV binds to RsbW in response to stress signals. To understand the regulation of SigB activity by RsbV and RsbW based on structural features, crystal structures and a small-angle X-ray scattering (SAXS) envelope structure of the RsbV–RsbW complex were determined. The crystal structures showed that RsbV and RsbW form a heterotetramer in a similar manner to a SpoIIAA–SpoIIAB tetramer. Multi-angle light scattering and SAXS revealed that the RsbV–RsbW complex is an octamer in solution. Superimposition of the crystal structure on the SAXS envelope structure showed that the unique dimeric interface of RsbW mediates the formation of an RsbV–RsbW octamer and does not prevent RsbV and SigB from binding to RsbW. These results provide structural insights into the molecular assembly of the RsbV–RsbW complex and the regulation of SigB activity.