Crystal Structure of the Bacillus stearothermophilus Anti-σ Factor SpoIIAB with the Sporulation σ Factor σF

Cell type-specific transcription during Bacillus sporulation is established by σF. SpoIIAB is an anti-σ that binds and negatively regulates σF, as well as a serine kinase that phosphorylates and inactivates the anti-anti-σ SpoIIAA. The crystal structure of σF bound to the SpoIIAB dimer in the low-affinity, ADP form has been determined at 2.9 Å resolution. SpoIIAB adopts the GHKL superfamily fold of ATPases and histidine kinases. A domain of σF contacts both SpoIIAB monomers, while 80% of the σ factor is disordered. The interaction occludes an RNA polymerase binding surface of σF, explaining the SpoIIAB anti-σ activity. The structure also explains the specificity of SpoIIAB for its target σ factors and, in combination with genetic and biochemical data, provides insight into the mechanism of SpoIIAA anti-anti-σ activity.