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