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

F is established, ␴ F (encoded by the SpoIIAC gene) is the first ␴ factor to establish forespore-specific gene expression by inducing expression of other sporulation-specific ␴ factors, which in turn direct the expression of downstream genes necessary for spore development SpoIIAB is also a Cell type-specific transcription during Bacillus sporu-lation is established by ␴ F. SpoIIAB is an anti-␴ that serine kinase that phosphorylates a third protein, F , as well as a serine kinase that phosphorylates and inactivates the anti-functions as an anti-anti-␴ by attacking and dissociating the SpoIIAB:␴ F complex (Alper et al., 1994; Duncan et al., anti-␴ SpoIIAA. The crystal structure of ␴ F bound to the SpoIIAB dimer in the low-affinity, ADP form has 1996). A model was proposed wherein SpoIIAA attack of the SpoIIAB:␴ F complex leads to the phosphorylation of been determined at 2.9 A ˚ resolution. SpoIIAB adopts the GHKL superfamily fold of ATPases and histidine the target serine of SpoIIAA. After phosphorylation and dissociation of SpoIIAA, SpoIIAB with ADP left in its kinases. A domain of ␴ F contacts both SpoIIAB mono-mers, while 80% of the ␴ factor is disordered. The binding pocket [SpoIIAB(ADP)] has a lower affinity for ␴ F , freeing ␴ F for transcription (Duncan et al., 1996). In interaction occludes an RNA polymerase binding surface of ␴ F , explaining the SpoIIAB anti-␴ activity. The addition, unphosphorylated SpoIIAA binds to SpoIIAB (ADP), antagonizing the role of SpoIIAB as an anti-␴ by structure also explains the specificity of SpoIIAB for its target ␴ factors and, in combination with genetic blocking the binding of ␴ SpoIIAA oscillates between the phosphory-and biochemical data, provides insight into the mechanism of SpoIIAA anti-anti-␴ activity. lated (P-SpoIIAA) and unphosphorylated forms by the actions of the kinase SpoIIAB and the phosphatase F. The structure explains the anti-␴ activity tion by the DNA-dependent RNA polymerase (RNAP) requires a single polypeptide, the ␴ factor, which binds of SpoIIAB and its specificity for ␴ F. Analysis of the structure, combined with previous genetic and bio-to the core RNAP (subunit composition ␣ 2 ␤␤Ј␻) to form holoenzyme (Burgess et al., 1969; Gross et al., 1998; chemical data, leads to a model explaining the anti-anti-␴ activity of SpoIIAA. Travers and Burgess, 1969). One primary ␴ factor (␴ A) directs the bulk of transcription during exponential growth. Specialized, alternative ␴ factors direct tran-Results and Discussion scription of specific regulons during …