Evidence in Support of a Docking Model for the Release of the Transcription Factor σF from the Antisigma Factor SpoIIAB in Bacillus subtilis

Abstract

Cell-specific activation of the transcription factor F during the process of sporulation in Bacillus subtilis is governed by an antisigma factor SpoIIAB and an antiantisigma factor SpoIIAA. SpoIIAB, which exists as a dimer, binds to F in a complex of stoichiometry F ⅐SpoIIAB 2 . Escape from the complex is mediated by SpoIIAA, which reacts with the complex to cause the release of free F . Previous evidence indicated that Arg-20 in SpoIIAB is a contact site for both F and SpoIIAA and that contact with F is mediated by Arg-20 on only one of the two subunits in the F ⅐SpoIIAB 2 complex. Here we report the construction of heterodimers of SpoIIAB in which one subunit is wild type and the other subunit is a mutant for Arg-20. We show that the dissociation constant for the binding of F to the heterodimer was similar to that for the wild type, a finding consistent with the idea that F contacts Arg-20 on only one of the two subunits. Although SpoIIAA was highly effective in causing the release of F from the wild type homodimer, the anti-antisigma factor had little effect on the release of F from the heterodimer. This finding is consistent with a model in which SpoIIAA docks on the F ⅐SpoIIAB 2 complex, making contact with the subunit in which Arg-20 is not in contact with F . SpoIIAB is both an anti-F factor and a protein kinase that phosphorylates and thereby inactivates SpoIIAA. We show that SpoIIAA effectively displaces F from a complex of F with a mutant (SpoIIAB R105A ) that is impaired in the kinase function of SpoIIAB. This result shows that SpoIIAAmediated displacement of F from SpoIIAB does not require concomitant phosphorylation of SpoIIAA.

Regulation of bacterial RNA polymerase σ factor activity: a structural perspective

“…Several structures of AB in complex with AA have now been solved [35]. These data provided structural evidence for the previously proposed docking model [34,36] that describes how AA induces σ F release from the σ F /AB complex ( Figure 2a). In this model, one AB protomer of the asymmetric σ F /AB 2 complex (AB1 of Figure 2a; for more details see Figures 7 and 9 of ref.…”

Compartmentalization of Gene Expression duringBacillus subtilisSpore Formation

“…Experiments were performed with SpoIIAB heterodimers consisting of wild-type SpoIIAB and SpoIIABR20E, a mutant deficient in binding SpoIIAA (92). The experiments revealed that SpoIIAA interacted with the molecule of SpoIIAB not bound to F and induced release of F from the other SpoIIAB molecule, most likely by steric displacement (115). Another interesting result from this study was that a mutant of SpoIIAB deficient in its kinase activity (SpoIIABR105A) resulted in excessive levels of F activity.…”

Structural and Functional Characterization of Partner Switching Regulating the Environmental Stress Response in Bacillus subtilis

“…Moreover, our analysis indicates that RsbT utilizes a highly overlapped set of amino acids in the N-terminal domains of both RsbR and RsbU, albeit with different chemical characteristics, to form its alternative complexes. (49,50). Therefore, one copy of SpoIIAB Arg 20 is always available for SpoIIAA to gain a "toehold" on SpoIIAB inducing release of F .…”