SpoIIAA is the anti-sigma-F factor antagonist (anti-anti-sigma factor) in the partner-switching module that gates activation of the forespore-specific sigma factor sigma F during Bacillus subtilis sporulation. In its unphosphorylated form SpoIIAA binds SpoIIAB and displaces sigma F from the SpoIIAB-sigma F complex, freeing sigma F to associate with core RNA polymerase; SpoIIAB then phosphorylates SpoIIAA on a serine residue, inactivating it and allowing SpoIIAB to re-bind sigma F. Dephosphorylation of phospho-SpoIIAA by the forespore-localized phosphatase SpoIIE regenerates active SpoIIAA and drives the asymmetric release of sigma F in the forespore compartment.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0043856 anti-sigma factor antagonist activity | IBA GO_REF:0000033 | ACCEPT | Summary: SpoIIAA is the anti-sigma factor antagonist for sigma F. In its unphosphorylated form SpoIIAA counteracts the anti-sigma factor SpoIIAB, releasing sigma F from inhibition; SpoIIAB-mediated phosphorylation on Ser-58 inactivates SpoIIAA. Reason: This is the core molecular function of SpoIIAA. UniProt describes SpoIIAA as an anti-anti-sigma factor that counteracts SpoIIAB to release sigma F. Note that the UniProt free-text hedge attributing the activity to "the phosphorylated form" predates the established partner-switch model, in which the unphosphorylated species is the active anti-anti-sigma factor and phosphorylation is the inactivating step; the term assignment itself is unaffected and aligns with the curated IBA annotation. Supporting Evidence: file:BACSU/spoIIAA/spoIIAA-uniprot.txt In the phosphorylated form it could act as an anti-anti-sigma factor that counteracts SpoIIAB and thus releases sigma f from inhibition. |
| GO:0045893 positive regulation of DNA-templated transcription | IEA GO_REF:0000108 | KEEP AS NON CORE | Summary: SpoIIAA indirectly promotes transcription by activating sigma F through antagonism of the anti-sigma factor SpoIIAB. Reason: SpoIIAA does not bind DNA or the RNA polymerase directly, but it positively regulates sigma F activity by counteracting SpoIIAB. The transcriptional effects are indirect, so this term is kept as a non-core downstream process. Supporting Evidence: file:BACSU/spoIIAA/spoIIAA-uniprot.txt In the phosphorylated form it could act as an anti-anti-sigma factor that counteracts SpoIIAB and thus releases sigma f from inhibition. |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000002 | MARK AS OVER ANNOTATED | Summary: SpoIIAA regulates transcription indirectly by controlling sigma F availability through SpoIIAB antagonism. Reason: This term is too general and redundant with the more specific positive regulation of transcription. SpoIIAA acts via sigma-factor control rather than serving as a direct transcriptional regulator, so this generic term overstates its role. Supporting Evidence: file:BACSU/spoIIAA/spoIIAA-uniprot.txt In the phosphorylated form it could act as an anti-anti-sigma factor that counteracts SpoIIAB and thus releases sigma f from inhibition. |
| GO:0030435 sporulation resulting in formation of a cellular spore | IEA GO_REF:0000120 | ACCEPT | Summary: SpoIIAA is a stage II sporulation protein required for the sigma F regulatory cascade that drives early sporulation gene expression. Reason: SpoIIAA is a core component of the sporulation regulatory network. UniProt explicitly annotates it as a stage II sporulation protein and tags it with the sporulation keyword, supporting its role in spore formation. Supporting Evidence: file:BACSU/spoIIAA/spoIIAA-uniprot.txt AltName: Full=Stage II sporulation protein AA; file:BACSU/spoIIAA/spoIIAA-uniprot.txt KW 3D-structure; Direct protein sequencing; Phosphoprotein; Reference proteome; Sporulation. |
| GO:0043856 anti-sigma factor antagonist activity | IEA GO_REF:0000002 | ACCEPT | Summary: SpoIIAA functions as an anti-sigma factor antagonist that releases sigma F from inhibition by SpoIIAB. Reason: The InterPro-based IEA aligns with the experimentally supported function of SpoIIAA as an anti-anti-sigma factor in the sigma F pathway, which is core to its molecular role. Supporting Evidence: file:BACSU/spoIIAA/spoIIAA-uniprot.txt In the phosphorylated form it could act as an anti-anti-sigma factor that counteracts SpoIIAB and thus releases sigma f from inhibition. |
| GO:0045152 antisigma factor binding | IEA GO_REF:0000002 | ACCEPT | Summary: SpoIIAA antagonizes the anti-sigma factor SpoIIAB, implying direct interaction with an anti-sigma factor to free sigma F. Reason: Antisigma factor binding is consistent with SpoIIAA's anti-anti-sigma role and is supported by UniProt functional annotation describing SpoIIAB antagonism. The term is specific and appropriate for this molecular function. Supporting Evidence: file:BACSU/spoIIAA/spoIIAA-uniprot.txt In the phosphorylated form it could act as an anti-anti-sigma factor that counteracts SpoIIAB and thus releases sigma f from inhibition. |
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Download this section (compressed HTML)Q: What is the precise cellular concentration and phosphorylation state of SpoIIAA on either side of the polar septum immediately before and after sigma F activation, and how quantitatively does SpoIIE phosphatase activity account for the observed asymmetric release of sigma F in the forespore?
Q: Does SpoIIAA participate in any regulatory circuits beyond the SpoIIAB/sigma F partner switch (for example, through phosphorylation by non-SpoIIAB kinases such as PrkC, or through interaction with paralogous anti-sigma-factor antagonists like RsbV during vegetative stress responses)?
Experiment: Combine live-cell FRET or bimolecular fluorescence complementation to monitor SpoIIAA-SpoIIAB and SpoIIAA-SpoIIE interactions in individual sporulating cells with quantitative Western/Phos-tag blotting of phospho-SpoIIAA in isolated mother-cell and forespore compartments. Test SpoIIE catalytic (phosphatase-dead) mutants and SpoIIAA phospho-mimetic/null substitutions at the Ser-58 target residue.
Hypothesis: Compartment-specific dephosphorylation of SpoIIAA by SpoIIE is both necessary and sufficient to explain the timing and asymmetry of sigma F activation in the forespore.
Type: Live-cell fluorescence imaging + compartment fractionation + phospho-blotting
Experiment: Screen for in vivo phosphorylation of SpoIIAA in wild-type and kinase-deletion backgrounds (e.g. prkC, prkD, yabT, rsbW) using targeted MS/MS phosphoproteomics on sporulating and vegetatively stressed cultures, and test SpoIIAA/RsbV cross- complementation by expressing each from its counterpart's promoter in the reciprocal deletion background.
Hypothesis: SpoIIAA function is modulated by other B. subtilis kinases or by paralogous anti-anti-sigma factors under specific physiological conditions.
Type: Phosphoproteomics + genetic complementation
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