SigK (also known as spoIVCB/spoIIIC) encodes the RNA polymerase sigma-K factor, the late mother cell-specific sigma factor of the sigma-70 family that directs transcription of late sporulation genes in B. subtilis. SigK has a unique feature: in strain 168, the sigK gene is interrupted by a ~48 kb prophage-like "skin" element (skinBs), and the functional sigK is only reconstituted late in sporulation when the SpoIVCA recombinase excises this element under sigE/SpoIIID control in the mother cell. SigK is synthesized as an inactive precursor (pro-sigK) with a 20-residue propeptide that must be removed by the intramembrane metalloprotease SpoIVFB for activation. SpoIVFB is held inactive by BofA and SpoIVFA until forespore-secreted proteases SpoIVB and CtpB cleave these inhibitors, thereby coupling sigK activation to developmental stage. The mature sigma-K factor contains conserved sigma-70 regions including a helix-turn-helix DNA-binding motif (residues 251-270) that recognizes sigK-dependent promoters. SigK activates the final phase of mother cell gene expression including coat proteins (cotA, cotB, cotD, cotE, cotH), cortex, and germination genes. SigK operates in a hierarchical feed-forward network with GerE, which both represses some sigK-activated genes and activates additional late genes.
| GO Term | Evidence | Action | Reason |
|---|---|---|---|
| GO:0000976 transcription cis-regulatory region binding | IBA GO_REF:0000033 | REMOVE | Summary: This annotation suggests sigK binds to transcription cis-regulatory regions independently. However, according to the GO definition of sigma factor activity (GO:0016987), sigma factors do not bind DNA on their own but when combined with the core polymerase to form the holoenzyme, they confer promoter specificity. This term is technically inaccurate for sigma factors as they require the core polymerase for DNA binding. Reason: Sigma factors do not bind DNA independently. The GO definition of sigma factor activity explicitly states that "sigma does not bind DNA on its own, when combined with the core to form the holoenzyme, the sigma factor binds specifically to promoter elements." This term implies autonomous DNA binding which is misleading for sigma factors. The correct molecular function is GO:0016987 sigma factor activity, which is already annotated for sigK. Supporting Evidence: UniProt:P12254 Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. |
| GO:0003899 DNA-directed RNA polymerase activity | IBA GO_REF:0000033 | REMOVE | Summary: This annotation incorrectly attributes catalytic RNA polymerase activity to sigK. Sigma factors are NOT enzymes and do not catalyze the polymerization reaction. The catalytic activity resides in the core polymerase subunits (beta, beta-prime), not in sigma factors. Reason: This is a clear over-annotation. GO:0003899 is defined as "Catalysis of the reaction: nucleoside triphosphate + RNA(n) = diphosphate + RNA(n+1)." Sigma factors do not possess catalytic activity. The catalytic activity of RNA polymerization is performed by the core enzyme subunits, not by sigma factors. SigK confers promoter specificity but does not catalyze nucleotide addition. UniProt describes sigma factors as "initiation factors that promote the attachment of RNA polymerase to specific initiation sites" - not enzymes. Supporting Evidence: UniProt:P12254 Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. |
| GO:0006355 regulation of DNA-templated transcription | IBA GO_REF:0000033 | ACCEPT | Summary: SigK regulates transcription by directing RNA polymerase to mother cell-specific promoters during late sporulation. The sigK regulon includes coat proteins, cortex genes, and germination factors. SigK operates in a feed-forward network with GerE to sculpt the late mother cell transcriptional program. Reason: Sigma factors inherently regulate transcription by determining which genes are transcribed. SigK specifically activates the late mother cell developmental program. UniProt states that "This sigma factor is responsible for the expression of sporulation specific genes in the mother cell." Deep research confirms sigK activates coat/cortex/germination genes. Supporting Evidence: UniProt:P12254 This sigma factor is responsible for the expression of sporulation specific genes in the mother cell. file:BACSU/sigK/sigK-deep-research-falcon.md sigK operates in a hierarchical, feed-forward mother-cell program following sigE and preceding GerE, activating coat/cortex/germination genes |
| GO:0016987 sigma factor activity | IBA GO_REF:0000033 | ACCEPT | Summary: This is the primary and most accurate molecular function annotation for sigK. SigK is a well-characterized sigma factor of the sigma-70 family. It contains conserved sigma-70 regions (regions 2 and 4) that mediate core RNAP binding and promoter recognition. UniProt explicitly classifies sigK as belonging to the sigma-70 factor family. Reason: This is the canonical molecular function for sigK. UniProt identifies sigK as "RNA polymerase sigma-K factor" belonging to the "sigma-70 factor family." The protein contains all characteristic sigma-70 family domains including the HTH DNA-binding motif (residues 251-270) and polymerase core binding motif (residues 79-92). Supporting Evidence: UniProt:P12254 Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. UniProt:P12254 Belongs to the sigma-70 factor family. |
| GO:1903865 sigma factor antagonist complex | IBA GO_REF:0000033 | ACCEPT | Summary: SigK (as pro-sigK) forms a complex with its regulatory proteins SpoIVFB, BofA, and SpoIVFA. GO:1903865 is defined as "A protein complex which is capable of sigma factor antagonist activity." The BofA/SpoIVFA complex inhibits SpoIVFB-mediated pro-sigK processing, thereby preventing premature sigK activation. Reason: UniProt documents the interaction between sigK and SpoIVFB (IntAct:EBI-15806052). PMID:19805276 demonstrates that "The CBS domain of SpoIVFB was shown to interact with Pro-sigma(K)". The pro-sigK-SpoIVFB-BofA-SpoIVFA complex represents a sigma factor antagonist complex that prevents premature sigK activity until the forespore signals for activation via SpoIVB and CtpB proteases. Supporting Evidence: UniProt:P12254 P12254; P26937: spoIVFB; NbExp=2; IntAct=EBI-15806052, EBI-15806037 PMID:19805276 The CBS domain of SpoIVFB was shown to interact with Pro-sigma(K) and ATP changed the interaction |
| GO:0003677 DNA binding | IEA GO_REF:0000043 | KEEP AS NON CORE | Summary: This generic DNA binding annotation is inferred from the UniProt DNA-binding keyword. SigK does contain a helix-turn-helix DNA-binding motif (residues 251-270) as documented in UniProt. However, sigma factors require the core polymerase for DNA binding - they do not bind DNA independently. Reason: SigK contains a documented HTH DNA-binding motif at residues 251-270 (UniProt feature). However, per the GO definition of sigma factor activity, "sigma does not bind DNA on its own." The more precise molecular function is GO:0016987 sigma factor activity. This annotation is kept as non-core since it reflects domain architecture but is less informative than the sigma factor activity annotation. Supporting Evidence: UniProt:P12254 DNA_BIND 251..270 /note="H-T-H motif" |
| GO:0003700 DNA-binding transcription factor activity | IEA GO_REF:0000002 | MODIFY | Summary: This annotation from InterPro domain mapping is incorrect for sigma factors. Sigma factors are not transcription factors in the classical sense; they are subunits of the RNA polymerase holoenzyme that confer promoter specificity rather than independent regulatory factors. Reason: GO:0003700 is defined as sequence-specific DNA binding that modulates transcription, applicable to classical transcription factors that independently bind DNA regulatory elements. Sigma factors function differently: they are dissociable subunits of RNA polymerase that determine promoter recognition as part of the holoenzyme complex. The correct term is GO:0016987 sigma factor activity, which is already annotated for sigK. Proposed replacements: sigma factor activity Supporting Evidence: UniProt:P12254 Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released |
| GO:0006351 DNA-templated transcription | IEA GO_REF:0000043 | ACCEPT | Summary: SigK is involved in DNA-templated transcription as a component of the RNA polymerase holoenzyme. This is an appropriate general biological process annotation inferred from the UniProt Transcription keyword. Reason: As a sigma factor, sigK is essential for transcription initiation from specific promoters. This general process term appropriately captures sigK's role in transcription. The annotation is supported by UniProt keywords (Transcription) and the well-characterized function of sigma factors in bacterial transcription. Supporting Evidence: UniProt:P12254 This sigma factor is responsible for the expression of sporulation specific genes in the mother cell. |
| GO:0006352 DNA-templated transcription initiation | IEA GO_REF:0000002 | ACCEPT | Summary: This is a highly appropriate annotation for sigma factors. Sigma factors specifically function in transcription initiation by directing RNA polymerase to promoters and are released after initiation. SigK acts at the initiation step to enable recognition of sigK-dependent promoters. Reason: UniProt states that "Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released." The specific role of sigma factors in initiation (as opposed to elongation or termination) makes this a precise and accurate annotation for sigK. Supporting Evidence: UniProt:P12254 Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released |
| GO:0006355 regulation of DNA-templated transcription | IEA GO_REF:0000120 | ACCEPT | Summary: Duplicate annotation (same term as the IBA annotation above) from automated inference. This accurately captures sigK's regulatory role in transcription of the late mother cell sporulation program. Reason: This is a duplicate of the IBA annotation for the same term. Both annotations are valid and supported by the same evidence. Sigma factors regulate transcription by determining which genes are expressed. SigK specifically regulates the late mother cell gene expression program during sporulation. Supporting Evidence: UniProt:P12254 This sigma factor is responsible for the expression of sporulation specific genes in the mother cell. |
| GO:0016987 sigma factor activity | IEA GO_REF:0000043 | ACCEPT | Summary: Duplicate annotation (same term as the IBA annotation above) from automated inference based on UniProt Sigma factor keyword. Reason: This is a duplicate of the IBA annotation for sigma factor activity. Both annotations are valid. This is the core molecular function of sigK, supported by the UniProt Sigma factor keyword and sigma-70 family domain architecture. Supporting Evidence: UniProt:P12254 Belongs to the sigma-70 factor family. |
| GO:0030435 sporulation resulting in formation of a cellular spore | IEA GO_REF:0000043 | ACCEPT | Summary: SigK is the late mother cell-specific sigma factor essential for sporulation in B. subtilis. It activates genes required for coat assembly, cortex formation, and germination, all essential for production of a mature spore. This biological process annotation appropriately captures the developmental context of sigK function. Reason: SigK is specifically active in the mother cell during late sporulation and is absolutely required for spore formation. UniProt states it is "responsible for the expression of sporulation specific genes in the mother cell." PMID:19805276 confirms that sigK directs "transcription of genes necessary for spore formation." Supporting Evidence: UniProt:P12254 This sigma factor is responsible for the expression of sporulation specific genes in the mother cell. PMID:19805276 releasing sigma(K) to direct transcription of genes necessary for spore formation |
| GO:2000142 regulation of DNA-templated transcription initiation | IEA GO_REF:0000108 | ACCEPT | Summary: This annotation is logically inferred from sigma factor activity (GO:0016987). Since sigma factors specifically function at the initiation step, regulation of transcription initiation is an appropriate process annotation for sigK. Reason: This is a more specific version of the transcription regulation annotation that correctly reflects that sigma factors act at the initiation stage. Since sigma factors "promote the attachment of RNA polymerase to specific initiation sites," they specifically regulate the initiation step of transcription. This logical inference from GO:0016987 is valid. Supporting Evidence: UniProt:P12254 Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites |
| GO:0005515 protein binding | IPI PMID:19805276 Intramembrane proteolytic cleavage of a membrane-tethered tr... | MODIFY | Summary: This annotation is based on the physical interaction between pro-sigK and the intramembrane metalloprotease SpoIVFB, demonstrated in PMID:19805276. The study shows that SpoIVFB engages pro-sigK as its substrate for regulated intramembrane proteolysis, releasing active sigK. Reason: While the annotation is not wrong (sigK does bind to SpoIVFB), "protein binding" is too vague to be informative. The specific interaction documented is between pro-sigK and its processing protease SpoIVFB. The CC term GO:1903865 (sigma factor antagonist complex) already captures this regulatory relationship more specifically. The protein binding annotation adds little value over the more specific complex annotation. Proposed replacements: sigma factor antagonist complex Supporting Evidence: PMID:19805276 The Bacillus subtilis SpoIVFB protein is a putative metalloprotease that cleaves membrane-tethered Pro-sigma(K), releasing sigma(K) to direct transcription of genes necessary for spore formation. PMID:19805276 The CBS domain of SpoIVFB was shown to interact with Pro-sigma(K) and ATP changed the interaction |
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Download this section (compressed HTML)Q: What is the complete SigK regulon and how does the feed-forward regulation with GerE sculpt the temporal expression pattern?
Q: How does SigK promoter recognition specificity differ from that of the early mother cell sigma factor SigE?
Q: Is skin element excision absolutely required for sigK function, or can strains without the skin element sporulate normally?
Experiment: ChIP-seq analysis of SigK binding during late sporulation would enable genome-wide identification of SigK binding sites, comprehensively defining the SigK regulon and validating the regulatory relationships with GerE.
Hypothesis: SigK binds to specific promoter sequences genome-wide to activate a defined late mother cell transcriptional program
Type: ChIP-seq
Experiment: Cryo-EM structural analysis of the full pro-sigK-SpoIVFB-BofA-SpoIVFA inhibitory complex would reveal the molecular mechanism of sigK sequestration prior to activation and how SpoIVB/CtpB-mediated cleavage relieves this inhibition.
Hypothesis: BofA and SpoIVFA form a steric block preventing pro-sigK from accessing the SpoIVFB active site
Type: Structural biology
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