The STAS (Sulphate Transporter and AntiSigma factor antagonist) domain is found in the C-terminal region of sulphate transporters and bacterial anti-sigma factor antagonists. It has been suggested that this domain may have a general NTP binding function. The establishment of differential gene expression in sporulating <taxon tax_id="1423">Bacillus subtilis</taxon> involves four protein components one of which is SpoIIAA (<db_xref db="SWISSPROT" dbkey="P10727"/>). The four components regulate the sporulation sigma factor F. Early in sporulation, SpoIIAA is in the phosphorylated state (SpoIIAA-P), as a result of the activity of the ATP-dependent protein kinase SpoIIAB (<db_xref db="SWISSPROT" dbkey="P10728"/>). The site at which this protein is a conserved serine. SpoIIAB is an anti-sigma factor that in its free form inhibits F by binding to it. Competition by SpoIIAA (the anti-anti-sigma factor) for binding to SpoIIAB releases Sigma F activity [<cite idref="PUB00004920"/>]. The STAS domain is found in the anti-sigma factor antagonist SpoIIAA.