An anti-sigma factor for extracytoplasmic function (ECF) sigma factor SigD. ECF sigma factors are held in an inactive form by an anti-sigma factor until released by regulated intramembrane proteolysis (RIP). RIP occurs when an extracytoplasmic signal triggers a concerted proteolytic cascade to transmit information and elicit cellular responses. The membrane-spanning regulatory substrate protein is first cut extracytoplasmically (site-1 protease, S1P), then within the membrane itself (site-2 protease, S2P), while cytoplasmic proteases finish degrading the regulatory protein, liberating the sigma factor. Neither S1P nor S2P proteases have been so far identified for this anti-sigma factor. Interacts with ECF RNA polymerase sigma factor SigD; this should inhibit the interaction of SigD with the RNA polymerase catalytic core. Positively regulated by alternative sigma factor SigD. The cytosolic domain interacts with ECF sigma factor SigD. The cytosolic fragment (residues 1-94) in both free and SigD-associated form, is degraded by a ClpP1-ClpP2-ClpX complex, as would be expected after S1P and S2P intramembrane proteolysis. This releases SigD so that it may bind to the RNA polymerase catalytic core.