Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. Extracytoplasmic function (ECF) sigma factors are held in an inactive form by a cognate anti-sigma factor until released, although no anti-sigma factor is known for this protein. May be involved in host intracellular survival after infection (strains H37Rv and CDC 1551). A role in the SOS response is controversial; it has been seen in strain CDC 1551 (PubMed:18039768) but not in H37Rv (PubMed:21169493). Interacts transiently with the RNA polymerase catalytic core formed by RpoA, RpoB, RpoC and RpoZ (2 alpha, 1 beta, 1 beta' and 1 omega subunit) to form the RNA polymerase holoenzyme that can initiate transcription. Very poorly expressed in exponential phase; further repressed at room temperature. Up-regulated 10-fold 7 days after infection of human macrophages. Induced by DNA-damaging agents but not by surface stress (detergent). The sigma-70 factor domain-2 mediates sequence-specific interaction with the -10 element in promoter DNA, and plays an important role in melting the double-stranded DNA and the formation of the transcription bubble. The sigma-70 factor domain 2 mediates interaction with the RNA polymerase subunits RpoB and RpoC (By similarity). The sigma-70 factor domain-4 contains a helix-turn-helix (H-T-H) motif that mediates interaction with the -35 element in promoter DNA. The domain also mediates interaction with the RNA polymerase subunit RpoA (By similarity). Not essential, no effect on the SOS response genes, no change in resistance to DNA-damaging agents including mitomycin C. Belongs to the sigma-70 factor family. ECF subfamily.