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 an anti-sigma factor until released, although no anti-sigma factor is known for this protein (By similarity). Regulates the promoter of SigI, may not be autoregulated. 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. Undetectable expression in log phase, 18-fold induced by 30 days, decreasing slightly after (at protein level). Up-regulated 2.4-fold 7 days after infection of human macrophages. 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. Interactions between sigma-70 factor domain-4 and anti-sigma factors prevents interaction of sigma factors with the RNA polymerase catalytic core (By similarity). No difference in growth in culture up to 120 days. Increased sensitivity to 10 and 20 mM H(2)O(2), no difference in response to growth at 53 degrees Celsius, pH 4.0, 10% ethanol or 0.05% sodium dodecyl sulfate (surface stress). No detectable difference in spleen or lungs of infected BALB/c mice during 15 weeks growth. Belongs to the sigma-70 factor family. ECF subfamily.