Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. Held in an inactive form by a cognate anti-sigma factor RsbW (UsfX) until released. Increased expression decreases growth rate, and after 3 days increases the expression of 51 loci encoding 33 protein-coding genes as well as some non-coding RNA (PubMed:22307756). Monomer. 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. Interacts (via sigma-70 factor domain 4) with anti-sigma-F factor RsbW (UsfX) with a possible 2:1 RbsW:SigF stoichiometry, which inhibits SigF activity. Constitutively expressed at a low level under all conditions tested, including stationary phase and several stresses. 3-fold induced during starvation (PubMed:11929527). Positively controls expression of its own operon (rsbW-sigF). 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). Infected guinea pigs mutant and wild-type bacteria grow comparably (PubMed:16735723). A double rsbW-sigF disruption shows no effect in the presence or absence of rifampicin (PubMed:20729364). Belongs to the sigma-70 factor family.