Sigma factors are initiation factors that promote the attachment of RNA polymerase to specific initiation sites and are then released. This sigma factor is the primary sigma factor during exponential growth. 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 (By similarity). Interacts (via sigma-70 factor domain 4) with the phage G1 protein gp67; this inhibits rRNA synthesis. Interaction with phage G1 protein gp67 does not inhibit transcription in general, but selectively inhibits transcription from promoters that require interaction of the RNA polymerase alpha subunit with DNA sequences upstream of the -35 promoter element. Contains 4 domains, connected by flexible linkers. In the active conformation, the domains are in an extended conformation, each making extensive interactions with the RNA polymerase catalytic core (By similarity). In the autoinhibited state, sigma-70 factor domain-1 packs closely together with sigma-70 factor domains-2 and -4, contrary to the extended conformation that is seen when the protein is part of the RNA polymerase holoenzyme. 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). Belongs to the sigma-70 factor family. RpoD/SigA subfamily.