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. Preferentially transcribes genes associated with fast growth, such as ribosomal operons, other protein-synthesis related genes, rRNA- and tRNA-encoding genes and prfB. 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. Identified in a complex containing RpoD, the RNA polymerase subunits RpoA, RpoB and RpoZ, CRP and DNA. Interacts with Rsd; this prevents interaction with the RNA polymerase catalytic core and with promoter DNA, and as a consequence, promotes transcription from promoters that require alternative sigma factors. Interacts with phage T4 AsiA; this interferes with binding to DNA and to the RNA polymerase. (Microbial infection) Interacts with Escherichia phage lambda antitermination protein Q. 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. 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. 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 (PubMed:19903881 and PubMed:21829166). This domain is probably also responsible for interaction with Crp (PubMed:10860740). Belongs to the sigma-70 factor family. RpoD/SigA subfamily.