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 (RsrA) until released. Responds to thiol-oxidative stress, involved in regulation of about 30 genes and operons, including the thioredoxin system (trxB-trxA, trxC), ribosomal protein L31, RNA polymerase-binding protein RbpA and mycothiol (MSH) biosynthetic (mshA) and recycling genes (mca). In conjunction with its cognate anti-sigma factor RsrA may sense the intracellular level of reduced MSH. 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 with cognate anti-sigma factor RsrA under reducing but not oxiding conditions, which prevents it binding to RNA polymerase. Treatment with the thiol-oxidzing agent diamide inhibits the interaction. Expressed from 2 promoters, 1 of which (sigRp2) positively regulates its own expression. The same promoter is transiently induced (about 70-fold) by the thiol-oxiding agent diamide. Also induced when mycothiol is oxidized or conjugated. 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). Interactions between sigma-70 factor domain-2 and anti-sigma factors prevents interaction of sigma factors with the RNA polymerase catalytic core (Probable). 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). SigR is present in multiple forms of different pI, suggesting other unknown post-translational modifications. SigR prime is probably partially degraded by ClpP1-ClpP2. Increased sensitivity to redox-cycling compounds menadione and plumbagin, and to diamide. Loss of expression of mycothiol biosynthetic and recycling genes. Constitutively expressed major isoform, not induced by diamide (at protein level), contributes about half of the diamide-induced expression of the regulon. Present in low amounts in unstressed cells, strongly but transiently produced upon diamide induction. Functions as a sigma factor, contributes about half of the diamide-induced expression of the regulon. Unstable, half-life of about 10 minutes. Belongs to the sigma-70 factor family. ECF subfamily.