Binds to RNA polymerase (RNAP), stimulating transcription from principal, but not alternative sigma factor promoters. Stimulates transcription from several principal sigma factor HrdB (SigA)-dependent promoters but not from a SigR-dependent promoter. Stimulation occurs in the presence of the transcription initiation inhibitor rifampicin (Rif). Bind 1 Zn(2+) per subunit. Homodimer (Probable). Forms a complex with the RNAP, and a complex with RNAP plus principal sigma factor HrdB associated with promoter. Binds to free principal sigma factors HrdB and HrdA, probably via the sigma-2 domain, but not to 6 other sigma factors tested. Constitutively expressed partially under control of SigR; both SigR-dependent and independent induction are further induced by the thiol-oxidant diamide (disulfide stress). Induced by Rif. Grows slowly, colonies are smaller. Not more diamide sensitive than wild-type, sporulation normal, 15-fold more sensitive to Rif. Belongs to the RNA polymerase-binding protein RbpA family.