Binds to RNA polymerase (RNAP), stimulating and stabilizing the formation of stable RNAP promoter complexes up to 2-fold from principal sigma factor SigA-dependent but not alternative sigma factor SigF-dependent promoters. Increases the affinity of core RNAP for SigA, increasing the transcriptional activity of RNAP. Unlike the case in M.smegmatis or S.coelicolor, has no effect on rifampicin inhibition of transcription. Has no effect on E.coli RNAP. Oligomerizes at more than 3 uM; this requires the C-terminal 32 residues. Forms a complex with the RNAP catalytic core, specifically with the beta subunit (rpoB). Interacts with free SigB, probably as a 1:1 complex, and also with free SigA, probably via its sigma-2 domain. Essential for growth, it cannot be disrupted. This protein is only found in actinomycetes; as it is essential in M.tuberculosis, it might make a potential drug target. Belongs to the RNA polymerase-binding protein RbpA family. Unlike its ortholog in S.coelicolor, it is not seen to bind zinc.