stress-response B subunit of RNA polymerase in promoter DNA unwinding

RbpA, a transcriptional activator that is essential for Mycobacterium tuberculosis replication and sur-vival during antibiotic treatment, binds to RNA poly-merase (RNAP) in the absence of promoter DNA. It has been hypothesized that RbpA stimulates house-keeping gene expression by promoting assembly of theA subunit with core RNAP. Here, using a purified in vitro transcription system of M. tuberculosis, we show that RbpA functions in a promoter-dependent manner as a companion of RNAP essential for pro-moter DNA unwinding and formation of the catalyti-cally active open promoter complex (RPo). Screening for RbpA activity using a full panel of the M. tuber-culosis subunits demonstrated that RbpA targets A and stress-response B, but not the alternative subunits from the groups 3 and 4. In contrast to A, the B subunit activity displayed stringent de-pendency upon RbpA. These results suggest that RbpA-dependent control of RPo formation provides a mechanism for tuning gene expression during the switch between different physiological states, and in the stress response