Region 3.2 of the σ factor controls the stability of rRNA promoter complexes and potentiates their repression by DksA

The sigma factor drives promoter recognition by bacterial RNA polymerase (RNAP) and is also essential for later steps of transcription initiation, including RNA priming and promoter escape. Conserved region 3.2 of the primary sigma factor ('sigma finger') directly contacts the template DNA strand in the open promoter complex and facilitates initiating NTP binding in the active center of RNAP. Ribosomal RNA promoters are responsible for most RNA synthesis during exponential growth but should be silenced during the stationary phase to save cell resources. In Escherichia coli, the silencing mainly results from the action of the secondary channel factor DksA, which together with ppGpp binds RNAP and dramatically decreases the stability of intrinsically unstable rRNA promoter complexes. We demonstrate that this switch depends on the sigma finger that destabilizes RNAP-promoter interactions. Mutations in the sigma finger moderately decrease initiating NTP binding but significantly increase promoter complex stability and reduce DksA affinity to the RNAP-rRNA promoter complex, thus making rRNA transcription less sensitive to DksA/ppGpp both in vitro and in vivo. Thus, destabilization of rRNA promoter complexes by the sigma finger makes them a target for robust regulation by the stringent response factors under stress conditions.