SRA

Cell cycle progression in most organisms requires tightly regulated programs of gene expression. The transcription factors involved typically stimulate gene expression by binding specific DNA sequences in promoters and recruiting RNA polymerase. Here, we find that the essential cell cycle regulator GcrA in Caulobacter crescentus activates the transcription of target genes in a fundamentally different manner. GcrA forms a stable complex with RNA polymerase and localizes to almost all active s70-dependent promoters in vivo, but activates transcription primarily at promoters harboring certain DNA methylation sites. Whereas most transcription factors that contact s70 interact with domain 4, GcrA interfaces with domain 2, the region that binds the -10 element during strand separation. Using kinetic analyses and a reconstituted in vitro transcription assay, we demonstrate that GcrA can stabilize RNA polymerase binding and directly stimulate open complex formation to activate transcription. Guided by these studies, we identify a regulon of ~200 genes, providing new insight into the essential functions of GcrA. Collectively, our work reveals a new mechanism for transcriptional regulation, and we discuss the potential benefits of activating transcription by promoting RNA polymerase isomerization rather than exclusively recruitment. Overall design: Examination of GcrA, RNAP, Sigma70 ChIP in PYE and in PYE + rifampicin-treated for 30 min; sigma32 and sigma54 in PYE + rifampicin-treated for 30 min