Structural basis for H-NS-mediated trapping of RNA polymerase in the open initiation complex at the rrnB P1

The Escherichia coli H-NS protein is a nucleoid-associated protein involved in both transcription regulation and DNA compaction. Each of these processes involves H-NS-mediated bridge formation between adjacent DNA helices. With respect to transcription regulation, preferential binding sites in the promoter regions of different genes have been reported, and generally these regions are curved. Often H-NS binding sites overlap with promoter core regions or with binding sites of other regulatory factors. Not in all cases, however, transcriptional repression is the result of preferential binding by H-NS to promoter regions leading to occlusion of the RNA polymerase. In the case of the rrnB P1, H-NS actually stimulates open complex formation by forming a ternary RNAP(.)H-NS(.)DNA complex, while simultaneously stabilizing it to such an extent that promoter clearance cannot occur. To define the mechanism by which H-NS interferes at this step in the initiation pathway, the architecture of the RNAP(.)H-NS(.)DNA complex was analyzed by scanning force microscopy (SFM). The SFM images show that the DNA flanking the RNA polymerase in open initiation complexes is bridged by H-NS. On the basis of these data, we present a model for the specific repression of transcription initation at the rrnB P1 by H-NS.