Combined transcriptomic and ChIPseq analyses of the RisA regulon.

The regulation of virulence is mediated by the two-component system BvgA/S, which activates the transcription of virulence-activated genes (s). In the avirulent phase, the s are not expressed, but instead, virulence-repressed genes (s) are expressed, under the control of another two-component system, RisA/K. Here, we combined transcriptomic and chromatin immunoprecipitation sequencing (ChIPseq) data to examine the RisA/K regulon. We performed RNAseq analyses of RisA-deficient and RisA-phosphoablative mutants cultivated in virulent and avirulent conditions. We confirmed that the expression of most s is regulated by phosphorylated RisA. However, the expression of some, including those involved in flagellum biosynthesis and chemotaxis, requires RisA independently of phosphorylation. Many RisA-regulated genes encode proteins with regulatory functions, suggesting multiple RisA regulation cascades. By ChIPseq analyses, we identified 430 RisA-binding sites, 208 within promoter regions, 201 within open reading frames, and 21 in non-coding regions. RisA binding was demonstrated in the promoter regions of most s and, surprisingly, of some s, as well as for other genes not identified as s or s. Unexpectedly, many genes, including some s, like , , and , contain a BvgA-binding site and a RisA-binding site, which increases the complexity of the RisAK/BvgAS network in virulence regulation.IMPORTANCEThe expression of virulence-activated genes (s) of , the etiological agent of whooping cough, is under the transcriptional control of the two-component system BvgA/S, which allows the bacterium to switch between virulent and avirulent phases. In addition, the more recently identified two-component system RisA/K is required for the expression of genes, collectively named s, that are repressed during the virulent phase but activated during the avirulent phase. We have characterized the RisA/K regulon by combined transcriptomic and chromatin immunoprecipitation sequencing analyses. We identified more than 400 RisA-binding sites. Many of them are localized in promoter regions, especially s, but some were found within open reading frames and in non-coding regions. Surprisingly, RisA-binding sites were also found in promoter regions of some s, illustrating the previously underappreciated complexity of virulence regulation in .