Stress response gene regulation in Chlamydia is dependent on HrcA-CIRCE interactions

HrcA is a transcriptional repressor that regulates stress response genes in many bacteria by binding to the CIRCE operator. We have previously shown that HrcA regulates the promoter for the dnaK heat shock operon in Chlamydia. Here we demonstrate that HrcA represses a second heat shock promoter that controls the expression of groES and groEL, two other major chlamydial heat shock genes. The CIRCE element of C. trachomatis groEL is the most divergent of known bacterial CIRCE elements, and HrcA had a decreased ability to bind to this nonconsensus operator and repress transcription. We demonstrate that the CIRCE element is necessary and sufficient for transcriptional regulation by chlamydial HrcA and that the inverted repeats of CIRCE are the binding sites for HrcA. Addition of a CIRCE element upstream of a non-heat-shock promoter allowed this promoter to be repressed by HrcA, showing in principle that a chlamydial promoter can be genetically modified to be inducible. These results demonstrate that HrcA is the regulator of the major chlamydial heat shock operons, and we infer that the mechanism of the heat shock response in Chlamydia is derepression. However, derepression is likely to involve more than a direct effect of increased temperature as we found that HrcA binding to CIRCE and HrcA-mediated repression were not altered at temperatures that induce the heat shock response.