Stress Response Gene Regulation in Chlamydia Is Dependent on HrcA-CIRCE Interactions

Abstract

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.

Chlamydial GroEL Autoregulates Its Own Expression through Direct Interactions with the HrcA Repressor Protein

“…We have previously shown that HrcA regulates both the dnaK and groE heat shock operons in Chlamydia trachomatis through binding to the CIRCE operator (50,51), but increased temperature within physiologic limits could not affect HrcA-mediated repression (51). We now show that HrcA, GroEL, and GroES can be isolated together from a chlamydial lysate as a complex bound to CIRCE.…”

“…Although elevated temperature has been shown to induce the heat shock response in Chlamydia (15) along with up-regulation of groE transcription (9,42), there are reasons to believe that it is not the main source of cellular stress. An increased temperature sufficient to induce the heat shock response in Chlamydia (9) could not relieve HrcA-mediated transcriptional repression in vitro (51). It is also worth noting that chlamydiae reside inside the relatively thermostable environment of a mammalian cell throughout the replicative cycle.…”

“…Chlamydial in vitro transcription reactions with recombinant proteins were performed as previously described (51).…”

“…Standard electrophoretic mobility shift assay (EMSA) reactions were performed as previously described (51).…”

The RpoH-Mediated Stress Response in Neisseria gonorrhoeae Is Regulated at the Level of Activity

“…The stress-responsive genes, such as dnaK and groE, are transcribed from conventional 70 promoters, but their expression is modulated by GroE-dependent binding of the HrcA repressor protein to the CIRCE element (54). This system is also found in some gram-negative bacteria, including Helicobacter pylori, a member of the epsilon subdivision of the proteobacteria (46), and Chlamydia species (49). Only the groE operon of the alpha subdivision of the proteobacteria retains a CIRCE operator sequence (54).…”