Feeling the Heat: The Campylobacter jejuni HrcA Transcriptional Repressor Is an Intrinsic Protein Thermosensor

The heat-shock response, a universal protective mechanism consisting of a transcriptional reprogramming of the cellular transcriptome, results in the accumulation of proteins which counteract the deleterious effects of heat-stress on cellular polypeptides. To quickly respond to thermal stress and trigger the heat-shock response, bacteria rely on different mechanisms to detect temperature variations, which can involve nearly all classes of biological molecules. In Campylobacter jejuni the response to heat-shock is transcriptionally controlled by a regulatory circuit involving two repressors, HspR and HrcA. In the present work we show that the heat-shock repressor HrcA acts as an intrinsic protein thermometer. We report that a temperature upshift up to 42 & DEG;C negatively affects HrcA DNA-binding activity to a target promoter, a condition required for de-repression of regulated genes. Furthermore, we show that this impairment of HrcA binding at 42 & DEG;C is irreversible in vitro, as DNA-binding was still not restored by reversing the incubation temperature to 37 & DEG;C. On the other hand, we demonstrate that the DNA-binding activity of HspR, which controls, in combination with HrcA, the transcription of chaperones' genes, is unaffected by heat-stress up to 45 & DEG;C, portraying this master repressor as a rather stable protein. Additionally, we show that HrcA binding activity is enhanced by the chaperonin GroE, upon direct protein-protein interaction. In conclusion, the results presented in this work establish HrcA as a novel example of intrinsic heat-sensing transcriptional regulator, whose DNA-binding activity is positively modulated by the GroE chaperonin.

Automated summary

The heat-shock response is a universal protective mechanism that involves transcriptional reprogramming of cellular transcriptome to counteract the deleterious effects of heat-stress. In Campylobacter jejuni, the response is controlled by two repressors, HspR and HrcA, with HrcA acting as an intrinsic protein thermometer. The DNA-binding activity of HrcA is negatively affected by temperature upshift, while HspR remains stable under heat-stress conditions.