Class I heat-inducible genes in Bacillus subtilis consist of the heptacistronic dnaK and the bicistronic groE operon and form the CIRCE regulon. Both operons are negatively regulated at the level of transcription by the HrcA repressor interacting with its operator, the CIRCE element. Here, we demonstrate that the DnaK chaperone machine is not involved in the regulation of HrcA and that the GroE chaperonin exerts a negative effect in the post-transcriptional control of HrcA. When expression of the groE operon was turned off, the dnaK operon was significantly activated and large amounts of apparently inactive HrcA repressor were produced. Overproduction of GroEL, on the other hand, resulted in decreased expression of the dnaK operon. Introduction of the hrcA gene and its operator into Escherichia coli was sufficient to elicit a transient heat shock response, indicating that no additional Bacillus-specific gene(s) was needed. As in B. subtilis, the groEL gene of E. coli negatively influenced the activity of HrcA. HrcA could be overproduced in E. coli, but formed inclusion bodies which could be dissolved in 8 M urea. Upon removal of urea, HrcA had a strong tendency to aggregate, but aggregation could be suppressed significantly by the addition of GroEL. Purified HrcA repressor was able specifically to retard a DNA fragment containing the CIRCE element, and the amount of retarded DNA was increased significantly in the presence of GroEL. These results suggest that the GroE chaperonin machine modulates the activity of the HrcA repressor and therefore point to a novel function of GroE as a modulator of the heat shock response.