Differential regulation of groESL operon expression in response to heat and light in Anabaena

The HrcA protein is known to bind the cis-element CIRCE and repress expression of hsp60 in certain bacteria. However, recent data from cyanobacteria have seriously questioned the HrcA/CIRCE interaction paradigm. A hrcA null mutant showed constitutive expression of Hsp60 proteins GroEL/Cpn60(GroEL2)], and an unexpected further increase in GroEL during temperature upshift, suggesting involvement of regulatory mechanisms other than HrcA in groESL expression in Anabaena. The negative regulation of both hsp60 genes groEL and cpn60 (groEL2)] at CIRCE element was established by: (1) constitutive expression of Green Fluorescent Protein gene, tagged to Anabaena hsp60 promoters, in E. coli, and its repression upon co-expression of Anabaena HrcA and (2) specific binding of Anabaena HrcA to the CIRCE element. Deletion analysis of other cis-elements further distinguished (a) a photo-regulation by the K-box and (b) thermoregulation from a novel H-box, over and above the negative regulation by HrcA at CIRCE.