A novel light- and heat-responsive regulation of the groE transcription in the absence of HrcA or CIRCE in cyanobacteria

The inactivation of the hrcA gene resulted in de-repression of the two CIRCE-containing groE genes in a cyanobacterium Synechocystis sp. strain PCC6803, indicating that the CIRCE operator/HrcA repressor system operates in the cyanobacterium. We found that the groE expression in the hrcA mutant is greatly induced by heat and/or light. Removal of a K-box containing and an N-box containing region upstream of the groESL1 promoter abolished light-induced transcription of a luxAB reporter gene fused with the groESL1 promoter. Similar sequences to the K-box, GTTCGG-NNAN-CCNNAC, were also found upstream of the dnaK2 genes. A specific binding of a protein(s) to the N-box, GATCTA, was detected by a gel mobility shift assay with using cell extracts. We propose that the cyanobacterial groEL expression is regulated by a putative positive mechanism mediated by these novel elements in addition to the HrcA/CIRCE system. The groEL2 genes from Synechococcus sp. strain PCC 7942 and Thermosynechococcus elongatus, which lack CIRCE, K-box, and N-box naturally, were also induced by heat and/or light, indicating that the control mechanism of the unique light-responsive groE expression is highly diversified in cyanobacteria.