Abstract
The CIRCE/HrcA system is highly conserved in cyanobacterial genomes. We have shown that heat-shock induction of the groESL1 operon in the cyanobacterium Synechocystis sp. PCC6803 is negatively regulated by the CIRCE/HrcA system. In Synechococcus elongatus PCC7942, a novel heat shock protein, Orf7.5, is involved in positive regulation of the groESL1 transcription. However, Orf7.5 is not conserved in some cyanobacteria, including Synechocystis sp. PCC6803. The purpose of this study is to evaluate the functional conservation of the CIRCE/HrcA system in S. elongatus PCC7942 and to understand the interplay between the CIRCE/HrcA system and the Orf7.5 regulatory system. We constructed single and double mutants of S. elongatus orf7.5, hrcA and orf7.5/hrcA and heat induction of the groESL1 transcription in these mutants was analyzed. Unexpectedly, derepression of the groESL1 transcription in an hrcA mutant was not observed. In all these mutants, the transcription was greatly suppressed under both normal and heat stress conditions, indicating that both HrcA and Orf7.5 are involved in regulation of the groESL1 transcription in a positive way. Consistent with the decrease in the groESL1 mRNA level, all the single and double mutants showed a great loss of acquired thermotolerance. Heat induction of the orf7.5 promoter activity was totally diminished in the orf7.5 mutant, indicating that Orf7.5 activates its own transcription. Yeast two hybrid analysis showed that the principle sigma factor RpoD1 interacts with Orf7.5. These results indicate that Orf7.5 enhances the transcription of groESL1 and orf7.5 by interacting with RpoD1.
