Roles of CmpR, a LysR family transcriptional regulator, in acclimation of the cyanobacterium Synechococcus sp. strain PCC 7942 to low-CO(2) and high-light conditions.

The cmp operon of Synechococcus sp. strain PCC 7942, encoding a high-affinity bicarbonate transporter, is induced under low CO(2) conditions by a LysR family protein CmpR. CmpR was found to be required also for induction of the operon by transfer of the cells from low-light to high-light conditions, indicating involvement of a common mechanism in the high-light- and low-CO(2)-responsive regulation. Expression of the high-light inducible genes psbAII and psbAIII, on the other hand, was found to be induced also by low-CO(2) conditions. A single promoter was responsible for the high-light and low-CO(2) induction of each of psbAII and psbAIII, suggesting involvement of a common regulatory mechanism in the light and CO(2) responses of the psbA genes. CmpR was, however, not required for the induction of psbAII and psbAIII, indicating the presence of multiple mechanisms for induction of genes under high-light and low-CO(2) conditions. The CmpR-deficient mutant nevertheless showed lower levels of the psbAII and psbAIII transcripts than the wild-type strain under all the light and CO(2) conditions examined. Gel shift assays showed that CmpR binds to the enhancer elements of psbAII and psbAIII, through specific interaction with a sequence signature conforming to the binding motif of similar LysR family proteins. These findings showed that CmpR acts as a trans-acting factor that enhances transcription of the photosystem II genes involved in acclimation to high light, revealing a complex network of gene regulation in the cyanobacterium.