Involvement of a CbbR homolog in low CO2-induced activation of the bicarbonate transporter operon in cyanobacteria

The cmpABCD operon of Synechococcus sp. strain PCC 7942, encoding a high-af finity bicarbonate transporter, is transcribed only under CO2-limited condi tions. In Synechocystis sp. strain PCC 6803, the slr0040, slr0041, slr0043, and slr0044 genes, forming an operon with a putative porin gene (slr0042), were identified as the cmpA, cmpB, cmpC, and cmpD genes, respectively, on the basis of their strong similarities to the corresponding Synechococcus c mp genes and their induction under low CO2 conditions. Immediately upstream of and transcribed divergently from the Synechocystis cmp operon is a gene (sll0030) encoding a homolog of CbbR, a LysR family transcriptional regula tor of the CO2 fixation operons of chemoautotrophic and purple photosynthet ic bacteria. Inactivation of sll0030, but not of another closely related cb bR homolog (sll1594), abolished low CO2 induction of cmp operon expression. Gel retardation assays showed specific binding of the SII0030 protein to t he sll0030-cmpA intergenic region, suggesting that the protein activates tr anscription of the cmp operon by interacting with its regulatory region. A cbbR homolog similar to sll0030 and sll1594 was cloned from Synechococcus s p. strain PCC 7942 and shown to be involved in the low CO2-induced activati on of the cmp operon. We hence designated the Synechocystis sIl0030 gene an d the Synechococcus cbbR homolog cmpR, In the mutants of the cbbR homologs, upregulation of ribulose-1,5-bisphosphate carboxylase/oxygenase operon exp ression by CO2 limitation was either unaffected (strain PCC 6803) or enhanc ed (strain PCC 7942), suggesting existence of other low CO2-responsive tran scriptional regulator(s) in cyanobacteria.