Involvement of a CbbR Homolog in Low CO 2 -Induced Activation of the Bicarbonate Transporter Operon in Cyanobacteria

Abstract

ABSTRACT The cmpABCD operon of Synechococcus sp. strain PCC 7942, encoding a high-affinity bicarbonate transporter, is transcribed only under CO 2 -limited conditions. 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 cmp genes and their induction under low CO 2 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 regulator of the CO 2 fixation operons of chemoautotrophic and purple photosynthetic bacteria. Inactivation of sll0030 , but not of another closely related cbbR homolog ( sll1594 ), abolished low CO 2 induction of cmp operon expression. Gel retardation assays showed specific binding of the Sll0030 protein to the sll0030-cmpA intergenic region, suggesting that the protein activates transcription of the cmp operon by interacting with its regulatory region. A cbbR homolog similar to sll0030 and sll1594 was cloned from Synechococcus sp. strain PCC 7942 and shown to be involved in the low CO 2 -induced activation of the cmp operon. We hence designated the Synechocystis sll0030 gene and the Synechococcus cbbR homolog cmpR . In the mutants of the cbbR homologs, upregulation of ribulose-1,5-bisphosphate carboxylase/oxygenase operon expression by CO 2 limitation was either unaffected (strain PCC 6803) or enhanced (strain PCC 7942), suggesting existence of other low CO 2 -responsive transcriptional regulator(s) in cyanobacteria.