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.Cyanobacteria fix CO 2 efficiently despite the low affinity and selectivity of their ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco) for CO 2 , because they possess a CO 2 -concentrating mechanism (CCM) to elevate the CO 2 concentration around the active site of Rubisco (11,21). The CCM involves the abilities to actively transport HCO 3 Ϫ into the cell, to convert CO 2 to HCO 3 Ϫ intracellularly, and to effectively convert HCO 3 Ϫ into CO 2 in carboxysomes, the polyhedral inclusion bodies to which Rubisco is localized. It is supposed that the conversion of CO 2 to HCO 3 Ϫ in the cytoplasm not only helps to maintain high intracellular HCO 3 Ϫ concentrations but also allows diffusion of CO 2 from external medium into the cytoplasm (10). Biosynthesis of the components of CCM, including Rubisco, is supposed to be controlled by CO 2 availability (11, 21); however, detailed studies on the transcriptional regulation of the CCM-related genes are yet to be performed, and the underlying molecular mechanism is unknown.Among the CCM-related genes, the cmp operon of Synechococcus sp. strain PCC 7942, encoding a high-affinity bicarbonate transporter (12,18), is known to be a typical low CO 2 -inducible transcription unit; the cmp operon mRNA and the CmpA protein, which is by far the most abundant protein among the proteins encoded by the operon, a...

An open reading frame (slr0899) on the genome of Synechocystis sp. strain PCC 6803 encodes a polypeptide of 149 amino acid residues, the sequence of which is 40% identical to that of cyanase from Escherichia coli. Introduction into a cyanase-deficient E. coli strain of a plasmid-borne slr0899 resulted in expression of low but significant activity of cyanase. Targeted interruption of a homolog of slr0899 from Synechococcus sp. strain PCC 7942, encoding a protein 77% identical to that encoded by slr0899, resulted in loss of cellular cyanase activity. These results indicated that slr0899 and its homolog in the strain PCC 7942 represent the cyanobacterial cyanase gene (designated cynS). While cynS of strain PCC 6803 is tightly clustered with the four putative molybdenum cofactor biosynthesis genes located downstream, cynS of strain PCC 7942 was found to be tightly clustered with the two genes located upstream, which encode proteins similar to the subunits of the cyanobacterial nitrate-nitrite transporter. In both strains, cynS was transcribed as a part of a large transcription unit and the transcription was negatively regulated by ammonium. Cyanase activity was low in ammonium-grown cells and was induced 7-to 13-fold by inhibition of ammonium fixation or by transfer of the cells to ammoniumfree media. These findings indicated that cyanase is an ammonium-repressible enzyme in cyanobacteria, the expression of which is regulated at the level of transcription. Similar to other ammonium-repressible genes in cyanobacteria, expression of cynS required NtcA, a global nitrogen regulator of cyanobacteria.Cyanase (EC 4.3.99.1), which catalyzes the decomposition of cyanate (NCO Ϫ ) into CO 2 and NH 3 , has been shown to be present in plants, some heterotrophic bacteria, and the cyanobacterium Synechococcus sp. strain PCC 6301 (6, 22). In living organisms, cyanate is formed by the decomposition of carbamoylphosphate (CP) or urea (2, 14). Cyanate reacts with nucleophilic groups of proteins and hence is a toxic compound. With cyanase, however, Escherichia coli cells can utilize cyanate as the sole source of nitrogen (11). Cyanate has been also shown to reversibly inhibit CP synthetase of E. coli (4). The proposed biological functions of cyanase in E. coli therefore include detoxification of endogenously formed cyanate, utilization of cyanate as the nitrogen source (11), and regulation of enzyme activities through modulation of the intracellular cyanate level (12). For the cyanobacterium Synechococcus sp. strain PCC 6301, Miller and Espie found a high activity of cyanase and estimated the rate of cyanate decomposition per cell to be 20 times that in E. coli (22). The high cyanase activity implies the importance of cyanate metabolism in cyanobacterial cells, but the physiological role of the enzyme remains to be clarified.In the investigation of the mechanism of ammonium-promoted activation and repression of the carbon and nitrogen assimilation genes, respectively, in Synechococcus sp. strain PCC 7942, we found that exogenously adde...