The three Synechocystis PCC6803 genes homologous to proteobacterial Calvin cycle regulators (cbbR) have been analysed. sll0998 appeared to be crucial to cell viability, whereas both sll0030 and sll1594 were found to be dispensable for cell growth. In spite of their sequence homology, Sll0030 and Sll1594 did not appear to regulate the transcription of Calvin cycle key genes. Further analysis of Sll1594 showed that this protein plays an important role in the adaptation to inorganic carbon starvation and osmotic stress. Sll1594 mediates the response to these stress conditions by regulating the transcription of a new regulon including the monocistronic genes sll1594 and slr1727 (encoding a presumptive Na+/H+ antiporter), as well as the ndh3 operon encoding the NAD(P)H-dehydrogenase subunits F3 and D3 and a protein of unknown function. The sll1594 gene and the ndh3 operon are negatively controlled by Sll1594, which also regulates the expression of the slr1727 gene. Sequence alignment of the diverse Sll1594 DNA binding sites led us to propose the TCAATG-(N10)-ATCAAT sequence as the consensus motif. To our knowledge, this is the first report on the characterization and analysis of a transcriptional regulator for ndh genes in a photoautotrophic organism.