In Synechocystis sp. strain PCC 6803, over 450 genes are upregulated following transfer of the cells from a high (1-5% CO(2) in air, HC) to a low level of CO(2) (as in air or lower, LC). This includes sbtA, ndhF3 and cmpA involved in inorganic carbon (Ci) uptake. Earlier studies implicated NdhR in the regulation of LC-induced genes but there are indications that additional components are involved. Following extraction of proteins from cells grown under HC and (NH4)(2)SO(4) fractionation, we have identified LexA and two AbrB-like proteins, Sll0359 and Sll0822, which bind to a fragment of the sbtA promoter. Using extracts prepared from LC-grown cells, Sll0822 did not bind to the sbtA promoter despite its presence in the cells, suggesting that it may serve as a repressor of LC-induced genes. This is supported by the fact that sbtA, ndhF3 and cmpA normally expressed only under LC in the wild-type are transcribed under both HC and LC in a Deltasll0822 mutant. When grown under HC this mutant exhibits an elevated apparent photosynthetic affinity to Ci, typically observed in the wild-type only under LC. Clearly, expression of genes essential for Ci uptake was sufficient to raise the apparent photosynthetic affinity for external Ci.