The role of cotA gene in proton extrusion of Synechocystis PCC6803

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Light-induced proton extrusion has been observed with various strains of cyanobacteria. We found that pxcA (formerly named as cotA) complemented a Synechocystis mutant unable to grow at acidic pH and that inactivation of this gene abolished the activity of light-induced proton extrusion. PxcA genes of Synechocystis PCC6803 and Synechococcus PCC7942 encode proteins of 440 and 427 amino acids, respectively, that showed significant sequence homology to a protein in the inner envelope membrane of chloroplasts encoded by cemA in chloroplast genome. We have demonstrated that PxcA is localized in the cytoplasmic membrane. The pxcA-inactivated mutant was unable to grow at acidic pHs or at low Na^+ concentrations. The activities of CO_2 uptake and NO_3 uptake by the mutant were inhibited under these conditions. There are two types of CemA/PxcA.CemA in chloroplasts of higher plants consists of 229 - 231 amino acids whereas CemA/PxcA in liverwort, Chlamydomonas and cyanobacteria are much larger and consist of 421-500 amino acids. Both short and long types of CemA/PxcA contain 4 membrane spanning domains. Phylogenetic tree for PxcA of cyanobacteria and CemA of plants and Chlamydomonas suggested that PxcA and CemA have the same ancestor.