ABSENCE OF LIGHT-INDUCED PROTON EXTRUSION IN A COTA-LESS MUTANT OF SYNECHOCYSTIS SP STRAIN PCC6803

cotA of Synechocystis sp. strain PCC6803 was isolated as a gene that c omplemented a mutant defective in CO2 transport and is homologous to c emA that encodes a chloroplast envelope membrane protein (A. Katoh, K. S. Lee, H. Fukuzawa, Ii. Ohyama, and T. Ogawa, Proc. Natl. Acad. Sci. USA 93:4006-4010, 1996), A mutant (M29) constructed by replacing cotA in the wild-type (WT) Synechocystis strain with the omega fragment wa s unable to grow in BG11 medium (similar to 17 mM Na+) at pH 6.4 or at any pH in a low-sodium medium (100 mu M Na+) under aeration with 3% ( vol/vol) CO2 in air, The WT cells grew well in the pH range between 6. 4 and 8.5 in BG11 medium but only at alkaline pH in the low-sodium med ium, Illumination of the WT cells resulted in an extrusion followed by an uptake of protons, In contrast, only proton uptake was observed fo r the M29 mutant in the light without proton extrusion, There was no d ifference in sodium uptake activity between the CST and mutant, The mu tant still possessed 51% of the WT CO2 transport activity in the prese nce of 15 mM NaCl. On the basis of these results we concluded that cot A has a role in light-induced proton extrusion and that the inhibition of CO2 transport in the M29 mutant is a secondary effect of the inhib ition of proton extrusion.