The expression and activity of type-1 NAD(P)H dehydrogenase (NDH-1) was compared between cells of Synechocystis PCC6803 grown in high (H-cells) and low (L-cells) CO 2 conditions. Western analysis indicated that L-cells contain higher amounts of the NDH-1 subunits, NdhH, NdhI and NdhK. An NADPH-specific subcomplex of NDH-1 showed higher NADPH-nitroblue tetrazolium oxidoreductase activity in L-cells. The activities of both NADPHmenadione oxidoreductase and light-dependent NADPH oxidation driven by photosystem I were much higher in Lcells than in H-cells. The initial rate of re-reduction of P700+ following actinic light illumination in the presence of DCMU under background far-red light was enhanced in Lcells. In addition, rotenone, a specific inhibitor of NDH-1, suppressed the relative rate of post-illumination increase in Chl fluorescence of L-cells more than that of H-cells, suggesting that the involvement of NDH-1 in cyclic electron flow around photosystem I was enhanced by low CO 2 . Taken together, these results suggest that NDH-1 complex and NDH-1-mediated cyclic electron transport are stimulated by low CO 2 and function in the acclimation of cyanobacteria to low CO 2 .

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