Thylakoid Membrane-Bound, NADPH-Specific Pyridine Nucleotide Dehydrogenase Complex Mediates Cyclic Electron Transport in the Cyanobacterium Synechocystis sp. PCC 6803

The donation of electrons from NADPH to the intersystem chain, as monitored by an increase in Chl fluorescence, occurred in the isolated thylakoid membranes of Synechocystis PCC 6803. The stimulation by NADPH of the methyl viologen-dependent photoreduction of dioxygen and of the reduction of P700+ after photooxidation in the presence of DCMU also confirmed the donation of electrons from NADPH to the electron carriers in the intersystem. These reactions were sensitive to rotenone, capsaicin, l-(2-thenoyl)-3,3,3-trifluoroacetone and HgCl2 but not to antimycin A or flavone. In contrast to the thylakoid membranes from the wild type, those from a mutant, designated M55, in which a gene of a subunit of the pyridine nucleotide dehydrogenase complex (NDH) had been inactivated, did not show evidence of such reactions. These results support our previous hypothesis that the transport of electrons from NADPH to the intersystem chain is mediated by NDH [Mi et al. (1994) Plant Cell Physiol. 35: 163] and indicate the binding of an NADPH-specific NDH to the thylakoid membranes. The Chl fluorescence was quenched transiently by addition of ferredoxin and NADP+ to the thylakoid membranes but showed a subsequent increase. This result suggests the reduction of plastoquinone by the photoreduced NADP+ and initiation of the NADPH-mediated cyclic flow of electrons around PSI. Furthermore, a similar response of Chl fluorescence was observed upon the addition of ferredoxin only, demonstrating the ferredoxin-dependent cyclic flow of electrons. Both pathways of cyclic electron transport were inhibited by rotenone, and were not detected in the NDH-defected thylakoid membranes from M55, indicating the participation of the NDH complex. These results confirm that, in Synechocystis, the thylakoid-bound NDH complex mediates the ferredoxin-dependent cyclic electron flow, as well as the NADPH-dependent cyclic electron flow.