The electron transfer pathway upon H₂oxidation by the NiFe bidirectional hydrogenase of Synechocystis sp. PCC 6803 in the light shares components with the photosynthetic electron transfer chain in thylakoid membranes

In anaerobic conditions the NiFe hydrogenase in the cyanobacterium Synechocystis sp. PCC 6803 catalyzes transient H₂production upon a darkness-to-light transition, followed by a rapid H₂uptake. We measured H₂uptake in Synechocystis mutants lacking photosystem I, photosystem II or terminal oxidases and in the wild-type strain with and without active cytochrome b₆f. Rapid light-induced H₂uptake was dependent on cytochrome b₆f and the presence of photosystem I. We propose light-dependent electron transport from H₂to plastoquinone, probably via NAD(P)H dehydrogenase, and on to cytochrome b₆f and photosystem I. In darkness H₂uptake is ∼10-fold slower than in the light and is independent of thylakoid redox components. The plastoquinone redox state may be key in determining the ultimate H₂redox partner. H₂uptake and production in darkness likely use the same redox partners. NADH and NADPH, but not reduced ferredoxin, were confirmed as hydrogenase redox donors in vitro.