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Reversible, NAD(P)+-reducing hydrogenase in Synechocystis sp. PCC 6803: Is there a structural and functional link to complex I and photosynthesis ? Jens Appel, Saranya Phunpruch and R ü diger Schulz Philipps-University, FB Biology/Botany, Karl-von-Frisch-Str., D-35032 Marburg, Germany Tel. +49-6421-282484, Fax +49-6421-282057, E-mail: mailto:schulzr@mailer.uni-marburg.de SUMMARY From the unicellular non-N2-fixing cyanobacterium Synechocystis sp. PCC 6803 we isolated and characterized a hox gene cluster encoding the large and the small subunit of a NAD(P)+-reducing NiFe-hydrogenase (reversible hydrogenase) and at least three additional polypeptides. Until now only 11 of the 14 polypeptides of the NADH-dehydrogenase were found in Synechocystis. By sequence homologies we suggested that the missing subunits of the peripheral part of the dehydrogenase, containing most of the FeS-clusters, are encoded by three ORFs of this gene-cluster. Since there are no other homologs of these subunits in the whole sequence of Synechocystis, which is now available in the internet, a joint use of these polypeptides as the diaphorase moiety of the complex I and the reversible hydrogenase seems obvious. Complex I of cyanobacteria is located at the interface of the photosynthetic and respiratory electron transport in the thylakoid membrane. Our working hypothesis is that the reversible hydrogenase could be an important controlling device of the redox conditions in the different membranes. It could for example function as an electron valve under high light intensities at the photosynthetic membrane. To prove this hypothesis, deletion mutants of the corresponding hox genes were generated. Besides confirmation of the suggested function of HoxH as the large subunit of the reversible hydrogenase, parts of the presented idea tested so far about the physiological role of the reversible hydrogenase in Synechocystis in relation to fermentation, respiration and photosynthesis were confirmed.