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The discovery of reversible hydrogenase genes in Synechocystis PCC 6803 [Appel & Schulz (1996) Biochem Biophys Acta 1298:141-147] was followed by a proposal that the diaphorase part of the enzyme could be shared both by respiratory complex I and hydrogenase [Appel & Schulz; Schmitz and Bothe (1996) Naturwiss 83:525]. During the Vienna meeting some important contributions were explicitly focused on this topic.
Crispin Howitt and Wim Vermaas (Arizona State U.) obtained hox- mutants in Synechocystis PCC 6803 that show almost the same oxygen consumption rates under photomixotrophic (PM) conditions as the wild type. Their conclusion was that HoxE, HoxF and HoxU polypeptides do not act as the NAD(P)H binding/oxidising module of NDH-1, the nature of which remains to be elucidated. Consistent with this, Hermann Bothe (U. Köln) presented evidence showing that a hoxU mutant performs respiratory oxygen-uptake unimpaired.
Jens Appel and Rüdiger Schulz (U. Marburg) put forward the hypothesis that during state transition the reversible hydrogenase could function as an electron valve by producing hydrogen, because autotrophic cultures permanently show a low hydrogenase activity. Genetic analysis of hoxE, hoxF, and hoxU mutants as well as their physiological and biochemical characterization are in progress to see if these genes, which show strong sequence similarities to genes of bacterial and mitochondrial NADH dehydrogeases, are really involved in NDH-1 in cyanobacteria.
In my opinion, the involvement of hydrogenase (as a whole or via one/more of its components) in either photosynthetic and/or respiratory electron transport is still an open question, as well as the biological significance of such an involvement. The reversible hydrogenase could serve as one more in a growing list of redox proteins involved in electron flow around PSI in Synechocystis PCC 6803 under normal or stressed conditions (light, anaerobiosis, salt, temperature, etc.), a topic abundently illustrated during the meeting.