The bidirectional hydrogenase of Synechocystis sp. PCC 6803 works as an electron valve during photosynthesis

Abstract

The activity of the bidirectional hydrogenase of the cyanobacterium Synechocystis sp. PCC 6803 was found not to be regulated in parallel to respiration but to photosynthesis. A mutant with a deletion in the large hydrogenase subunit gene (hoxH), which contains the active site, was impaired in the oxidation of photosystem I (PSI) when illuminated with light, which excites either PSI alone or both photosystems. The fluorescence of photosystem II (PSII) of this mutant was higher than that of wild-type cells. The transcript level of the photosynthetic genes psbA, psaA and petB was found to be different in the hydrogenase-free mutant cells compared to wild-type cells, which indicates that the hydrogenase has an effect on the regulation of these genes. Collectively, these results suggest that the bidirectional hydrogenase functions as a valve for low-potential electrons generated during the light reaction of photosynthesis, thus preventing a slowing down of electron transport. This conclusion is supported by growth curves demonstrating that the mutant cells need more time to adapt to changing light intensities. Investigations of the wild-type and deltahoxH strains strongly suggest that Synechocystis contains only the bidirectional hydrogenase, which seems to be essentially insensitive to oxygen.

Identification of Novel Ssl0352 Protein (NdhS), Essential for Efficient Operation of Cyclic Electron Transport around Photosystem I, in NADPH:plastoquinone Oxidoreductase (NDH-1) Complexes of Synechocystis sp. PCC 6803

“…P700 ϩ reduction was monitored as an absorption change at 820 nm as described by Appel et al (24). The first order kinetics of P700 ϩ rereduction was determined in the dark after illumination of cells with 26.5 watts m Ϫ2 FR with a maximum at 715 nm.…”

Induction of Photosynthetic Carbon Fixation in Anoxia Relies on Hydrogenase Activity and Proton-Gradient Regulation-Like1-Mediated Cyclic Electron Flow in Chlamydomonas reinhardtii

“…At light onset after a prolonged period in dark anoxic conditions, the photosynthetic electron flow is mainly a LEF toward hydrogenase , and lack of hydrogenase activity in hydrogenase maturation factor EF (hydEF) mutant strain deficient in hydrogenases maturation (Posewitz et al, 2004) induces a lag in induction of PSII activity . In cyanobacteria, the bidirectional Ni-Fe hydrogenase might also work as an electron valve for disposal of electrons generated at the onset of illumination of cells (Cournac et al, 2004) or when excess electrons are generated during photosynthesis, preventing the slowing of the electron transport chain under stress conditions (Appel et al, 2000;Carrieri et al, 2011). The bidirectional Ni-Fe hydrogenase could also dispose of excess of reducing equivalents during fermentation in dark anaerobic conditions, helping to generate ATP and maintaining homeostasis (Barz et al, 2010).…”

Rewiring hydrogenase-dependent redox circuits in cyanobacteria

“…Native cyanobacterial hydrogenases are predominantly thought to function in the recapture of hydrogen that is created as a byproduct of nitrogenase activity within nitrogen-fixing microbes (3). Hydrogenases may further function to provide a terminal electron sink in the absence of oxygen or during changes in flux through the photosynthetic machinery (4,5).…”