Abstract
The cyanobacterium Synechocystis sp. PCC 6803 has a bidirectional [NiFe]-hydrogenase (Hox hydrogenase) which reversibly reduces protons to H2. This enzyme is composed of a hydrogenase domain and a diaphorase moiety, which is distinctly homologous to the NADH input module of mitochondrial respiratory Complex I. Hox hydrogenase physiological function is still unclear, since it is not required for Synechocystis fitness under standard growth conditions. We analyzed the phenotype under prolonged darkness of three Synechocystis knock-out strains, lacking either Hox hydrogenase (ΔHoxE-H) or one of the proteins responsible for the assembly of its NiFe active site (ΔHypA1 and ΔHypB1). We found that Hox hydrogenase is required for Synechocystis growth under this condition, regardless of the functional status of its catalytic site, suggesting an additional role beside hydrogen metabolism. Moreover, quantitative proteomic analyses revealed that the expression levels of several subunits of the respiratory NADPH/plastoquinone oxidoreductase (NDH-1) are reduced when Synechocystis is grown in the dark. Our findings suggest that the Hox hydrogenase could contribute to electron transport regulation when both photosynthetic and respiratory pathways are down-regulated, and provide a possible explanation for the close evolutionary relationship between mitochondrial respiratory Complex I and cyanobacterial [NiFe]-hydrogenases.
Highlights
Hydrogen metabolism is one of the most ancient and crucial processes of life
All Synechocystis mutant strains lacking one or more of the hox genes from the hoxE-H operon show no significant differences from the wild type strain. This observation raises the question of what exactly is the role of the Hox hydrogenase. This enzyme is constitutively expressed under both aerobic and anaerobic conditions, suggesting that it likely provides an evolutionary advantage that would manifest under non-standard growth conditions, and that it could have an additional role besides H2 metabolism
It is well known that some cyanobacterial communities can be exposed and survive to natural environments where regular prolonged periods of complete darkness occur, for example in lake sediments, soil water or in dense aquatic accumulations produced at the surface
Summary
Hydrogen metabolism is one of the most ancient and crucial processes of life. Today, it is at the center of increasing attention in the context of bioenergy production technologies, that will be of critical importance in the near future. The precise physiological function of the Hox bidirectional hydrogenase in Synechocystis (and in cyanobacteria in general) is still under debate[8] It is unknown what role this protein can have in oxygen-rich environments; the enzyme is active only under anoxic or micro-oxic conditions as it is temporary inactivated by molecular oxygen (O2)[1]. The hox and hyp genes are constitutively expressed in Synechocystis under both anaerobic and aerobic conditions[17,18], even if the Hox hydrogenase is active only under strict anaerobiosis. This supports the hypothesis that this protein could have an additional function in oxygen-rich environments. Our results indicate that the Hox hydrogenase could be involved in the adaptive response of Synechocystis to prolonged darkness, a harsh growth condition frequently occurring in natural environments
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