Physiological Reactions of the Reversible Hydrogenase from Anabaena 7120

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Abstract
The reversible hydrogenase from Anabaena 7120 appeared when O2 was continuously removed from a growing culture. Activity increased further when cells were incubated under Ar in the dark or in the light plus 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Hydrogenase existed in an inactive state during periods of O2 evolution. It could be reductively activated by exposure to reduced methyl viologen or by dark, anaerobic incubation. Hydrogenase-containing cells evolved H2 slowly during dark anaerobic incubations, and the rate of H2 evolution was increased by illumination with low intensity light. Light enhancement of H2 evolution was of short duration and was eliminated by the ferredoxin antagonist disalicylidene diaminopropane. Physiological acceptors that supported H2 uptake included NO3-, NO2- and HSO3-, and light had a slight influence on the rate of H2 uptake with these acceptors. Low levels of O2 supported H2 uptake, but higher concentrations of O2 activated the hydrogenase. Hydrogen uptake with HCO3- as acceptor was the most rapid reaction measured, and was strictly light-dependent. It occurred only at low light intensities, and higher light intensities restored normal O2-evolving photosynthesis. Evidently, hydrogenase is present to capture exogenous H2 as a source of reducing equivalents during growth in anaerobic environments.