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The reversible hydrogenase from Anabaena 7120 appeared when O(2) was continuously removed from a growing culture. Activity increased further when cells were incubated under argon in the dark or in the light plus 3-(3,4-dichlorophenyl)-1,1-dimethylurea. Hydrogenase existed in an inactive state during periods of O(2) evolution. It could be reductively activated by exposure to reduced methyl viologen or by dark, anaerobic incubation. Hydrogenase-containing cells evolved H(2) slowly during dark anaerobic incubations, and the rate of H(2) evolution was increased by illumination with low intensity light. Light enhancement of H(2) evolution was of short duration and was eliminated by the ferredoxin antagonist disalicylidene diaminopropane. Physiological acceptors that supported H(2) uptake included NO(3) (-), NO(2) (-), and HSO(3) (-), and light had a slight influence on the rate of H(2) uptake with these acceptors. Low levels of O(2) supported H(2) uptake, but higher concentrations of O(2) inactivated the hydrogenase. Hydrogen uptake with HCO(3) (-) as acceptor was the most rapid reaction measured, and it was strictly light-dependent. It occurred only at low light intensities, and higher light intensities restored normal O(2)-evolving photosynthesis. It is suggested that hydrogenase is present to capture exogenous H(2) as a source of reducing equivalents during growth in anaerobic environments.

Physiological Reactions of the Reversible Hydrogenase from Anabaena 7120