Net aerobic H2 production and the induction of uptake hydrogenase activity in the nitrogenfixing Anabaena strains CA and 1F were strictly dependent upon the Ni concentration in the growth medium. Ni concentrations as low as 10 nM blocked H2 production and stimulated an uptake hydrogenase activity in whole cells. Two types of uptake hydrogenase activity were seen: a dark aerobic uptake approximately 30% as active as the H2 production rate, and a lightdependent activity in the presence of DCMU [3-(3,4-dichlorophenyl)-l, 1-dimethylurea] at low oxygen concentrations, amounting to about 50% of the H2 production rate. Together these activities may account for the strong control of net aerobic H2 production by nickel. A significant fraction of the Ni-stimulated uptake hydrogenase activity formed during the transition from nickel deficiency to nickel sufficiency was blocked by chloramphenicol. Nickel may be required for activation of an uptake hydrogenase, or for hydrogenase synthesis, or for synthesis of another protein which is involved in H uptake.


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