Effect of molecular hydrogen on histidine utilization by Alcaligenes eutrophus

The effect of molecular hydrogen on heterotrophic metabolism of the facultative chemolithoautotrophic bacterium Alcaligenes eutrophus strain H 16 was representatively investigated on histidine utilization. The presence of hydrogen in a histidine or urocanate-containing medium had two effects (i) growth of the cells was inhibited, and (ii) formation of histidase was repressed. Both effects were relieved by supplying the cells with exogenous carbon dioxide. Studies on mutants defective in chemolithoautotrophic metabolism revealed that growth inhibition by hydrogen was exclusively mediated by the catalytic function of the soluble hydrogenase. Mutants containing only particulate hydrogenase activity did not exhibit growth inhibition. Repression of histidase formation, however, was mediated by the catalytic activity of the soluble as well as the particulate hydrogenase. Unexpectedly, mutants defective in autotrophic carbon dioxide fixation but unaffected in hydrogen oxidation showed an inhibition of growth by hydrogen but no repression of histidase synthesis. Mutants which formed histidase constitutively were still sensitive to repression in the presence of hydrogen. The results indicate that repression of enzyme synthesis by hydrogen is dependent on the function of both, the hydrogen-oxidizing and the carbon dioxide-fixing system. It is concluded that the hydrogen effect is a transient regulatory mechanism and only relevant for unbalanced conditions of growth.