p H-Conditional, Ammonia Assimilation-Deficient Mutants of Hydrogenomonas eutropha : Evidence for the Nature of the Mutation

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Abstract
Two amination-deficient mutants of Hydrogenomonas eutropha , characterized by p H-dependent linear growth on non-amino acid substrates, were investigated to determine the exact nature of the mutation. Glutamate dehydrogenase, the only aminating enzyme found in wild-type cells, was present at similar levels in mutant cells. Phenylalanine and aspartate, which allowed normal growth of the mutants, could transaminate 2-oxoglutarate to glutamate, whereas alanine, which does not support normal growth, could not transfer its amino nitrogen to form glutamate. In H. eutropha , l -alanine is apparently synthesized by β-decarboxylation of aspartate. Studies with NH 4 + ions as the sole nitrogen source demonstrated that growth rates of the mutant strains were dependent on both extracellular p H and NH 4 + ion concentration. Comparison of these results revealed that the growth rate of mutant cultures was proportional to the concentration of extracellular NH 3 . Wild-type cultures were not dependent on extracellular NH 3 since exponential growth rates did not vary with p H or NH 4 + ion concentration. The results suggest that the mutant strains lack an NH 4 + ion transport system and consequently are dependent on NH 3 diffusion which does not support optimal amination rates. The significance of the findings for the amino acid metabolism of H. eutropha is discussed.

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