pH-Conditional, Ammonia Assimilation-Deficient Mutants of Hydrogenomonas eutropha: Evidence for the Nature of the Mutation1

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Two amination-deficient mutants of Hydrogenomonas eutropha, characterized by pH-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 NH4+ ions as the sole nitrogen source demonstrated that growth rates of the mutant strains were dependent on both extracellular pH and NH4+ ion concentration. Comparison of these results revealed that the growth rate of mutant cultures was proportional to the concentration of extracellular NH3. Wild-type cultures were not dependent on extracellular NH3 since exponential growth rates did not vary with pH or NH4+ ion concentration. The results suggest that the mutant strains lack an NH4+ ion transport system and consequently are dependent on NH3 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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