OBSERVATIONS ON THE EFFECT OF AMINO ACIDS ON THE GROWTH INITIATION OF RHIZOBIUM MELILOTI, WITH SPECIAL REFERENCE TO THE SYNTHESIS OF ALANINE FROM PYRUVATE AND AMMONIUM IONS

Washed cells of Rhizobium meliloti were capable of forming pyruvate from glucose and, in addition, washed cells and sonic extracts possessed a reversible alanine dehydrogenase, capable of forming alanine from pyruvate and NH4+. This synthesis of alanine was optimum at an alkaline pH and at a substrate concentration of 0.025 M and was stimulated by diphosphopyridine nucleotide but not by triphosphopyridine nucleotide. Sonic extracts in the presence of NH4+ also formed glutamate from α-ketoglutarate and aspartate from fumarate. Nevertheless, washed cells did not initiate growth in a 0.25% carbohydrate medium, containing NH4Cl, unless amino acids were present. These requisite acids either could be supplied in the medium, or the cells could be forced to synthesize them by addition to the medium of increased levels of certain compounds, such as 0.9% glucose, from which NH4+-accepting compounds could be produced. If the stimulative effect of amino acids in low-carbohydrate media were a result of an increase in the accumulation of such NH4+-acceptors such an accumulation did not apparently result from increased carbohydrate oxidation or decreased "oxidative" assimilation, since NH4+ and α-dinitrophenol, which do not initiate growth, were more active, respectively, in these two latter aspects than the amino acid (histidine) tested. On the basis of several considerations it is hypothesized that the primary effect of the growth-initiating amino acid may be directed toward the synthesis of a labile protein, intimately connected with growth, which is destroyed in resting cells.