Effect of Amino Acids on the Nitrogenase System of Klebsiella pneumoniae

Abstract

The effect of exogenous amino-acids and the free amino-acid pool on the synthesis of the nitrogenase system of K. pneumoniae M5al (formerly Aerobacter aerogenes M5al) was investigated. When an actively N2 -fixing culture was used to inoculate a medium containing a limiting concentration of NH4+, an induction lag period was observed. When either a single amino-acid or a mixture of amino-acids was substituted at the same nitrogen concentration, growth was uninterrupted by the induction period. It appears that a step or steps in the formation of the nitrogenase system are repressed by NH4 and are not affected by amino-acid N. The amino acids, far from repressing formation of nitrogenase as does NH4, actually stimulate its formation. It appears that both free and amino nitrogen are used simultaneously. The amino-acids that served concomitantly with N2 as a source of nitrogen were: aspartic acid, serine, threonine, leucine and histidine. Of these amino-acids, it was shown that aspartic acid is readily taken up by the cells. Of the amino-acids not serving as an immediate nitrogen source, isoleucine is not taken up by the cells. The free amino-acid pool of the cells was measured at the onset and termination of the induction period. Ninhydrin-positive material in the amino-acid pool was depleted by 35% during the induction period.