ASSIMILATORY NITRATE REDUCTASE OF SOME SOIL BACTERIA

Abstract

To elucidate the nature of bacterial assimilatory nitrate reductase, we attempted to use bacteria which contain assimilatory but not dissimilatory nitrate reductase. Four strains of bacteria (M-1, M-2, M-3 and J-1), isolated in this study, and Nocardia globerula IFO 13509 had such characteristics; they grew in a medium containing nitrate as the sole nitrogen source and did not accumulate nitrite, ammonia or molecular nitrogen during growth in nitrate synthetic or nitrate complex media with or without aeration. Strains M-1, M-2 and M-3 are Bacillus spp., which are either identical to or closely resemble B. megaterium, and strain J-1 is a Pseudomonas sp. which resembles P. hydrogenovora. Resting cells of these bacteria assimilated nitrate in the presence of glucose or pyruvate without accumulation of nitrite. Assimilatory nitrate reductase in cell-free extracts from the bacteria reduced nitrate if NADPH or NADH was added as an electron donor. Both NADPH and NADH served as the donor for the enzymes from M-1 and M-2, while only NADPH was the effective donor for the enzymes from M-3, J-1 and N. globerula. All the enzymes from these bacteria were predominantly localized in the soluble cytoplasmic fraction. Effects of inhibitors, such as KClO₃, NaN₃, NH₂OH and p-chloromercuri benzoate, were variable when one of the inhibitors was tested for the enzymes from different strains, but inhibition was similar when tested on the enzyme from the same strain (M-1 or M-2) with either NADPH or NADH. The enzymes from M-1, M-3 and J-1 were inhibited by 1mM KClO₃, but the enzymes from M-2 and N. globerula were not inhibited; the enzyme from M-2 was not inhibited by 1mM NaN₃, but the enzymes from the other strains were inhibited.