Assimilation of Inorganic Nitrogen Compounds by Nitrobacter agilis

Abstract

Nitrobacter agilis, a chemolithotrophic bacterium, utilizes ammonia as well as nitrite as a nitrogen source for growth. The growth yields were increased about twofold by growing the bacterium in a nitrite medium supplemented with 2 mm-ammonium chloride. Higher concentrations of ammonium chloride, however, competitively inhibited nitrite oxidation and growth of the bacterium. Washed cells readily incorporated 15NH4 +, 15NH2OH, 15NO2 − and 15NO3 − respectively (in decreasing order) into cell nitrogen. Enzyme activities in cell extracts of ammonia-supplemented cultures, compared to those without ammonia, were greater for glutamate dehydrogenase (EC 1.4.1.2) and similar for glutamine synthetase (EC 6.3.1.2), whereas glutamate synthase (EC 1.4.1.14) was barely detectable. Inhibitors of glutamine synthetase (l-methionine dl-sulphoximine) and glutamate synthase (azaserine) did not affect the incorporation of 15NH4 − and 15NO2 − into cell nitrogen of washed cells. The results indicate that glutamate dehydrogenase is a key enzyme in N. agilis for the assimilation of either nitrite or ammonia. Glutamine synthetase, which was also active in cell extracts, is probably required for the production of glutamine. Glutamate synthase, however, does not appear to be an important enzyme for ammonia assimilation.