Microbial production of ammonium ion from nitrogen.

Genetic manipulation of nitrogenase and key glutamate-forming enzymes can provide mutants that excrete fixed N2 as NH4+. A derepressed N2 fxation mutant (SK-24) has been isolated , which excretes up to 20.2 mumol of fixed N2 as NH4+ per mg of cell protein in 24 hr at room temperature. Biochemical analysis shows that this mutant, which requires glutamate for growth, releases fixed N2 as NH4+ into the environment because of (i) constitutive synthesis of nitrogenase and (ii) genetic blocks resulting in losses of glutamate synthase [L-glutamine:2-oxoglutarate aminotransferase (NADPH oxidizing), EC 2.6.1.53] and glutamate dehydrogenase [L-glutamate:NADP oxidoreductase (deaminating), EC 1.4.1.4] activities, enzymes essential for NH4+ assimilation into cell material. The parent strain (asm-1), missing only glutamate synthase activity, also actively excretes NH4+ during early phases of its growth but eventually reutilizes the NN4+. A miximum yield of 4.0 mumol of NH4+/ml per 24 hr has been noted for asm-1 only during the growth period. Biosynthesis of NH4+ PROCEEDS AT THE EXPENSE OF A Variety of fermentable sugars, such as sucrose or glucose, with a maximum energy conversion efficiency of about 5 glucose degraded per NH4+ formed. The use of microbes for production of NH4+ fertilizer is discussed.