Nitrogen Fixation (NIF) Regulatory Mutants of Klebsiella: Determination of the Energy Cost of N₂ Fixation In Vivo

Abstract

Experiments carried out in a number of laboratories have established that biological nitrogen fixation requires a large input of metabolic energy (see Burns & Hardy, 1975; Winter and Burris, 1976; Zumft & Mortenson, 1975). Evidence has been presented by Hardy & Havelka (1973) that the supply of energy may often be a rate-limiting step in symbiotic N₂ fixation. It has, however, proved difficult to determine exactly how much energy is required. In vitro experiments with nitrogenase from various organisms have established that reduction of N₂ or alternate substrates requires ATP in addition to a suitable reductant. Most recent in vitro studies have reported a minimal requirement of 12–15 moles ATP per mole N₂ reduced to 2NH ⁺₄ , but the values vary considerably with the experimental conditions (see Burns & Hardy, 1975; Winter and Burris, 1976; Zumft & Mortenson, 1975).