Rhizobium japonicum mutants unable to use hydrogen

Abstract

FACTORS which limit nitrogen fixation by the legume nodule symbiosis may be environmental (such as illumination, water or CO₂ supply), or intrinsic (such as supply of photosynthate to the nodule¹, effectiveness of rhizobial strain). An important intrinsic feature seems to be ATP-dependent hydrogen evolution by nitrogenase, which leads to loss of energy that would otherwise be available for nitrogen fixation². Some strains of Rhizobium japonicum possess a unidirectional hydrogenase, capable of hydrogen oxidation but not hydrogen evolution, which recycles the hydrogen evolved by nitrogenase, regenerating ATP as well as protecting the nitrogenase from oxidative damage^3–5. The physiological importance of hydrogenase in this context, with its obvious implications for agriculture, has so far been deduced from studies on naturally occurring strains possessing or lacking hydrogenase^2,6. A more accurate assessment requires the isolation of isogeneic hydrogen uptake-negative (Hup⁻) mutants from hydrogenase-positive (Hup⁺) parents, and their comparison in physiological and agronomic studies. Such mutants would also be valuable for gene transfer and biochemical studies. We now report the isolation of Hup⁻ mutants.