Role of uptake hydrogenase in providing reductant for nitrogenase in Rhizobium leguminosarum bacteroids

Abstract

The role of uptake hydrogenase in providing reducing power to nitrogenase was investigated in Rhizobium leguminosarum bacteroids from nodules of Pisum sativum L. (cv. Homesteader). H 2 increased the rate of C 2H 2 reduction in the absence of added substrates. Malate also increased nitrogenase (C 2H 2) activity while decreasing the effect of H 2. At exogenous malate concentrations above 0.05 mM no effect of H 2 was seen. Malate appeared to be more important as a source of reductant than of ATP. When iodoacetate was used to minimize the contribution of endogenous substrates to nitrogenase activity in an isolate in which H 2 uptake was not coupled to ATP formation, H 2 increased the rate of C 2H 2 reduction by 77%. In the presence of iodoacetate, an ATP-generating system did not enhance C 2H 2 reduction, but when H 2 was also included, the rate of C 2H 2 reduction was increased by 280% over that with the ATP-generating system alone. The data suggest that, under conditions of substrate starvation, the uptake hydrogenase in R. leguminosarum could provide reductant as well as ATP in an isolate in which the H 2 uptake is coupled to ATP formation, to the nitrogenase complex.