Ethane formation from acetylene as a potential test for vanadium nitrogenase in vivo

Abstract

Azotobacter chroococcum and A. vinelandii, both obligately aerobic diazotrophic bacteria, have at least two genetically distinct systems for nitrogen fixation1,2. One is the well-characterized molybdenum nitrogenase, whereas in a second system (the vanadium nitrogenase) the conventional molybdoprotein is replaced by a vanadoprotein and a second iron protein replaces the one normally found with the molybdoprotein3–5. A third system may exist in A. vinelandii6'7. Reiterations of DNA encoding nitrogenase structural genes (nifHDK) are found in several genera of diazotrophs (see ref. 1). In the azotobacters there is evidence, based on physiological and biochemical properties of strains carrying deletions of nifHDK, that the reiterated genes are involved in the synthesis of alternative nitrogenases including the vanadium (V)-nitrogenase. The screening of other genera for the presence of a functional V-nitrogenase would be greatly facilitated by a specific test for it which could be applied in vivo. The reduction of acetylene to ethylene8,9 by the molybdenum (Mo)-nitrogenase has been widely used in ecological, genetic and physiological studies of biological nitrogen fixation. We demonstrate here that purified V-nitrogenase catalyses the reduction of acetylene not only to ethylene, but also to ethane, which the Mo-nitrogenase does not10. This property distinguishes between these two systems in A. chroococcum and A. vinelandii in vivo. Ethane formation also occurred in cultures of Clostridium pasteurianum W5, suggesting that V-nitrogenase is not restricted to the Azotobacteriaceae.