Requirement of nifV gene for production of wild-type nitrogenase enzyme in Klebsiella pneumoniae

Abstract

Klebsiella pneumoniae nitrogenase is a complex enzyme consisting of two component proteins, the tetrameric MoFe protein (Kp1) and the dimeric Fe protein (Kp2)¹. Together these catalyse the ATP-dependent reduction of N₂ and of several triple-bonded substrates such as C₂H₂, CN⁻, N⁻₃ and CH₃NC (refs 2, 3). In the absence of substrate, protons are reduced to H₂ and all substrates compete with protons for reduction. Carbon monoxide inhibits all reductions except H₂ evolution⁴. Seventeen genes are required for the synthesis of nitrogenase and expression of its activity in K. pneumoniae^5–8, some of which are believed to be involved in processing of the nitrogenase proteins during synthesis^9,10. Here we describe studies of the nitrogenase of bacteria mutated at one of these genes—nifV. These mutants produce nitrogenase which does not reduce N₂ in vivo, although it does in vitro in conditions of high electron flux. We show that the defect is in the Kp1 component, and suggest that the nifV gene product modifies the Kp1 component so that N₂ reduction will occur at the lower electron fluxes present in vivo, and that this might have been involved in the possible evolutionary development of the nitrogenase from a cyanide-detoxifying enzyme.