Regulation of Expression of Genes for Three Nitrogenases in Azotobacter Vinelandii

Abstract

Three genetically distinct nitrogenases can be synthesized in A. vinelandii: conventional molybdenum nitrogenase encoded by nifHDK, a vanadium nitrogenase encoded by vnfHDGK, and a third enzyme which contains neither Mo nor V encoded by anfHDGK. A complex array of regulatory proteins controls expression of the three sets of nitrogenase structural genes. These include the three specific activators NIFA, VNFA, and ANFA; NIFL, which as in K. pneumoniae is necessary for ammonium repression of Mo nitrogenase genes; and NFRX which is required for expression from the nifH and anfH promoters. The nfrX gene has been found to be similar to an E. coli gene, glnD, which encodes uridylyl transferase. A model is presented showing the way these interacting regulatory products are thought to regulate synthesis of the three nitrogenases. Two other observations are of more practical significance. Firstly, the A. vinelandii nifL mutants excreted more than 5mM ammonium when fixing nitrogen. Secondly, Mo fully repressed the alternative nitrogenase genes at 30°, repressed much less at 20°, and not at all at 14°. This result suggests that A. vinelandii has the capacity to synthesize non-molybdenum nitrogenases regardless of environmental Mo content at temperate soil temperatures.