Regulation of nitrogenase-2 in Azotobacter vinelandii by ammonium, molybdenum, and vanadium

Under diazotrophic conditions in the absence of molybdenum and in the presence of vanadium, Azotobacter vinelandii reduces N2 to NH4+ by using nitrogenase-2, a V-containing enzyme complex encoded by vnfH (the gene for dinitrogenase reductase-2), and vnfDGK (the genes for dinitrogenase-2 subunits). Accumulation of the vnfHorfFd and vnfDGK transcripts occurred under Mo-deficient conditions in the presence and absence of V; however, in the case of vnfDGK, the protein products only accumulated in the presence of V. This suggests that V is required for translation of the vnfDGK transcripts. In addition, expression of vnfH-lacZ and vnfD-lacZ transcriptional fusions was only partially repressed in the presence of NH4+. Transcripts hybridizing with vnfH (1.4 and 1.0 kb), vnfDG (3.4 and 1.8 kb), and vnfK (3.4 kb) were detected in RNA extracted from wild-type cells cultured with NH4+ in the presence or absence of V. However, nitrogenase-2 subunits were not detected in extracts of cells derepressed for nitrogenase-2 in the presence of NH4+. These results indicate that this nitrogen source acts at the posttranscriptional level as well as at the transcriptional level. vnf transcripts were not detected in the presence of Mo (with or without NH4+).