Activation of nif gene expression in Azotobacter by the nifA gene product of Klebsiella pneumoniae.

Structural similarity of nitrogenase, the enzyme responsible for biological nitrogen fixation, from various diazotrophic bacteria has been shown by intergeneric biochemical complementation of component proteins in vitro, DNA and protein sequence analysis, and DNA hybridization between nif (nitrogen fixation) structural genes from Klebsiella pneumoniae and genomic sequences from other nitrogen-fixing bacteria. Despite nitrogenase homologies, little is known about similarities among nif regulatory mechanisms although repression of nitrogenase synthesis by NH4+ and O2 occurs in most diazotrophs. In K. pneumoniae, the ntr (gln) genes concerned with regulation of nitrogen metabolism control expression of the nifLA operon whose products act as repressor (nifL) and activator (nifA) of the seven other nif transcriptional units. Here we report that the nifA gene product of K. pneumoniae can activate expression of nif genes in both Azotobacter vinelandii and Azotobacter chroococcum, organisms whose aerobic physiology contrasts with that of the facultative K. pneumoniae.