Regulation of nitrogen metabolism in Azotobacter vinelandii: isolation of ntr and glnA genes and construction of ntr mutants.

The ntrA, ntrB and ntrC products are responsible for regulating the transcription of many genes involved in the assimilation of poor nitrogen sources in enteric bacteria. The presence of a similar system in the non-enteric bacterium Azotobacter vinelandii is reported here. Genes analogous to ntrA and ntrC were isolated from an A. vinelandii gene library by complementation of Escherichia coli mutants. The gene encoding glutamine synthetase, glnA, was also isolated and found to be adjacent to ntrC but distant from ntrA, as it is in enteric organisms. The cloned Azotobacter genes also complemented Klebsiella pneumoniae mutants and hybridized to K. pneumoniae ntrA, ntrC and glnA gene probes. The role of ntrA and ntrC in A. vinelandii was established by using Tn5 insertions in the cloned genes to construct mutants by marker exchange. These mutants show that both ntrA and ntrC are required for the utilization of nitrate as a nitrogen source. However, ntrC is not required for nitrogen fixation by A. vinelandii, in contrast with K. pneumoniae where both ntrA and ntrC are essential.