Molecular analysis of the Azotobacter vinelandii glnA gene encoding glutamine synthetase

The gene encoding glutamine synthetase (GS), glnA, was cloned from Azotobacter vinelandii on a 6-kb EcoRI fragment that also carries the ntrBC genes. The DNA sequence of 1,952 bp including the GS-coding region was determined. An open reading frame of 467 amino acids indicated a gene product of Mr 51,747. Transcription of glnA occurred from a C residue located 32 bases upstream of an ATG considered to be the initiator codon because (i) it had a nearby potential ribosome-binding site and (ii) an open reading frame translated from this site indicated good N-terminal homology to 10 other procaryotic GSs. Sequences similar to the consensus RNA polymerase recognition sites at -10 and -35 were present at the appropriate distance upstream of the transcription initiation site. As expected from earlier genetic studies indicating that expression of A. vinelandii glnA did not depend on the rpoN (ntrA; sigma 54) gene product, no sigma 54 recognition sequences were present, nor was there significant regulation of glnA expression by fixed nitrogen. Repeated attempts to construct glutamine auxotrophs by recombination of glnA insertion mutations were unsuccessful, Although the mutated DNA could be found by hybridization experiments in drug-resistant A. vinelandii transformants, the wild-type glnA region was always present. These results suggest that glnA mutations are lethal in A. vinelandii. In [14C]glutamine uptake experiments, very little glutamine was incorporated into cells, suggesting that glutamine auxotrophs are nonviable because they cannot be supplied with sufficient glutamine to support growth.