The glnA gene of the cyanobacterium Agmenellum quadruplicatum PR-6 (Synechococcus sp. strain PCC 7002) was isolated by complementing an Escherichia coli strain auxotrophic for glutamine (YMC11) with a PR-6 cosmid library. PR-6 glnA is a single-copy gene that encodes a deduced amino acid sequence that is highly homologous to the deduced glnA amino acid sequences reported for other bacteria. No homology was found between the PR-6 glnA flanking sequences and the ntrB, ntrC, or glnB genes of other bacteria. Northern (RNA) and primer extension analyses of PR-6 RNA revealed one predominant and several minor glnA transcripts of about 1.5 to 1.7 kb. The steady-state amounts of these transcripts increased three- to fivefold when the cells were starved for nitrogen. However, we found that mutant PR-6 cells lacking glnA were still able to use nitrate or ammonium as a sole nitrogen source. Although no RNA homologous to an internal fragment of the glnA gene could be detected in the mutant cells, they retained about 60% of wild-type glutamine biosynthetic activity. The mutant cells were more sensitive than the wild-type cells to methionine sulfoximine, a transition state analog of glutamate, a result that might indicate the presence of an additional glutamine synthetase; however, cell extracts of wild-type PR-6 cells and those lacking glnA were both able to use carbamyl phosphate instead of ammonium as a nitrogen donor for the synthesis of glutamine, a result that indicates the use of carbamyl phosphate synthetase to assimilate ammonium and produce glutamine.