Sequences of seven tryptic peptides derived from Saccharomyces cerevisiae glutamine synthetase have been determined. The amino terminus of yeast enzyme is acetylated and has the following sequence: acetyl-Ala-Glu-Ala-Ser-Ile-Glu-Lys. Neither higher eukaryotic nor bacterial glutamine synthetase contain sequences homologous to this yeast amino terminus. 8-Azidoadenosine 5'-triphosphate [( alpha-32P]8-N3ATP) has been used to photolabel the ATP-binding site in yeast glutamine synthetase. Only one 32P-labeled tryptic peptide was obtained as a major fraction and its sequence is Glu-Gly-Tyr-Gly-X-Phe-Glu-Asp-Arg. Similar photolabeling experiments with bovine glutamine synthetase yielded a tryptic peptide whose sequence is Gly-X-Phe-Glu-Asp-Arg, where X is likely Tyr covalently attached by nitrene derived from [alpha-32P]8-N3ADP. Sequences very homologous to this nucleotide-binding site can be found in other eukaryotic enzymes but not in prokaryotic enzymes. In addition, the sequences of two cysteine-containing peptides and three other tryptic peptides were established. Sequences homologous to all these five peptides can be found in mammalian and plant enzymes. The homology between yeast and higher eukaryotic glutamine synthetases was sufficiently strong to suggest that the overall tertiary and quaternary structures of these enzymes must be similar. The sequences presented here, particularly the amino terminus sequence will be valuable in identifying the structural gene of yeast glutamine synthetase, thereby making it possible to study its transcriptional regulation. In addition, the sequences of the cysteine-containing peptides will be useful in determining whether or not the covalent modification of a sulfhydryl group(s) is responsible for the modulation of glutamine synthetase activity.