Glutamine synthetase in marine algae: New surprises from an old enzyme

Glutamine synthetase (GS), which catalyzes the formation of glutamine from ammonium and glutamate in the presence of ATP, is encoded by three distinct gene families: GSI, GSII, and GSIII. Genes encoding GSI are found in the B acteria and Archaea, whereas GSII genes are found in eukaryotes and a few s pecies of Bacteria. Members of the third family, GSIII, have been described from a limited number of bacteria; however, recent biochemical and molecul ar data suggest that this type of enzyme is broadly distributed among the a lgae. Peptide fragments obtained from GS purified from the marine diatom Sk eletonema costatum (Greville) Cleve are 77% identical to a partial sequence of GSIII from Chaetoceros compressum Lauder, which permits the unambiguous assignment of the biochemically characterized enzyme to the GSIII gene fam ily. The N-terminal sequence was 43% identical to the GSIII-like enzyme pur ified from the haptophyte Emiliania huxleyi (Lohm.) Hay et Miller and sever al residues were conserved among bacterial and eukaryotic GSIII enzymes. Th e presence of genes encoding GSIII in diatoms and haptophytes indicates tha t this enzyme family is more broadly distributed in eukaryotes than previou sly suspected.