ISOFORMS OF GLUTAMINE-SYNTHETASE IN THE MARINE COCCOLITHOPHORID EMILIANIA-HUXLEYI (PRYMNESIOPHYCEAE)

Two isoenzymes of glutamine synthetase (EC 6.3.1.2), GS(1) and GS(2), have been purified from cells of Emiliania huxleyi using Cibacron blue dye ligand chromatography and gel filtration, separated by ion-exchan ge chromatography on Mono a and partly characterized. Each enzyme is a homohexamer with a molecular mass of 402 kDa for GS(1) and 501 kDa fo r GS(2). The molecular mass of the subunits of GS(1) and GS(2) was est imated to be 61 and 78 kDa, respectively. As in higher plants, GS(1) i s slightly more thermostable than GS(2) and much less stimulated by th iols than GS(2). For these reasons, GS(1) was designated as the cytoso lic enzyme and GS(2) as the chloroplastic one. Although the K(m)s for NH2OH are about the same, GS(2) possesses a much higher affinity for g lutamine than GS(1). As in bacteria, ATP appears to pray an important role in the allosteric regulation of GS(2). L-Ala and CTP are potent i nhibitors of GS(1) activity. CTP, carbamoyl-phosphate and L-Ala exert a cumulative inhibitory effect on GS(1) activity. GS(2) is also inhibi ted to some extent by L-Ala and L-His. NH2-terminal sequence analysis of GS(2) did not show any homology with bacteria, cyanobacteria or hig her plants. (C) 1997 Elsevier Science Inc.