Two genes encoding distinct cytosolic glutamine synthetases are closely linked in the pine genome

The major isoenzyme of glutamine synthetase found in leaves of angiosperms is the chloroplastic form. However, pine seedlings contain two cytosolic gl utamine synthetases in green cotyledons: GS1a, the predominant isoform, and GS1b, a minor enzyme whose relative amount is increased following phosphin otricin treatment. We have cloned a GS1b cDNA, and comparison with the prev iously reported GS1a cDNA sequence indicated that they correspond to separa te cytosolic GS genes encoding distinct protein products. Phylogenetic anal ysis showed that the newly reported sequence is closer to cytosolic angiosp erm GS than to GS1a, suggesting therefore that GS1a could be a divergent gy mnospermous CS1 gene. Gene mapping using a F2 family of maritime pine showe d co-localization of both GS genes on group 2 of the genetic linkage map. T his result supports the proposed origin of different members of the GS1 fam ily by adjacent gene duplication. The implications for gymnosperm genome or ganization are discussed, (C) 2000 Federation of European Biochemical Socie ties. Published by Elsevier Science B.V, All rights reserved.