Evolution of the glutamine synthetase gene in plants

Evolutionary comparison of glutamine synthetase (GS) genes encoding cytosolic and chloroplast izozymes from different plants have been performed. Nucleotide sequences reveal high ratio of the synonymous/nonsynonymous substitutions rates (varying from 5 to 12, average 7.5). The analysis confirmed that cytosolic and chloroplast GS genes form sister groups, and provided arguments that they arose as a result of gene duplication. Some properties of the evolution of the cytosolic GS family in legumes were revealed: there are three groups of orthologous genes which do not fully coincide with tissue expression patterns: thus suggesting that the genes may have acquired their biological function independently of the originating processes. It may include the modification of promoter and regulatory regions of the already existing genes, as the result of the appearance of the nodulation phenomenon. Time of divergence of monocots, Solanaceae, Brassicaceae and Fabaceae have been estimated, based mainly on chloroplast sequences. (C) 1997 Elsevier Science Ireland Ltd.