EVOLUTION OF THE GLUTAMINE-SYNTHETASE GENE IN PLANTS

Evolutionary comparison of glutamine synthetase (GS) genes encoding cy tosolic and chloroplast izozymes from different plants have been perfo rmed. Nucleotide sequences reveal high ratio of the synonymous/nonsyno nymous substitutions rates (varying from 5 to 12, average 7.5). The an alysis confirmed that cytosolic and chloroplast GS genes form sister g roups, and provided arguments that they arose as a result of gene dupl ication. Some properties of the evolution of the cytosolic GS family i n legumes were revealed: there are three groups of orthologous genes w hich do not fully coincide with tissue expression patterns: thus sugge sting that the genes may have acquired their biological function indep endently 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 div ergence of monocots, Solanaceae, Brassicaceae and Fabaceae have been e stimated, based mainly on chloroplast sequences. (C) 1997 Elsevier Sci ence Ireland Ltd.