Regulation of Genes for Enzymes Along a Common Nitrogen Metabolic Pathway

Abstract

We have characterized the multigene families encoding glutamine synthetase (GS) and asparagine synthetase (AS) in Pisum sativum. The isolated GS and AS genes have been used as probes to study the expression of individual members of these gene families during various aspects of plant development. These studies have shown that chloroplast GS2 and cytosolic GS are encoded by homologous nuclear genes which are differentially expressed in vivo (18,19). The nuclear gene for chloroplast GS2 is regulated by light, phytochrome, and photorespiration (4). In contrast, two nearly identical genes for cytosolic GS (GS3A and GS3B) are expressed at highest levels in developmental contexts where large amounts of nitrogen are mobilized in plants (22). Analysis of transgenic tobacco plants containing the pea GS promoters fused to a GUS reporter gene has shown that the genes for chloroplast GS2 and cytosolic GS3A are expressed in distinct cell types (5). These transgenic experiments have demonstrated that the chloroplast GS2 and cytosolic GS3A isoforms serve distinct, non-overlapping roles in plant nitrogen metabolism. Parallel studies on the gene family for plant AS have shown that peas contain two AS genes (AS1 and AS2) (20). The AS1 gene shows a dramatic dark-induced expression, which reflects the role of asparagine as the preferred nitrogen transport compound in dark-grown plants (20). Both AS1 and AS2 are expressed coordinately with genes for cytosolic GS during germination and nitrogen-fixation (20). Our combined studies on the gene families for GS and AS should uncover the molecular basis for the coordinate regulation of genes for enzymes along a common nitrogen metabolic pathway in plants.