Carbon and amino acids reciprocally modulate the expression of glutamine synthetase in arabidopsis

In bacteria and yeast, glutamine synthetase (CS) expression is tightly regu lated by the metabolic status of the cell, both at the transcriptional and posttranscriptional levels. We discuss the relative contributions of light and metabolic cues on the regulation of members of the GS gene family (chlo roplastic CS2 and cytosolic CS1) in Arabidopsis. These studies reveal that the dramatic induction of mRNA for chloroplastic GS2 by light is mediated i n part by phytochrome and in part by light-induced changes in sucrose (Suc) levels. In contrast, the modest induction of mRNA for cytosolic CS1 by lig ht is primarily mediated by changes in the levels of carbon metabolites. Su c induction of mRNA for CS2 and CS1 occurs in a time- and dose-dependent ma nner. Sue-induced changes in GS mRNA levels were also observed at the level of CS enzyme activity. In contrast, amino acids were shown to antagonize t he Suc induction of CS, both at the level of mRNA accumulation and that of enzyme activity. For CS2, the gene whose expression was the most dramatical ly regulated by metabolites, we used a GS2 promoter-beta-glucuronidase fusi on to demonstrate that transcriptional control is involved in this metaboli c regulation. Our results suggest that the metabolic regulation of GS expre ssion in plants is controlled by the relative abundance of carbon skeletons versus amino acids. This would allow nitrogen assimilation into glutamine to proceed (or not) according to the metabolic status and biosynthetic need s of the plant. This type of GS gene regulation is reminiscent of the nitro gen regulatory system in bacteria, and suggests an evolutionary link betwee n metabolic sensing and signaling in bacteria and plants.