Ic. Oliveira et Gm. Coruzzi, Carbon and amino acids reciprocally modulate the expression of glutamine synthetase in arabidopsis, PLANT PHYSL, 121(1), 1999, pp. 301-309
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.