ACR11 is an Activator of Plastid-Type Glutamine Synthetase GS2 in Arabidopsis thaliana

Glutamine synthetase (GS) is an important enzyme for nitrogen assimilation, and GS2, encoded by GLN2, is the only plastid-type GS in Arabidopsis thaliana. A co-expression analysis suggested that the expression level of the gene encoding a uridylyltransferase-like protein, ACR11, is strongly correlated with GLN2 expression levels. Here we showed that the recombinant ACR11 protein increased GS2 activity in vitro by reducing the Km values of its substrate glutamine. A T-DNA insertion mutant of ACR11 exhibited a reduced GS activity under low nitrate conditions and reduced glutamine levels. Biochemical analyses revealed that ACR11 and GS2 interacted both in vitro and in vivo. These data demonstrate that ACR11 is an activator of GS2, giving it a mechanistic role in the nitrogen assimilation of A. thaliana.