Micromonas commoda N-Acetyl-L-Glutamate Kinase Reflects Specificity in the Control of Arginine Synthesis at the Base of the Green Line

N-Acetyl-L-glutamate kinase (NAGK) catalyzes the first committed step in arginine biosynthesis in organisms that perform the cyclic pathway of ornithine synthesis. In cyanobacteria and most Archaeplastida, the activity of NAGK is controlled by the PII signal transduction protein. During evolution, representatives of the class Mamiellophyceae, <i>Ostreococcus</i> and <i>Bathycoccus</i> lost the gene encoding PII, while <i>Micromonas</i> retained this gene. Here, we perform coupled enzyme and pull-down assays and show that <i>M. commoda</i> NAGK is activated by N-acetyl-L-glutamate and inhibited by arginine but is not controlled by PII proteins. This loss may have been compensated for by the enzyme's low sensitivity to arginine. In contrast, <i>M. commoda</i> PII relieved <i>Chlamydomonas reinhardtii</i> NAGK from feedback inhibition by arginine. These observations suggest that <i>M. commoda</i> NAGK possesses a unique feature: it has lost the ability to interact with PII protein. The findings are discussed in the context of the relationship between NAGK control and the PII role in Mamiellophyceae.