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

## TL;DR

The study explores how the enzyme NAGK in the algae Micromonas commoda functions without a regulatory protein called PII, which is unusual compared to other algae.

## Contribution

The paper reveals that M. commoda NAGK is not regulated by PII proteins, a novel finding in the context of arginine biosynthesis control.

## Key findings

- M. commoda NAGK is activated by N-acetyl-L-glutamate and inhibited by arginine but not by PII proteins.
- M. commoda PII can relieve feedback inhibition of NAGK in Chlamydomonas reinhardtii.
- The enzyme's low sensitivity to arginine may have compensated for the loss of PII regulation.

## Abstract

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, Ostreococcus and Bathycoccus lost the gene encoding PII, while Micromonas retained this gene. Here, we perform coupled enzyme and pull-down assays and show that M. commoda 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, M. commoda PII relieved Chlamydomonas reinhardtii NAGK from feedback inhibition by arginine. These observations suggest that M. commoda 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.