Nitrogen storage regulation by PII protein: lessons learned from taxonomic outliers

The paper ‘Interaction of N ‐acetyl‐ l ‐glutamate kinase with the PII signal transducer in the non‐photosynthetic alga Polytomella parva : Co‐evolution towards a hetero‐oligomeric enzyme’ by Selim et al . highlights how the study of a true taxonomic oddity, the heterotrophic unicellular alga P. parva , has been instrumental in uncovering the large potential for adaptive variation in the signaling complex of PII with the enzyme N ‐acetylglutamate kinase (NAGK). This complex modifies the regulatory properties of NAGK, allowing nitrogen stockpiling as arginine. In P. parva , a stable PII‐NAGK complex is formed which lacks regulation by canonical PII effectors but which exhibits novel adaptive responses to nitrogen abundance mediated by glutamine, a neo‐effector of PII proteins of photosynthetic eukaryotes.