Coordination of central metabolism

Carbon/Nitrogen (C/N) assimilation reactions via the Calvin-Benson and Glutamine-synthetase/Glutamate synthetase (GS/GOGAT) cycles require a tight regulation and a constant sensing of the quantity and quality of carbon and nitrogen availability. This dynamic C/N balance is maintained through a complex crosstalk of different input signals, which primarily converge on the signal transduction proteins of the PII signaling superfamily, which senses the energy/carbon/nitrogen/redox status of the cell through binding small metabolites.

Evolution of PII signaling (from cyanobacteria to Archaeplastida)

The ancestor of Archaeplastida inherited its PII signal transduction protein from an ancestral cyanobacterial endosymbiont. Over the course of evolution, plant PII proteins acquired a glutamine-sensing C-terminal extension, called Q-loop, subsequently present in all Chloroplastida PII proteins.

Over the past years, we systematically investigated different PII proteins from various algal strains (red, green and nonphotosynthetic algae) and from higher plant (Oryza sativa and Arabidopsis thaliana) with respect to their sensory and regulatory properties. The highly conserved role of the controlling of nitrogen metabolism via regulating enzyme of arginine biosynthesis, N-acetyl-l-glutamate kinase (NAGK), as a main PII-interactor has been demonstrated across oxygenic phototrophs of cyanobacteria and Archaeplastida. A hallmark of PII-NAGK interaction in the non-photosynthetic alga Polytomella parva is their co-evolution towards a stable hetero-oligomeric complex enzyme, irrespective of effector molecules. In addition, the PII signaling system of red algae has been identified as an evolutionary intermediate between that of Cyanobacteria and Chloroplastida.