Chlorosis as a Developmental Program in Cyanobacteria: The Proteomic Fundament for Survival and Awakening.

Cyanobacteria that do not fix atmospheric nitrogen gas survive prolonged periods of nitrogen starvation in a chlorotic, dormant state where cell growth and metabolism are arrested. Upon nutrient availability, these dormant cells return to vegetative growth within 2-3 days. This resuscitation process is highly orchestrated and relies on the stepwise reinstallation and activation of essential cellular structures and functions. We have been investigating the transition to chlorosis and the return to vegetative growth as a simple model of a cellular developmental process and a fundamental survival strategy in biology. In the present study, we used quantitative proteomics and phosphoproteomics to describe the proteomic landscape of a dormant cyanobacterium and its dynamics during the transition to vegetative growth. We identified intriguing alterations in the set of ribosomal proteins, in RuBisCO components, in the abundance of central regulators and predicted metabolic enzymes. We found O-phosphorylation as an abundant protein modification in the chlorotic state, specifically of metabolic enzymes and proteins involved in photosynthesis. Nondegraded phycobiliproteins were hyperphosphorylated in the chlorotic state. We provide evidence that hyperphosphorylation of the terminal rod linker CpcD increases the lifespan of phycobiliproteins during chlorosis.