Revisiting cyanobacterial state transitions

CALZADILLA P.I.; ZHAN, JIAO; SÉTIF, PIERRE; KIRILOVSKY, DIANA

Paris

Jornada; Sfphi2019 : Journées de la Société Française de Photosynthèse; 2019

Société Française de Photosynthèse

Photosynthetic organisms must sense and respond to uctuating environmental condi-tions in order to perform ecient photosynthesis and to avoid the formation of dangerousreactive oxygen species. The excitation energy arriving at each photosystem permanentlychanges due to variations in the intensity and spectral properties of the absorbed light.Cyanobacteria, like plants and algae, have developed a mechanism, named state transitions,that balances photosystem activities. Here, we characterize the role of the cytochrome b6fcomplex and phosphorylation reactions in cyanobacterial state transitions using Synechococ-cus elongatus PCC 7942 and Synechocystis PCC 6803 as model organisms. First, largePhotosystem II uorescence quenching was observed in State II, which does not appearto be related to energy transfer from Photosystem II to Photosystem I (spillover). Thismembrane-associated process was inhibited by betaine, sucrose and high concentrations ofphosphate. Then, using dierent chemicals aecting the plastoquinone pool redox state andcytochrome b6f activity, we demonstrate that this complex is not involved in state transi-tions in S. elongatus or Synechocystis PCC6803. Finally, by constructing and characterizing21 protein kinase and phosphatase mutants and using chemical inhibitors, we demonstratethat phosphorylation reactions are not essential for cyanobacterial state transitions. Thus,signal transduction is completely dierent in cyanobacterial and plant (green alga) statetransitions