The cytochrome b6f complex is not involved in cyanobacterial state transitions

Abstract

Photosynthetic organisms must sense and respond to fluctuating environmental conditions in order to perform efficient photosynthesis and to avoid the formation of dangerous reactive oxygen species. The excitation energy arriving at each photosystem permanently changes 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 b6f complex and phosphorylation reactions in cyanobacterial state transitions using Synechococcus elongatus PCC 7942 and Synechocystis PCC 6803 as model organisms. First, large photosystem II (PSII) fluorescence quenching was observed in State II, a result that does not appear to be related to energy transfer from PSII to PSI (spillover). This membrane-associated process was inhibited by betaine, Suc, and high concentrations of phosphate. Then, using different chemicals affecting the plastoquinone pool redox state and cytochrome b6f activity, we demonstrate that this complex is not involved in state transitions in S. elongatus or Synechocystis PCC6803. Finally, by constructingandcharacterizing21proteinkinaseandphosphatasemutantsandusingchemicalinhibitors,wedemonstrate that phosphorylation reactions are not essential for cyanobacterial state transitions. Thus, signal transduction is completely different in cyanobacterial and plant (green alga) state transitions.