Shedding new light on cyanobacterial state transitions

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

Newry

Conferencia; Gordon Research Conference - Photosynthesis 2019; 2019

Gordon Research Conference

Photosynthetic organisms need to sense and respond to light fluctuating environmental conditions in order to maximize photosynthetic efficiency while avoiding the formation of dangerous reactive oxygen species. The specific illumination of either photosystem creates an energy imbalance, leading to the reduction or oxidation of the intersystem electron transport chain. Cyanobacteria, like plants and algae, have developed a mechanism, named state transitions, that balances photosystem activities under illuminations leading to the reduction or oxidation of the intersystem electron transport chain. State transitions are triggered by changes in the redox state of the membrane-soluble plastoquinone (PQ) pool. In plants and green algae, these changes in redox potential are sensed by cyt b6f, which interacts with a specific kinase that alters the phosphorylation status of light-harvesting complex II (LHCII). This phosphorylation allows LHCII to shuttle between PSI and PSII, thereby redistributing excitation energy between the photosystems. Using Synechococcus elongatus PCC 7942 and Synechocystis PCC 6803 as model organisms, we have just discovered that in cyanobacteria, this process, occur via a process quite different from that of plants and green algae. First, a process in PSII associated with fluorescence quenching independent of PSI plays an important role in cyanobacterial state transitions. This process appears to be more relevant than fluorescence changes coming from the cyanobacterial extramembrane antenna, the phycobilisome, at least in S. elongatus. Second, by characterizing the role of the cytochrome b6f in cyanobacterial state transitions using different chemicals affecting the PQ pool redox state and the activity of the cytochrome b6f, we demonstrated that this complex is not involved in S. elongatus and Synechocystis PCC6803 state transitions. Then, by constructing and characterizing 21 protein kinase and phosphatase mutants and using chemical inhibitors, it was clearly shown that phosphorylation reactions are not essential in cyanobacterial state transitions. In conclusion signal transduction is completely different in cyanobacterial and plant (green alga) state transitions opening a new view about the signal sensing and state transitions mechanism.