Structural dynamics essential for photosynthetic adaptation and survival of cyanobacteria in fluctuating light intensities

2026-07-22

Like most photosynthetic organisms, cyanobacteria are vulnerable to fluctuations in light intensity, which can damage their photosynthetic machinery. To protect themselves against such fluctuations, they use a photoprotective mechanism called non-photochemical quenching (NPQ), i.e. the dissipation of excess absorbed photo-energy as heat. NPQ in cyanobacteria is triggered by orange carotenoid protein (OCP) light activation. Based on spectroscopic and diffraction studies of OCP in Synechocystis 6803 (gene slr1963), it was suggested that OCP light activation occurs through light-induced movement of a carotenoid causing movement and/or dissociation of OCP N- and C-terminal domains. However, the exact structural dynamics of OCP light-activation need to be unravelled. Furthermore, the growing availability of cyanobacterial genomes allowed identification of additional OCP subfamilies (OCP2, OCPX) in different cyanobacteria. The first results demonstrating different kinetics of light-activation in the different OCP paralogs raised questions about differences in their photoprotective roles and in photoactivation mechanisms. This topic has not been studied to date. Here I propose to resolve structural changes during photoprotection-related transitions of OCP in different OCP subfamilies using time-resolved X-ray crystallography. X-ray crystallography of OCP1 encoded by slr1963, the best-characterized OCP protein, as well as its paralogs from the OCP2 and OCPX subfamilies will be performed. This approach will be combined with ultrafast transient (polarised) absorption spectroscopy on isolated proteins and oriented single crystals to describe the structure-dependent flow of photoenergy in the proteins. This study has several potential applications ranging from enhancing cyanobacterial light harvesting to improve biofuel production, to better understanding of carotenoid-protein interactions in artificial photosynthesis systems, and for optogenetics.

United Kingdom (UK)

2026-02-01