Orange Carotenoid Protein (OCP) Related NPQ in Synechocystis PCC 6803 OCP-Phycobilisomes Interactions

Abstract

The Orange-carotenoid-protein (OCP) is essential for a photoprotective non-photochemical fluorescence quenching (NPQ) mechanism in cyanobacteria. This mechanism decreases the energy arriving from the phycobilisomes (PBS), at the reaction centres of Photosystem II under strong illumination. Studies were performed to elucidate the interactions between the OCP and the phycobilisomes. We first determined that OCP did not co-isolate with phycobilisomes. Then, purified OCP was incubated with whole phycobilisomes from the WT or from a mutant lacking phycocyanin. The phycobilisomes were also dissociated and then reconstructed in the presence of OCP. In both cases, the OCP co-migrates with the phycobilisomes in a sucrose gradient suggesting a relationship between the OCP and the PBS. The OCP-PBS fractions presented a lower fluorescence level than the PBS fractions without OCP.