The linker protein ApcI regulates light harvesting under red light in Synechocystis sp. PCC 6803

Phycobilisomes are versatile cyanobacterial antenna complexes that harvest light energy to drive photosynthesis. They can adapt to various light conditions; for example, dismantling under high light to prevent photo-oxidation and arranging in rows under low light to increase light harvesting efficiency. Light quality also influences phycobilisome structure and function, as observed under far-red light exposure. Here, we describe a phycobilisome linker protein, ApcI (previously hypothetical protein Sll1911), expressed specifically under red light (620 nm) or upon chemically induced reduction of the plastoquinone pool. We characterized ApcI in Synechocystis sp. PCC 6803 using mutant analyses, phycobilisome binding experiments, and protein interaction studies. Deletion of apcI conferred high light tolerance on Synechocystis sp. PCC 6803 compared to the wild-type strain, leading to reduced energy transfer from phycobilisomes to the photosystems under high light. Binding experiments revealed that ApcI replaces the linker protein ApcG at the membrane-facing side of the phycobilisome core via a paralogous C-terminal motif. Additionally, the N-terminal region of ApcI interacts with photosystem II. Our findings highlight the importance of phycobilisome remodeling for adaptation to different light conditions. The characterization of ApcI provides insight into the mechanisms by which cyanobacteria optimize light harvesting in response to varying light conditions.