The new phycobilisome linker protein ApcI regulates high light adaptation in Synechocystis sp. PCC 6803

Phycobilisomes are versatile cyanobacterial antenna complexes that harvest light energy to drive photosynthesis. These complexes can also adapt to various light conditions, 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 new, phycobilisome linker protein, ApcI (previously hypothetical protein sll1911), expressed specifically under red light. We characterized ApcI in Synechocystis sp. PCC 6803 using mutant strain analyses, phycobilisome binding experiments, and protein interaction studies. Mutation of apcI conferred high light tolerance to Synechocystis sp. PCC 6803 compared to wild type with reduced energy transfer from phycobilisomes to the photosystems. Binding experiments revealed that ApcI replaces the linker protein ApcG at the membrane-facing side of the phycobilisome core using a paralogous C-terminal domain. Additionally, the N-terminal extension of ApcI was found to interact with photosystem II. Our findings highlight the importance of phycobilisome remodeling for adaptation under different light conditions. The characterization of ApcI provides new insights into the mechanisms by which cyanobacteria optimize light-harvesting in response to varying light environments.