Allophycocyanin A is a carbon dioxide receptor in the cyanobacterial phycobilisome.

Light harvesting is fundamental for production of ATP and reducing equivalents for CO 2 fixation during photosynthesis. However, electronic energy transfer (EET) through a photosystem can harm the photosynthetic apparatus when not balanced with CO 2 . Here, we show that CO 2 binding to the light-harvesting complex modulates EET in photosynthetic cyanobacteria. More specifically, CO 2 binding to the allophycocyanin alpha subunit of the light-harvesting complex regulates EET and its fluorescence quantum yield in the cyanobacterium Synechocystis sp. PCC 6803. CO 2 binding decreases the inter-chromophore distance in the allophycocyanin trimer. The result is enhanced EET in vitro and in live cells. Our work identifies a direct target for CO 2 in the cyanobacterial light-harvesting apparatus and provides insights into photosynthesis regulation.