Evidence for a cysteine-mediated mechanism of excitation energy regulation in a photosynthetic antenna complex

Significance All photosynthetic organisms face the challenge of absorbing solar energy and regulating its flow through their light-harvesting antennas across widely varying photic conditions. For anoxygenic phototrophs, this process is complicated by the need to downregulate photosynthetic output when oxygen is encountered. The Fenna–Matthews–Olson protein from green sulfur bacteria is able to quench excitations in aerobic conditions effectively despite its apparent lack of photoprotective accessory molecules, indicating a previously unidentified type of energy transfer regulation. In this study, we provide evidence for a novel energy-quenching mechanism involving cysteine–bacteriochlorophyll photochemistry. This interaction should be able to be programed into other natural or bio-inspired antennas, opening new possibilities for regulating these systems in response to excess light.