All photosynthetic organisms require mechanisms to regulate the amount of energy they absorb for photosynthesis to avoid damage from excess irradiance.  In cyanobacteria, photosynthetic bacteria that are the ancestors of plant chloroplasts, the structure and positioning of the light-harvesting phycobilisome (PBS) structure provides an example of a molecular control mechanism that governs energy flow.  When the amount of captured light energy in the PBS exceeds what can be used for photosynthesis, the rapid conversion of excess excitation energy into heat before it causes harm is known as non-photochemical quenching or photoprotection.  In cyanobacteria this is mediated by a water-soluble photoreceptor protein, the Orange Carotenoid Protein (OCP).  We have recently determined molecular structures of the PBS from the cyanobacterium Synechocystis sp. PCC6803 in both a light harvesting and OCP-bound (photoprotected) state.  Identification of the OCP binding site and the discovery of two proteins that we propose mediate positioning of the PBS provide the foundation for goals of this project--gaining an understanding of the structural basis of PBS attachment to the membrane/photosystems for the directing of energy flow.  Moreover, our data suggests that there may be another layer of regulation mediated by linking phosphate groups to both the OCP and the PBS attachment proteins.   We expect that outcomes from determining the mechanistic details of PBS positioning, our model system will become a key platform for fundamental research on energy transfer in pigment-protein complexes. Likewise, it will serve as inspiration for synthetic biologists, chemists and materials scientists to design new sustainable technologies for harnessing the clean and abundant energy in sunlight.