To regulate the effectiveness of photosynthesis and photoprotection cyanobacteria utilize a system
consisting of only few components. Photoactivation of the orange carotenoid protein (OCP) enables its
interaction with a specific, yet controversial site in the core of the light-harvesting antenna, the phycobilisome
(PBS). The resulting delivery of a quenching carotenoid molecule to the antenna pigments
leads to thermal dissipation of the excitation energy absorbed by the latter, and, consequently, to
depression of the photosynthetic activity. The nature of the OCP-induced PBS fluorescence quenching
mechanism remains debatable, however, specific protein-protein interactions between PBS and photoactivated
OCP should provide a unique environment for interactions between the excitation energy
donor and acceptor. Here we questioned whether the F€orster theory of resonance energy transfer can
explain PBS quenching by OCP even at their very small spectral overlap and whether in model systems,
the absence of specific protein-protein interactions of OCP with a donor of energy can be compensated by
a better spectral overlap. Hybridization of algal R-phycoerythrin with cyanobacterial OCP by chemical
crosslinking results in a significant decrease of R-phycoerythrin fluorescence lifetime, irrespective of the
OCP photoactivation status. Supported by structural considerations, this indicates that FRET may be the
essence of cyanobacterial photoprotection mechanism.