The chlorophyll-protein CP43' (isiA gene) induced by stress conditions in cyanobacteria is shown to serve as an antenna for Photosystem II (PSII), in addition to its known role as an antenna for Photosystem I (PSI). At high light intensity, this antenna is converted to an efficient trap for chlorophyll excitations that protects system II from photo-inhibition. In contrast to the 'energy-dependent non-photochemical quenching' (NPQ) in chloroplasts, this photoprotective energy dissipation in cyanobacteria is triggered by blue light. The induction is proportional to light intensity. Induction and decay of the quenching exhibit the same large temperature-dependence.