FLUORESCENCE LIFETIME STUDIES OF CYANOBACTERIAL PHOTOSYSTEM-II MUTANTS

Time-resolved and steady-state fluorescence spectra have been determined in intact cells of wild type and selected Photosystem II mutants of the cyanobacterium Synechocystis sp. PCC 6803. Upon excitation of phycobilisome components, in wild type fluorescence lifetimes and spectra are observed that are compatible with energy transfer within the phycobilisome, between the phycobilisome and Photosystem II, and within Photosystem II (PS II). In a mutant carrying a spinach/Synechocystis CP47 hybrid protein (one of the chlorophyll-binding antennae within PS II), energy transfer between phycobilisomes and PS II has been modified, suggesting an involvement of CP47 in energy transfer between phycobilisomes and PS II. In a mutant containing some CP43 (another chlorophyll-binding antenna protein in PS II) but lacking all other major PS II proteins, chlorophyll can still be bound to CP43, but significant long-lived fluorescence from this chlorophyll could not be observed. In a mutant lacking all major PS II components, phycobilisomes displayed a fluorescence yield larger than in wild type, but considerably smaller than observed in isolated systems. The conformation of one or more phycobilins in or near the anchor protein most likely is important in determining the fluorescence yield of phycobilisomes.