EXCITATION-ENERGY TRANSFER IN THYLAKOID MEMBRANES FROM CHLAMYDOMONAS-REINHARDTII LACKING CHLOROPHYLL-B AND WITH MUTANT PHOTOSYSTEM-I

Energy trapping in Photosystem I (PS I) was studied by time-resolved f luorescence spectroscopy of PS II-deleted Chi b-minus thylakoid membra nes isolated from site-directed mutants of Chlamydomonas reinhardtii w ith specific amino acid substitutions of a histidine ligand to P-700 i n vivo the fluorescence of the PS I core antenna in mutant thylakoids with His-656 of PsaB replaced by asparagine, serine or phenylalanine i s characterized by an increase in the lifetime of the fast decay compo nent ascribed to the energy trapping in PS I (25 ps in wild type PS I with intact histidine-656, 50 ps in the mutant PS I with asparagine-65 6 and 70 ps in the mutant PS I with phenylalanine-656). Assuming that the excitation dynamics in the PS I antenna are trap-limited, the incr ease in the trapping time suggests a decrease in the primary charge se paration rate. Western blot analysis showed that the mutants accumulat e significantly less PS I than wild type. Spectroscopically, the mutat ions lead to a decrease in relative quantum yield of the trapping in t he PS I core and increase in relative quantum yield of the fluorescenc e decay phase ascribed to uncoupled chlorophyll-protein complexes whic h suggests that improper assembly of PS I and LHC in the mutant thylak oids may result in energy uncoupling in PS I.