Chlorophyll fluorescence lifetime studies of greening in yellow mutants of Chlamydomonas reinhardtii: Assembly of the Photosystem I core complex

Changes in the functional organization of chlorophyll occur during light-dependent chlorophyll accumulation in yellow mutants of Chlamydomonas reinhardtii . These changes were studied using single-photon counting techniques to determine the chlorophyll fluorescence decay kinetics at various times after transfer of de-greened cultures to the light Several different yellow mutants were analyzed: y-1, a photoautotrophic strain; LM18-al2b, a Photosystem-II (PS II) -deficient derivative of y- 1; and C2, a strain deficient in both Photosystem I (PS I) and PS II. The results demonstrate that the residual chlorophyll present in de-greened cultures has a similar organization, independent of the chlorophyll-protein composition of the fully greened strains. Relative to the composition in light-grown cultures, the chlorophyll present in de-greened cells is enriched in chlorophyll b and has fluorescence decay kinetics similar to monomeric chlorophyll in solution. The reaction-center core complexes accumulate preferentially during the first 2 h of greening, and the light-harvesting complexes beginto accumulate more rapidly thereafter. After 5–6 h, during the stage of maximal chlorophyll accumulation rate, reaction centers and antenna complexes accumulate in constant proportions. The PS I reaction-center core complex has a constant apparent size of about 130 Chl/P-700 at all stages of greening under the conditions we examined.