BIOGENESIS OF THYLAKOID MEMBRANES IN CHLAMYDOMONAS-REINHARDTII Y1 - AKINETIC-STUDY OF INITIAL GREENING

Initiation of thylakoid membrane assembly was examined in degreened ce lls of Chlamydomonas reinhardtii y1 cells depleted of thylakoid membra nes and photosynthetic activity by growth in the dark for 3 to 4 d. Ph otoreductive activities of photosystem II (PSII) and photosystem I (PS I) increased with no apparent lag when degreened cells were exposed to light at 38 degrees C. However, fluorescence transients induced by ac tinic light, which reflect the functional state of PSII, changed only slightly during the first 2 h of greening. When these cells were treat ed with 3-(3,4-dichlorophenyl)-1,1-dimethyl urea (DCMU) or saturating light, fluorescence increased commensurate with the cellular content o f chlorophyll. In similar experiments with greening cells of C. reinha rdtii CC-2341 (ac-u-g-2.3), a PSI-minus strain, fluorescence increased with chlorophyll without treatment with DCMU. These data suggested th at fluorescence of initial PSII centers in greening y1 cells was quenc hed by activity of PSI. Continuous monitoring of fluorescence in the p resence or absence of DCMU showed that assembly of quenched PSII cente rs occurred within seconds after exposure of y1 cells to light. These results are consistent with initial assembly of PSI and PSII within lo calized domains, where their proximity allows efficient energy couplin g.