CHLOROPHYLL SYNTHESIS MODULATES RETENTION OF APOPROTEINS OF LIGHT-HARVESTING COMPLEX-II BY THE CHLOROPLAST IN CHLAMYDOMONAS-REINHARDTII

Localization of apoproteins of the major light-harvesting complex (LHC II) in Chi b-less cells of Chlamydomonas reinhardtii cbn1-113 was dete rmined by immunoelectron microscopy. In dark-grown cells, a low amount of apoproteins was detected in cytoplasmic vacuoles. The amount in va cuoles, and in the cytosol, increased dramatically when the rate of pr otein synthesis was enhanced in the dark by raising the temperature to 38 degrees C. After exposure of cells to light, the apoproteins accum ulated also in the chloroplast. Mature-sized apoproteins were recovere d in an alkali-soluble fraction of cellular proteins commensurate with accumulation in the cytoplasm. At 25 degrees C, content of apoprotein s in the chloroplast of pale-green cells grown in medium lacking aceta te was one-half of the amount in cells grown with acetate, yet the tot al amount remained similar. Cytoplasmic vacuoles, which were nearly fi lled with immunoreactive, electron-opaque material, were more abundant in cells grown without acetate as compared with cells grown with acet ate. Accumulation of apoproteins outside of the chloroplast suggested that translocation into the organelle of a portion of the apoproteins, apparently synthesized in excess of the amount accommodated by Chi sy nthesis, was aborted after processing of precursors. These results sug gested that assembly of LHCII was required for retention of apoprotein s by the chloroplast.