FORMATION OF LIGHT-HARVESTING COMPLEXES OF PHOTOSYSTEM-II IN SCENEDESMUS .2. DIFFERENT PATTERNS OF LIGHT-REGULATION OF LHC-GENE EXPRESSION IN GREEN AND GREENING CELLS

Time courses of the accumulation of chlorophyll (Chl) a, Chl b, of mRN A transcribed from the genes encoding the Chl a/b-binding proteins of the light-harvesting complexes (LHCs) (Lhc mRNAs) and LHC apoproteins have been investigated in Scenedesmus wild type (WT) and the mutant C- 2A' upon transfer from darkness to light. The WT develops the complete photosynthetic apparatus in darkness, whereas C-2A' exhibits a light- dependent formation of Chls and functional chloroplasts. Accumulation kinetics of Chls and LHC apoproteins were paralleled in green (WT) and greening (C-2A') cells. Illumination induced a steady increase in the se compounds which was only moderate in the WT but pronounced in C-2A' . Accumulation of Lhc mRNAs, however, was strongly enhanced by light i n both WT and C-2A' cells and showed a transient time course with a ma ximum after 4 h illumination. Amounts of Lhc mRNAs and LHC approprotei ns were correlated only during the initial 4 h of greening of C-2A'. I n green cells (WT and later stages of greening of C-2A'), amounts of L hc mRNAs and LHC apoproteins diverged markedly. These findings and res ults provided by in-vitro translation of polysomal RNA lead to the fol lowing conclusion: the light-regulation of Lhc-gene expression in Scen edesmus takes place mainly at the transcriptional level in greening ce lls and at the post-translational level in green cells.