FORMATION OF LIGHT-HARVESTING COMPLEXES OF PHOTOSYSTEM-II IN SCENEDESMUS .1. CORRELATIONS BETWEEN AMOUNTS OF PHOTOSYNTHETIC PIGMENTS, LHC MESSENGER-RNAS AND LHC APOPROTEINS DURING CONSTITUTIONAL DARK-DEPENDENTAND LIGHT-DEPENDENT LHC-GENE EXPRESSION

The patterns of accumulation of photosynthetic pigments, mRNAs transcr ibed from the genes encoding chlorophyll a/b-binding proteins (Lhc mRN As), and light-harvesting complex (LHC) apoproteins have been compared in dark- and light-grown cells of wild-type (WT) Scenedesmus obliquus and the mutants WT-LHC1, C-6D, C-6E, C-2A', C-2A'-LHC1 and C-2A'-LHC2 . In contrast to the WT, which developed the complete photosynthetic a pparatus in darkness, mutant strains exhibited either a lack of chloro phyll (Chl) b, carotenoids or LHC formation, or a light requirement fo r the synthesis of Chls or carotenoids. Accumulation of LHC apoprotein s strictly depended on the presence of Chl a and carotenoids. The form ation of a functional light-harvesting complex additionally required C hl b. Amounts of Lhc mRNAs of dark-grown mutant cells exceeded those o f the WT. Thus, accumulation of Lhc mRNAs was independent of Chl and c arotenoid synthesis and of chloroplast development. Transcripts of 1.4 and 1.6 kb accumulated constitutionally during growth in darkness. Il lumination increased the amounts of these mRNAs and, additionally, a 1 .8-kb Lhc mRNA appeared. Hence, light altered the expression of Lhc ge nes both quantitatively and qualitatively. Ratios of Chl a/Lhc mRNA, C hl a/LHC apoprotein and Lhc mRNA/LHC apoprotein were identical in the greening mutants C-6D, C-2A', C-2A'-LHC, and C-2A'-LHC, after 8 h illu mination. Light-independent accumulation of Lhc mRNAs, by contrast, wa s not correlated with the amounts of either Chl a or LHC apoproteins.