Formation of light-harvesting complexes of photosystem II in Scenedesmus

Abstract

The patterns of accumulation of photosynthetic pigments, mRNAs transcribed from the genes encoding chlorophyll a/b-binding proteins (Lhc mRNAs), 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 apparatus in darkness, mutant strains exhibited either a lack of chlorophyll (Chl) b, carotenoids or LHC formation, or a light requirement for the synthesis of Chls or carotenoids. Accumulation of LHC apoproteins strictly depended on the presence of Chl a and carotenoids. The formation of a functional light-harvesting complex additionally required Chl b. Amounts of Lhc mRNAs of dark-grown mutant cells exceeded those of the WT. Thus, accumulation of Lhc mRNAs was independent of Chl and carotenoid synthesis and of chloroplast development. Transcripts of 1.4 and 1.6 kb accumulated constitutionally during growth in darkness. Illumination increased the amounts of these mRNAs and, additionally, a 1.8-kb Lhc mRNA appeared. Hence, light altered the expression of Lhc genes both quantitatively and qualitatively. Ratios of Chl a/Lhc mRNA, Chl a/LHC apoprotein and Lhc mRNA/LHC apoprotein were identical in the greening mutants C-6D, C-2A′, C-2A′-LHC1, and C-2A′-LHC2 after 8 h illumination. Light-independent accumulation of Lhc mRNAs, by contrast, was not correlated with the amounts of either Chl a or LHC apoproteins.