title: Biosynthesis of the light-harvesting chlorophyll a/b protein. Polypeptide turnover in darkness.
abstract: 1. When etiolated pea seedlings were exposed to continuous light for 24 h and then returned to darkness, 38% of the chlorophyll a, 74% of the chlorophyll b and 84% of the light-harvesting chlorophyll a/b protein that had accumulated under illumination proved to be unstable in darkness. The unstable chlorophyll displayed a half-life of about 90 min. In contrast, alpha and beta subunits of the chloroplast coupling factor and the large and small subunits of ribulose 1,5-biphosphate carboxylase continued to accumulate in darkness, although at a slower rate than in plants maintained under light. 2. Short-term labelling in vivo with L-[35S]methionine showed that leaves continued to synthesize the light-harvesting protein and the small subunit of ribulose 1,5-biphosphate carboxylase for up to 48 h after transfer of plants from light and darkness. However, after long-term labelling (16 h), the light-harvesting chlorophyll a/b protein was found to be labelled to high specific activity only in illuminated leaves. 3. I conclude that the light-harvesting chlorophyll a/b protein is subject to turnover after transfer of plants from light to darkness. The site of breakdown appears to be the photosynthetic membrane. I suggest that turnover of the protein is part of the normal physiological mechanism for co-ordinating the accumulation of the pigment and protein components of the light-harvesting chlorophyll a/b complex.
mesh_terms: D002734:Chlorophyll / Q000096:biosynthesis*; D003580:Cytochromes / Q000096:biosynthesis*; D003624:Darkness; D010450:Endopeptidases; D007700:Kinetics; D008027:Light; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D010940:Plant Proteins / Q000096:biosynthesis*; D010944:Plants / Q000378:metabolism*; D012273:Ribulose-Bisphosphate Carboxylase / Q000378:metabolism; D012697:Serine Endopeptidases
chemical_list: D003580:Cytochromes; D045342:Light-Harvesting Protein Complexes; D045322:Photosynthetic Reaction Center Complex Proteins; D010940:Plant Proteins; D002734:Chlorophyll; D010450:Endopeptidases; D012697:Serine Endopeptidases; C028344:glutamyl endopeptidase; D012273:Ribulose-Bisphosphate Carboxylase