Biosynthesis of the Light‐Harvesting Chlorophyll a/b Protein

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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 abdüt 90 min. In contrast, the α and β subunits of the chloroplast coupling factor and the large and small subunits of ribulose 1,5‐bisphosphate 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‐bisphosphate carboxylase for up to 48 h after transfer of plants from light to 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.