The origin of thylakoid membranes was studied in Chlamydomonas reinhardtii y-1 cells during greening at 380C. Previous studies showed that, when dark-grown cells are exposed to light under these conditions, the initial rates of accumulation of chlorophyll and the chlorophyll a/b-binding proteins in membranes are maximal (MA Maloney JK Hoober, DB Marks [1989] Plant Physiol 91: 1100-1106; JK Hoober MA Maloney, LR Asbury, DB Marks [1990] Plant Physiol 92: 419-426). As shown in this paper, photosystem 11 activity, which was nearly absent in dark-grown cells, also increased at a linear rate in parallel with chlorophyll. As compared with those made at 250C, photosystem 11 units assembled during greening at 380C were photochemically more efficient, as judged by saturation at a lower fluence of light and a negligible loss of excitation energy as fluorescence. Electron microscopy of cells in light for 5 or 15 minutes at 380C showed that these initial, functional thylakoid membranes developed in association with the chloroplast envelope.

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