Phycobilisome-thylakoid Topography on Photosynthetically Active Vesicles of Porphyridium cruentum1

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Conditions are described for isolating functional phycobilisome-thylakoid vesicles from the red alga Porphyridium cruentum. Phycobilisome-thylakoid vesicles were prepared by brief sonication and centrifugation in a medium containing 0.5 molar sucrose, 0.5 molar potassium phosphate, and 0.3 molar sodium citrate (pH 7.0). They required ferricyanide as an oxidant and had O2 evolution rates (about 450 micromoles O2 per hour per milligram chlorophyll) higher than whole cells (about 250 micromoles O2 per hour per milligram chlorophyll). Energy transfer to photosystem II chlorophyll was evident from a high F695 nanometer (−196 C) emission peak. Preparations could be stored for over 24 hours and were considerably more stable than those from the cyanobacterium Anabaena variabilis (Katoh T, E Gantt 1979 Biochim Biophys Acta 546: 383-393). In electron micrographs of negatively stained material, the active thylakoid vesicles were found covered by closely spaced phycobilisomes on their external surface. The phycobilisome number in negatively stained vesicles was 450 per square micrometer, which was in the same range as the 400 per square micrometer observed in surface sections. A cell containing 1.5 × 10−6 micrograms phycoerythrin and 1.3 × 10−6 micrograms chlorophyll was found to contain 5 to 7 × 105 phycobilisomes on a thylakoid area of 1.1 to 1.6 × 103 square micrometers.

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