The accumulation of sulfate-35S by Porphyridium aerugineum cells and subsequent appearance of solubilized capsular polysaccharide-35S in the growth medium were examined The uptake of label by the cells was largely light dependent. Pulse-chase experiments using log phase cells revealed a rapid labeling of solubilized capsular polysaccharide, recovered from the medium as the cetylpyridinium chloride precipitate Polyacrylamide gel electrophoresis of the polysaccharide-35S showed the sulfate to be firmly bound to an immobile fraction. Sephadex chromatography revealed the molecular weight of the polysaccharide to be in excess of 2 x 105. Acid hydrolysis of the polysaccharide-35S released sulfate-35S ion as evidenced by radioautography of thin layer chromatographs Preliminary electron microscope evidence suggests that the synthesis, movement, and deposition of the capsular polysaccharide on the cell surface are Golgi complex-mediated processes

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