Particulate preparations from the Chlorophyta Prototheca zopjii catalyze the incorporation of ['4C]glucose from UDP-[14C]glucose into lipids. These lipids have been characterized as 1ipid-Pglucose, lipid-PP-glucose, and lipid-PP-oligosaccharides. The lipid-linked oligosaccharides were a mixture ranging from a disaccharide to approximately a decasaccharide. Cellulase digestion and periodate oxidation showed that the oligosaccharides seem to be built of P-1,4-linked glucoses. The lipid moiety had the properties of dolichol.The glucolipids described appeared as precursors of a water-soluble polymer. Treatments of this polymer with hydrolytic enzymes and periodate oxidation indicate that it could be a glycoprotein containing 8-1,4-linked glucoses. When GDP-glucose was added to the incubation mixture, the C-labelled soluble polymer became insoluble in hot alkali. This insoluble polymer had the properties expected for cellulose. A scheme is proposed with the reactions involved in the initiation of cellulose biosynthesis. 14

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