Tracking the sterol biosynthesis pathway of the diatom Phaeodactylum tricornutum

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Journal Article
New Phytologist, 2014, 204 (3), pp. 521 - 535
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Diatoms are unicellular photosynthetic microalgae that play a major role in global primaryproduction and aquatic biogeochemical cycling. Endosymbiotic events and recurrent genetransfers uniquely shaped the genome of diatoms, which contains features from severaldomains of life. The biosynthesis pathways of sterols, essential compounds in all eukaryoticcells, and many of the enzymes involved are evolutionarily conserved in eukaryotes. Althoughwell characterized in most eukaryotes, the pathway leading to sterol biosynthesis in diatomshas remained hitherto unidentified.Through the DiatomCyc database we reconstructed the mevalonate and sterol biosyntheticpathways of the model diatom Phaeodactylum tricornutum in silico. We experimentally veri-fied the predicted pathways using enzyme inhibitor, gene silencing and heterologous geneexpression approaches.Our analysis revealed a peculiar, chimeric organization of the diatom sterol biosynthesispathway, which possesses features of both plant and fungal pathways. Strikingly, it lacks aconventional squalene epoxidase and utilizes an extended oxidosqualene cyclase and a multi-functional isopentenyl diphosphate isomerase/squalene synthase enzyme.The reconstruction of the P. tricornutum sterol pathway underscores the metabolic plastic-ity of diatoms and offers important insights for the engineering of diatoms for sustainable pro-duction of biofuels and high-value chemicals.
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