title: Evolutionarily conserved Delta(25(27))-olefin ergosterol biosynthesis pathway in the alga Chlamydomonas reinhardtii.
abstract: Ergosterol is the predominant sterol of fungi and green algae. Although the biosynthetic pathway for sterol synthesis in fungi is well established and is known to use C24-methylation-C24 (28)-reduction (Δ(24(28))-olefin pathway) steps, little is known about the sterol pathway in green algae. Previous work has raised the possibility that these algae might use a novel pathway because the green alga Chlamydomonas reinhardtii was shown to possess a mevalonate-independent methylerythritol 4-phosphate not present in fungi. Here, we report that C. reinhardtii synthesizes the protosterol cycloartenol and converts it to ergosterol (C24β-methyl) and 7-dehydroporiferasterol (C24β-ethyl) through a highly conserved sterol C24- methylation-C25-reduction (Δ(25(27))-olefin) pathway that is distinct from the well-described acetate-mevalonate pathway to fungal lanosterol and its conversion to ergosterol by the Δ(24(28))-olefin pathway. We isolated and characterized 23 sterols by a combination of GC-MS and proton nuclear magnetic resonance spectroscopy analysis from a set of mutant, wild-type, and 25-thialanosterol-treated cells. The structure and stereochemistry of the final C24-alkyl sterol side chains possessed different combinations of 24β-methyl/ethyl groups and Δ(22(23))E and Δ(25(27))-double bond constructions. When incubated with [methyl-(2)H(3)]methionine, cells incorporated three (into ergosterol) or five (into 7-dehydroporiferasterol) deuterium atoms into the newly biosynthesized 24β-alkyl sterols, consistent only with a Δ(25(27))-olefin pathway. Thus, our findings demonstrate that two separate isoprenoid-24-alkyl sterol pathways evolved in fungi and green algae, both of which converge to yield a common membrane insert ergosterol.
authors.affiliation: Department of Chemistry and Biochemistry, Texas Tech University, Lubbock, TX 79409, USA.
authors.forename: Matthew B
authors.initials: MB
authors.lastname: Miller
authors.forename: Brad A
authors.initials: BA
authors.lastname: Haubrich
authors.forename: Qian
authors.initials: Q
authors.lastname: Wang
authors.forename: William J
authors.initials: WJ
authors.lastname: Snell
authors.forename: W David
authors.initials: WD
authors.lastname: Nes
mesh_terms: D000475:Alkenes / Q000737:chemistry*; D000818:Animals; D016825:Chlamydomonas reinhardtii / Q000378:metabolism*; D004875:Ergosterol / Q000096:biosynthesis* / Q000737:chemistry* / Q000378:metabolism; D019143:Evolution, Molecular; D008715:Methionine / Q000378:metabolism
chemical_list: D000475:Alkenes; D008715:Methionine; D004875:Ergosterol