title: Selenocysteine's mechanism of incorporation and evolution revealed in cDNAs of three glutathione peroxidases.
abstract: The nonsense codon, UGA, has for the first time recently been shown to encode selenocysteine in two proteins, mouse glutathione peroxidase (GSH-Px) (EC 1.11.1.9) and bacterial formate dehydrogenase. A co-translational rather than post-translational selenium-incorporation mechanism has been implicated. Furthermore, high expression levels of GSH-Px have suggested that suppression of termination is efficient and specific. We have isolated and characterized pituitary, kidney and placenta cDNAs for bovine, human and mouse GSH-Px respectively. It is demonstrated that this novel suppression event occurs in diverse tissues, in at least three mammalian species and at the translational step. Surprisingly, GSH-Px is shown to be extramitochondrially encoded, indicating a cytosolic suppression event rather than one utilizing the mitochondria's well-documented extended codon-reading ability. Sequence analysis reveals that a simple proximal contextual pattern responsible for readthrough does not exist. Analysis of predicted secondary structures of mRNAs, however, has revealed a conformation which may be unique to selenocysteine proteins and may prove useful as a tool for artificial incorporation of selenocysteines. A human intron for GSH-Px from an unspliced mRNA has been isolated whose position indicates an ancient, divergent evolutionary relationship with thioredoxin-S2, rather than an independent convergent one.
authors.affiliation: Chiron Research Laboratories, Chiron Corporation, Emeryville, CA 94608.
authors.forename: G T
authors.initials: GT
authors.lastname: Mullenbach
authors.forename: A
authors.initials: A
authors.lastname: Tabrizi
authors.forename: B D
authors.initials: BD
authors.lastname: Irvine
authors.forename: G I
authors.initials: GI
authors.lastname: Bell
authors.forename: J A
authors.initials: JA
authors.lastname: Tainer
authors.forename: R A
authors.initials: RA
authors.lastname: Hallewell
mesh_terms: D000818:Animals; D001483:Base Sequence; D002417:Cattle; D003545:Cysteine / Q000031:analogs & derivatives* / Q000378:metabolism; D004247:DNA / Q000378:metabolism*; D005979:Glutathione Peroxidase / Q000378:metabolism*; D006801:Humans; D051379:Mice; D008969:Molecular Sequence Data; D015394:Molecular Structure; D012643:Selenium / Q000378:metabolism*; D017279:Selenocysteine
chemical_list: D017279:Selenocysteine; D004247:DNA; D005979:Glutathione Peroxidase; D012643:Selenium; D003545:Cysteine