title: Identification and expression of an atypical isoform of metallothionein in the African clawed frog Xenopus laevis.
abstract: Exploiting the annotation of the western clawed frog Silurana tropicalis genome, we identified a new metallothionein (MT) gene, exhibiting all the features to be considered an active gene, but with an atypical coding region, showing only 17 cysteine residues instead of the canonical 20 cysteines of vertebrate metallothioneins and two anomalous cysteine triplets. However, the presence of a gene in the genome does not ensure its effective expression. By using conventional and Real-Time PCR analyses, we demonstrated that this atypical MT is constitutively expressed throughout the life cycle of the African clawed frog Xenopus laevis; moreover, this gene is highly expressed in the adult liver, the major site of MT expression and synthesis in vertebrates. To our knowledge, the X. laevis MT described in this paper is the first sequence of a vertebrate MT showing only 17 cysteine residues, arranged in two Cys-Cys-Cys motifs. Phylogenetic analyses also demonstrated that the atypical X. laevis MT merges in the anuran clade, but is the most derived sequence among tetrapods MTs. Finally, Tajima's Relative Rate Test suggested a different evolutionary rate between the canonical X. laevis MT and this novel isoform.
authors.affiliation: Dipartimento di Biologia, Università Federico II, via Mezzocannone 8, 80134 Napoli, Italy. Electronic address: rosaria.scudiero@unina.it.
authors.forename: Rosaria
authors.initials: R
authors.lastname: Scudiero
authors.affiliation: Dipartimento di Biologia, Università Federico II, via Mezzocannone 8, 80134 Napoli, Italy.
authors.forename: Margherita
authors.initials: M
authors.lastname: Tussellino
authors.affiliation: Dipartimento di Biologia, Università Federico II, via Mezzocannone 8, 80134 Napoli, Italy.
authors.forename: Rosa
authors.initials: R
authors.lastname: Carotenuto
mesh_terms: D000375:Aging; D000595:Amino Acid Sequence; D000818:Animals; D001483:Base Sequence; D005075:Biological Evolution; D003001:Cloning, Molecular; D019295:Computational Biology; D003545:Cysteine / Q000737:chemistry; D018076:DNA, Complementary / Q000096:biosynthesis / Q000235:genetics; D005786:Gene Expression Regulation / Q000235:genetics*; D008668:Metallothionein / Q000737:chemistry / Q000235:genetics*; D008969:Molecular Sequence Data; D010802:Phylogeny; D020033:Protein Isoforms / Q000235:genetics; D012313:RNA / Q000096:biosynthesis / Q000235:genetics; D060888:Real-Time Polymerase Chain Reaction; D014982:Xenopus laevis / Q000235:genetics*
chemical_list: D018076:DNA, Complementary; D020033:Protein Isoforms; D012313:RNA; D008668:Metallothionein; D003545:Cysteine
keywords: Gene expression; Metallothionein; Phylogenetic analysis; Real-time PCR; Xenopus laevis