The zebrafish is widely used as a model system for studying mammalian developmental genetics and more recently, as a model system for carcinogenesis. Since there is mounting evidence that selenium can prevent cancer in mammals, including humans, we characterized the selenocysteine tRNA[Ser]sec gene and its product in zebrafish. Two genes for this tRNA were isolated and sequenced and were found to map at different loci within the zebrafish genome. The encoding sequences of both are identical and their flanking sequences are highly homologous for several hundred bases in both directions. The two genes likely arose from gene duplication which is a common phenomenon among many genes in this species. In addition, zebrafish tRNA[Ser]sec was isolated from the total tRNA population and shown to decode UGA in a ribosomal binding assay.
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"Many duplicated genes were lost during evolution, while other duplicated zebrafish genes were retained. Two identical selenocysteine tRNA genes were found encoded in the zebrafish genome, which most likely evolved by duplication of the entire genome (Xu et al. 1999). However, since we found three homologues of human SelT and SelW, genome duplication could not be solely responsible for the evolution of these selenoprotein genes. "
[Show abstract][Hide abstract] ABSTRACT: Fish are an important source of selenium in human nutrition and the zebrafish is a potentially useful model organism for the study of selenium metabolism and its role in biology and medicine. Selenium is present in vertebrate proteins in the form of selenocysteine (Sec), the 21st natural amino acid in protein which is encoded by UGA.
We report here the detection of 18 zebrafish genes for Sec-containing proteins. We found two zebrafish orthologs of human SelT, glutathione peroxidase 1 and glutathione peroxidase 4, and single orthologs of several other selenoproteins. In addition, new zebrafish selenoproteins were identified that were distant homologues of SelP, SelT and SelW, but their direct orthologs in other species are not known. This multiplicity of selenoprotein genes appeared to result from gene and genome duplications, followed by the retention of new selenoprotein genes. We found a zebrafish selenoprotein P gene (designated zSelPa) that contained two Sec insertion sequence (SECIS) elements and encoded a protein containing 17 Sec residues, the largest number of Sec residues found in any known protein. In contrast, a second SelP gene (designated zSelPb) was also identified that contained one SECIS element and encoded a protein with a single Sec. We found that zSelPa could be expressed and secreted by mammalian cells.
The occurrence of zSelPa and zSelPb suggested that the function of the N-terminal domain of mammalian SelP proteins may be separated from that of the C-terminal Sec-rich sequence: the N-terminal domain containing the UxxC motif is likely involved in oxidoreduction, whereas the C-terminal portion of the protein may function in selenium transport or storage. Our data also suggest that the utilization of Sec is more common in zebrafish than in previously characterized species, including mammals.
[Show abstract][Hide abstract] ABSTRACT: The discovery of two atypical amino acids, selenocysteine and pyrrolysine, in the genetic code is discussed. These findings
have expanded our understanding of the genetic code, since the repertoire of amino acids in the genetic code was supplemented
by two novel ones, in addition of the standard 20 amino acids. Current views on specific mechanisms of selenocysteine insertion
in forming selenoproteins are considered, as well as the results of studies of new translational components involved in biosynthesis
and incorporation of selenocysteine at different stages of translation. Similarity in the strategies of decoding UGA and UAG
as codons for respectively selenocysteine and pyrrolysine is discussed. The review also presents evidence on the medical and
biological role of selenium and selenoproteins containing selenocysteine as the main biological form of selenium.
No preview · Article · Aug 2010 · Russian Journal of Genetics
[Show abstract][Hide abstract] ABSTRACT: Aminoacyl-tRNAs are substrates for translation and are pivotal in determining how the genetic code is interpreted as amino acids. The function of aminoacyl-tRNA synthesis is to precisely match amino acids with tRNAs containing the corresponding anticodon. This is primarily achieved by the direct attachment of an amino acid to the corresponding tRNA by an aminoacyl-tRNA synthetase, although intrinsic proofreading and extrinsic editing are also essential in several cases. Recent studies of aminoacyl-tRNA synthesis, mainly prompted by the advent of whole genome sequencing and the availability of a vast body of structural data, have led to an expanded and more detailed picture of how aminoacyl-tRNAs are synthesized. This article reviews current knowledge of the biochemical, structural, and evolutionary facets of aminoacyl-tRNA synthesis.
Full-text · Article · Feb 2000 · Annual Review of Biochemistry