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J.Health Sci., 46(6), 399-404, 2000
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Mammalian Selenocysteine tRNA, Its Enzymes and Selenophosphate
Takaharu Mizutani,*, a Chiharu Goto,a and Tsuyoshi Totsuka b
aDepartment of Drug Metabolism and Disposition, Faculty of Pharmaceutical Sciences, Nagoya City University, Nagoya 467-8603 Japan and bDepartment of Physiology, Aichi Prefectural Institute for Developmental Research, Kasugai 480-0392 Japan
Selenocysteine (Sec) is the 21st amino acid, because Sec has a specific tRNA and codon UGA, and shares a major stop codon UGA. The present article surveys the structure of mammalian tRNASec, the identity elements, the enzymes to synthesize Sec, and selenophosphate as a Se donor. Sec-tRNA is converted from Ser-tRNA by Sec synthase. tRNASec has a long 9 bp AA-stem, 6 bp D-stem and 4 bp T-stem, differing from Ser tRNAs. The 9 bp AA-stem and 6 bp D-stem were identity elements necessary for selenylation, however, the 4 bp T-stem is not essential. This finding was supported by the active mutants derived from major tRNASer by changing the AA-stem to 9 bp and the D-stem to 6 bp. There are many enzymes and factors that produce and bring Sec-tRNA to ribosomes. The reaction proceeds as follows. The first step is Ser-tRNA synthetase which recognizes the discrimination base G73 and the long extra arm. Bovine Sec synthase, composed of two protein species, recognizes Ser-tRNASec among three Ser-tRNAs and changes Ser-tRNASec to Sec-tRNASec, by the addition of Se from selenophosphate (SeP). The stability of SeP and some properties of the synthetase are discussed. Sec-tRNA is recognized by an elongation factor specific to Sec-tRNA, and is brought to Se-protein mRNA. The discrimination mechanism of the Sec UGA codon from a major stop codon UGA remains to be resolved.
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