Selenium (Se) is an essential trace element. Se is found as selenocysteine (Sec) in Se-proteins. Sec is the 21
st amino acid, because Sec has its tRNA, the codon UGA and those components in its translational machinery. Sec UGA codon shares with major stop codon UGA. We purified Sec synthesizing enzymes, such as seryl-tRNA synthetase (SerRS), Sec synthetase (SecS) and selenophosphate synthetase (SePS). I described the procedures to prepare Sec tRNA, SerRS, SecS, SePS and [
75Se]H
2Se in detail. We clarified that SecS composed of two proteins, SecSα and SecSβ. Sec synthesizing and incorporating systems present in
Monela,
Animalia and
Protoctista but not in
Plantae and
Fungi. We showed that protozoa had Sec tRNA on which Sec was synthesized from Ser-tRNA by bovine and protozoa SecS. Some worms, such as
Caenorhabditis elegans and
Fasiola gigantica, also had Sec tRNA on which Sec was synthesized by bovine liver SecS or
C. elegans enzymes. We showed recognition sites of mammalian Sec tRNA by SecS. The identitiy units of Sec tRNA are 9 bp aminoacyl- and 6 bp D-stems. This recognition is not the base-specific manner but the length-specific manner. From comparison of the phylogeny trees of Sec synthesizing system and translation system, we concluded that the evolution of Sec synthesizing system is older than that of the translation system.
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