Chlamydomonas reinhardtii selenocysteine tRNA[Ser]Sec

Eukaryotic selenocysteine (Sec) protein insertion machinery was thought to be restricted to animals, but the occurrence of both Sec-containing proteins and the Sec insertion system was recently found in Chlamydomonas reinhardtii, a member of the plant kingdom. Herein, we used RT-PCR to determine the sequence of C. reinhardtii Sec tRNASer]Sec, the first non-animal eukaryotic Sec tRNASer]Sec sequence. Like its animal counterpart, it is 90 nucleotides in length, is aminoacylated with serine by seryl-tRNA synthetase, and decodes specifically UGA. Evolutionary analyses of known Sec tRNAs identify the C. reinhardtii form as the most diverged eukaryotic Sec tRNASer]Sec and reveal a common origin for this tRNA in bacteria, archaea, and eukaryotes.