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 See 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 See tRNA([Ser]Sec), the first non-animal eukaryotic See tRNA([Ser]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 See tRNAs identify the C. reinhardtii form as the most diverged eukaryotic Sec tRNA([Ser]Sec) and reveal a common origin for this tRNA in bacteria, archaea, and eukaryotes.