The 21st amino acid, selenocysteine (sec), is distinct
from other amino acids because it lacks its own tRNA synthetase and
is the only amino acid synthesized on its cognate tRNA. Synthesis
of sec begins with acylation of tRNA(sec) (TRSP; MIM 165060) by
seryl-tRNA synthetase (SARS; MIM 607529) to give ser-tRNA(sec),
which is subsequently phosphorylated by O-phosphoseryl-tRNA kinase
(PSTK; MIM 611310) to give O-phosphoseryl-tRNA(sec). SEPSECS
catalyzes the final step of sec synthesis by converting
O-phosphoseryl-tRNA(sec) to selenocysteinyl-tRNA(sec) using
selenophosphate as the selenium donor (Palioura et al., 2009
[PubMed 19608919]).
