Abstract
The trace element selenium is found in proteins as selenocysteine (Sec), the 21st amino acid to participate in ribosome-mediated translation. The substrate for ribosomal protein synthesis is selenocysteinyl-tRNA
Sec
. Its biosynthesis from seryl-tRNA
Sec
has been established for bacteria, but the mechanism of conversion from Ser-tRNA
Sec
remained unresolved for archaea and eukarya. Here, we provide evidence for a different route present in these domains of life that requires the tRNA
Sec
-dependent conversion of
O
-phosphoserine (Sep) to Sec. In this two-step pathway,
O
-phosphoseryl-tRNA
Sec
kinase (PSTK) converts Ser-tRNA
Sec
to Sep-tRNA
Sec
. This misacylated tRNA is the obligatory precursor for a Sep-tRNA:Sec-tRNA synthase (SepSecS); this protein was previously annotated as SLA/LP. The human and archaeal SepSecS genes complement
in vivo
an
Escherichia coli
Sec synthase (SelA) deletion strain. Furthermore, purified recombinant SepSecS converts Sep-tRNA
Sec
into Sec-tRNA
Sec
in vitro
in the presence of sodium selenite and purified recombinant
E. coli
selenophosphate synthetase (SelD). Phylogenetic arguments suggest that Sec decoding was present in the last universal common ancestor. SepSecS and PSTK coevolved with the archaeal and eukaryotic lineages, but the history of PSTK is marked by several horizontal gene transfer events, including transfer to non-Sec-decoding Cyanobacteria and fungi.
