RNA-dependent conversion of phosphoserine forms selenocysteine in eukaryotes and archaea

doi:10.1073/pnas.0609703104

RNA-dependent conversion of phosphoserine forms selenocysteine in eukaryotes and archaea

Departments of *Molecular Biophysics and Biochemistry and
^‡Chemistry, Yale University, New Haven, CT 06520-8114; and
^†Department of Microbiology, University of Georgia, Athens, GA 30602-2605

Contributed by Dieter Söll, November 1, 2006 (received for review October 30, 2006)

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.

Footnotes

^§To whom correspondence should be addressed. E-mail: soll{at}trna.chem.yale.edu
Author contributions: J.Y. and S.P. contributed equally to this work; J.Y., S.P., J.C.S., D. Su, P.O., W.B.W., and D. Söll designed research; J.Y., S.P., J.C.S., D. Su, and P.O. performed research; M.J.H., A.M.C., and W.B.W. contributed new reagents/analytic tools; J.Y., S.P., D. Su, P.O., and D. Söll analyzed data; and J.Y., S.P., P.O., and D. Söll wrote the paper.
The authors declare no conflict of interest.
Abbreviations:

aa-tRNA,

aminoacyl-tRNA;

BV,

benzyl viologen;

FDHH,

formate dehydrogenase H;

IPTG,

isopropyl β-d-thiogalactoside;

PLP,

pyridoxal phosphate;

PSTK,

phosphoseryl-tRNASec kinase;

Sec,

selenocysteine;

SelA,

selenocysteine synthase;

SelB,

elongation factor SelB;

SelD,

selenophosphate synthetase;

Sep,

O-phosphoserine;

SepSecS,

Sep-tRNA:Sec-tRNA synthase;

SerRS,

seryl-tRNA synthetase.