R. Amberg et al., SELENOCYSTEINE SYNTHESIS IN MAMMALIA - AN IDENTITY SWITCH FROM TRNA(SER) TO TRNA(SEC), Journal of Molecular Biology, 263(1), 1996, pp. 8-19
The mechanism of selenocysteine insertion into proteins is distinct fr
om ail other amino acids in all lines of descent in that it needs spec
ific protein cofactors and a structurally unique tRNA(Sec). It is firs
t aminoacylated with serine and further recognized among all other ser
ylated serine isoacceptors by a selenocysteine synthase and is convert
ed to selenocysteyl-tRNA(Sec). We present here the complete set of ide
ntity elements for selenylation of mammalian seryl-tRNA(Sec) and show
that the transplantation of these elements into normal serine tRNA all
ows its selenylation. Four particular structural motifs differentiate
eukaryotic tRNA(Sec) from normal tRNA(Ser):the orientation of the extr
a arm, the short 4 bp T Psi C-stem, the extra long 9 bp acceptor-stem
and the elongated 6 bp dihydrouridine-stem. Only the last two are esse
ntial and only together sufficient for selenocysteine synthesis, where
by the additional base-pairs of the acceptor-stem may be replaced by n
on-paired nucleotides, Each exchange of the first three structural mot
ifs mentioned above between tRNA(Ser) and tRNA(Sec) resulted in a sign
ificant loss of serylation, indicating that the overall composition of
particular structure elements is necessary to maintain normal functio
ns of tRNA(Sec). Since we find that all seryl-tRNAs which are selenyla
ted are also substrates for serine phosphorylation we propose that pho
sphoseryl-tRNA(Sec) is a storage form of seryl-tRNA(Sec). (C) 1996 Aca
demic Press Limited