SELENOCYSTEINE SYNTHESIS IN MAMMALIA - AN IDENTITY SWITCH FROM TRNA(SER) TO TRNA(SEC)

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