Selenocysteine Synthesis in Mammalia: An Identity Switch From tRNA to tRNA

Author： Amberg R. Mizutani T. Wu X.Q. Gross H.J.

ISSN： 0022-2836

Source： Journal of Molecular Biology, Vol.263, Iss.1, 1996-10, pp. : 8-19

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Abstract

The mechanism of selenocysteine insertion into proteins is distinct from all other amino acids in all lines of descent in that it needs specific protein cofactors and a structurally unique tRNA . It is first aminoacylated with serine and further recognized among all other serylated serine isoacceptors by a selenocysteine synthase and is converted to selenocysteyl-tRNA We present here the complete set of identity elements for selenylation of mammalian seryl-tRNA and show that the transplantation of these elements into normal serine tRNA allows its selenylation. Four particular structural motifs differentiate eukaryotic tRNA from normal tRNA : the orientation of the extra arm, the short 4 bp TPsiC-stem, the extra long 9 bp acceptor-stem and the elongated 6 bp dihydrouridine-stem. Only the last two are essential and only together sufficient for selenocysteine synthesis, whereby the additional base-pairs of the acceptor-stem may be replaced by non-paired nucleotides. Each exchange of the first three structural motifs mentioned above between tRNA and tRNA resulted in a significant loss of serylation, indicating that the overall composition of particular structure elements is necessary to maintain normal functions of tRNA . Since we find that all seryl-tRNAs which are selenylated are also substrates for serine phosphorylation we propose that phosphoseryl- tRNA is a storage form of seryl-tRNA .