The length and the secondary structure of the D-stem of human selenocysteine tRNA are the major identity determinants for serine phosphorylation.

Selenocysteine tRNA [tRNA(Ser)Sec] has been shown to be serylated by tRNA(Ser) synthetase. The serine moiety of seryl-tRNA(Ser)Sec in vertebrates is further phosphorylated by a kinase, in addition to being converted into selenocysteine. Using site-directed mutagenesis we have introduced a number of mutations into T7 RNA polymerase transcripts of human tRNA(Ser)Sec. Our results show that most of the unique structural features of tRNA(Ser)(Sec), like the 5'-triphosphate, the 9 bp long acceptor stem and the anticodon, are not identity elements for phosphorylation of human seryl-tRNA(Ser)Sec. However, the length and secondary structure of the D-stem (6 bp in contrast with 4 bp in the canonical serine tRNA) of human tRNA(Ser)Sec, but not its sequence, are the major identity determinants which discriminate this tRNA from common tRNA(Ser) and identify it as the substrate for phosphorylation by seryl-tRNA(Ser)Sec kinase. This notion is confirmed by the fact that normal seryl-tRNA(Ser), which is not a substrate for serine phosphorylation, becomes a substrate if two additional base pairs are introduced into its D-stem.