We have recently found an unusual class of tRNAs in bovine liver. We have to date characterized and determined the nucleotide sequence of two tRNAs in this class. Both of these tRNAs are aminoacylated with serine yet they bind to the termination codon UGA in a ribosome binding assay and show little or no binding in response to serine codons. In in vitro protein synthesis these two tRNAs suppress UGA termination. Moreover, we have found that these two serine tRNAs can be specifically phosphorylated to form phosphoseryl-tRNA with homologous kinase. In addition to these unusual characteristics these two tRNAs also have unusual structural features. They have two extra nucleotides, compared to other tRNAs, between the universal residues U at position 8 and A at position 14, and they contain an extra unpaired nucleotide within the double-stranded loop IV stem. These two tRNAs are the largest eukaryotic tRNAs sequenced to date (90 nucleotides). Despite their large size, they contain very few modified residues (4 and 6, respectively). Our research goals are to study the functional role of these seryl-tRNAs in protein synthesis, the isolation and sequence analysis of the genes coding for these tRNAs, the isolation and characterization of the kinase(s) that phosphorylate these specific serine tRNAs and the effect of phosphorylation on their function, and the isolation and characterization of other members of this unusual class of tRNAs (tRNAs which do not read a codon corresponding to their amino acid).
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