Evidence for the Occurrence of a Common Pentanucleotide Sequence in the Structures of Transfer Ribonucleic Acids

Abstract

Transfer ribonucleic acids are a mixture of different molecular species of similar size (1). They can be distinguished by the ability of each of them to accept only one amino acid and transfer it to protein (1). The elucidation of the structures of tRNAsl is of considerable interest in view of the highly specific nature of these reactions. It has been shown that the 3’.hydroxyl terminal sequence, the amino acid acceptor end, of all functional tRNA molecules is -pCpCpA (2, 3). The 5’-phosphate terminal was found to be predominantly pGp(4). Aside from these similarities, the structures of different tRNA molecules seem to vary significantly, as indicated by structural studies performed on bulk (I) as well as purified tRNA preparations (5). The nucleotide composition of tRNA is characterized by the presence of several minor components in addition to the four major nucleotides. The minor nucleotides include pseudouridylic acid (tip) and ribothymidylic acid (Tp), as well as other methylated derivatives of purine and pyrimidine nucleotides (6, 7). Each of the purified RNAs studied has a specific content of minor nucleotides, which occupy definite positions along the RNA chain (5). The role of the minor nucleotides in the structure and function of tRNA is still little understood. This paper presents evidence that strongly suggests that the sequence -GpTp$pCpGp-, including two minor nucleotides, is part of the structure of all tRNh molecules. This sequence gives the tetranucleotide Tp$pCpGp on digestion of the RNAs with RNase Tl. The isolation of this tetranucleotide from digests of purified alanine-, valine-, and tyrosine-tRNAs, as well as from bulk tRNA of yeast, Escherichia co& and rat liver, will be described.