The incorporation of methyl groups into amino acid transfer ribonucleic acid

Abstract

The presence of methylated purine and pyrimidine bases in the amino acid transfer or soluble RNA (S-RNA) of both mammalian and bacterial cells has been known for several years ( Dunn, 1959). The methylated bases, which occur in different relative concentrations in each species ( Smith and Dunn, 1959), are not incorporated as such ( Starr, 1962) but apparently are formed by methylation of the preformed polynucleotide chain ( Fleissner and Borek, 1962). Methionine acts as the methyl donor in the intact cell ( Biswas et al, 1961; Mandel and Borek, 1961) as well as in the in vitro cell-free preparations ( Fleissner and Borek, 1962). The function of the methyl groups is not known. Fleissner and Borek (1962) have suggested that the methylation alters the secondary and tertiary structures of the polynucleotides, or reduces the number of sites of hydrogen bonding, thereby facilitating transient attachment to binding sites. The studies reported in this and the following paper were undertaken in order to confirm the presence of the methyl groups in the soluble RNA fraction and to determine whether the pattern of methylation is involved in the determination of amino acid specificity of S-RNA.