Abstract
A new chemical method for radioactive labeling of single-stranded regions of RNA has been used to probe the three-dimensional structure of E. coli tRNA(fMet) in solution. The procedure involves conversion of cytosine residues to N(4)-[(14)C]methylcytosines by treatment with (14)CH(3)NH(2) and sodium bisulfite at pH7. Ribonuclease digestion of the modified tRNA produces (14)C-labeled oligonucleotides which comigrate with the corresponding unlabeled oligonucleotides, facilitating structural analysis. By this procedure, E. coli tRNA(fMet) has been found to contain only six reactive cytosines: C(1), C(16), C(17), C(35), C(75) and C(76). In addition, slow reaction at C(m) (33) was observed. These results are in excellent agreement with previously reported data on the sites of exposed cytosine residues in tRNA(fMet) obtained by two other chemical methods. The methylamine-bisulfite procedure is recommended for studying the ordered structure of more complex polyribonucleotides such as viral and ribosomal RNAs.