Sequence specificity of internal methylation in B77 avian sarcoma virus RNA subunits

Following ribonuclease digestion of methyl-3H-labeled B77 avian sarcoma virus RNA subunits, methylated oligonucleotides were isolated by diethylaminoethylcellulose chromotogrpahy. Partial nucleotide sequences were deduced from the known enzymatic specificities of the ribonucleases. In addition to methylated nucleosides in the 5'-terminal cap structure, m7G(5')GmpCp, N6-methyladenosine(m6A) was found to be present in only two internal sequences of the RNA molecule, Gpm6ApC and Apm6ApC. The average numbers of methylated nucleosides per RNA subunit are about 12-13 in Gpm6ApC, 1-2 in Apm6ApC, and 2 in m7GpppGmpCp. The sequences containing m6A in B77 sarcoma virus RNA are identical to m6A-containing sequences previously reported for the bulk mRNA from HeLa cells (Wei, C.M., Gershowitz, A., and Moss, B. (1976), Biochemistry 15, 397-401). Analysis of the oligonucleotides produced by RNase A digestion indicated that the sequence of bases on the 5' side of these trinucleotides is not specific. The oligonucleotide profile, however, was highly reproducible in different virus preparations. This suggests that the methylations occur at specific positions on the RNA molecule. Some of the methylated oligonucleotides produced by RNase A digestion appear to be present in less than molar amounts. Several hypotheses are proposed to explain this result.