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Late SV40 16S and 19S mRNAs were found to contain an average of three m6A residues per mRNA molecule. The methylated residues of both the viral and cellular mRNAs occur in two sequences; Gpm6ApC and (Ap)nm6ApC, where n = 1-4. More than 60% of the m6A residues in SV40 16S and 19S mRNAs occur in Gpm6ApC even though there are twice as many (A)nAC than GAC sequences in these messengers. The m6A containing oligonucleotides of late SV40 MRNAs were localized in the viral messengers. In the 16S mRNA two m6A oligonucleotides were located at the 5' coding region between 0.95--0.0 map units. The third m6A residue was mapped between 0.0--0.14 map units in the translated portion of this mRNA. The overall pattern of internal methylation in the 19S mRNA is similar. However, some differences between 16S and 19S mRNAs were observed in both the content and location of the longer (Ap)n m6AC nucleotides. These results provide the first example of precise localization of internal methylation sequences in mRNA species with defined coding specificity. It implies that a) location of m6A residues is not random but specific to a particular region of the RNA, b) apart from sequence specificity other structural features of the mRNA may influence internal methylation and c) m6A residues are present in coding regions of SV40 mRNAs.
Canaani et al. (Mon,) studied this question.