N6, O2′-dimethyladenosine a novel methylated ribonucleoside next to the 5′ terminal of animal cell and virus mRNAs

MODIFIED 5′-terminal structures of the type m7G(5′)ppp(5′)Nm have been identified in various viral1–4 and cellular5–8 mRNAs. The 5′-terminal modification of mRNA may be carried out by specific guanylyl- and methyltransferases9 and the structure seems to be required for efficient in vitro translation10. Studies5 of the 5′-terminal sequences of HeLa cell mRNAs revealed a methylated nucleoside that chromatographed slightly ahead of 2′-O-methyladenosine (Am). We have now identified this novel nucleoside and found that it is common to mRNAs of human and mouse cells as well as at least one virus.

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