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Poly(A)-containing HeLa cell mRNA prepared from cells labeled with methyl-(3)Hmethionine or (32)Pphosphate was found to contain a variety of methylated, blocked 5'-terminal structures of two general types: m(7)GpppN(7)-Np and m(7)GpppN(m)-N(m)-Np. In addition, about one-third of the (3)Hmethyl label was present in the N(6)-methyladenosine; this labeled nucleoside was not found in the 3'-terminal one-third of the mRNA chain and thus may also be in the 5' portion of the mRNA.
Furuichi et al. (Thu,) studied this question.