Methylated constituents of heterogeneous nuclear RNA: presence in blocked 5' terminal structures.

A substantial portion of the hnRNA of mouse L cells contains internal residues of N6-methyl adenylate and blocked 5' terminal sequences which are apparently of the type m7G5' ppp5' YmpZp..., in which 7-methyl guanosine is joined by a 5'-5' pyrophosphate linkage to a 2'-0-methylated residue, Ym. These sequences are indistinguishable from those comprising one of the two classes of blocked 5' sequences found in mRNA, and are quite distinct from those comprising the other class. The remarkable similarity in 5' terminal methylated sequences of hnRNA and a major fraction of mRNA appears to extend even to the relative occurrence of each of the four 2'-0-methylated species in position Ym.