Nucleotide sequences at the 5' termini of reovirus mRNA's.

During in vitro synthesis of reovirus mRNA by viral cores, methyl groups from S-adenosylmethionine are incorporated only into 5'-terminal cap structures, i.e., m7GpppGmCp.... Thus, mRNA synthesized in the presence of S-adenosyl-[methyl-3H]methionine is 3H labeled specifically at the 5' terminus. This circumstance was exploited in the determination of 5'-terminal nucleotide sequences. Seven 5'-terminal fragments derived by complete RNase T1, digestion of methyl-3Hlabeled mRNA were partially degraded with RNase T2, and the products were separated by electrophoresis-homochromatography. From the patterns formed by the methyl-3H-labeled RNase T2 products, the sequences of the seven RNase T1-generated fragments were deduced. All seven fragments started with the sequence m7GpppGmCUA, after which the sequences diverged, with a tendency to be either U-rich or A-rich. Their chain lengths ranged from 7 to 10 nucleotides (excluding the m7G residue), and none of them contained an initiator AUG triplet. The sequences obtained support the hypothesis that virion-associated oligonucleotides arise through abortive transcription of the viral genome. There is no apparent 5'-terminal sequence feature distinctive of early versus late mRNA species within the small-mRNA size class.