Differential synthesis of blocked and unblocked 5'-termini in reovirus mRNA: effect of pyrophosphate and pyrophosphatase.

Viral mRNA synthesized in vitro by reovirusassociated enzymes contained 5'-terminal ppG, GpppG, or m7GpppGm. Selective synthesis of one type of 5'-end was obtained by adjusting the composition of the transcription reaction mixture based on a series of reactions proposed for formation of blocked, methylated 5'-terminal sequences of mRNA (capformation). The presence of inorganic pyrophosphate increased the proportion of ppG 5'-ends, while addition of pyrophosphatase yielded mRNA with 5'-terminal GpppG, which in the presence of a methyl donor was converted to m7GpppGm.