Purified reovirus synthesizes in vitro a mixture of mRNA molecules that contain 5’terminal structures of the type ppG..., GpppG..., and m7GpppGm, the relative amount of each depending upon the presence of S-adenosylmethionine (SAM) or S-adenosylhomocysteine(SAH) in the transcription incubation mixture. Reovirus mRNAs containing 5’termini of the type ppG... and GpppG... can be specifically modified by wheat germ extracts in the presence of SAM to yield the structure m7GpppG...(Muthukrishnan et al., 1975). The mRNA methylase activity is ribosome-independent and is recovered almost entlrely in the high speed supernatant fraction of wheat germ extracts. Its activity is inhibited by aurintrlcarboxylic acid.

Ribosome binding experiments with reovirus mRNA and wheat germ extract indicate that only those mRNA molecules containing 5’terminal m7G are capable of participating in the initiation of protein synthesis and subsequent polysome formation. mRNA with unmethylated 5’termini do not form a complex with 40s ribosomal subunits. Discrimination between unmethylated and methylated reovirus mRNA, active in protein synthesis, apparently occurs at or before the formation of 4OS-mRNA complexes.