Ribosome binding to reovirus mRNA in protein synthesis requires 5' terminal 7-methylguanosine

Abstract

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 entirely in the high speed supernatant fraction of wheat germ extracts. Its activity is inhibited by aurintricarboxylic 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 40S-mRNA complexes. T1 or pancreatic RNAase digestion of methylated mRNA bound to 80S ribosomes yields 5' terminal fragments of apparent chain lengths about 32 and 36 nucleotides, respectively. A portion of the T1 RNAase-resistant fragments rebinds to ribosomes to form a nuclease-resistant complex. In contrast, the shorter 5' terminal oligonucleotide m7GpppGmpCpUp(Np)3Gp derived by RNAase T1 digestion of purified reovirus mRNA does not bind to ribosomes. The results suggest that 5' terminal m7G may function as the primary recognition signal for ribosome binding and that wheat germ ribosomes bind very close to the 5' termini of some of the species of methylated reovirus mRNAs.