The effect of "cap" analogs on reovirus mRNA binding to wheat germ ribosomes. Evidence for enhancement of ribosomal binding via a preferred cap conformation.

A variety of compounds related to the 5'-terminal "cap" (m7GpppN) of eukaryotic mRNA's were chemically synthesized and tested as inhibitors of reovirus mRNA binding to wheat germ ribosomes. Under our conditions of mRNA binding to ribosomes, 7-methyl-, 7-ethyl-, and 7-benzyl-GDP, but not GDP, decreased stable initiation complex formation by 70 to 80% at a concentration of 0.1 mM indicating that 7-substitution, but not a specific substituent, was required for the effect. Elimination of the positive charge on the imidazole of the 7-substituted compounds by treatment with alkali destroyed their inhibitory activity. Similarly, reduction to 8-hydro-m7GDP reversibly decreased the activity of m7GDP. The results were consistent with the hypothesis that the positive charge resulting from 7-alkylation provides an active cap conformer for binding via interaction of phosphate oxygens with the positively charged imidazole moiety. In accord with this suggestion, 7-carboxymethyl GDP and 7,8-dimethyl GDP were found to be less inhibitory than m7GDP. A 2-amino group was also important since m7IDP was less effective than m7GDP and 0.1 mM m7XDP did not inhibit ribosome binding. Other poor inhibitors were 6-Cl-m7GDP and 1,7-dimethyl GDP but N2,7-dimethyl GDP, 2'-deoxy-m7GDP, and m7GppI had essentially the same activity as m7GDP.