5'-Terminal 7-methylguanosine and mRNA function: influence of potassium concentration on translation in vitro.

Vaccinia mRNAs containing either 5'-terminal m7G or unmethylated 5'-terminal structures were synthesized in vitro and their relative efficiencies of translation were compared in wheat germ and reticulocyte cell-free protein-synthesizing systems. The importance of the m7G group for efficient translation increases as the K+ concentration is raised. At K+ concentrations optimal for translation of mRNA containing m7G, unmethylated mRNA is translated at the same relative low efficiency in both cell extracts. The rate of binding of mRNA to ribosomes at K+ concentrations close to those found in intact cells is strongly influenced by the presence of m7G regardless of the source of the cell extract.