Competition between α- and β-Globin Messenger Ribonucleic Acids for Eucaryotic Initiation Factor 2

Synthesis of α- and β-globin was studied in a micrococcal nuclease treated reticulocyte lysate programmed with rabbit globin mRNA. The products of translation were analyzed by cellulose acetate electrophoresis under denaturing conditions. Nearly equimolar synthesis of α and β chains occurs at low mRNA concentrations, but the α/β synthetic ratio decreases drastically as more mRNA is present. Several lines of evidence suggest that direct mRNA competition for initiation factor eIF-2, the protein that binds methionyl-tRNAfMet, is involved in the regulation of α- and β-globin synthesis. The translational competition between α- and β-globin mRNA can be relieved by addition of highly purified eIF-2, even though total protein synthesis is not stimulated. The extent of relief by a given amount of eIF-2 declines with increasing mRNA concentration. At greater than optimal concentrations of salt, the α/ β synthetic ratio declines in parallel with total globin synthesis, inhibition by KOAc occurring at higher concentrations than in the case of KCl. Addition of excess eIF-2 leads to effective relief of translational competition when it is sharpened by elevated concentrations of KCl and raises the α/β synthetic ratio fivefold (from 0.16 to 0.8). The behavior of globin mRNA translation as a function of increasing concentrations of KCl or KOAc matches exactly with that observed for the direct binding of 125I-labeled globin mRNA to eIF-2, both with respect to the salt concentration giving 50% inhibition and to the displacement between the responses to KCl and KOAc. Binding of purified a-globin mRNA to eIF-2 is more sensitive to KC1 than the binding of unfractionated globin mRNA. These findings suggest strongly that mRNA interacts directly with eIF-2 during protein synthesis and that a-globin mRNA has a lower affinity for eIF-2 than does β-globin mRNA.