Competition between globin messenger ribonucleic acids for a discriminating initiation factor

Translation of messenger ribonucleic acids for α- and β-globin chains was analyzed in an mRNA-dependent fractionated protein-synthesizing system derived from rabbit reticulocytes. The α/β chain synthesis ratio is highly dependent on the concentration of unfractionated globin mRNAs; the ratio is 1.5 at low mRNA concentration and declines to 0.03 at a high concentration. Several lines of evidence support the conclusion that this effect is caused by competitive binding of the messengers to an initiation factor which preferentially associates with the β-mRNA. Such a discriminating factor is present in the 0.5 M KCl wash fraction from ribosomes and it elutes from a diethylaminoethyl-cellulose column between 0.10 and 0.21 M KCl. Studies using purified preparations of initiation factors suggest that IF-M₃ and IF-M₄ may act synergistically to produce the activity of the discriminating initiation factor. Although the discriminating factor is required for translation of both messengers, its apparent binding constant to β-mRNA is 50 times larger than to α-mRNA. The concentration of discriminating factor mRNA complex does not limit the overall rate of synthesis in this cell-free system. Nevertheless, the relative effectiveness of different messengers is determined by the relative concentrations of their complexes with the discriminating factor.