Alpha and Beta Globin Messenger Ribonucleic Acid

Abstract This paper demonstrates that regulation of synthesis of the two globin chains in the rabbit reticulocyte is complex: cells contain about 1.5 times more ribosome-associated α mRNA than β, but ribosomes initiate translation of each α mRNA only about 65% as frequently as a β mRNA. Hence regulation of globin synthesis occurs not only as a result of mRNA availability, but also as a consequence of selective ratios of initiation. These experiments utilize seven antibiotics—emetine, anisomycin, sparsomycin, blasticidin, cycloheximide, fusidic acid, and streptovitacin. We show that at concentrations which inhibit protein synthesis in reticulocyte extracts 60 to 80%, all reduce the rate of propagation of nascent globin chains, but do not directly inhibit initiation of new chains. This results in accumulation of large polyribosomes, in which the mRNA molecules are saturated with ribosomes. Under these conditions protein synthesis is not limited by chain initiation and the relative amount of the two globin chains produced should be proportional to the relative amount of the mRNA; indeed we find that all drugs induce a change in the ratio of α:β globin synthesized from 1.0 to about 1.5. Using two techniques to determine the classes of polyribosomes which are synthesizing each globin chain, we have confirmed the finding of Hunt et al., (Hunt, T., Hunter, T., and Munro, A., Nature, 220, 481 (1968)) that β chains are made on larger polyribosomes than α chains. We believe that this is due solely to the decreased rate of ribosome attachment to an α mRNA relative to a β mRNA. In accord with this notion, we show that in the presence of any of the seven translation inhibitors, both classes of globin chains are made on the same classes of very large polyribosomes, and that these ribosomes contain relatively more nascent α chains than β chains.