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In human reticulocytes, the critical balancing of a-and fl-globin synthesis may be controlled in part by differential translation of the three major adult globin messenger RNAs (mRNAs), al, a2, and ft. In this study, we determined, as a parameter of translational efficiency, the relative ribosome loading of these three mRNAs. Using oligonucleotide probes specific for the al-and a2-globin mRNAs, we find that these two mRNAs have identical translational profiles. Their distribution contrasts with that of j3-globin mRNA, which is present on heavier polyribosomes and is less prevalent in pre-8OS messenger ribonucleoprotein fractions. The relative distribution of a-vs. 13-globin mRNA is consistent with more efficient ft-globin translation. In contrast, the parallel distributions of al-and a2-globin mRNAs suggests they are translated with equal efficiencies. Considering the relative concentrations of the two a-globin mRNAs in normal reticulocytes, this result predicts a dominant role for the a2-globin locus in human a-globin expression.

The translational efficiency of an mRNA may be determined at the step of translational initiation by the efficiency of its interaction with the cap binding protein complex. To further investigate the role of these interactions in translational control, we compare in vitro the relative sensitivities of rabbit and human alpha- and beta-globin mRNAs to translational inhibition by cap analogues. We find that rabbit beta-globin mRNA is more resistant to translational inhibition by cap analogues than rabbit alpha-globin mRNA, while in contrast, human beta-globin mRNA is more sensitive to cap analogue inhibition than human alpha-globin mRNA. This opposite pattern of translational inhibition by cap analogues of the rabbit and human alpha- and beta-globin mRNAs is unexpected as direct in vivo and in vitro comparisons of polysome profiles reveal parallel translational handling of the alpha- and beta-globin mRNAs from these two species. This discordance between the relative translational sensitivities of these mRNAs to cap analogues and their relative ribosome loading activities suggests that cap-dependent events may not be rate limiting in steady-state globin translation.