Opposite responses of rabbit and human globin mRNAs to translational inhibition by cap analogues

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.