Features in the 5′ non-coding sequences of rabbit α and β-globin mRNAs that affect translational efficiency

The 5′ non-coding sequence of rabbit β-globin mRNA was mutagenized in an attempt to identify structural features that might contribute to the ability to support translation in an homologous rabbit reticulocyte lysate. Translational efficiency was not reduced by substitutions introduced in nearly every position of the β-globin leader sequence, suggesting that the 5′ non-coding domain of this highly efficient mRNA contains no special effector motifs. Instead, efficient translation appears to require only a moderately long leader sequence devoid of secondary structure, especially near the 5′ end. Consistent with that interpretation, substitutions in several positions actually improved translation relative to the wild-type β-globin leader sequence; experimental assessment of the secondary structure of these derivatives revealed a perfect inverse correlation between secondary structure content and translational efficiency. Other experiments probed the structural basis for the long-noted difference in translational efficiency between rabbit α and β-globin mRNAs, a difference that was reproduced here using only the 5′ non-coding domains of those mRNAs. The possibility that translation of ribosomal protein mRNAs might be modulated by a mechanism similar to that of α-globin mRNA is discussed. Because the β-globin leader sequence has been incorporated into some popular expression vectors, and because globin genes are targets for gene therapy, this analysis of how globin mRNA leader sequences function in translation and how they can be improved may have practical applications.