The intergenic spacer of the Drosophila Adh-Adhr dicistronic mRNA stimulates internal translation initiation

The eukaryotic ribosome normally cannot be recruited upstream of internal ORFs and therefore polycistronic mRNAs are not efficiently translated in eukaryotic cells. However, examples of dicistronic mRNAs have been reported in Drosophila and other eukaryotes, and it was proposed that the intergenic spacers might contain internal ribosome entry site (IRES) elements. Here we have investigated the translation mechanism of the dicistronic Adh-Adhr mRNA of Drosophila melanogaster. The data indicate that the full-length intergenic spacer strongly enhances translation of the internal ORF of dicistronic reporters, both in S2 cells and adult flies. Interestingly, transcripts derived from intron-containing constructs gave rise to higher translation yields, suggesting a link between pre-mRNA splicing and efficient internal translation initiation.