Internal ribosome entry site drives cap-independent translation of reaper and heat shock protein 70 mRNAs in Drosophila embryos

Abstract

Translation is a sensitive regulatory step during cellular stress and the apoptosis response. Under such conditions, cap-dependent translation is reduced and internal ribosome entry site (IRES)-dependent translation plays a major role. However, many aspects of how mRNAs are translated under stress remain to be elucidated. Here we report that <i>reaper</i> mRNA, a pro-apoptotic gene from <i>Drosophila melanogaster</i>, is translated in a cap-independent manner. In <i>Drosophila</i> mutant embryos devoid of the eukaryotic initiation factor 4E (eIF4E), <i>reaper</i> transcription is induced and apoptosis proceeds. In vitro translation experiments using wild-type and <i>eIF4E</i> mutant embryonic extracts show that reporter mRNA bearing <i>reaper</i> 5′ untranslated region (UTR) is effectively translated in a cap-independent manner. The 5′UTR of <i>reaper</i> exhibits a high degree of similarity with that of <i>Drosophila</i> <i>heat shock protein 70</i> mRNA, and both display IRES activity. Studies of mRNA association to polysomes in embryos indicate that both <i>reaper</i> and <i>heat shock protein 70</i> 70 mRNAs are recruited to polysomes under apoptosis or thermal stress. Our data suggest that <i>heat shock protein 70</i> 70 and <i>reaper</i>, two antagonizing factors in apoptosis, use a similar mechanism for protein synthesis.