Internal-ribosome-entry-site functional activity of the 3′-untranslated region of the mRNA for the β subunit of mitochondrial H+-ATP synthase

Translation in vitro of the mammalian nucleus-encoded mRNA for the β subunit of mitochondrial H⁺-ATP synthase (β-mRNA) of oxidative phosphorylation is promoted by a 150 nt translational enhancer sequence in the 3ʹ-untranslated region (3ʹ UTR). Titration of the eukaryotic initiation factor eIF4E with cap analogue revealed that translation of capped β-mRNA was pseudo-cap independent. The 3ʹ UTR of β-mRNA stimulates the translation of heterologous uncapped mRNA species, both when the 3ʹ UTR is placed at the 3ʹ end and at the 5ʹ end of the transcripts. The 3ʹ UTRs of the α subunit of mitochondrial H⁺-ATP synthase (α-F1-ATPase) and subunit IV of cytochrome c oxidase (COX IV) mRNA species, other nucleus-encoded transcripts of oxidative phosphorylation, do not have the same activity in translation as the 3ʹ UTR of β-mRNA. On dicistronic RNA species, the 3ʹ UTR of β-mRNA, and to a smaller extent that of COX IV mRNA, is able to promote the translation of the second cistron to a level comparable to the activity of internal ribosome entry sites (IRESs) described in picornavirus mRNA species. These results indicate that the 3ʹ UTRs of certain mRNA species of oxidative phosphorylation have IRES-like functional activity. Riboprobes of the active 3ʹ UTRs on dicistronic assays formed specific RNA-protein complexes when cross-linked by UV to proteins of the lysate, suggesting that cytoplasmic translation of the mRNA species bearing an active 3ʹ UTR is assisted by specific RNA-protein interactions.