Translation in vitro of the mammalian nucleus-encoded mRNA for the beta subunit of mitochondrial H(+)-ATP synthase (beta-mRNA) of oxidative phosphorylation is promoted by a 150 nt translational enhancer sequence in the 3'-untranslated region (3' UTR). Titration of the eukaryotic initiation factor eIF4E with cap analogue revealed that translation of capped beta-mRNA was pseudo-cap independent. The 3' UTR of beta-mRNA stimulates the translation of heterologous uncapped mRNA species, both when the 3' UTR is placed at the 3' end and at the 5' end of the transcripts. The 3' UTRs of the alpha subunit of mitochondrial H(+)-ATP synthase (alpha-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 3' UTR of beta-mRNA. On dicistronic RNA species, the 3' UTR of beta-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 3' UTRs of certain mRNA species of oxidative phosphorylation have IRES-like functional activity. Riboprobes of the active 3' 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 3' UTR is assisted by specific RNA-protein interactions.