Involvement of the aphthovirus RNA region located between the two functional AUGs in start codon selection.

Initiation of translation in picornavirus RNAs occurs internally, mediated by an element termed internal ribosome entry site (IRES). In the aphthovirus RNA, the IRES element directs translation initiation at two in-frame AUGs separated by 84 nucleotides. We have found that bicistronic constructs that contained the IRES element followed by the fragment including the aphthovirus start codons in front of the second gene mimicked the translation initiation pattern of viral RNA observed in infected cells. In those constructs, the frequency of initiation at the first AUG was increased by a sequence context that resembled the favorable consensus for cap-dependent translation, although initiation at the second site was always preferred. In addition, we have found that initiation at the second start codon was not diminished under conditions in which the first initiation codon was blocked by antisense oligonucleotide interference. Interestingly, mutations that positioned the second AUG out-of-frame with the first AUG did not interfere with the frequency of initiation at the second one. On the contrary, IRES-dependent translation initiation in bicistronic constructs lacking the sequences present between functional AUGs in the viral RNA was sensitive to the presence of out-of-frame initiator codons and hairpins in the spacer region. This remarkable difference in start codon recognition was due to the nucleotide composition of the RNA that separated the IRES from the initiator codon. Thus our results indicate that the region located in the aphthovirus RNA between functional AUGs is involved in start codon recognition, strongly favoring selection of the second start AUG as the main initiator codon.