Mutagenesis of the large untranslated sequence at the 5' end of the genome of foot-and-mouth disease virus revealed that a region of approximately 450 nucleotides preceding the open reading frame of the viral polyprotein is involved in the regulation of translation initiation at two internal start sites. Variations in two domains of this region reduced the translation efficiency up to 10-fold, whereas an intermediate segment seemed to be less essential. A pyrimidine-rich sequence preceding the start codon was most sensitive in that conversion of single pyrimidine residues to purines decreased the translation efficiency strongly. The data are in agreement with a recently proposed general structural model for the internal ribosome entry site of the cardiovirusaphthovirus subgroup of picornaviruses (E. V.

A cellular 57-kDa protein (p57) that binds specifically to the internal translation initiation site in the 5' untranslated region of foot-and-mouth disease virus RNA was detected in cell extracts of different mammalian species by UV cross-linking. The protein binds to two distinct sites of the translation control region which have as the only common sequence a UUUC motif. The first binding site consists of a conserved hairpin structure, whereas the second binding site contains an essential pyrimidine-rich region without obvious secondary structure. Competition experiments indicate that the complexes with the two binding sites were formed by a single p57 species. The protein binds also to the 5' untranslated region of other picornaviruses. Results from footprint analyses with foot-and-mouth disease RNA suggest the participation of additional cellular factors in the translation initiation complex.

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