A CONSERVED HELICAL ELEMENT IS ESSENTIAL FOR INTERNAL INITIATION OF TRANSLATION OF HEPATITIS-C VIRUS-RNA

Translation of hepatitis C virus (HCV) RNA is initiated by cap-indepen dent internal ribosome binding to the 5' noncoding region (NCR). To id entify the sequences and structural elements within the 5' NCR of HCV RNA that contribute to the initiation of translation, a series of poin t mutations was introduced within this sequence. Since the pyrimidine- rich tract is considered a characteristic feature of picornavirus inte rnal ribosome entry site (IRES) elements, our mutational analysis focu sed on two putative pyrimidine tracts (Py-I and Py-II) within the HCV 5' NCR, Translational efficiency of these mutant RNAs was examined by in vitro translation and after RNA transfection into liver-derived cel ls. Mutational analysis of Py-I (nucleotides 120 to 130), supported by compensatory mutants, demonstrates that the primary sequence of this motif is not important but that a helical structural element associate d with this region is critical for HCV IRES function. Mutations in Py- II (nucleotides 191 to 199) show that this motif is dispensable for IR ES function as well. Thus, the pyrimidine-rich tract motif, which is c onsidered as an essential element of the picornavirus IRES elements, d oes not appear to be a functional component of the HCV IRES. Further, the insertional mutagenesis study suggests a requirement for proper sp acing between the initiator AUG and the upstream structures of the HCV IRES element for internal initiation of translation.