The 5' nontranslated region (NTR) of hepatitis C virus (HCV) consists of 341 nucleotides (nt). This region comprises the majority of the internal ribosome entry site (IRES) which controls the efficiency of viral translation. Previous studies of the 3' boundary of the HCV IRES yielded conflicting data regarding the involvement of viral coding sequences in IRES activity. We therefore studied the functional significance of the 5' proximal coding sequences of the HCV core gene on IRES activity. We constructed monocistronic and bicistronic DNAs that contained either a chloramphenicol acetyl transferase (CAT) gene or a luciferase (Luc) gene as the reporter. Results from both in vitro and in vivo experiments indicated that the optimal IRES ranged within nt 1-371. Further mutational analyses of sequences surrounding the initiation codon revealed that primary sequences downstream of the AUG initiator rather than the secondary structure are important in regulating optimal IRES function. We are also able to demonstrate that a non-AUG codon could be used to initiate the synthesis of a reporter protein, albeit with lower efficiency. These findings bear important implications for the HCV IRES secondary structures.
Copyright 1998 Academic Press.