Predicted secondary structure of the hepatitis G virus and GB virus-A 5'untranslated regions consistent with an internal ribosome entry site.

We describe comparative sequence analysis of 20 isolates of the recently discovered hepatitis G virus (HGV) and propose a model of the RNA secondary structure at the 3' end of the 5' untranslated region (UTR) of this virus. A single AUG starting at nucleotide position 552, which was in-frame and continuous with the putative polyprotein, was conserved in all 20 isolates and appeared to be the most likely site for the initiation of polyprotein synthesis. This consensus AUG was 14 amino acid residues upstream of a sequence with identity to the envelope protein E1 of hepatitis C virus (HCV), but the actual function of this N-terminal peptide of HGV is still not certain. None of the isolates encoded a sequence with similarity to the nucleocapsid protein of any known virus. The RNA secondary structure of the fragment under study was determined using thermodynamic modelling and validated using the results of comparative sequence analysis. Further support for the model was gained from the prediction of significant sequence and structural homology in the RNA secondary structure of the complete 5'-UTR of GB virus-A (GBV-A). A possible mechanism for translation initiation in HGV and GBV-A was suggested by the identification of features in common with the internal ribosome entry sites (IRESs) of HCV and picornaviruses.