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Indeed, RdRp activity has been demonstrated with recombinant NS5B ( 3). The HCV NS5B protein, located at the extreme C terminus of the polyprotein, contains motifs shared by all RdRps, such as the GDD motif ( 2). The key enzyme involved in both of these steps is a virally encoded RNA-dependent RNA polymerase (RdRp). Genome replication proceeds in two steps: synthesis of complementary minus-strand RNA using the genome as template and the subsequent synthesis of genomic RNA using this minus-strand RNA template. The viral genome contains a single ORF encoding a polyprotein of ≈3,000 amino acids ( 1) that is the precursor to all viral proteins.
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HCV is a positive-strand RNA virus in the Flaviviridae family. There is thus an urgent need to develop HCV-specific antiviral agents to counteract this important public health problem. There is no vaccine available against HCV, and current therapies are effective only in a minority of cases. The virus is capable of establishing a persistent infection in the majority of cases, leading to chronic hepatitis that often develops into cirrhosis and, in many cases, into hepatocellular carcinoma. Hepatitis C virus (HCV) is a major human pathogen that has infected an estimated 3% of the population worldwide. It also suggests a rearrangement of the thumb domain as well as a possible concerted movement of thumb and fingertips during translocation of the RNA template-primer in successive polymerization rounds. This superposition reveals the majority of the amino acid residues of the hepatitis C virus enzyme that are likely to be implicated in binding to the replicating RNA molecule and to the incoming NTP. In addition, it shows that the hepatitis C virus polymerase was crystallized in a closed fingers conformation, similar to HIV-1 reverse transcriptase in ternary complex with DNA and dTTP. Superposition to the available structures of the latter shows that residues from the palm and fingertips are structurally equivalent. The fingers subdomain contains a region, the “fingertips,” that shares the same fold with reverse transcriptases. The structure of the catalytic domain contains 531 residues folded in the characteristic fingers, palm, and thumb subdomains. This enzyme is a key target for developing specific antiviral therapy. We report the crystal structure of the RNA-dependent RNA polymerase of hepatitis C virus, a major human pathogen, to 2.8-Å resolution.