In vivo analysis of the 3 ' untranslated region of the hepatitis C virus after in vitro mutagenesis of an infectious cDNA clone

Large sections of the 3' untranslated region (UTR) of hepatitis C virus (HC V) were deleted from an infectious cDNA clone, and the RNA transcripts from seven deletion mutants were tested sequentially for infectivity in a chimp anzee. Mutants lacking all or part of the 3' terminal conserved region or t he poly(U-UC) region were unable to infect the chimpanzee, indicating that both regions are critical for infectivity in vivo. However, the third regio n, the variable region, was able to tolerate a deletion that destroyed the two putative stem-loop structures within this region. Mutant VR-24 containi ng a deletion of the proximal 24 nt of the variable region of the 3' UTR wa s viable in the chimpanzee and seemed to replicate as well as the undeleted parent virus. The chimpanzee became viremic 1 week after inoculation with mutant VR-24, and the HCV genome titer increased over time during the early acute infection. Therefore, the poly(U-UC) region and the conserved region , but not the variable region, of the 3' UTR seem to be critical for in viv o infectivity of HCV.