An unexpected RNA distal interaction mode found in an essential region of the hepatitis C virus genome

The 3'X tail is a functionally essential 98-nt sequence located at the 3'-end of the hepatitis C virus (HCV) RNA genome. The domain contains two absolutely conserved dimer linkage sequence (DLS) and k nucleotide segments involved in viral RNA dimerization and in a distal base-pairing interaction with stem-loop 5BSL3.2, respectively. We have previously shown that domain 3'X forms an elongated structure comprising two coaxially stacked SL1' and SL2' stem-loops. This conformation favors RNA dimerization by exposing a palindromic DLS segment in an apical loop, but buries in the upper stem of hairpin SL2' the k nucleotides involved in the distal contact with 5BSL3.2. Using nuclear magnetic resonance spectroscopy and gel electrophoresis experiments, here we show that the establishment of the complex between domain 3'X and stem-loop 5BSL3.2 only requires a rearrangement of the nucleotides forming the upper region of subdomain SL2'. The results indicate that the interaction does not occur through a canonical kissing loop mechanism involving the unpaired nucleotides of two terminal loops, but rather involves a base-paired stem and an apical loop and may result in a kissing three-way junction. On the basis of this information we suggest how the 3'X tail switches between monomer, homodimer and heterodimer states to regulate the HCV viral cycle.