HIV-1 can escape from RNA interference by evolving an alternative structure in its RNA genome

HIV-1 replication can be efficiently inhibited by intra-cellular expression of an siRNA targeting the viral RNA. However, HIV-1 escape variants emerged after prolonged culturing. These RNAi-resistant viruses contain nucleotide substitutions or deletions in or near the targeted sequence. We observed an inverse correlation between the level of resistance and the stability of the siRNA/target-RNA duplex. However, two escape variants showed a higher level of resist-ance than expected based on the duplex stability. We demonstrate that these mutations induce alternative folding of the RNA such that the target sequence is occluded from binding to the siRNA, resulting in reduced RNAi efficiency. HIV-1 can thus escape from RNAi-mediated inhibition not only through nuc-leotide substitutions or deletions in the siRNA target sequence, but also through mutations that alter the local RNA secondary structure. The results highlight the enormous genetic flexibility of HIV-1 and provide detailed molecular insight into the sequence specifi-city of RNAi and the impact of target RNA secondary structure