Short Double-stranded RNAs with an Overhanging 5′ ppp-Nucleotide, as Found in Arenavirus Genomes, Act as RIG-I Decoys*

Arenavirus RNA genomes are initiated by a “prime and realign” mechanism, such that the initiating GTP is found as a single unpaired (overhanging) nucleotide when the complementary genome ends anneal to form double-stranded (ds) RNA panhandle structures. dsRNAs modeled on these structures do not induce interferon (IFN), as opposed to blunt-ended 5′ pppdsRNA. This study examines whether these viral structures can also act as decoys, by trapping RIG-I in inactive dsRNA complexes. We examined the ability of various dsRNAs to activate the RIG-I ATPase (presumably a measure of helicase translocation on dsRNA) relative to their ability to induce IFN. We found that there is no simple relationship between these two properties, as if RIG-I can translocate on short dsRNAs without inducing IFN. Moreover, we found that 5′ pppdsRNAs with a single unpaired 5′ ppp-nucleotide can in fact competitively inhibit the ability of blunt-ended 5′ pppdsRNAs to induce IFN when co-transfected into cells and that this inhibition is strongly dependent on the presence of the 5′ ppp. In contrast, 5′ pppdsRNAs with a single unpaired 5′ ppp-nucleotide does not inhibit poly(I-C)-induced IFN activation, which is independent of the presence of a 5′ ppp group.