Interaction of West Nile virus NS5 with orthoflavivirus SLA RNAs and their effects on viral replication and inhibition

West Nile virus (WNV) is a single-stranded, positive-sense RNA virus in the Orthoflavivirus genus, within Flaviviridae. The genus encompasses numerous pathogens of public health importance, including WNV, dengue virus (DENV), Zika virus (ZIKV), and Japanese encephalitis virus (JEV). Orthoflavivirus replication depends on the presence of the stem-loop A (SLA) structure in the 5′ untranslated region of the genome. The viral polymerase, NS5, interacts with the SLA and initiates synthesis of the negative-strand RNA. The sequences and secondary structures of SLA and NS5 are highly conserved across orthoflaviviruses, suggesting that the viruses utilize a similar SLA-mediated replication mechanism. Here, we determined the molecular shapes of WNV and JEV SLAs and investigated WNV NS5 interaction with orthoflavivirus SLAs. Although WNV NS5 interacts with DENV, ZIKV, and JEV SLAs in binding assays, only DENV and ZIKV SLAs could replace WNV SLA for viral replication. Next, we found that the top and side loops of SLA are important regions for WNV NS5 interaction. Consequently, when these SLA mutations were introduced into a WNV replicon, genomic replication was greatly reduced. Finally, we tested whether the WNV SLA mimic could inhibit viral replication. The addition of exogenous SLA reduces replication of both WNV replicon and infectious virus, suggesting that exogenous SLA can outcompete the viral SLA for NS5 interaction. Next-generation sequencing data indicate that the presence of exogenous SLA during infection increased the genetic diversity of WNV.