Identification of cis-Acting Nucleotides and a Structural Feature in West Nile Virus 3′-Terminus RNA That Facilitate Viral Minus Strand RNA Synthesis

The 3'-terminal nucleotides (nt) of West Nile virus (WNV) genomic RNA form a penultimate 16-nt small stem-loop (SSL) and an 80-nt terminal stem-loop (SL). These RNA structures are conserved in divergent flavivirus genomes. A previous in vitro study using truncated WNV 3' RNA structures predicted a putative tertiary interaction between the 5= side of the 3=-terminal SL and the loop of the SSL. Although substitution or deletion of the 3'G (nt 87) within the SSL loop, which forms the only G-C pair in the predicted tertiary interaction, in a WNV infectious clone was lethal, a finding consistent with the involvement in a functionally relevant pseudoknot interaction, extensive mutagenesis of nucleotides in the terminal SL did not identify a cis-acting pairing partner for this SSL 3'G. However, both the sequence and the structural context of two adjacent base pairs flanked by symmetrical internal loops in the 3'-terminal SL were shown to be required for efficient viral RNA replication. Nuclear magnetic resonance analysis confirmed the predicted SSL and SL structures but not the tertiary interaction. The SSL was previously reported to contain one of three eEF1A binding sites, and G87 in the SSL loop was shown to be involved in eEF1A binding. The nucleotides at the bottom part of the 3'-terminal SL switch between 3' RNA-RNA and 3'-5' RNA-RNA interactions. The data suggest that interaction of the 3' SL RNA with eEF1A at three sites and a unique metastable structural feature may participate in regulating structural changes in the 3'-terminal SL.