Figure 1

Model for NRS effects on splicing and polyadenylation. Schematic of RSV RNA with exons depicted as boxes and introns shown as thin lines. The light shading represents the upstream SR protein binding region of the bipartite NRS, and the darker shading depicts the region that binds U1 snRNP. SR proteins promote U1 binding, which initiates early interactions with factors associated with the viral 3' splice site (env in this example), and this is thought to mature into a spliceosome-like NRS inhibitory complex (indicated by the large oval) that forms between the NRS and the viral 3' splice site but which is catalytically inactive (an X over the intron); a possible role for U6 snRNP is indicated by the question mark. The NRS complex sequesters the 3'ss from interacting with the authentic viral 5'ss to block splicing. The NRS complex may influence polyadenylation by serving to stabilize the binding of splicing factors to the weak viral 3' splice site, which can then either recruit or stabilize the polyadenylation complex (arrow) and thereby enhance polyadenylation of viral unspliced RNA. U11 snRNP modulates NRS function by antagonizing U1 binding and assembly of the NRS inhibitory complex. A downstream region (intermediate shading) binds hnRNP H, which recruits U11 to a site that overlaps the U1 binding site.