Figure 3.

Dynamic network of RNA-RNA interactions in the spliceosome. (A) Exon sequences are indicated by grey boxes and intron sequences by a thin black line. snRNAs are shown schematically (secondary structure as observed in mammals) in grey or black, with those regions engaging in base pairing interactions (indicated by short lines) highlighted in color (not drawn to scale). The 5′ end of the snRNAs is indicated by a black dot. Solely loop 1 of the U5 snRNA is shown. During the transition from a precatalytic spliceosome (upper diagram) to a catalytically activated spliceosome (lower diagram) U1 and U4 are displaced, and U6 and U2 engage in novel base pairing interactions. (B) Conformational toggling of the yeast U2 snRNA. Two mutually exclusive stem structures (stem IIa and stem IIc) are thought to form within the U2 snRNA at different stages of splicing. Solely the 5′ end of the U2 snRNA is shown schematically.