Figure 1. Secondary structures of the spliceosomal snRNAs. Close-set parallel lines indicate RNA stem structures held together by Watson–Crick base pairing. The 5¢ end cap structures of the RNAs are indicated by blue balls for 2,2,7-trimethylguanosine and pink balls for -monomethylguanosine caps. Conserved regions of the snRNAs that are involved in RNA–RNA interactions in the spliceosome are shaded in yellow and the sites of Sm protein binding are shaded in pink.

Figure 2. The pre-mRNA splicing reaction and snRNA interactions in the spliceosomes. (a) Schematic of the two-step spliceosomal splicing reaction described in the text. (b) RNA–RNA interactions in the major, U2-type spliceosome. Left: the initial interactions of the pre-mRNA with U1 and U2 snRNPs. Right: the interactions in the mature spliceosome. Yellow bars indicate regions that interact by base pairing. The adenosine residue at the branch site is circled. (c) RNA–RNA interactions in the minor, U12-type spliceosome.