Fig. 1.

Schematic representation of the first four steps of pre-mRNA splicing by the major spliceosome, with PRPF31 shown in red. In step 1, U1snRNP recognises and binds the splice donor site (the 5′ splice site). In step 2, binding of U1snRNP to the splice donor site promotes the binding of U2snRNP to the branch site. Independently of this, the U4/U6.U5 tri-snRNP forms in the cell. In step 3, the U4/U6.U5 tri-snRNP is recruited to the pre-mRNA, where U6snRNP replaces U1snRNP. This forms the catalytically active spliceosome, which in step 4 cuts away the intron and joins the exons through two transesterification reactions. (For interpretation of the references to colour in this figure legend, the reader is referred to the Web version of this article.)