Structure of a yeast step II catalytically activated spliceosome

Abstract

Poised for the second step of splicing In eukaryotes, noncoding sequences in transcribed precursor mRNA are cut out by a dynamic macromolecular machine, the spliceosome. This involves two sequential reactions. The first cuts one end of the noncoding intron and loops it back on itself to form an intron lariat, and the next excises the intron and ligates the coding mRNA. Insights into the first step of splicing have come from the structures of two intermediates: the Bact complex, which is primed for catalysis, and the C complex, which is formed after the first splicing reaction. Yan et al. now report a high-resolution structure of the step II catalytically activated spliceosome (the C* complex). This structure shows conformational changes that position catalytic motifs to accomplish the second splicing reaction. Science , this issue p. 149