The 3.8 Å structure of the U4/U6.U5 tri-snRNP: Insights into spliceosome assembly and catalysis

Structure of a key spliceosomal complex In eukaryotes, when DNA is transcribed into RNA, the primary transcript has protein-coding sequences interrupted by noncoding sequences called introns. Introns are removed by a complex molecular machine, the spliceosome. Wan et al. determined the structure of a key subcomplex, the tri-SNP, that comprises three small nuclear RNAs and more than 30 proteins. The structure, determined by electron microscopy at 3.8 Å resolution, unexpectedly shows a primary RNA transcript bound in the tri-SNP. Further analysis revealed how the spliceosome assembles to achieve its complex functions. Science , this issue p. 466