Structure of a human catalytic step I spliceosome

Structure of the human spliceosome Catalyzed by the spliceosome, precursor mRNA splicing proceeds in two steps: branching and exon ligation. Transition from the C (catalytic post-branching spliceosome) to the C* (catalytic pre-exon ligation spliceosome) complex is driven by the adenosine triphosphatase/helicase Prp16. Zhan et al. report the cryo-electron microscopy structure of the human C complex, showing that two step I splicing factors stabilize the active site and link it to Prp16. Science , this issue p. 537