Figure 1. Pre-mRNA splicing by the spliceosome. (A) Model of pre-mRNA splicing by the spliceosome depicting known assembly and catalysis intermediates. ATPases/RNA helicases and a G-protein (Snu114) are shown where their activities are required (red and pink, respectively). Red traffic lights indicate stages at which Brr2 is expected to be shut off, whereas a green traffic light indicates Brr2 being switched on during catalytic activation. Presently, it is not clear if Brr2 is actively shut off again after spliceosome catalytic activation. (B) Changes in the snRNA–pre-mRNA interaction network during spliceosome catalytic activation, highlighting the presumed substrates of the Prp28 and Brr2 RNA helicases.