Figure 4. Spliceosome dynamics during the catalytic steps.

Both catalytic steps can be divided into ATP-dependent and ATP-independent steps. Prp2 and Prp16 are required in ATP-dependent steps. Hydrolysis of ATP results in the release of SF3a/b and Yju2/Cwc25 in the first and second step respectively. This allows Yju2/Cwc25 and Slu7/Prp18/Prp22 to bind to the spliceosome and promote the first and second transesterification reactions respectively in an ATP-independent manner. Yellow circles with different colours in the centre represent spliceosomes with different sets of splicing factors binding at the catalytic centre.