Figure 5.

RNAP IIO facilitates, but IIA inhibits, spliceosome formation. Standard splicing reaction mixtures with β-globin pre-mRNA and S100 plus 250 ng of SR proteins in the presence of buffer (lanes 1–5), 80 ng of RNAP IIO (lanes 7–12) or 80 ng of RNAP IIA (lanes 13–18) were incubated for the time indicated above each lane, and splicing complexes were processed and fractionated by native gel electrophoresis as described. The positions of the free precursor RNA, nonspecific complex H, prespliceosomal complex A, early spliceosomal complex B, and catalytically active late spliceosomal complex C are indicated at right.