Fox-1 blocks formation of the pre-spliceosomal early (E) complex on intron 9 via binding to GCAUG element in intron 8. (A) Spliceosomal complex formation on the Ex9-10 + 3GCAUG (lanes 1–4) and the Ex9-10 ΔGCAUG (lanes 1, 2, 5 and 6) transcripts in Mock nuclear extracts (lanes 1 and 2) or nuclear extracts containing Flag-tagged mFox-1 (lanes 3, 4, 7 and 8) under normal splicing condition in the presence of ATP for the indicated time above each lane. Spliceosomal complexes were separated by 4% native polyacrylamide gels and position of each complex is indicated on the left. (B) Spliceosomal complex formation on the hF1γEx 9-10 transcripts in the absence of ATP. Transcripts of the Ex9-10 + 3GCAUG (lanes 1–4) and the Ex9-10 ΔGCAUG (lanes 5–8) were incubated in Mock nuclear extracts (lanes 1, 2, 5 and 6) or Fox-1 nuclear extracts (lanes 3, 4, 7 and 8) in the absence of ATP and separated on a 1.5% native agarose gel. Positions of pre-spliceosomal complexes E and H are indicated on the left.