SF2/ASF UV cross-linked to the sequences at the junction of exon 2′/2. (A) UV cross-linking templates consisting of the wild-type (WT) or mutated (Mu1, Mu2) junction of exon 2′/2 and its flanking sequences. (B) Purified SF2/ASF used in the cross-linking experiments. (C) WT or mutant transcripts cross-linked to HeLa nuclear extract or SF2/ASF. ³²P-labeled transcripts were subjected to UV cross-linking by the use of indicated nuclear extracts or purified SF2/ASF in the presence of tRNA as a nonspecific competitor. NE indicates nuclear extracts. (D) For competition assay, 25-fold molar excess of unlabeled WT or Mu1 and Mu2 transcripts was added in binding reactions. − indicates that the probe was incubated in the absence of competitors.