Figure 2. SR proteins bind exon 23 enhancer elements.

³²P labeled D90 RNA was incubated at 30°C for 10 min with purified SR proteins prepared from calf thymus as previously described [63]. Binding reactions performed in the absence or in the presence of increasing concentrations of cold competitor RNAs (with molar excess of 30, 60, 90 and 120 fold, as indicated by the gradients above the gel lanes) were fractionated on a 4% non-denaturing polyacrylamide gel. The schematic representation of the RNAs is depicted as in Figure 1. The positions of free RNA and bound complexes are shown. Comparable results were obtained in two independent experiments.