Fig. 1.

The effect of U1C on U1 snRNA–5′ss base-pairing at normal and low temperatures. (A) RNA–RNA base-pairing assayed by psoralen cross-linking at both high and low temperatures. Standard commitment complex formation assays were performed in WT, U1C L13 mutant, and U1C depletion extracts with ³²P-radiolabeled WT-72 at the two temperatures. Psoralen cross-linking was carried out after 20 min of incubation as described (20). U1 snRNA–5′ss interactions and free RNA substrates are indicated by arrows. (B) Percentage of low-temperature base-pairing. The relative intensity of lanes 4–6 of A was measured, and the base-pairing in the U1C-depleted extract at low temperature was arbitrarily set to 100%.