FIG. 6.

hU3-55K binding to the U3 box B/C motif requires a distinct region on the surface of 15.5K. HeLa nuclear extract was preincubated with either 800 pmol of U4-SL1 RNA oligonucleotide (+) or buffer (−). Radiolabeled U3 box B/C motif RNA was subsequently added in the presence (lanes 3 to 8) or absence (lanes 1 and 2) of recombinant 15.5K or mutant 15.5K. The binding of hU3-55K was then assayed by immunoprecipitation (see Materials and Methods). Bound RNAs were recovered and then separated on an 8% polyacrylamide-7 M urea gel. Five percent of the RNA after incubation in nuclear extract was used as input in the bottom gel. The 15.5K protein used is indicated above each lane (15.5K-Wt, wild-type 15.5K). α-hU3-55K, anti-hU3-55K antibody.