Fig. 3. The mutations that inhibit polyadenylation in vivo affect U1A binding in vitro. Electrophoretic mobility shift assays showing the binding of increasing amounts of recombinant U1A to (A) pre-cleaved immunoglobulin substrates containing either the wild-type sequence or multiple mutations as indicated; and (B) the wild-type or mutated PIE element in the 3′-UTR of U1A mRNA.