Fig. 4.

Competition binding studies demonstrate that a pyrimidine-to-purine substitution in the core UCCCCU binding site does not efficiently bind MN9D proteins. MN9D cytoplasmic proteins were incubated with radiolabeled RNA probe encoding TH mRNA 3′-UTR sequences between the KpnI and SphI sites. Different amounts of wild-type or mutant RNAs encoding the same sequence as the radiolabeled probe were incubated with the proteins for 15 min before the addition of probe. A 160-nt RNA encoding 28S rRNA sequences was used as a nonspecific competitor. The sequence of the probe and the competitor RNAs are depicted in the figure.