FIG. 3.

CELF proteins bind specifically to a downstream MSE-containing region. UV cross-linking experiments were performed using purified recombinant GST-CELF fusion proteins and uniformly ³²P-labeled RNA (G and U) containing partial or full-length segments of the cTNT-regulated region. Proteins and RNA were preincubated in HeLa nuclear extract to increase the specificity of binding. Similar results were obtained when tRNA, bovine serum albumin, and heparin were used instead of HeLa nuclear extract (not shown). Each lane contains equal molar amounts of RNA. An approximately 100-kDa protein that bound nonspecifically to all three RNA substrates in HeLa cell nuclear extracts is also shown.