Identification of a binding site for USF in FP18. (A) Cross competition between an oligonucleotide containing a known USF binding site (Table 1) and the FP18 oligonucleotide (K562 extract). Probes and competitors are indicated above the lanes. The amount of competitor added in lanes marked a, b, and c was 20, 40, and 80 ng, respectively. The band marked with the arrowhead is USF; the band marked with the small arrow is CSBP-2. The faster migrating complex formed on the USF probe was not identified, but this complex is also competed efficiently by FP18. (B) Antibody supershifts of the USF complex on the USF oligonucleotide and on FP18. Assays were performed as described in Materials and Methods. In each set of three lanes, lane 1 contains a standard binding assay, lane 2 contains 0.3 mL of anti USF antibody, and lane 3 contains a similar amount of pre-immune serum. Arrowhead, USF complex.