A consensus AP1 site forms DNA-protein complexes. (A, B, and C) Gel mobility shift experiments were performed with³²P end-labeled double-stranded oligonucleotide containing a single AP1 consensus binding site and 1 μg of nuclear extract from uninduced K562 cells (K562) or phorbol ester-induced K562 cells (4d Ind) (A, B, and C). A DNA-protein complex formed in the absence of competitive inhibitor (ø) (A, B, and C), in the presence of an identical oligonucleotide with a 2-bp mutation in the consensus binding site (AP1m) (A), in the presence of the unlabeled specific 75-bp enhancer region (bp −1503/−1578) with two tandem AP1 binding sites (C), or in the presence of an irrelevant double-stranded oligonucleotide containing the consensus binding site for transcription factors of the early growth response family (Egr) (B). The unlabeled AP1 oligonucleotide inhibited DNA-protein complex formation (A, B, and C). Monoclonal or polyclonal antibodies against either c-fos or c-jun (at the indicated concentration) were preincubated with nuclear extracts for 18 hours at 4°C before the addition of ³²P end-labeled probe, as described in Materials and Methods. Antisera against both c-fos and anti–c-jun inhibited DNA-protein complex formation in a concentration-dependent manner. The molar excess of unlabeled competitor or the concentration of antibody is indicated. Gel mobility shift analyses were performed as described in Materials and Methods.