Gel mobility shift analysis of the megakaryocyte enhancer. (A and B) Gel mobility shift analysis was performed with³²P end-labeled double-stranded DNA fragment containing the 75-bp enhancer region (bp −1503/−1578) with two tandem AP1 binding sites and 1 μg of nuclear protein from either uninduced K562 cells (K562) or K562 cells induced for 4 days (4d Ind) with phorbol. Three major DNA protein complexes are indicated as A, B, and C. DNA-protein complexes were formed in the absence of competitive inhibitor (ø) (A and B), in the presence of a non-specific (NS) inhibitor consisting of 119-bp double-stranded DNA fragment from the coding region of the ₂ cDNA (A), or in the presence of 75-bp DNA fragment containing mutations to both AP1 binding sites (Dbl AP1m) (B). The presence of an unlabeled specific inhibitor (75 bp) inhibited DNA-protein complex formation (A and B). Gel mobility shift analysis was performed as described in Materials and Methods.