GABP and PU.1 compete for binding to the mNE promoter ets site. (A) EMSA was performed with the mNE promoter probe and purified bacterially expressed GST-GABPα and GST-PU.1. Filled arrow, binding by GABPα (lanes 2 to 5). Addition of increasing amounts of PU.1 generated a new binding complex (open arrow, lanes 3 to 5) that competed with GABPα for the radiolabeled mNE promoter probe. No novel complex that might represent a direct physical interaction of GABPα and PU.1 was seen. (B) Open arrow indicates binding by PU.1 (lanes 2 to 5). Addition of increasing amounts of GABPα (filled arrow, lanes 3 to 5) competed with PU.1 for the radiolabeled mNE promoter probe.