Cooperation of NF-Y and USF1/2 in binding to the HOXB4 promoter in vitro. Varying concentrations of recombinant GST-NF-Y or GST-USF1/2 protein, alone or in combination, were incubated with HOXB4 core promoter, and their interactions were tested in vitro by EMSA. Lanes 1 and 2 show the incubation of 2 and 0.7 ng GST-NF-Ya/b/c with ³²P-labeled HOXB4 promoter probe; in lane 3, GST-NF-Ya/b/c used in lane1 plus 1 μg NF-Ya/c antibodies was added to the reaction. Lanes 4, 5, and 6 show the addition of 6, 2, and 0.7 ng GST-USF1/2, respectively, in the reaction; in lane 7, 2 ng GST-USF1/2 and 1 μg USF1/2 antibodies were added. In lanes 8 and 9, the same amounts of GST-NF-Ya/b/c protein were added to reaction as in lanes 1 and 2, together with 2 ng GST-USF1/2.