Binding of recombinant C/EBPβ fusion protein and GATA-1 protein to the MBP promoter. A double-stranded MBP promoter oligonucleotide extending from bp −93 to −58 was end-labeled with [γ-³²P] ATP and incubated with 1 μg of double-stranded poly(dI-dC) in the presence of 1 μg maltose-binding protein (MBP)-C/EBPβ fusion protein (lanes 2 through 6 and 8 through 12) and 8 μg nuclear protein from COS7 cells that were transiently transfected with a GATA-1 expression vector (lanes 7 through 12). Unlabeled double-stranded competitor oligonucleotides, shown schematically in the lower panel, were added at a 100-fold molar excess over the labeled probe oligonucleotide, MBP bp −93 to −58 (competitor A; lanes 3 and 9), mutated GATA-consensus site oligonucleotide (competitor B; lanes 4 and 10), mutated C/EBP-consensus site oligonucleotide (competitor C; lanes 5 and 11), and mutated GATA-1 and C/EBP-consensus site oligonucleotide (competitor D; lanes 6 and 12).