v-Myb binds to the Mcm4 promoter region in vitro. A. Electrophoretic mobility shift experiments were performed using bacterially expressed full-length v-Myb protein and radiolabeled oligonucleotides corresponding to Myb binding sites MBS-1, MBS-4 and MBS-6. Binding reactions were performed without Myb protein (lanes 1) or with increasing amounts of v-Myb (lanes 2 to 4). Complexes of full-length v-Myb and the oligonucleotides are marked by an arrowhead. The band at the bottom corresponds to unbound oligonucleotides. B. Radiolabeled oligonucleotide containing the Myb binding site A of the mim-1 promoter were incubated with bacterially expressed full-length v-Myb and excess of unlabeled competitor oligonucleotides, as indicated at the top. Competitors were used at 40-fold or 80-fold molar excess of unlabeled over labeled oligonucleotide, respectively. Only the upper part of the gel containing the complex of v-Myb and the radioactive oligonucleotide is shown.