FIG. 1.

(A and B) Mnt-Max is the predominant complex detected in EMSA using either Odc E-box1 (A) or E-box2 (B and C) probes, using extracts from proliferating FDC-P1 myeloid cells (A) growing (+ Serum) or quiescent (− Serum) BALB/c 3T3 fibroblasts (B). Arrowheads, diamonds, and circles indicate Mnt-Max, USF, and Max-Max complexes, respectively, as established by supershift analyses. Results using E-box probe 1 or 2 were comparable for all cell extracts (data not shown). (C) c-myc loss enhances Mnt-Max DNA binding activity in ES cells. EMSA was performed with extracts from the indicated ES cells. ✧ indicates a complex of unknown origin that is augmented in c-myc^−/− ES cells and which contains Max, since it is supershifted by an antiserum against Max. (D). c-myc loss compromises Odc expression in ES cells. RNA was prepared from exponentially growing ES cell cultures, and levels of c-myc and Odc transcripts were determined by real-time PCR. Oligonucleotides and probes for the detection of acidic ribosomal protein P0 mRNA (arp p0) were used as an internal control, since this gene is not regulated by Myc (17).