DNA binding of USF is required for specific E-box dependent gene activation in vivo.

Although USF-1 and -2 are the major proteins that bind to Myc-regulated E-box (CACGTG) elements in many cells, there is no clear role for USF during Myc-dependent gene regulation. Using dominant negative alleles of USF-1 we now show that DNA binding by USF at a Myc-regulated E-box limits the ability of another E-box binding factor, TFE-3, to activate a target gene of Myc in vivo and to stimulate S phase entry in resting fibroblasts. Similarly, dominant negative alleles of USF-1 relieve the restriction that prevents activation of the IgH enhancer by TFE-3 in non B-cells. DNA binding activity of USF complexes is abundant in primary human B-cells and is significantly downregulated during B-cell immortalization. Re-expression of USF-1 in immortalized B-cells retards proliferation. Our data establish an essential role for USF in restricting E-box dependent gene activation in vivo and suggest that this control is relaxed during cellular immortalization.