Lymphocytes regulate their responsiveness to IL-2 through the transcriptional control of the IL-2Rα gene, which encodes a component of the high affinity IL-2 receptor. In the mouse IL-2Rα gene this control is exerted via two regulatable elements, a promoter proximal region, and an IL-2–responsive enhancer (IL-2rE) 1.3 kb upstream. In vitro and in vivo functional analysis of the IL-2rE in the rodent thymic lymphoma-derived, CD4−CD8− cell line PC60 demonstrated that three separate elements, sites I, II, and III, were necessary for IL-2 responsiveness; these three sites demonstrate functional cooperation. Site III contains a consensus binding motif for members of the Ets family of transcription factors. Here we demonstrate that Elf-1, an Ets-like protein, binds to site III and participates in IL-2 responsiveness. In vitro site III forms a complex with a protein constitutively present in nuclear extracts from PC60 cells as well as from normal CD4−CD8− thymocytes. We have identified this molecule as Elf-1 according to a number of criteria. The complex possesses an identical electrophoretic mobility to that formed by recombinant Elf-1 protein and is super-shifted by anti–Elf-1 antibodies
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