The same nuclear proteins bind the proximal CACCC box of the human β-globin promoter and a similar sequence in the enhancer

Using in vitro assays, we show that nuclear proteins related to the Sp1 and GT-l factors bind to a CACCC box sequence in the human β-globin enhancer, adjacent to binding sites for the erythroid-specific factor NFE1 and the ubiquitous factor CP1. The same proteins are known to bind to the proximal, but not to the distal, CACCC box in the human β-globin promoter. A C G mutation in the promoter CACCC box, known to cause β-thalassemia, greatly decreases protein binding to the CACCC box; the same effect is obtained when this mutation is introduced into the enhancer CACCC box.