Discrimination among potential activators of the beta-globin CACCC element by correlation of binding and transcriptional properties.

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Abstract
Adult beta-globin-like promoters contain a cis-acting element, CCACACCC, that is conserved across species and is required for wild-type levels of transcription. We have studied the contribution of this element and proteins that interact with it to activate beta-globin transcription. We found that an erythroid-like cell line, MEL, contains several proteins that specifically bind the CACCC element. By comparing the DNA-binding properties of promoters with mutations in the CACCC element with the transcriptional activities of these mutant promoters, we found that two CACCC-binding proteins did not bind to mutant promoters that direct decreased levels of transcription. One of these proteins is the transcriptional activator Sp1, and the other we have designated CACD (CACCC-binding species D). We subjected CACD to a binding site selection procedure and obtained high-affinity CACD binding sites that are identical to that of the beta-globin CACCC element. This result, combined with our finding that CACD binds the CACCC element with a higher affinity than does Sp1, argues that the CACCC element is a target of CACD rather than Sp1. The strategy of correlating the results of a binding site selection experiment with those of in vivo expression and in vitro binding studies may allow evaluation of the relative potential of different proteins to activate transcription through a single cis-acting site.