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The E-box of the human glycophorin B promoter is involved in the erythroid-specific expression of the GPB gene.

Previous studies performed on the glycophorin B (GPB) expression demonstrated that this gene is expressed in erythroid cells only and that the ubiquitous factor Ku70 is involved in the process. Here, we investigated the contribution of the -70 E-box sequence toward the GPB promoter expression. We found that the E-box bound two factors, the USF1/USF2 protein and an unidentified ubiquitous protein which was named factor U. Site-directed mutagenesis performed on the -70 E-box showed that the USF factor had an activating effect in CAT assays. Conversely, mutation of the -70 E-box that impaired the binding of factor U led to a positive CAT activity in nonerythroid cells and thus to the loss of the erythroid-specific expression of the GPB gene. This indicates that, in addition to the Ku70 factor, the extinction of the GPB promoter expression in nonerythroid cells depends also on the repressing effect of the factor U.