Promoter activation by enhancer-dependent and -independent loading of activator and coactivator complexes.

Activation of the pDbeta1 promoter at the TCRbeta locus requires a functional distal enhancer, Ebeta. Here, we have analyzed the mechanism of promoter activation in thymocytes from mice containing or lacking Ebeta. We found that pDbeta1 shows a complex profile of transcription factor and chromatin remodeling complex occupancy even at Ebeta(-) alleles. The presence of Ebeta, however, results in a few specific changes in factor occupancy at the promoter. These differences correlate with localized alterations in histone modifications and in the recruitment of the basal transcriptional machinery. In addition, Ebeta is also bound by CBP and Pol II, suggesting a mechanism for delivery of a holoenzyme complex to the pDbeta1 promoter. These results illustrate a specialized, long-range function of an enhancer in the hierarchical events that regulate assembly of a cell type-specific promoter.