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Boundary elements interfere with communication between enhancers and promoters, but only when interposed. Understanding this activity will require identifying the proteins involved. The boundary element-associated factor BEAF is one protein that is implicated in boundary element function. Three genomic fragments (scs', BE76 and BE28) containing BEAF binding sites function as boundary elements in transgenic Drosophila, suggesting that this is an intrinsic property of the numerous genomic regions to which BEAF binds. To characterize additional proteins that interact with boundary elements, we have isolated a protein that binds to two of these boundary elements (BE76 and BE28) and have identified it as the transcription factor DREF. We present evidence that BEAF and DREF compete for binding to overlapping binding sites, and that this competition occurs in vivo. DREF is believed to regulate genes whose products are involved in DNA replication and cell prolife ration, suggesting that the activation of transcription predicted to result from the displacement of BEAF by DREF might be limited to certain rapidly proliferating tissues. This is the first suggestion that the activity of a subset of boundary elements might be regulated.

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