Function and factor interactions of a locus control region element in the mouse T cell receptor-alpha/Dad1 gene locus.

Locus control regions (LCRs) refer to cis-acting elements composed of several DNase I hypersensitive sites, which synergize to protect transgenes from integration-site dependent effects in a tissue-specific manner. LCRs have been identified in many immunologically important gene loci, including one between the TCRdelta/TCRalpha gene segments and the ubiquitously expressed Dad1 gene. Expression of a transgene under the control of all the LCR elements is T cell specific. However, a subfragment of this LCR is functional in a wide variety of tissues. How a ubiquitously active element can participate in tissue-restricted LCR activity is not clear. In this study, we localize the ubiquitously active sequences of the TCR-alpha LCR to an 800-bp region containing a prominent DNase hypersensitive site. In isolation, the activity in this region suppresses position effect transgene silencing in many tissues. A combination of in vivo footprint examination of this element in widely active transgene and EMSAs revealed tissue-unrestricted factor occupancy patterns and binding of several ubiquitously expressed transcription factors. In contrast, tissue-specific, differential protein occupancies at this element were observed in the endogenous locus or full-length LCR transgene. We identified tissue-restricted AML-1 and Elf-1 as proteins that potentially act via this element. These data demonstrate that a widely active LCR module can synergize with other LCR components to produce tissue-specific LCR activity through differential protein occupancy and function and provide evidence to support a role for this LCR module in the regulation of both TCR and Dad1 genes.