Tcra Enhancer Activation by Inducible Transcription Factors Downstream of Pre-TCR Signaling

The Tcra enhancer (E alpha) is essential for pre-TCR-mediated activation of germline transcription and V(D)J recombination. E alpha is considered an archetypical enhanceosome that acts through the functional synergy and cooperative binding of multiple transcription factors. Based on dimethylsulfate genomic footprinting experiments, there has been a long-standing paradox regarding E alpha activation in the absence of differences in enhancer occupancy. Our data provide the molecular mechanism of E alpha activation and an explanation of this paradox. We found that germline transcriptional activation of Tcra is dependent on constant phospholipase C gamma, as well as calcineurin- and MAPK/ERK-mediated signaling, indicating that inducible transcription factors are crucially involved. NFAT, AP-1, and early growth response factor 1, together with CREB-binding protein/p300 coactivators, bind to E alpha as part of an active enhanceosome assembled during pre-TCR signaling. We favor a scenario in which the binding of lymphoid-restricted and constitutive transcription factors to E alpha prior to its activation forms a regulatory scaffold to recruit factors induced by pre-TCR signaling. Thus, the combinatorial assembly of tissue-and signal-specific transcription factors dictates the E alpha function. This mechanism for enhancer activation may represent a general paradigm in tissue-restricted and stimulus-responsive gene regulation. The Journal of Immunology, 2012, 188: 3278-3293.