Regulation of pT alpha gene expression by a dosage of E2A, HEB, and SCL.

The expression of the pT alpha gene is required for effective selection, proliferation, and survival of beta T-cell receptor (beta TCR)-expressing immature thymocytes. Here, we have identified two phylogenetically conserved E-boxes within the pT alpha enhancer sequence that are required for optimal enhancer activity and for its stage-specific activity in immature T cells. We have shown that the transcription factors E2A and HEB associate with high affinity to these E-boxes. Moreover, we have identified pT alpha as a direct target of E2A-HEB heterodimers in immature thymocytes because they specifically occupy the enhancer in vivo. In these cells, pT alpha mRNA levels are determined by the presence of one or two functional E2A or HEB alleles. Furthermore, E2A/HEB transcriptional activity is repressed by heterodimerization with SCL, a transcription factor that is turned off in differentiating thymocytes exactly at a stage when pT alpha is up-regulated. Taken together, our observations suggest that the dosage of E2A, HEB, and SCL determines pT alpha gene expression in immature T cells.