LEF-1 contains an activation domain that stimulates transcription only in a specific context of factor-binding sites

Lymphoid enhancer factor 1 (LEF-1) is a member of the high mobility group (HMG) family of proteins and participates in the regulation of the T cell receptor (TCR) alpha enhancer. We have previously shown that DNA binding by the HMG domain of LEF-1 induces a sharp bend in the DNA helix. Together with the dependence of LEF-1 on other factor-binding sites to regulate gene expression, DNA bending induced by the HMG domain suggested an 'architectural' role for LEF-1. In this study, we performed experiments to distinguish between a model in which the HMG domain is the only functional determinant of LEF-1 and a model in which additional domains of LEF-1 are involved in the regulation of gene expression. First, we show that the HMG domain alone is not sufficient to stimulate TCR alpha enhancer function. Second, we replaced the HMG domain of LEF-1 with the DNA-binding domain of the bacterial repressor LexA, which binds a specific nucleotide sequence without inducing a sharp bend in the DNA helix. The chimeric LEF-LexA protein increased the activity of a TCR alpha enhancer in which the LEF-1-binding site had been replaced with a LexA recognition sequence. Transcriptional stimulation by LEF-LexA, however, was less efficient than that observed with endogenous LEF-1. The LEF-LexA-mediated activation of gene expression was dependent upon an amino-terminal region of LEF-1 and a specific context of factor-binding sites in the TCR alpha enhancer. Neither multimerized LexA-binding sites, nor TCR alpha enhancers with altered spatial arrangements of factor-binding sites, were functional for regulation by LEF-LexA. Together, these data suggest that an aminoterminal region in LEF-1 contributes to the context-dependent regulation of the TCR alpha enhancer by LEF-1, presumably by interacting with other enhancer-bound proteins.