Abstract
MyoD is a muscle-specific transcriptional activator; E12 is a B-cell activator. An IgH enhancer is activated almost 100-fold by E12 but not at all by Myo; an MCK enhancer is activated almost 1000-fold by MyoD and not at all by E12. MyoD and E12 are both basic helix-loop-helix proteins that bind to similar E-box sequences (CANNTG); the IgH enhancer contains the same E boxes as the MCK enhancer, yet each retains exclusive specificity for either E12 or MyoD, respectively. We show that the IgH enhancer contains a cis-acting negative element that is directed at MyoD, but not at E12. This repression requires the mu E5 E box within the IgH enhancer; however, the specificity for repression, as opposed to activation, is associated with 2 bp flanking each side of the mu E5 E box. The target for repression of MyoD in the IgH enhancer is the bHLH region of MyoD. Our results suggest that MyoD only activates myogenic genes because nonmuscle enhancers that contain E boxes also contain negative elements that prevent MyoD activity.
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