TISSUE-SPECIFIC GENE ACTIVATION BY MYOD - DETERMINATION OF SPECIFICITY BY CIS-ACTING REPRESSION ELEMENTS

MyoD is a muscle-specific transcriptional activator; E12 is a B-cell a ctivator. An IgH enhancer is activated almost 100-fold by E12 but not at all by MyoD; 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 pr oteins that bind to similar E-box sequences (CANNTG); the IgH enhancer contains the same E boxes as the MCK enhancer, yet each retains exclu sive specificity for either E12 or MyoD, respectively. We show that th e 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 wit hin the IgH enhancer; however, the specificity for repression, as oppo sed to activation, is associated with 2 bp flanking each side of the m u E5 E box. The target for repression of MyoD in the IgH enhancer is t he bHLH region of MyoD. Our results suggest that MyoD only activates m yogenic genes because nonmuscle enhancers that contain E boxes also co ntain negative elements that prevent MyoD activity.