Domains outside of the DNAbinding domain impart target gene specificity to myogenin and

Myogenin and MRF4 belong to the MyoD famiiy of muscle-specific transcription factors, which can activate myogenesis when introduced into nonmyogenic celis. These proteins share homology within a basic-helix-loop-helix motif that mediates DNA binding and dimerization, but they are divergent in their amino and carboxyl termini. Although myogenin and MRF4 bind the same sequence within the muscle creatine kinase enhancer, only myogenin efficiently transactivates this enhancer. By creating chimeras of myogenin and MRF4, we show that the specificities of these factors for transactivation of the muscle creatine kinase enhancer can be interchanged by swapping their amino and carboxyl termini. Within these chimeras, strong cooperation between the amino and carboxyl termini was observed. These findings suggest that myogenin and MRF4 discriminate between muscle-specific enhancers and that target gene specificity is determined by domains surrounding the basic-helix-loop-helix region. The MyoD family includes MyoD (12), myogenin (13, 35), myf5 (2), and MRF4 (1, 19, 24), each of which is expressed exclusively in skeletal muscle and can activate myogenesis when introduced into a variety of nonmyogenic cell types (reviewed in references 22 and 31). These factors share extensive homology within a segment of about 70 amino acids that encompasses a basic-helix-loop-helix (bHLH) motif that mediates DNA binding and dimerization (4, 11, 17, 20); however, these factors show little or no homology outside of this region. Members of the MyoD family bind DNA preferentially as heterooligomers with the widely expressed HLH proteins E12 and E47 (3, 5, 8, 9, 11, 20, 21). These heterooligomers bind with high affinity to the se-quence CANNTG, referred to as an E-box, which is present in the control regions of numerous muscle-specific gene