Differential trans-activation of a muscle-specific enhancer by myogenic helix-loop-helix proteins is separable from DNA binding.

The muscle creatine kinase (MCK) enhancer was used as a target to study the specificity of DNA binding and trans-activation by members of the helix-loop-helix (HLH) family of myogenic regulatory factors, MyoD1, myogenin, myf-5, and MRF4. Whereas all four myogenic factors bound with similar affinities to the MCK enhancer in the presence of the widely expressed HLH protein E12, only MyoD1, myogenin, and myf-5 efficiently trans-activated the enhancer in transiently transfected 10T1/2 and 3T3 cells. That MRF4 binds the MCK enhancer without activating transcription suggests that domains in addition to those required for DNA binding are important for transcriptional activation and supports the notion that the different members of the HLH family of myogenic regulatory factors may selectively regulate unique sets of muscle-specific genes.