Differential trans-activation of muscle-specific regulatory elements including the mysosin light chain box by chicken MyoD, myogenin, and MRF4.

We have isolated cDNAs encoding a chicken homologue of MRF4 (cMRF4) in addition to chicken MyoD (CMD1) and myogenin (c-myogenin) described previously. In an attempt to understand the roles that cMRF4, CMD1, and c-myogenin play in chicken myogenesis, the effects of these factors on muscle-specific cis-elements identified in regulatory regions of myosin alkali light chain (MLC) genes were examined. The promoter analysis of some of MLC genes has revealed two sorts of muscle-specific positive regulatory elements to date, an enhancer located upstream of the adult type LC1 gene and a cis-element, termed an MLC box, conserved among promoters of various MLC genes. The LC1 enhancer was exclusively trans-activated by CMD1. Although c-myogenin also activated transcription driven by the LC1 promoter, it was suggested that c-myogenin requires a cis-element(s) other than the CMD1-responsive enhancer. Chicken MRF4 could not trans-activate any of the constructs containing the LC1 promoter. In contrast, the promoter of the embryonic L23 gene was trans-activated by all of the three factors. From deletion and mutation analysis, the MLC box was shown to be involved in their positive regulation. These results extend previous observations that individual myogenic regulatory factors exhibit different capabilities in transcriptional activation of muscle-specific genes by acting distinctively upon their regulatory elements.