MYOD AND MYOGENIN ACT ON THE CHICKEN MYOSIN LIGHT-CHAIN 1 GENE AS DISTINCT TRANSCRIPTIONAL FACTORS

Expression of MyoD, myogenin, MRF4, and Myf-5 converts nonmuscle cells to muscle cells. In an attempt to analyze the roles of these factors, we have investigated their effects on transcription driven by the pro moter of the chicken myosin alkaline light-chain (MLC1) gene. The acti vation by CMD1 or c-myogenin (chicken MyoD or myogenin, respectively) was dependent on the existence of a muscle-specific regulatory region located from positions -2096 to -1743. Its distal half containing a pa ir of E boxes (CANNTG), had been previously characterized as an enhanc er responsive to CMD1 but not to c-myogenin. In this study, we report the identification of another enhancer in the muscle-specific regulato ry region which is preferentially responsive to c-myogenin. Deletion a nd mutation analyses indicated that this enhancer requires a single E box and its flanking sequences. Furthermore, analysis of chimeric prot eins of CMD1 and c-myogenin indicated that regions outside the basic h elix-loop-helix domain of c-myogenin are involved in the specificity o f the enhancer. These results show that CMD1 and c-myogenin act on the MLC1 gene by recognizing different upstream DNA sequences and that di rect or indirect interactions between the regions outside the basic he lix-loop-helix domain and flanking sequences of E boxes are involved i n the target sequence specificity.