Regulatory elements governing transcription in specialized myofiber subtypes

Skeletal myofibers of vertebrates acquire specialized metabolic and physiol ogical properties as a consequence of developmental cues in the embryo and different patterns of contractile activity in the adult. The myoglobin gene is regulated stringently in muscle fibers, such that high myoglobin expres sion is observed in mitochondria-rich, oxidative myofibers (Types I and IIa ) compared with glycolytic fibers (Type IIb). Using germline transgenesis a nd somatic cell gene transfer methods, we defined discrete regions of the m urine and human genes encoding myoglobin that are sufficient to confer musc le- and fiber type-specific expression to reporter genes. Mutational analys is confirms the importance of APT-rich, MEF2-binding motifs in myoglobin ge ne regulation, as suggested by previous studies using different experimenta l approaches. In addition, we demonstrated a previously unsuspected role fo r an intragenic E-box motif as a negative regulatory element contributing t o the tightly regulated variation in myoglobin gene expression among partic ular myofiber subtypes.