MEF-2 AND OCT-1 BIND TO 2 HOMOLOGOUS PROMOTER SEQUENCE ELEMENTS AND PARTICIPATE IN THE EXPRESSION OF A SKELETAL MUSCLE-SPECIFIC GENE

The murine adult IIB myosin heavy chain (IIB MyHC) gene is expressed o nly in certain skeletal muscle fibers. Within the proximal promoter ar e two A + T-rich motifs, mAT1 and mAT2, which greatly enhance muscle-s pecific transcription; myogenic cells contain proteins that bind to th ese sequences. MEF-2 binds to both mAT1 and mAT2; a mutation abolishin g its binding to mAT1 greatly diminishes the activity of the promoter. Both mAT motifs also form complexes with a protein requiring a target sequence typical of POU domain proteins, which migrate in electrophor etic mobility shift assays to the same position as a complex containin g purified Oct-1 and which are supershifted by an antibody specific to Oct-1; this protein is therefore probably Oct-1. Footprinting experim ents demonstrate that mAT1 is preferentially occupied by MEF-2 and mAT 2 by Oct-1 and that these two proteins appear to bind cooperatively to their respective sites. Although the two mAT motifs have sequences th at are very similar, they nonetheless exhibit distinct behaviors and p erform differ entry in the activation of the promoter. The contributio n. of the IIB MyHC gene to specification of the myogenic phenotype is thus at least in part regulated by MEF-2 and Oct-1.