USE OF A CONDITIONAL MYOD TRANSCRIPTION FACTOR IN STUDIES OF MYOD TRANSACTIVATION AND MUSCLE DETERMINATION

DNA sequences encoding the hormone-binding domains of several steroid hormone receptors were fused in frame to the MyoD gene. When the gene for this chimeric protein was expressed in NIH 3T3 or 10T1/2 fibroblas ts, these cells displayed hormone-dependent induction of myogenesis. O ur experiments focused on cell lines expressing estrogen receptor-MyoD chimeras. Induction of these lines in the presence of estradiol and a n inhibitor of protein synthesis, cycloheximide, resulted in the activ ation of the endogenous myogenin gene but did not activate the muscle- specific creatine kinase or cardiac alpha-actin gene. This result sugg ests that MyoD is not a ''direct'' activator of these downstream myoge nic genes but must first activate myogenin as an intermediary. Once mu scle is induced by estrogen receptor-MyoD the muscle phenotype is very stable and does not need the continued presence of estradiol for its maintenance.