E-BOX SITES AND A PROXIMAL REGULATORY REGION OF THE MUSCLE CREATINE-KINASE GENE DIFFERENTIALLY REGULATE EXPRESSION IN DIVERSE SKELETAL-MUSCLES AND CARDIAC-MUSCLE OF TRANSGENIC MICE

Previous analysis of the muscle creatine kinase (MCK) gene indicated t hat control elements required for transcription in adult mouse muscle differed from those required in cell culture, suggesting that distinct modes of muscle gene regulation occur in vivo. To examine this furthe r, we measured the activity of MCK transgenes containing E-box and pro moter deletions in a variety of striated muscles. Simultaneous mutatio n of three E boxes in the 1,256-bp MCK 5' region, which abolished tran scription in muscle cultures, had strikingly different effects in mice . The mutations abolished transgene expression in cardiac and tongue m uscle and caused a reduction in expression in the soleus muscle (a mus cle with many slow fibers) but did not affect expression in predominan tly fast muscles: quadriceps, abdominals, and extensor digitorum longu s. Other regulatory sequences with muscle-type-specific activities wer e found within the 358-bp 5'-flanking region. This proximal region con ferred relatively strong expression in limb and abdominal skeletal mus cles but was inactive in cardiac and tongue muscles. However, when the 206-bp 5' enhancer was ligated to the 358-bp region, high levels of t issue-specific expression were restored in all muscle types. These res ults indicate that E boxes and a proximal regulatory region are differ entially required for maximal MCK transgene expression in different st riated muscles. The overall results also imply that within skeletal mu scles, the steady-state expression of the MCK gene and possibly other muscle genes depends on transcriptional mechanisms that differ between fast and slow fibers as well as between the anatomical and physiologi cal attributes of each specific muscle.