Regulatory regions of the mouse muscle creatine kinase (MCK) gene, previously discovered by analysis in cultured muscle cells, were analyzed in transgenic mice. The 206-bp MCK enhancer at nt ؊1256 was required for high-level expression of MCK-chloramphenicol acetyltransferase fusion genes in skeletal and cardiac muscle; however, unlike its behavior in cell culture, inclusion of the 1-kb region of DNA between the enhancer and the basal promoter produced a 100-fold increase in skeletal muscle activity. Analysis of enhancer control elements also indicated major differences between their properties in transgenic muscles and in cultured muscle cells. Transgenes in which the enhancer right E box or CArG element were mutated exhibited expression levels that were indistinguishable from the wild-type transgene. Mutation of three conserved E boxes in the MCK 1,256-bp 5 region also had no effect on transgene expression in thigh skeletal muscle expression. All of these mutations significantly reduced activity in cultured skeletal myocytes. However, the enhancer AT-rich element at nt ؊1195 was critical for expression in transgenic skeletal muscle. Mutation of this site reduced skeletal muscle expression to the same level as transgenes lacking the 206-bp enhancer, although mutation of the AT-rich site did not affect cardiac muscle expression. These results demonstrate clear differences between the activity of MCK regulatory regions in cultured muscle cells and in whole adult transgenic muscle. This suggests that there are alternative mechanisms of regulating the MCK gene in skeletal and cardiac muscle under different physiological states.The muscle creatine kinase (MCK) gene is transcriptionally activated during striated muscle differentiation and is expressed at high levels in adult heart and skeletal muscles (11,39). Previous cell culture analyses of MCK gene regulation have implicated both a 5Ј muscle-specific enhancer (bp Ϫ1256 to Ϫ1050) and the adjacent 1-kb region of DNA (bp Ϫ1049 to ϩ7) as playing important roles in expression of the MCK gene in skeletal and cardiac muscle (24,29,63). The MCK enhancer contains a number of conserved DNA motifs, which are also found in the regulatory regions of many other muscle-specific genes (reviewed in references 12, 22, and 49). These motifs bind trans-acting factors in vitro and are critical control elements in cultured muscle cells (2,3,8,9,24). The MCK enhancer sites include E boxes, which contain the core consensus binding sequence CANNTG for the myogenic basic helix-loop-helix (bHLH) proteins (MyoD, Myf-5, myogenin, and MRF-4) (for reviews, see references 17, 22, 49, and 72); a CArG element, containing the consensus serum response factor-binding sequence CC(A/T) 6 GG (68); and an AT-rich site, which has been shown in gel shift assays to bind ubiquitously expressed factors (1, 15a, 25), as well as MHox and MEF-2 (14). The 1-kb region of DNA between the enhancer and the basal promoter exhibits low-level activity in cultured skeletal myocytes and cardiomyocytes (2).Mutation of eac...