cis-acting elements responsible for muscle-specific expression of the myosin heavy chain beta gene.

The 5' flanking region of the rabbit myosin heavy chain (HC) beta gene extending 295 bp upstream from the cap site provides muscle-specific transcriptional activity. In this study, we have identified and functionally characterized cis-acting elements that regulate the muscle-specific expression within this region. By using linker-scanner (LS) mutants between -295 bp and a putative TATA box, we found five distinct positive cis-acting sequences necessary for transcription: element A, the sequences between -276 and -263, which contains a putative M-CAT motif in an inverted orientation; B, the sequences between -207 and -180; C, the sequences between -136 and -127; D, the sequences between -91 and -80; and E, a TATA consensus sequence at -28. The fragment containing both A and B elements dramatically enhanced the expression of the chloramphenicol acetyltransferase (CAT) gene driven by a heterologous promoter in differentiated muscle cells, whereas fragments containing either A or B elements alone had little or no effect in either muscle or nonmuscle cells. Therefore, these two elements appear to act cooperatively in determining a high level of muscle- and stage-specific expression. Unlike the typical enhancer element, this region functions in an orientation-dependent manner. In contrast, the fragment containing C and D elements activates the heterologous promoter in both muscle and nonmuscle cells in an orientation-independent manner.