CArG elements control smooth muscle subtype-specific expression of smooth muscle myosin in vivo

Expression of smooth muscle myosin heavy chain (SM-MHC) is tightly controlled depending on the differentiated state of smooth muscle cells (SMCs). To better understand the mechanisms that regulate transcription of the SM-MHC gene in vivo, we tested the function of several conserved CArG elements contained within the -4200 to +11,600 region of this gene that we had previously shown to drive SMC-specific expression in transgenic mice. CArG1 in the 5'-flanking sequence was required for all SMCs, while CArG2 and a novel intronic CArG element were differentially required in SMC subtypes. Of particular note, mutation of the intronic CArG selectively abolished expression in large arteries. A promoter construct containing three repeats of a conserved 227-bp intronic CArG-containing region was sufficient to direct transcription in vascular SMCs in transgenic mice, although this construct was also expressed in skeletal and cardiac muscle. These results support a model in which transcriptional regulation of SM-MHC is controlled by multiple positive and negative modular control regions that differ between SMCs and non-SMCs and among SMC subtypes. We also demonstrated that the CArG elements of the endogenous SM-MHC gene were bound by SRF in chromatin.