Positive and negative gene regulation in muscle

By the analysis of cis and trans-acting element involved in transcriptional regulation of chicken myosin alkali light chain genes, we have identified the MLC box, muscle specific enhancer element and negative regulatory element. The MLC box is an essential element for the expression of MLC genes located at approximately 100 bp upstream from mRNA start sites. The core sequence of MLC box is similar to the consensus of actin gene CArG box and SRE of c-fos oncogene. In vitro DNA-protein binding assay has revealed that the MLC box, CArG box and SRE might bind to a common or a similar protein complex. CMD1, cMyogenin, and cMRF4 transactivate the promorter with an intact MLC box, but not the promoter lacking MLC box, indicating that the MLC box itself is transactivated by the myogenic regulatory factors. This transactivation must have been due to the indirect effect of the myogenic regulatory factors, because chicken myogenic factors do not bind to the MLC box. A cis element identified at about 150 bp upstream from the cap site of cardiac MLC gene suppresses the cardiac MLC gene expression in skeletal muscle cells but not in cardiac muscle cells. The protein(s) bound to NRE might be identical with one of proteins bound to SRE. NRE may block the function of MLC box and resultantly inhibits the expression of cardiac MLC1 gene in skeletal muscle cells. Skeletal muscle enhancer at -2 kb of skeletal MLC1f gene is composed of two subelements P and D, cooperative action between them is required for sufficient enhancer activity. CMD1 and myogenin bind to the enhancer sequences of skeletal MLC1 gene and MCK gene and transactivate these genes preferentially in skeletal muscle cells. In addition to the CMD1 responsible enhancer, another cis-element is required for transactivation of the MLC1f gene by cMyogenin. An E-box adjacent to MLC box may co-work with the enhancer to increase the expression of MLC1f gene. Muscle specific and developmentally regulated expression of MLC gene family is regulated by the combination of these cis and trans-acting elements.