The promoter of the chicken cardiac myosin light chain 2 gene shows cell-specific expression in transfected primary cultures of chicken muscle.

Transcriptional regulation of the chicken cardiac myosin light chain 2 (MLC2-A) gene was investigated in chicken primary myoblast and fibroblast cultures transfected with vector constructs containing the bacterial marker gene for chloramphenicol acetyltransferase (CAT) under the control of the MLC2-A promoter. We here demonstrate that sequences close to the TATA box are sufficient to direct muscle specific and regulated expression of the MLC2-A mRNA. Transcription from MLC2-A promoter/CAT hybrids in myocytes starts from the authentic cap site that is also used in vivo. In primary breast muscle cells, bromodeoxyuridine (BUdR), a reversible blocking agent of cell differentiation, suppresses transcription from the MLC2-A promoter whereas nonmuscle promoters like the RSV- or the cytoplasmic beta-actin promoter are unaffected in their transcriptional capacity. Although the endogenous cardiac MLC2-A gene in chicken is exclusively active in heart, the transfected MLC 2-A promoter escapes this cell type control in primary cultures of breast muscle. These results demonstrate that although muscle specificity of the MLC2-A gene and its transcriptional up-regulation during differentiation is maintained in a rather short promoter segment, restrictive elements determining the muscle cell type specificity in vivo are either not present in our constructs or are not acting under the conditions of transient transfection.