Slow troponin C gene expression in chicken heart and liver is regulated by similar enhancers.

Two isoforms of troponin C (TnC) are encoded by distinct single copy genes. Expression of fast TnC is restricted to the skeletal muscle, whereas the slow isoform is expressed in both skeletal and cardiac muscle. Chicken slow TnC (cTnC) gene is also expressed in some non-muscle tissues like the liver and the brain. Expression of cTnC gene is regulated by two distinct enhancers in cardiac and skeletal muscles. The cardiac specific enhancer is located in the immediate 5' flanking region (bp-124 to -79) of the murine cTnC gene whereas the skeletal enhancer is located within the first intron (bp 997 to 1141). In the present study we have examined how cTnC gene expression is regulated in the chicken liver. Transient transfection of liver cells with CTnC-CAT reporter constructs containing various regions of the murine cTnC gene showed that its expression in chicken liver is regulated by the cardiac specific enhancer. Furthermore, electrophoretic mobility shift assays using synthetic oligonucleotides corresponding to both CEF-1 and CEF-2 regions of the murine cardiac enhancer revealed formation of specific DNA-protein complexes. Ultraviolet light induced covalent linking of nuclear proteins to CEF-1 and CEF-2 oligomers were used to examine the nature of the cardiac enhancer binding polypeptides; one polypeptide of 48 kDa appeared to bind to both CEF-1 and CEF-2 sequences.