ISOLATION AND CHARACTERIZATION OF AN AVIAN SLOW MYOSIN HEAVY-CHAIN GENE EXPRESSED DURING EMBRYONIC SKELETAL-MUSCLE FIBER FORMATION

We have isolated and begun characterization of the quail slow myosin h eavy chain (My HC) 3 gene, the first reported avian slow MyHC gene. Ex pression of slow MyHC 3 in skeletal muscle is restricted to the embryo nic period of development, when the fiber pattern of future fast and s low muscle is established. In embryonic hind limb development, slow My HC 3 gene expression coincides with slow muscle fiber formation as di stinguished by slow MyHC-specific antibody staining. In addition to ex pression in embryonic appendicular muscle, slow MyHC 3 is expressed co ntinuously in the atria. Transfection of slow My HC 3 promoter-reporte r constructs into embryonic myoblasts that form slow MyHC expressing f ibers identified two regions regulating expression of this gene in ske letal muscle, The proximal promoter, containing potential muscle speci fic regulatory motifs, permits expression of a reporter gene in embryo nic slow muscle fibers, while a distal element, located greater than 2 600 base pairs upstream, further enhances expression 5-fold. The slow muscle fiber-restricted expression of slow MyHC 3 during embryonic dev elopment, and expression of slow MyHC 3 promoter-reporter constructs i n embryonic muscle fibers in vitro, makes this gene a useful marker to study the mechanism establishing the slow fiber lineage in the embryo .