Very little is known of the molecular mechanisms involved in heart muscle differentiation, one of the primary early events of embryonic development. I propose to study the mechanism by which the expression of specific cardiac genes are regulated during heart muscle development, using the chick embryo as a model system. My basic strategy will be to initially isolate two chicken genes, the cardiac actin and calmodulin genes, and to measure the mRNA levels expressed during the differentiation of cardiac muscle cells in culture. The cardiac actin gene will be isolated from a chicken gene library and its sequence organization determined. This structure will be compared with the structure of the alpha-actin gene, and the possible linkage of these two genes and the other actin genes will be investigated. A knowledge of the organization of the actin gene family in cardiac muscle will be important in understanding the differential regulation of individual members of the gene family. The expression of the cardiac actin gene will be monitored during cardiomyogenesis and compared with other tissue-specific actin genes to determine the mechanisms involved in its selective expression in cardiac muscle. Furthermore, the sequence organization of the cardiac calmodulin gene and its physical relationship to the other calmodulin gene(s) will be determined, so as to understand the organization of the calmodulin gene family. Then, the expression of the muscle-specific calmodulin gene will be examined during cardiomyogenesis and contrasted with the other calmodulin gene(s) and the actin gene family to further our understanding of the regulation of tissue-specific selective gene expression. The results of these experiments should also yield knowledge fundamental to our understanding of an extremely important process: cardiac muscle development.