This proposal deals with muscle (M) and brain (B) specific creatine kinases. The B-isozyme is the fetal form and during muscle differentiation there is a switch from the B-specific to the M-specific isozyme. There is also evidence that the B-isozyme is a major protein induced by estrogen in the uterus. The purpose of the proposal is to study the tissue specific expression of creatine kinase genes and the estrogen induction of the B-isozyme. The starting point for the work is the M- and B-isozyme cDNA clones which we have isolated and sequenced. These are being used to isolate rabbit creatine kinase genes. The genes are to be introduced into mammalian cells and manipulated through a series of mutagenic procedures in order to learn about sequences which control tissue specific expression and estrogen induction of the B-isozyme. Two mammalian gene transfer systems are under investigation. Preliminary experiments have detected a potential creatine kinase homologue in yeast. Further experiments will attempt to determine whether authentic creatine (or guanidino) kinase occurs in yeast and, if so, whether it is an essential protein. The system may offer an additional route to explore cellular roles of creatine kinases.