Both neural and weight-bearing activity have been shown to be important physiological factors regulating slow skeletal muscle phenotype in the adult animal. While these phenotypic changes have been well characterized, there is not much known about the molecular mechanisms through which neural or weight-bearing activity regulate the slow contractile protein isoform genes. The long-term objectives are to understand mechanisms through which contractile protein gene families are regulated in the acquisition and maintenance of diverse skeletal muscle fiber types. The specific aims of this proposal are: 1) To define the region of the MLC2 slow gene that is necessary for slow muscle specific expression in vivo: 2) To determine cis-acting sequences of the MLC 2 slow gene that are necessary for induction in response to slow innervation. 3) To determine the cis-acting sequences of the MLC 2 slow gene that are necessary for high level expression in response to weight-bearing activity. 4) To identify transcription factor(s) binding to the gene regions identified in Specific Aims 2 and 3. 5) To isolate and characterize factor(s) identified in Specific Aims 4. This proposal combines the use of a novel biological assay (muscle regeneration) with which induction and high level expression of slow contractile protein genes are discernable by their dependence on the slow nerve and weight- bearing activity with in vivo DNA injection of MLC2 slow luciferase fusion gene constructs. This will allow for a relatively rapid in vivo analyses of different promoter constructs which will lead to the identification of specific elements and the transcription factors binding to those elements. This work has broad application to basic and applied areas of biomedical and health science fields. At the basic science level, new insight will be gained into DNA elements and factors regulating a specific contractile protein isoform gene. Contractile protein genes are a major topic of investigation not only for muscle biology, but their function and regulation are also implicated in maintenance of cell architecture and morphology, cell cycle events, and cell transformation/cancer. At the more applied level, because of the common occurrence of muscle regeneration in humans, this study will have wide clinical application. Muscle regeneration occurs following muscle transplant surgery for correction of facial paralysis or anal incontinence; following muscle damage due to mechanical, thermal, or metabolic stress; as well as its association with dystrophic muscle pathologies.