The synthesis and accumulation of myofibrillar proteins will be studied in cell cultures of skeletal muscle myoblasts and in developing muscle of Japanese quail embryos (Coturnix coturnix). These studies examine the control and coordination of myosin, actin tropomyosin and troponin synthesis during myoblast differentiation and the expression of fiber type-specific myofibrillar protein synthesis. The synthesis, stabilities and accumulation of the mRNAs for myofibrillar proteins also will be studied in differentiating myoblasts. RNA isolated from muscle cells at different stages of differentiation will be translated in wheat germ and reticulocyte lysates to measure the accumulation of myofibrillar protein mRNA during myoblast differentiation. The synthesis and stabilities of these specific mRNAs will be examined by pulse-chase precursor labeling experiments utilizing specific DNA hybridization probes of myofibrillar protein mRNAs prepared by recombinant DNA cloning. These studies examine whether the synthesis of myofibrillar proteins during myoblast differentiation is regulated by control of the transcription or the translation of their mRNAs. BIBLIOGRAPHIC REFERENCES: Bowman, L. and Emerson, C.P., Jr. Post-Transcriptional Regulation of Ribosome Accumulation during Myoblasts Differentiation. Cell, April, 1977, in press. Bowman, L. and Emerson, C.P., Jr. (Abstract). Post-Transcriptional Regulation of Ribosomal RNA Accumulation during Myogenesis. J. Cell Biol. 70, 332 (1977).