Terminal differentiation of chick muscle in terms of cellular mechanisms underlying the synthesis of the myosin heavy chain (MHC) proteins will be studied. We will isolate the myosin genes from a library of recombinant DNA, consisting of chick DNA fragments joined to bacteriophage lambda Charon 4A fragments. The identity of the recombinants containing the MHC genes will be confirmed by a Hybrid-Arrested-RNA-Translation Assay. The organization of the structural and non-structural myosin gene sequences, and the number of distinct myosin genes will be determined by restriction endonuclease mapping. The clone-derived myosin DNA will be used as a probe to map any structural differences that are present in myosin mRNA derived from two cellular compartments: (1) The heavy polysomes which are synthesizing protein. (2) The messenger ribonucleoprotein particles, or mRNP's, which are inactive in protein synthesis. The data will indicate whether the processing of translationally inactive myosin mRNA (mRNA mRNA) to active mRNA (polys.mRNA) is accompanied by sequence removal. Clone-derived myosin DNA will also be used to determine the myosin mRNA pools during myogenesis in primary chick cultures. The data will be correlated with the rate of myosin synthesis and the translational utilization of the myosin mDNA during development will be determined.