In rat fast skeletal muscle, two light chain myosin proteins, LC1 and LC3, are expressed from a single genetic unit. The unique structures of the LC1 and LC3 proteins are generated by differential transcription from two promoters separated by 10kb, accompanied by two alternate splicing pathways. In fetal muscle development, and in differentiating myogenic tissue culture cells, accumulation of the LC1 mRNA precedes that of LC3, suggesting that differential transcription and splicing of LC1 and LC3 messages is developmentally regulated. Our objective is to characterize the mechanisms by which differentiating muscle tissue generates two different myosin light chain transcripts from the same genetic locus. By testing isolated LC1 and LC3 promoters in available tissue culture systems for the ability to express heterologous test genes upon cellular differentiation, we will determine the extent to which upstream regulatory elements play a role in the selective transcription of LC1 and LC3 mRNAs during the differentiation process.