Muscle-specific RNA processing continues in the absence of myogenin expression

During muscle development, muscle-specific gene expression is achieved by a complex set of transcriptional and post-transcriptional mechanisms. We examined the requirement of new protein synthesis and myogenin expression for muscle-specific tropomyosin expression during myogenesis of the rat muscle cell line L6E9. The results show that new protein synthesis is required for both muscle gene transcription and muscle-specific alternate RNA processing. However, once initiated, the synthesis of muscle tropomyosin mRNA continued in the presence of cycloheximide, while the expression of muscle actin, myosin heavy chain, and myogenin mRNA was abolished. These results suggest that muscle-specific processing of tropomyosin transcripts can continue to occur in the absence of myogenin expression unlike the expression of muscle actin and myosin heavy chain mRNAs. However, the transfection of myogenin cDNA into nonmuscle cells induced muscle-specific RNA processing of tropomyosin, suggesting indirect involvement of myogenic factors in the initiation of muscle-specific RNA processing pathways.