A nonmuscle tropomyosin is encoded by the smooth/skeletal beta-tropomyosin gene and its RNA is transcribed from an internal promoter.

The smooth/skeletal muscle beta-tropomyosin gene contains an additional exon (exon 1') which is located between exons 2 and 3 and which is used to generate a 1.3-kb transcript expressed in undifferentiated muscle as well as nonmuscle cells. This mRNA, besides exon 1', corresponds to exons 3, 4, 5, 6A, 7, 8, and 9B of the gene and codes for a 247-amino acid low molecular weight tropomyosin. Exon 1' contains the coding sequence for the first 44 amino acids of the protein as well as the whole 5'-untranslated region. During the transition from myoblasts to myotubes, initiation of transcription continues from the internal promoter but also occurs from the distal promoter in order to give rise to the skeletal beta-tropomyosin-specific transcript.