A single gene codes for the β subunits of smooth and skeletal muscle tropomyosin in the chicken

A chicken genomic DNA library was screened with a full length cDNA corresponding to the beta subunit of smooth muscle tropomyosin. When hybridized with RNAs isolated from various tissues, this cDNA recognizes two mRNA species: one of 1.3 kilobase pairs present only in smooth muscle and one of 1.6 kilobase pairs present only in skeletal muscle. Two overlapping recombinant phages were shown to contain the entire locus and were further characterized. This locus contains 11 exons and spans approximately 13 kilobase pairs. Exon 1 (amino acids 1-38) contains the 5'-untranslated region which is common to the two mRNAs. Exons 6 (amino acids 189-213) and 11 (amino acids 258-284) contain sequences which are present exclusively in the 1.3-kilobase pair smooth muscle mRNA while exons 7 and 10, which code for an analogous region, contain sequences which are present exclusively in the 1.6-kilobase pair skeletal muscle mRNA (exons 10 and 11 also contain the entire 3'-untranslated regions of the corresponding mRNAs). Other exons, 2 to 5 (amino acids 39-188) and 8 and 9 (amino acids 214-257), contain sequences which are present in both mRNAs. Our results indicate that both the smooth and skeletal beta-tropomyosin mRNAs are derived from transcripts of a single gene with a unique promoter by a differential splicing mechanism.