| Abstract: | We describe the analysis of two quail cDNA clones representing distinct but closely related alpha-tropomyosin mRNAs. cDNA clone cC101 corresponds to a 1.2-kilobase RNA which accumulates to high levels during myoblast differentiation and which encodes the major isoform of skeletal muscle alpha-tropomyosin. cDNA clone cC102 corresponds to a 2-kilobase RNA which is abundant in cultured embryonic skin fibroblasts and which encodes one of two alpha-tropomyosin-related fibroblast tropomyosins of 35,000 and 34,000 daltons apparent molecular mass (class 1 tropomyosins). The cC102 protein is unique among reported nonstriated-muscle tropomyosins in being identical in amino acid sequence to the major isoform of skeletal muscle alpha-tropomyosin over an uninterrupted stretch of at least 183 amino acids (residues 75-257). The two protein sequences differ in the COOH-terminal region beginning with residue 258. Because the cC101 and cC102 RNAs share an extensive region (at least 373 nucleotides) of nucleotide sequence identity upstream of the codon for residue 258, they are likely derived from a single gene by alternative RNA splicing, as was recently proposed in the case of related beta-tropomyosin mRNAs in human fibroblasts and skeletal muscle (MacLeod, A. R., Houlker, C., Reinach, R. C., Smillie, L. B., Talbot, K., Modi, G., and Walsh, F. S. (1985) Proc. Natl. Acad. Sci. U.S.A. 82, 7835-7837). No alpha-tropomyosin-related RNAs are abundant in undifferentiated myoblasts. This suggests the possibility of a fibroblast-specific function, as opposed to a general nonmuscle-cell function for class 1 tropomyosins and also has implications for the regulation of alpha-tropomyosin gene expression during embryonic development. |
