Generation of troponin T isoforms by alternative RNA splicing in avian skeletal muscle. Conserved and divergent features in birds and mammals.

We describe the isolation and sequence analysis of quail muscle cDNA clones encoding two closely related isoforms of the striated muscle contractile protein, troponin T. The cDNAs represent two troponin T mRNAs that exhibit an unusual sequence relationship. The two mRNAs have identical sequences over hundreds of nucleotides including 3' untranslated regions, but they differ dramatically in a discrete, internally located block of 38 nucleotides. The two alternative sequences of this 38-nucleotide block encode two different but related versions of amino acid residues 230-242, near the C terminus of the protein. These results are consistent with a novel mechanism of troponin T isoform generation by alternative mRNA splicing pathways from a single gene containing two different exons corresponding to amino acids 229-242, as recently proposed by Medford et al. (Medford, R. M., Nguyen, H. T., Destree, A. T., Summers, E., and Nadal-Ginard, B. (1984) Cell 38, 409-421). This proposal was based on analysis of a rat troponin T genomic DNA clone and a cDNA clone corresponding to one of the two alternatively spliced mRNAs. Our analysis of quail troponin T cDNA clones, apparently corresponding to two alternatively spliced mRNA species, provides important new evidence for this novel mechanism of troponin T isoform generation and reveals the differential splicing mechanism to be of great antiquity, antedating the bird-mammal divergence. One of the quail alternative isoform sequences clearly corresponds to one of the rat sequences, but the other quail alternative sequence does not correspond to either of the rat sequences. This result suggests a greater complexity of troponin T gene structure or a greater diversity of troponin T isoform genes than is currently known, and also has implications for the functional significance of the troponin T protein isoform heterogeneity. Comparison of quail and mammal alternative isoform sequences also reveals strongly conserved features which suggest that all the isoform alternative amino acid sequences are variations on a common structural theme.