Origin of Fetal Troponin T: Developmentally Regulated Splicing of a New Exon in the Fast Troponin T Gene

We have identified a new exon in the fast troponin T (TnT) gene whose alternative splicing is developmentally regulated. In previous studies three novel isoforms of TnT were identified in fetal and neonatal rabbit skeletal muscles (Briggs et al., 1990, Dev. Biol. 140, 253-260). These proteins are recognized by an antibody against fast TnT, but no combination of the previously identified exons could explain their protein chemical properties. To determine whether an additional exon is present in the fast TnT gene, we have cloned and sequenced cDNAs from fetal rabbit muscle RNA. Reverse transcription coupled to PCR was used with primers from conserved regions of the fast TnT sequence that span the 5′ alternatively spliced exons. Sequences were obtained for each of the fetal TnTs. All included known fast TnT coding sequences plus an additional 36 nucleotides between exons 8 and 9. The peptide predicted from this sequence is highly acidic and accounts for the chemical properties of the fetal proteins. Similar experiments analyzing neonatal rat TnT cDNAs identified an homologous sequence, and comparison with the genomic sequence revealed that it is encoded by a single exon. Thus, fetal TnTs are generated from the fast TnT gene by the inclusion of a new, alternatively spliced exon, which we have designated f (for fetal), whose developmentally regulated splicing appears to be a common feature of mammalian skeletal muscle development.