Identification and analysis of teleost slow muscle troponin T (sTnT) and intronless TnT genes

In the present study cDNA clones representing two slow skeletal muscle troponin T genes (sTnT1sb and sTnT2sb) in the sea bream (Sparus auratus), an important aquaculture species, were isolated and characterised. A third, intronless, TnT gene (iTnTsb), which is an apparent orthologue of a previously described zebrafish TnT, was also isolated. In adult sea bream sTnT expression was restricted to red muscle and, using northern blotting, a single low abundance transcript was identified for sTnT1sb (1260 nucleotides) and a single high abundance transcript was identified for sTnT2sb (1000 nucleotides). In contrast, iTnTsb is predominantly expressed in adult fast muscle. All three TnT genes are also expressed during larval development. Phylogenetic analysis of sea bream sTnT proteins to identify maximum parsimony showed that iTnTsb, sTnT1sb and sTnT2sb each cluster in independent groups. sTnT1sb clustered with other vertebrate sTnTs, while sTnT2 clustered with a group of fish specific sequences (from Fugu rubripes, Oryzia latipes and Salmo trutta). The teleost sTnT2 and iTnT each constitute new, apparently teleost specific, TnT groups. Analysis of the corresponding Fugu scaffold indicates that sTnT2sb is encoded by a gene with twelve exons. The two sTnT cDNAs isolated in sea bream probably arose by duplication of an ancestral gene, and iTnT by reverse transcription. It remains to be established if the encoded proteins have different structural and mechanistic roles in fish muscle.