STRUCTURE AND REGULATION OF THE MOUSE CARDIAC TROPONIN-I GENE

The gene coding for mouse cardiac troponin I (TnI) has been cloned and sequenced. The cardiac TnI gene contains 8 exons and has an exon-intr on organization similar to the quail fast skeletal TnI gene except for the region of exons 1-3, which is highly divergent. Comparative analy sis suggests that cardiac TnI exon 1 corresponds to fast TnI exons 1 a nd 2 and that cardiac exon 3, which codes for most of the cardiac-spec ific amino-terminal extension and has no counterpart in the fast gene, evolved by exon insertion/deletion. The amino acid sequence of cardia c TnI exon 4 shows limited homology (36% identity) with fast TnI exon 4 but is remarkably similar (79% identity) to the corresponding sequen ce of slow TnI, possibly reflecting an isoform-specific TnC-binding si te. The cardiac TnI gene is one of the very few contractile protein ge nes expressed exclusively in cardiac muscle. To identify the regulator y sequences responsible for the cardiac-specific expression of this ge ne we transfected cultured cardiac and skeletal muscle cells with frag ments up to 4.0 kilobases of the 5'-flanking region linked to a report er gene. Deletion analysis reveals four major regions in the 5'-flanki ng sequence, a minimal promoter region, which directs expression at lo w level in cardiac and skeletal muscle cells, and two upstream cardiac -specific positive regions separated by a negative region.