Abstract
The organization of troponin I (TnI) genes from the ascidian Halocynthia roretzi have been determined. Halocynthia possesses roughly two types of TnI isoforms. One type is a single-copied adult TnI (adTnI) gene, which contains eight exons and seven introns. adTnI expresses two isoforms, the shorter body wall muscle TnI and the longer cardiac Tnl, through alternative splicing. The mRNAs of these TnI isoforms may undergo transsplicing of the 5'-leader sequences, like the Tnl mRNA of another ascidian species, Ciona intestinalis. The other type comprises multi-copied larval TnI (laTnl) genes. Halocyn-thia has at least three laTnIs (α, β, and γ), which are composed of five exons and four introns, and two of them (α and γ) are clustered in tandem. All laTnIs have B- and M-regions within their 5'-upstream regions, which have been discovered to be the regula-tory elements of Halocynthia larval actin genes. The expression of Halocynthia laTnIs and larval actins may be regulated in the same manner. It is known that Ciona does not possess a larva-specific TnI isoform. The phylogenetic tree of ascidian TnIs suggests that laTnIs might have only been generated within the Pleurogona lineage after Enterogona/Pleurogona divergence, and this scenario well agrees with the absence of laTnIs in Ciona.
