The solitary ascidian Halocynthia roretzi possesses three types of mus
cle: the larval tail striated muscle, the adult heart striated muscle,
and the adult body wall smooth muscle, The troponin complex is observ
ed in ail types of muscle, and the isoform sequences and expression pa
tterns of two of the three troponin components, troponins C and T, hav
e been reported, In this study, we have determined cDNA sequences of t
he three TnI isoforms from H. roretzi. One of the three isoforms (adul
t TnI), expressed in adult body wall smooth muscle and heart muscle, w
as composed of 173 amino acids, being similar to vertebrate fast and s
low skeletal TnIs in length, The other two isoforms (larval TnI alpha
and TnI beta) were isolated from a cDNA library of larvae. Both larval
TnIs were composed of 142 amino acids, with truncation amounting to c
a, 30 amino acid residues at the C-termini, These larval TnIs are the
smallest known TnIs. The position of the last intron of these TnIs was
also determined. When compared with vertebrate TnI genes, the last in
tron of the ascidian adult TnI gene is located at 6 nucleotides downst
ream, and the introns of the two larval TnIs are positioned at 9 nucle
otides upstream, These results suggest that H. roretzi TnI is encoded
by at least three genes.