The oxidized nucleotide precursors 7, 8-dihydro-8-oxo-dGTP (8-oxo-dGTP) and 1, 
2-dihydro-2-oxo-dATP (2-oxo-dATP) are readily incorporated into nascent DNA 
strands during replication, which would cause base substitution mutations. E. 
coli MutT and human homologue hMTH1 hydrolyze 8-oxo-dGTP, thereby preventing 
mutations. In this study, we searched for hMTH1 homologues in the ascidian Ciona 
intestinalis using the NCBI-BLAST database. Among several candidates, we focused 
on one open reading frame, designated as CiMutT, because of its high degree of 
identity (41.7%) and similarity (58.3%) to the overall amino acid sequence of 
hMTH1, including the Nudix box. CiMutT significantly suppressed the mutator 
activity of E. coli mutT mutant. Purified CiMutT had a pyrophosphohydrolase 
activity that hydrolyzed 8-oxo-dGTP to 8-oxo-dGMP and inorganic pyrophosphate. 
It had a pH optimum of 9.5 and Mg(++) requirement with optimal activity at 5 mM. 
The activity of CiMutT for 8-oxo-dGTP was comparable to that of hMTH1, while it 
was 100-fold lower for 2-oxo-dATP than that of hMTH1. These facts indicate that 
CiMutT is a functional homologue of E. coli MutT. In addition, the enzyme 
hydrolyzed all four of the unoxidized nucleoside triphosphates, with a 
preference for dATP. The specific activity for 8-oxo-dGTP was greater than that 
for unoxidized dATP and dGTP. These results suggest that CiMutT has the 
potential to prevent mutations by 8-oxo-dGTP in C. intestinalis.