Escherichia coli MutT protein hydrolyzes 8-oxo-7,8-dihydro-2'-dGTP (8-oxo-dGTP) 
to the monophosphate, thus avoiding the incorporation of 
8-oxo-7,8-dihydroguanine (8-oxo-G) into nascent DNA. Bacterial and mammalian 
homologs of MutT protein share the phosphohydrolase module (MutT: 
Gly37-->Gly59). By saturation mutagenesis of conserved residues in the MutT 
module, four of the 10 conserved residues (Gly37, Gly38, Glu53 and Glu57) were 
revealed to be essential to suppress spontaneous A:T-->C:G transversion mutation 
in a mutT(-) mutator strain. For the other six residues (Lys39, Glu44, Thr45, 
Arg52, Glu56 and Gly59), many positive mutants which can suppress the 
spontaneous mutation were obtained; however, all of the positive mutants for 
Glu44 and Arg52 either partially or inefficiently suppressed the mutation, 
indicating that these two residues are also important for MutT function. Several 
positive mutants for Lys39, Thr45, Glu56 and Gly59 efficiently decreased the 
elevated spontaneous mutation rate, as seen with the wild-type, hence, these 
four residues are non-essential for MutT function. As Lys38 and Glu55 in human 
MTH1, corresponding to the non-essential residues Lys39 and Glu56 in MutT, could 
not be replaced by any other residue without loss of function, different 
structural features between the two modules of MTH1 and MutT proteins are 
evident.