MutT protein of Escherichia coli hydrolyzes oxidized guanine nucleotides, 
8-oxo-dGTP and 8-oxoGTP, to the corresponding monophosphates, thereby preventing 
misincorporation of 8-oxoguanine into DNA and RNA, respectively. Although the 
biological significance of the MutT has been established, how MutT protein 
actually works in vivo remains to be elucidated. The current study shows the 
molecular behavior of the MutT protein in vivo and in vitro with special 
reference to control of spontaneous mutagenesis. A single E. coli cell carries 
about 70-75 molecules of the MutT protein and that this number does not change 
even when the cells were cultured in anaerobic and hyper-oxidative conditions. 
Conditional gene silencing analyses revealed that about a half number of MutT 
molecules are needed for keeping the spontaneous mutation frequency at the 
normal level. The MutT functions are not needed under anaerobic condition, yet 
the level of the MutT protein in cell is kept constant, probably for preparing 
for sudden changes of oxygen pressure. There is a possibility that MutT 
functions in close association with other proteins, and evidence is presented 
that MutT protein can interact with some proteins in vivo.