In Escherichia coli the mutT gene is one of several that acts to minimize 
mutagenesis by reactive oxygen species. The bacterial MutT protein and its 
mammalian homolog have been shown to catalyze in vitro the hydrolysis of the 
oxidized deoxyguanosine nucleotide, 8-oxo-dGTP, to its corresponding 
monophosphate. Thus, the protein is thought to "sanitize" the nucleotide pool by 
ridding the cell of a nucleotide whose incorporation into DNA would be intensely 
mutagenic. However, because others have shown mutT mutations to be mutagenic 
under some conditions of anaerobic growth, and have shown 8-oxo-dGTP to be a 
poor DNA polymerase substrate, there is reason to question this model. We have 
devised an assay for 8-oxo-dGTP in bacterial extracts. Using this assay, which 
involves reversed-phase high-performance liquid chromatography and 
electrochemical detection, we have been unable to detect 8-oxo-dGTP in extracts 
of three different mutT mutants of E. coli, even after growth of the bacteria in 
the presence of hydrogen peroxide. Our estimated upper limit for 8-oxo-dGTP 
content of these bacteria is about 200 molecules/cell, corresponding to a 
concentration of about 0.34 microm. When 8-oxo-dGTP was added at 0.34 microm to 
an in vitro DNA replication system primed with a DNA template that permits 
scoring of replication errors and with the four normal dNTPs at their estimated 
intracellular concentrations, there was no detectable effect upon the frequency 
of replication errors. These findings lead us to question the conclusion that 
8-oxo-dGTP is the most significant physiological substrate for the MutT protein.