We have investigated in detail the interactions between the Escherichia coli 
mutT, mutM, and mutY error-prevention systems. Jointly, these systems protect 
the cell against the effects of the oxidative stress product, 8-oxoguanine 
(8-oxoG), a base analog with ambiguous base-pairing properties, pairing with 
either A or C during DNA synthesis. mutT mutator strains display a specific 
increase in A.T-->C.G transversions, while mutM and mutY mutator strains show 
specific G.C-->T.A increases. To study in more detail the in vivo processing of 
the various mutational intermediates leading to A.T-->C.G and G.C-->T.A 
transversions, we analyzed defined A.T-->C.G and G.C-->T.A events in strains 
containing all possible combinations of these mutator alleles. We report three 
major findings. First, we do not find evidence that the mutT allele 
significantly increases G.C-->T.A transversions in either mut(+), mutM, mutY or 
mutMmutY backgrounds. We interpret this result to indicate that incorporation of 
8-oxodGTP opposite template C may not be frequent relative to incorporation 
opposite template A. Second, we show that mutT-induced A.T-->C.G transversions 
are significantly reduced in strains carrying mutY and mutMmutY deficiencies 
suggesting that 8-oxoG, when present in DNA, preferentially mispairs with dATP. 
Third, the mutY and mutMmutY deficiencies also decrease A.T-->C.G transversions 
in the mutT(+) background, suggesting that, even in the presence of functional 
MutT protein, A.T-->C.G transversions may still result from 8-oxodGTP 
misincorporation.