Interactions among the Escherichia coli mutT, mutM, and mutY damage prevention pathways
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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.