Specific mutator effects of ung (uracil-DNA glycosylase) mutations in Escherichia coli

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Abstract
Studies of trpA reversions revealed that G:C .fwdarw. A:T transitions were stimulated about 30-fold in E. coli ung mutants, whereas other base substitutions were not affected. A dUTPase (dut) mutation, which increases the incorporation of uracil into DNA in place of thymine, had no significant effect on the rate of G:C .fwdarw. A:T transitions. The glycosylase may reduce the mutation rate in wild-type cells by acting in the repair of DNA cytosine residues that have undergone spontaneous deamination to uracil. When .lambda. bacteriophages were treated with bisulfite, an agent known to produce cytosine deamination, the frequency of clear-plaque mutants was increased an additional 20-fold by growth on an ung host. Bisulfite-induced mutations of the cellular chromosome were about equal in ung+ and ung strains; during the treatment of ung+ cells with bisulfite, the glycosylase was inactivated.

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