title: Deoxyuridine misincorporation causes site-specific mutational lesions in the lacI gene of Escherichia coli.
abstract: Spontaneous forward mutation in lacI was analyzed by DNA sequencing in a Dut- strain of E. coli. Hyperuracil incorporation into DNA due to the defect in deoxyuridinetriphosphatase caused a 5-fold increase in mutation frequency. Deletion, duplication and base-substitution frequencies were all enhanced in the Dut- strain. However, the analysis of the specificity of mutation revealed a remarkable site- and class-specificity. For example, base substitutions at a single site, a G:C = greater than A:T transition (Ochre 34) accounted for 55% of the base substitutions recovered. The spontaneous A:T = greater than G:C hotspot at position +6 at the lac operator was also recovered at an enhanced frequency in the Dut- strain where it accounted for 25% of the base substitutions. Many of the deletion and duplication events were recovered more than once; most had endpoints in A/T rich regions. The spontaneous frameshift hotspot involving the gain or loss of 5'-CTGG-3' in a region where this tetramer is tandemly repeated 3 times, was also greatly enhanced. No frameshifts involving a single base pair nor IS1 insertions were identified among the 86 lacI mutants sequenced. The analysis of these events reveals them to be generally consistent with a mechanism involving AP sites generated by the removal of misincorporated uracil by uracil-N-glycosylase. Considering the number of potential AP sites (approximately 1 per 170 base pairs) E. coli is remarkably refractory to mutational consequences of deoxyuridine misincorporation in place of thymidine.
mesh_terms: D010582:Bacteriophage lambda / Q000235:genetics; D001483:Base Sequence; D004269:DNA, Bacterial / Q000235:genetics; D003857:Deoxyuridine / Q000235:genetics*; D004926:Escherichia coli / Q000235:genetics*; D005798:Genes, Bacterial; D007763:Lac Operon; D009154:Mutation; D011995:Recombination, Genetic
chemical_list: D004269:DNA, Bacterial; D003857:Deoxyuridine