The incision pattern and incision efficiency of (+)anti-BPDE-DNA adduct by the Escherichia coli UvrABC excision repair system

The major objective of this research was to study how the DNA sequence surrounding a DNA adduct affects the efficiency of repair of the adduct by the Escherichia coli UvrABC repair system. The principal adduct formed by the reaction of (7R,8S)-dihydroxy-(9S,10R)-epoxy-7,8,9,10-tetrahydro-benzo (a) pyrene ((+)anti-BPDE) with DNA is a relatively stable adduct attached to the N2 position of guanine (N2-dG adduct). Using $\rm\lbrack H\sp3\rbrack$-(+)anti-BPDE, adduct stability was determined. Results showed that 98% of the (+)anti-BPDE N2-dG adduct was stable at 25$\sp\circ$C over a period of two weeks. The effect of the surrounding sequence on the recognition of damage and the efficiency of incision by the UvrABC repair enzymes were studied using high resolution gel electrophoresis followed by densitometric scanning of the autoradiograms. The UvrABC enzyme generally incised the 8th phosphodiester bond 5$\sp\prime$ and the 5th phosphodiester bond 3$\sp\prime$ to the adducted base. Aberrant cutting was also observed. The UvrABC incision efficiency was only weakly correlated to the BPDE binding. At some sites, such as the G doublets and the NarI recognition sequence (GGCGCC), the efficiency of incision varied markedly, showing a large effect of the surrounding DNA sequence. At some sites, such as the second G of the TGG, in which the second G was a mutation hotspot and also a poorly repaired site, poor efficiency of repair might increase the mutation rate. However, in general, there was no clear connection between the mutation rate and the efficiency of repair.