Table 2.

Gaps opposite the bulky benzo[a]pyrene-guanine adduct are efficiently repaired by homologous recombination

Donor plasmid	Gap repair, %
None	0.8±0.3
pFGPB-cm	68.5±7.1
pFGP20/T-amp	27.2±3.3

E. coli WBY100 cell where transformed with either pFGPBP-cm or pFGP20/T-amp, and selected on LB-cm or LB-amp plates, respectively. Cells carrying the donor plasmid were then, re-transformed with the gap-lesion plasmid GP-BPG1 carrying a benzo[a]pyrene-G adduct. BP-G1 Tansformants where assayed for their ability to survive on kanamycin plats relative to GP20 transformants. The efficiency of gap repair was calculated by dividing the number of colonies obtained with the gap-lesion plasmid, by the number of colonies obtained with GP20. Typically plates with 50–200 colonies were counted, except for the background plates, where colony counts were lower. The results are the averages of 3 independent experiments.

Plasmid pFGPBP-cm is a descendent of the gap-lesion plasmid GP-BPG1-cm obtained by introducing the latter into E. coli and selecting for cm^R colonies. It has a G at the position corresponding to the lesion in GP-BPG1–cm. FGP20/T-amp was previously described ²⁰. It contains a short stretch of 12 nucleotides that is heterologous to the cognate sequence surrounding the lesion in the gap-lesion plasmid GP-BPG1, and contains a T at the position corresponding to the lesion in plasmid GP-BPG1-cm.