Mutagenic specificity of the (+) anti ‐diol epoxide of dibenz[ a,j ]anthracene in the supF gene of an Escherichia coli plasmid

This study was designed to examine the mutagenic specificity of (+) anti ‐dibenz[ a,j ]anthracene 3,4‐diol‐1,2‐epoxide ((+) anti ‐DB[ a,j ]A‐DE) in SOS‐induced repair‐proficient Escherichia coli ES87 (Δ pro‐lac, strA )/F′( pro + , lacl Q , lacl am26 , lacZΔM 15 ). The plasmid pUB3, which contains the mutation target gene, supF , was modified with (+) anti ‐DB[ a,j ]A‐DE in vitro (two to five adducts/plasmid) and then transformed into bacteria by electroporation. The spontaneous mutation frequency for unmodified pUB3 in uninduced cells was about 2 x 10 ‐6 and for SOS‐induced cells, about 8 x 10 ‐6 . The spontaneous supF ‐ mutations were primarily insertions, deletions, and frameshifts. The mutation frequency for (+) anti ‐DB[ a,j ]A‐DE‐modified pUB3 was about 8 x 10 ‐6 and about 32 x 10 ‐6 for uninduced cells and SOS‐induced cells, respectively. (+) anti ‐DB[ a,j ]A‐DE induced primarily point mutations in supF in SOS‐induced cells. GC→ transitions were the major mutations observed in SOS‐induced cells (37%). GC→T́ (21%) and GC→C̀ (8.6%) transversion mutations were also observed, whereas mutations at AT base pairs were rare (1.9%). Furthermore, a large number of tandem GC/GC→Ã/AT transition mutations were also observed (about 15% of all mutations in SOS‐induced cells). Taken together, single and tandem GC→Ã mutations accounted for slightly over half (about 51%) of the mutations observed in SOS‐induced cells. These results demonstrated that (+) anti ‐DB[ a,j ]A‐DE was mutagenic in repair‐proficient E. coli ; however, unlike other polycyclic aromatic hydrocarbons that induce primarily transversion mutations, (+) anti ‐DB[ a,j ]A‐DE caused mostly GC→Ã transitions.