The effect of neighbouring bases on G-specific DNA cleavage mediated by treatment with the anti -diol epoxide of benzo(a)pyrene in vitro

: Three 5'-end-labelled double-stranded linear DNA fragments of defined sequence were treated with r-7, t-8-dihydroxy-t-9, 10-oxy-7,8,9,10-tetrahydrobenzo[a]pyrene (anti-BPDE). The DNA samples were then examined by gel electrophoresis both before and after denaturation and treatment with alkali. The extent of modification of deoxyguanosine (dG) residues was estimated from changes in electrophoretic mobility: at saturation less than 25% of the dG residues appeared to be modified by reaction with anti-BPDE. The determination of the sites of G-specific strand cleavage in a total of 0.5 kbp of DNA by sequencing gel electrophoresis showed that scission at dG residues is sequence specific and that whilst, for example, cleavage occurred at the central dG residues of all 5'-CGG-3' (21/21), of all 5'-TGG-3' (14/14), of all 5'-TGT-3' (7/7) and of all 5'-CGT-3' (5/5) sequences examined, it did not occur in any of the 5'-GGA-3' (0/12) or 5'-GGC-3' (0/15) sequences and only occurred rarely in the 5'-GGG-3' (1/48) and 5'-GGT-3' (2/11) sequences. No cleavage was found at internal dG residues within poly(dG)9 or poly(dG)18 sequences. The data may permit prediction of the sites of strand scission in DNA molecules of known sequence that have been modified by diol-epoxides of polycyclic hydrocarbons.