Interference of benzo[a]pyrene diol epoxide-deoxyguanosine adducts in a GC box with binding of the transcription factor Sp1

Previous studies indicated that DNA adducts formed by the carcinogenic diol epoxide 7r,8t-dihydroxy-9t,10t-epoxy-7,8,9,10-tetrahydrobenzo[a]pyrene (BPDE) can increase the affinity of the transcription factor Sp1 for DNA sequences that are not normally specific binding sites. Whether adducts that form in the normal binding site, the GC box sequence, increase the affinity of Sp1 for the modified GC-box was not determined. Starting with a 23-nt sequence that contains two natural GC box sequences, site-specifically modified oligonucleotides were prepared with a single (+)-BPDE-deoxyguanosine adduct at one of three positions: the center of each GC box or in between the two boxes. Four modified oligonucleotides were studied, two derived from cis addition of BPDE to the exocyclic amino group and two from tyrans addition. For three of these site-specifically modified oligonucleotides, there was a diminution in Sp1 affinity, whereas Sp1 binding to the fourth modified oligonucleotide was abolished. Furthermore, random modification of the oligonucleotide to a level of about 1 BPDE adduct per fragment slightly decreased the affinity for Sp1, and no evidence was found for a subpopulation of molecules with high affinity. These findings suggest that BPDE modification of the GC box does not lead to an increased affinity for Sp1. This is consistent with a model in which aBPDE-induced bend in the DNA mimics the conformation of the normal GC box:Sp1 complex, leading to high-affinity binding of Sp1 to non-GC box sites.