The Escherichia coli O6-methylguanine-DNA methyltransferase does not repair promutagenic O6-methylguanine residues when present in Z-DNA.

The repair of O6-methylguanine present in N-methylnitrosourea (MNU)-treated alternating polynucleotides MNU-poly(dG-dC) X poly(dG-dC) and MNU-poly(dG-me5dC) X poly(dG-me5dC] was investigated using O6-methylguanine-DNA methyltransferase purified from Escherichia coli. Both modified polynucleotides are equally good substrates for the DNA methyltransferase when they are in the B-form. The substrate properties of the MNU-treated polynucleotides do not differ from those of MNU-treated DNA. One of these modified polynucleotides, MNU-poly(dG-me5dC) X (dG-me5dC), can adopt the Z-conformation under physiological conditions. The conformational transition of the poly(dG-me5dC) X poly(dG-me5dC) from the B-form to the Z-form was monitored by the modification of its spectroscopic properties and by the specific binding of antibodies raised against Z-DNA. The O6-methylguanine residues are repaired in MNU-poly(dG-me5dC) X poly(dG-me5dC) in B-form. At variance, the conversion of this template to the Z-form completely inhibits the repair of the O6-methylguanine residues. The cooperative transition from the Z- to the B-form of MNU-poly(dG-me5dC) X poly(dG-me5dC), mediated by intercalating drugs such as ethidium bromide, restores the ability of MNU-poly(dG-me5dC) X poly(dG-me5dC) to be substrate for the transferase. These results imply that the promutagenic DNA lesion O6-methylguanine persists in Z-DNA fragments and suggest that DNA conformation modulates the extent of DNA repair and, as a result, plays an important role in determining the mutagenic potency of chemical carcinogens.