Targeted base stacking disruption by the EcoRI DNA methyltransferase.

We describe a novel fluorescence-based assay for detecting DNA conformational alterations within enzyme-DNA complexes. The target adenine for EcoRI DNA methyltransferase (GAATTC) was replaced with 2-aminopurine, which fluoresces upon excitation at 310 nm. Addition of the methyltransferase to the duplex binding site results in a 14-fold increase in fluorescence intensity with a 10 nm blue shift. The fluorescence is approximately 50% of that observed with equimolar free nucleoside, consistent with extrahelical stabilization of the target base in the enzyme-DNA complex. The shift in lambda max further implies the base is placed into a low dielectric environment. For adenine-specific DNA methyltransferases, a hydrophobic pocket composed of highly conserved amino acids lies proximal to the cofactor binding site. Substitution of 2-aminopurine adjacent to the target base also results in detectable changes in fluorescence emission following complex formation with the methyltransferase. Thus, other classes of enzymes hypothesized to utilize base flipping can be investigated by this method.