Quantitative Labeling of Long Plasmid DNA with Nanometer Precision

On target. DNA methyltransferases deliver the methyl group from the natural cofactor S-adenosyl-L-methionine to defined DNA sequences. Engineered aziridine cofactors were used to trick these enzymes into labeling DNA. Sequence specificity was directly visualized by using electron microscopy. The figure shows streptavidin molecules (encircled) complexed to linearized plasmid DNA that was biotinylated at four positions with the DNA methyltransferase M.TaqI and an aziridine cofactor.