DNATopoisomerase I-Mediated Nicking of Circular Duplex DNA

Type I topoisomerases catalyze the reversible nicking of duplex DNA (for review, see ref. 1 ). In the nicked state, the enzyme is attached to the DNA by way of a phosphodiester bond between a tyrosine residue in the protein and the end of the broken strand. The polarity of attachment depends on the source of the enzyme; the bacterial enzyme is linked to the 5′-end, whereas the eukaryotic enzyme is attached to the 3′-end of the DNA.