DNA topoisomerase I (EC 18.104.22.168) [1,2,3,4] is one of the two types of enzyme
that catalyze the interconversion of topological DNA isomers. Type I
topoisomerases act by catalyzing the transient breakage of DNA, one strand at
a time, and the subsequent rejoining of the strands. When a eukaryotic type 1
topoisomerase breaks a DNA backbone bond, it simultaneously forms a protein-DNA link where the hydroxyl group of a tyrosine residue is joined to a 3'-phosphate on DNA, at one end of the enzyme-severed DNA strand. In eukaryotes
and poxvirus topoisomerases I, there are a number of conserved residues in the
region around the active site tyrosine.
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