This subfamily of topoisomerase is divided on the basis that these enzymes preferentially relax negatively supercoiled DNA, from a 5' phospho- tyrosine linkage in the enzyme-DNA covalent intermediate and has high affinity for single stranded DNA.

1: Three-dimensional structure of the 67K N-terminal fragment of E. coli DNA topoisomerase I. Lima CD, Wang JC, Mondragon A; Nature 1994;367:138-146. PMID:8114910