Topoisomerase

**Semantic type:** Amino Acid, Peptide, or Protein|Enzyme

**Definition:** Enzymes that change the degree of supercoiling in DNA by cutting one or both strands. Type I topoisomerases cut only one strand of DNA; type I topoisomerase of E. coli (omega protein) relaxes negatively supercoiled DNA and does not act on positively supercoiled DNA. Type II topoisomerases cut both strands of DNA; type II topoisomerase of E. coli (DNA gyrase) increases the degree of negative supercoiling in DNA and requires ATP. It is inhibited by several antibiotics, including nalidixic acid and ovobiocin.