Escherichia coli DNA topoisomerase I is a zinc metalloprotein with three repetitive zinc-binding domains.

Escherichia coli DNA topoisomerase I catalyzes interconversions of different DNA topological isomers by the breakage and rejoining of DNA phosphodiester bonds. It has a crucial role in maintaining an optimal DNA superhelicity in E. coli. It is a single polypeptide of 864 amino acids. Analysis of the amino acid sequence reveals three tandem repeat units each containing two pairs of cysteines suggesting that each unit may form a zinc-binding domain. We have determined that each enzyme molecule contains three to four zinc atoms using inductively coupled plasma-atomic emission analysis. Modification of the cysteine residues and removal of the zinc from the enzyme result in loss of activity. Zinc ions are needed for full recovery of enzyme activity when the cysteine modification is reversed. Comparison with the zinc-binding domains of the sequence-specific DNA-binding proteins shows significant differences.