Only Escherichia coli DNA topoisomerase I requires a cofactor. Magnesium II is not required for DNA cleavage by topoisomerase or formation of the covalent protein-DNA intermediate. In fact, no magnesium was found in the crystal structures. But Mg(II) is required for relaxation activity of the topoisomerase IA. Its role is not fully understood, but there are several theories as to why magnesium is required.

First, magnesium has been detected in complex with the topoisomerase IA with approximately two Mg(II) bound to each enzyme molecule. The activity of the topoisomerase was greatest when the Mg(II) concentrations were higher than that needed for 2 Mg(II) per enzyme, but near full activity was observed with only the concentration necessary for just two Mg(II) per molecule of topoisomerase. This amount of magnesium was enough for relaxation activity to be observed.

According to fluorescence measurements, the binding of Mg(II) does alter the enzyme conformation. This alteration in conformation may be required for either strand passage or the rejoining of the 5' and 3' ends of the cleaved DNA. A second possible role is to position the 3'OH for DNA religation after strand passage.

There are a number of conserved acidic amino acid residues that are possibilities for binding sites for Mg(II). The binding of the magnesium would change the conformation by interfering with the existing network of hydrogen bonds and possibly creating new hydrogen bonds. This is currently being studied by site-directed mutagensis (Zhu, 1997).

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Updated:  December 10, 1998

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