Function
Once cut, the ends of the DNA are separated, and a second DNA duplex is passed through the break. Following passage, the cut DNA is religated. This reaction allows type II topoisomerases to increase or decrease the linking number of a DNA loop by 2 units, and it promotes chromosome disentanglement. Reactions involving the increase in supercoiling require two molecules of ATP. Janet Lindsley has done much work to examine how the hydrolysis of ATP translates to topoisomerase function. For example, DNA gyrase, a type II topoisomerase observed in E. coli and most other prokaryotes, introduces negative supercoils and decreases the linking number by 2. Gyrase is also able to remove knots from the bacterial chromosome. Along with gyrase, most prokaryotes also contain a second type IIA topoisomerase, termed topoisomerase IV. Gyrase and topoisomerase IV differ by their C-terminal domains, which is believed to dictate substrate specificity and functionality for these two enzymes. Footprinting indicates that gyrase, which forms a 140-base-pair footprint and wraps DNA, allowing it to introduce negative supercoils, while topoisomerase IV, which forms a 28-base-pair footprint, does not wrap DNA.
Eukaryotic type II topoisomerase cannot introduce supercoils; it can only relax them.
The role of type IIB topoisomerase is less understood. Unlike type II topoisomerases, it cannot simplify DNA topology (see below), but it shares several structural features with type IIA topoisomerases.
Read more about this topic: Type II Topoisomerase
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