Catenated and Knotted DNA Must Be Corrected

Circular molecules of DNA may become interlocked during replication or recombination. Such structures are called catenanes. The circles may be liberated by certain type II topoisomerases, such as topoisomerase IV (Fig. 4.18) of E. coli and related enzymes. Circular DNA molecules may also form knots. Type II topoisomerases can both create and untie knots. Like DNA gyrases, these enzymes are tetramers of two different subunits, one for cutting the DNA and the other for energy coupling. Like gyrase, topoisomerase IV is inhibited by quinolone antibiotics.

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