Escherichia coli Type-1 Topoisomerases: Identification, Mechanism, and Role in Recombination
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Excerpt
Topoisomerases catalyze the concerted breakage and reunion of DNA strands. The energy of the broken phosphodiester bond is conserved in a covalent linkage of the enzyme to DNA, allowing the break to be resealed in the absence of an external energy source. Topoisomerases carefully control the breakage and reunion process, never leaving the DNA while it is broken. These enzymes carry out diverse reactions including supercoiling, catenation, knotting, recombination, and aspects of DNA replication. Type-2 topoisomerases make a transient double-stranded break and pass duplex DNA through the break. This changes the topological linking number of DNA (Lk) in steps of two, as explained by the sign-inversion mechanism (Brown and Cozzarelli 1979; Liu et al. 1980; Gellert 1981). Type-1 topoisomerases, on the other hand, break only one strand at a time and alter the Lk in steps of one (Champoux 1978; Wang and Liu 1979; Cozzarelli 1980b).
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