Kinetic pathways of topology simplification by Type-II topoisomerases in knotted supercoiled DNA.

Abstract

The topological state of covalently closed, double-stranded DNA is defined by the knot type n}{}K and the linking-number difference n}{}Delta Lk relative to unknotted relaxed DNA. DNA topoisomerases are essential enzymes that control the topology of DNA in all cells. In particular, type-II topoisomerases change both n}{}K and n}{}Delta Lk by a duplex-strand-passage mechanism and have been shown to simplify the topology of DNA to levels below thermal equilibrium at the expense of ATP hydrolysis. It remains a key question how small enzymes are able to preferentially select strand passages that result in topology simplification in much larger DNA molecules. Using numerical simulations, we consider the non-equilibrium dynamics of transitions between topological states n}{}(K,Delta Lk) in DNA induced by type-II topoisomerases. For a biological process that delivers DNA molecules in a given topological state n}{}(K,Delta Lk) at a constant rate we fully characterize the pathways of topology simplification by type-II topoisomerases in terms of stationary probability distributions and probability currents on the network of topological states n}{}(K,Delta Lk). In particular, we observe that type-II topoisomerase activity is significantly enhanced in DNA molecules that maintain a supercoiled state with constant torsional tension. This is relevant for bacterial cells in which torsional tension is maintained by enzyme-dependent homeostatic mechanisms such as DNA-gyrase activity.