DNA supercoiling determines the activation energy barrier for site specific recombination by Tn21 resolvase.

A kinetic analysis of site specific recombination by Tn21 resolvase has been carried out using DNA substrates of varying superhelicities. The rates for the formation of the recombinant product increased with increasing superhelicity up to a maximum value, after which further increases in superhelicity caused no further increase in rate. The reactions with DNA of reduced superhelicity were extremely slow, yet they eventually led to virtually all of the substrate being converted to product. Hence, the level of DNA superhelicity must determine the activation energy barrier for at least one of the steps within the reaction pathway that can be rate-limiting. In the presence (but not in the absence) of Mg2+ ions, the DNA was fully saturated with resolvase whenever the protein was in stoichiometric excess over resolvase binding sites on the DNA. Thus the process affected by DNA supercoiling cannot be coupled to the binding of resolvase. Instead, the step whose rate is determined by supercoiling seems to be located within the reaction pathway after the synapse. However, these reactions may involve two forms of the synaptic complex that are converted to the recombinant product at different rates.