  The protein recombinase can change the knot type of circular DNA. The action
of a recombinase converting one knot into another knot is normally
mathematically modeled by band surgery. Band surgeries on a 2-bridge knot
N((4mn-1)/(2m)) yielding a (2,2k)-torus link are characterized. We apply this
and other rational tangle surgery results to analyze Xer recombination on DNA
catenanes using the tangle model for protein-bound DNA.
