Configuration of DNA strands and mechanism of strand exchange in the Hin invertasome as revealed by analysis of recombinant knots.

The Hin recombinase of Salmonella normally catalyzes a site-specific DNA inversion reaction that is very efficient when the Fis protein and a recombinational enhancer sequence are present. The mechanism of this recombination reaction has been investigated by analyzing the formation and structure of knots generated in different plasmid substrates in vitro. Hin seldom knots the wild-type substrate under standard recombination conditions. However, we show that increasing the length of DNA between the recombination sites and the enhancer and changing the sequence of the core nucleotides where strand exchange occurs increases the efficiency of the knotting reaction. The structure of the knots generated by different mutant substrates strongly supports a model involving a unique configuration of DNA strands at synapsis and DNA strand exchange mediated by rotation of one set of Hin subunits after DNA cleavage. Analysis of the stereostructure of the knots by electron microscopy of RecA-coated DNA molecules demonstrates that the direction of subunit rotation is exclusively clockwise. Because multiple subunit rotations generating knotted molecules do not occur efficiently when the enhancer is located in its native position, we suggest that the enhancer normally remains associated with the two recombination sites in the invertasome structure during strand exchange to limit strand rotation once it has been initiated. Under certain conditions, however, complex knots are formed that are probably the result of the premature release of the enhancer and multiple, unrestrained subunit exchanges.