Intermediates in Hin-mediated DNA inversion: a role for Fis and the recombinational enhancer in the strand exchange reaction.

The site-specific inversion reaction controlling flagellin synthesis in Salmonella involves the function of three proteins: Hin, Fis and HU. The DNA substrate must be supercoiled and contain a recombinational enhancer sequence in addition to the two recombination sites. Using mutant substrates or modified reaction conditions, large amounts of complexes can be generated which are recognized by double-stranded breaks within both recombination sites upon quenching. The cleaved molecules contain 2-bp staggered cuts within the central dinucleotide of the recombination site. Hin is covalently associated with the 5' end while the protruding 3' end contains a free hydoxyl. We demonstrate that complexes generated in the presence of an active enhancer are intermediates that have advanced past the major rate limiting step(s) of the reaction. In the absence of a functional enhancer, Hin is also able to assemble and catalyze site-specific cleavages within the two recombination sites. However, these complexes are kinetically distinct from the complexes assembled with a functional enhancer and cannot generate inversion without an active enhancer. The results suggest that strand exchange leading to inversion is mediated by double-stranded cleavage of DNA at both recombination sites followed by the rotation of strands to position the DNA into the recombinant configuration. The role of the enhancer and DNA supercoiling in these reactions is discussed.