Step-arrest mutants of FLP recombinase: implications for the catalytic mechanism of DNA recombination.

The site-specific recombinase (FLP) encoded by the yeast plasmid 2 micron circle belongs to the integrase (of phage lambda) family of recombinases. The sparse homology within the members of this family contrasts with the invariance of three residues, His-396, Arg-399, and Tyr-433 (the numbers correspond to the family alignment positions), among them. We report here results on substrate recognition and catalysis by FLP proteins altered at these residues. Mutations of the conserved His and Tyr that aborted the reaction at specific steps of catalysis permitted genetic dissection of the possible biochemical steps of recombination. We provide indirect evidence that recombination by FLP proceeds through a Holliday junction intermediate.