Fig. 13

Site-specific recombination as catalyzed by a Tyr-recombinase. (a) Four recombinase monomers (ovals) are bound to the recognition sites in two DNA molecules (black and red lines). The active site tyrosine-OH groups in each monomer are shown. “p” represents the phosphodiester bonds that are cleaved during the reactions. (b) The active site Tyr in two recombinase monomers attack phosphodiester bonds in two DNA strands, displacing a 5′-OH and forming a phosphodiester between the 3′-end and the protein. (c) A few base pairs of DNA unwind and the strands switch to form base pairs with the other duplex (red with black). (d) The DNA OH groups react with the DNA-protein phosphodiester, displacing the protein and rejoining the DNA ends. The DNA is in the form of a Holliday junction (see Fig. 3c). The steps shown in (e–g) are chemically the same as those in (a–c): (e) The Tyr residues in the other two monomers react with the DNA. (f) The broken DNA strands switch base pairs to the other strand. (g) The OH at the DNA end reacts with the protein-DNA phosphodiester to rejoin the ends. The red and black DNA strands are joined covalently in a continuous DNA molecule