Figure 3.

Twofold symmetric catalytic domain interactions of Int, Cre and Flp. (a) Domain structure of a single active monomer overlaid on the Int, Cre and Flp tetramers bound to their respective HJs (shown in the background). The active site tyrosine nucleophiles of Int, Cre and Flp are Y342, Y324 and Y343, respectively. The CB-domain of Int is the equivalent of the N-domains of Cre and Flp. Int has an additional domain, the N-domain (black), which binds to arm DNA sites. The C-terminal β9 sheet and the N-helix of Int and Cre, respectively, interact with neighboring subunits within the tetramer. The Y343 nucleophile of Flp is supplied in trans by a neighboring subunit of the tetramer (shown in blue). The linker regions between the N-domains and catalytic domains of Cre and Flp (orange) have an additional helix involved in intermolecular interaction that is not present in the equivalent linker region of Int (orange). (b) Cartoons illustrating how the twofold symmetric intermolecular interactions of the catalytic domains of Int, Cre and Flp tetramers bound to DNA affect the position of the active site tyrosine with respect to the active (red circles) and inactive (white circles) scissile phosphates. The view is from the top (i.e. the N-domains would be above the drawn tetramers). These twofold symmetric interactions probably also influence the positions of other active site residues and thus further contribute to the activation of two out of four monomers, as has been most clearly seen in the Cre crystal structures [10]. The cis interaction of β9 and β6 in the unliganded Int monomer removes Y342 from the active site pocket and inactivates Int (small diagram on the left [41]).