Fig. 7.

A model of the λ Int dimer showing a possible intermolecular interaction between R169 and E153. The λ Int C75 structure (1P7D) (7) was fitted manually on the crystal structure of a Cre–DNA complex (4CRX) (9), such that the two λ Int molecules pack reasonably well. The side-chain torsion angles of R169 were adjusted manually. The CB domains of λ Int are shown at the top with part of the catalytic domains shown at the bottom. The bound DNA, clamped by the two domains, was omitted for clarity. In this model, the intermolecular distance between R169 and E153 on adjacent protomers is 4 Å, consistent with the ion pair identified by the biochemical experiments reported here. Also illustrated here is the large distance, which precludes intramolecular interactions, between R169 and E153 on the same protomer (labeled with or without a prime). The cyclic pattern of interactions between R169 and E153 is not immediately obvious because only two of the protomers in the tetrameric complex are shown.