Dimerization of the operator binding domain of phage λ repressor

Dimerization of λ repressor is required for its binding to operator DNA. As part of a continuing study of the structural basis of the coupling between dimer formation and operator binding, the authors have undertaken 1H NMR and gel filtration studies of the dimerization of the N-terminal domain of λ repressor. Five protein fragments have been studied: three are wild-type fragments of different length (1-102, 1-92, and 1-90), and two are fragments bearing single amino acid substitutions in residues involved in the dimer interface (1-102, Tyr-88 → Cys; 1-92, Ile-84 → Ser). The tertiary structure of each species is essentially the same, as monitored by the 1H NMR resonances of internal aromatic groups. However, significant differences are observed in their dimerization properties. 1H NMR resonances of aromatic residues that are involved in the dimer contact allow the monomer-dimer equilibrium to be monitored in solution. The structure of the wild-type dimer contact appears to be similar to that deduced from X-ray crystallography and involves the hydrophobic packing of symmetry-related helices (helix 5) from each monomer. Removal of two contact residues, Val-91 and Ser-92, by limited proteolysis disrupts this interaction and also prevents crystallization. The Ile-84 → Ser substitution also disrupts this interaction, which accounts for the severely reduced operator affinity of this mutant protein