Mutations in λ Repressor's Amino-Terminal Domain: Implications for Protein Stability and DNA Binding

The DNA binding properties of 52 different single-amino acid substitutions in λ repressor's amino-terminal domain have been characterized. Seven proteins bearing mutations that change solvent-exposed side chains have been purified. The amino-terminal domains of these mutant repressors are folded and are comparable to the wild-type amino-terminal domain in thermal stability. In contrast, a purified mutant repressor bearing a substitution in a buried side chain contains an amino-terminal domain with decreased thermal stability. We argue that mutations that alter solvent-exposed wild-type side chains define residues that form the operator DNA binding surface of λ repressor whereas completely or partially buried mutations exert their effect by decreasing protein stability.