Amino acid substitutions that increase the thermal stability of the lambda Cro protein

A mutant Cro protein, which bears the Ile-30----Leu substitution, is thermally unstable and degraded more rapidly than wild-type Cro in vivo. Using an antibody screen, we have isolated five different second site suppressor substitutions that reduce the proteolytic hypersensitivity of this mutant Cro protein. Two of the suppressor substitutions increase the thermal stability of Cro by 12 degrees C to 14 degrees C. These amino acid substitutions affect residues 16 and 26, which are substantially exposed to solvent in the crystal structure of wild-type Cro.