Mutational study at Ser300 position of the Escherichia coli lactose repressor.

We have previously reported that a Ser300Asn mutant of the Escherichia coli lactose repressor protein produced a temperature-sensitive phenotype. In order to analyze the structure-function relationship of the lactose repressor protein, we conducted 18 amino acid substitutions at this Ser 300 site by using in vitro mutagenesis. The substitutions at this position that exhibited repressors with the wild-type phenotype in vivo were Gly, Ala, Ile, Thr, Met and Cys; the other 10 substitutions examined (Leu, Val, Tyr, Trp, Asp, Glu, Gln, His, Lys and Arg) resulted in the lacI- phenotype. In addition, the Ser300Phe mutation resulted in the lacIts phenotype, while the Ser300Pro mutation resulted in lacIts,s. It seems likely that the Ser300 position plays an important role in oligomer formation and inducer binding.