Lactose repressor protein reaction with 2-chloromercuri-4-nitrophenol

Reaction of the lactose repressor protein with 2-chloromercuri-4-nitrophenol resulted in modification of two of the three cysteine residues per monomer at all pH values tested. At pH 8.1 it was possible to detect two equivalence points, corresponding to one and two mercurinitrophenol residues introduced per monomer. Inducer and operator binding activities were not affected by the introduction of the mercurial into the protein structure. Inducer binding, but not anti-inducer binding, produced a change in the spectrum of the mercurinitrophenol, apparently due to alterations in the environment of the chromophores as a result of shifts in the structure of the repressor protein.