Lac Repressor

Abstract Lac repressor is characterized by the amino acid sequence of its first 59 amino acid residues and the sequence of additional peptides, accounting for a total of 104 residues, or about 30% of the molecule. Trypsin and chymotrypsin readily inactivate the DNA binding of repressor without a loss in binding properties for the inducer isopropyl-β-d-thiogalactoside. Characterization of the trypsinized repressor shows that digestion under native conditions yields a trypsin-resistant core molecule, which is still a tetramer and exhibits full inducer binding. The inducer binding activity as well as the tetrameric structure can be recovered after denaturation in a random coil solvent. The monomer molecular weight of the tryptic core is approximately 28,000, compared to 38,000 for the intact repressor polypeptide chain indicating the release of about 80 to 90 amino acid residues in the form of tryptic peptides. Characterization of these peptides indicates that they include the NH2-terminal 59 residues of the lac repressor and that they all probably come from the NH2-terminal region of the molecule. Thus, trypsin or chymotrypsin is able to attack the NH2-terminal regions of the repressor tetramer without inducing any further internal nicks in the polypeptide chain. These findings provide substantial support for the hypothesis that the NH2-terminal region of lac repressor is required for binding to the lac operator, but not for the binding of inducer or for the folding of the subunit and its association into a tetrameric structure.