PHOTOCHEMICAL MODIFICATIONS OF lac REPRESSOR: EFFECT OF EFFECTORS BINDING ON TRYPTOPHAN PHOTOOXIDATION

UV irradiation of lac repressor modifies the fluorescence of the protein and its binding to the inducer and to the operator. It has been previously shown that the total loss of fluorescence is due to photooxidation of, on average, one of the two tryptophyl residues of each protomer. The present work explains this observation by showing that N‐formylkynurenine formed at one site is responsible for the quenching of fluorescence of the other tryptophan via an energy transfer process. Consequently, no photoreaction occurs for the second tryptophyl residue. Photodamage of the two tryptophyl residues (in position 201 and 220) of each protomer were assayed by spectrofluorometric titration in the pH range from 8.5 to 5. For repressor alone, both residues are equally photodamaged. In the presence of the inducer isopropyl‐β‐D‐thio‐galactoside, IPTGt, residue 220 is completely protected, and tryptophan 201 is slightly more exposed to photooxidation. In the presence of antiinducer, residue 220 is only partially protected. Our results are discussed in terms of conformational changes triggered by the two types of ligands.