Lac Repressor. Fluorescence of the two tryptophans

Area of Science:

• Biochemistry
• Molecular Biology
• Protein Structure-Function

Background:

• The lac repressor protein regulates gene expression in E. coli.
• Understanding inducer binding is crucial for deciphering gene regulation mechanisms.

Purpose of the Study:

• To investigate the role of tryptophan residues in lac repressor function.
• To elucidate the conformational changes associated with inducer binding.

Main Methods:

• Comparative fluorescence emission spectroscopy of wild-type and mutant lac repressors.
• Potassium iodide quenching of tryptophan fluorescence.
• Chemical modification of tryptophan residues.

Main Results:

• Tryptophan residue 209's environment is altered upon inducer binding.
• Mutations at tryptophan 209 reduce inducer binding affinity.
• Tryptophan 209 is implicated in conformational changes but not direct inducer contact.

Conclusions:

• The geometry at lac repressor residue 209 is critical for inducer-binding site geometry.
• Tryptophan 209 plays a key role in the allosteric conformational change induced by ligand binding.