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Ion effects on the lac repressor--operator equilibrium

M D Barkley, P A Lewis, G E Sullivan

    Biochemistry
    |June 23, 1981
    PubMed
    Summary
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    The binding of lac repressor protein to operator DNA is highly sensitive to ions. Researchers quantified ionic interactions and binding energy, revealing insights into this crucial molecular recognition process.

    Area of Science:

    • Molecular Biology
    • Biophysics
    • Biochemistry

    Background:

    • The lac repressor protein regulates gene expression by binding to operator DNA.
    • Understanding the influence of the ionic environment on protein-DNA interactions is crucial for molecular biology.

    Purpose of the Study:

    • To investigate the effects of various ions on the interaction between lac repressor protein and operator DNA.
    • To quantify the number of ionic interactions and estimate the binding free energy in the repressor-operator complex.

    Main Methods:

    • Membrane filter technique was employed to study the lac repressor-operator interaction.
    • Equilibrium association constants were determined as a function of ionic conditions (cations, anions, pH).
    • Analysis of salt concentration dependence using binding theory (Record et al., 1976).

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    Main Results:

    • Repressor-operator binding is extremely sensitive to the ionic environment.
    • Approximately 9-11 ionic interactions occur between repressor and DNA phosphates at physiological pH.
    • A favorable nonelectrostatic binding free energy of 9-12 kcal/mol was estimated.
    • Anion type significantly impacts the equilibrium constant, with a 10000-fold decrease observed across different anions.

    Conclusions:

    • The ionic environment plays a critical role in modulating lac repressor-operator DNA binding affinity.
    • The results provide quantitative data on ionic interactions and binding energy, aligning with previous estimates.
    • The study highlights the utility of varying ionic conditions for in vitro manipulation of repressor-operator interactions.