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Specific binding of lac repressor to linear versus circular polyoperator molecules.

H M Sasmor¹, J L Betz

¹Department of Biochemistry, Biophysics and Genetics, University of Colorado School of Medicine, Denver 80262.

|September 25, 1990

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Lactose repressor binds DNA operator sites. While linear DNA allows full binding, circular DNA saturation depends on its topological state, suggesting DNA conformation impacts repressor accessibility.

Area of Science:

Molecular Biology
Genetics
Biochemistry

Background:

The lactose (lac) repressor protein regulates gene expression by binding to operator DNA sequences.
Understanding repressor-DNA interactions is crucial for deciphering gene regulation mechanisms.

Purpose of the Study:

To investigate the binding of the lac repressor to polyoperator DNA molecules.
To determine how DNA topology (linear, circular, covalently closed) affects repressor binding affinity and saturation.

Main Methods:

Gel shift assays were employed to analyze repressor-operator DNA complex formation.
Specific binding was distinguished from non-specific DNA association using controlled repressor concentrations and the inducer isopropyl beta-D-thiogalactoside (IPTG).

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Main Results:

All eight operator sites on linear polyoperator DNA could be simultaneously bound by lac repressor.
Repressor binding to circular polyoperator DNA was saturable only in the nicked circular state, not in the covalently closed circular form.
No evidence of bifunctional repressor binding or DNA loop formation was observed.

Conclusions:

DNA topology significantly influences lac repressor binding to multiple operator sites.
The conformational changes induced by repressor binding to one operator site may propagate through topologically constrained DNA, affecting accessibility of other sites.