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A study of energetics of cooperative interaction using a mutant lambda-repressor.
N K Jana1, S Deb, B Bhattacharyya
1Department of Biochemistry and Department of Biophysics, Bose Institute, Acharya J. C. Bose Birth Centenary Building, P 1/12 C.I.T Scheme VII M, Calcutta 700 054, India.
Protein Engineering
|October 31, 2000
Summary
A lambda-repressor mutant (S228N) shows normal protein structure and stability, despite defects in tetramer formation. This suggests loop energetics are key to its cooperative DNA binding.
Area of Science:
- Molecular Biology
- Biochemistry
- Structural Biology
Background:
- The lambda repressor protein regulates gene expression through cooperative DNA binding.
- Understanding the structural basis of this cooperativity is crucial for deciphering gene regulation mechanisms.
Purpose of the Study:
- To investigate the structural and functional properties of a lambda-repressor mutant (S228N) defective in free-state tetramer formation.
- To elucidate the role of protein structure and energetics in the cooperative binding of the lambda repressor to its operator DNA.
Main Methods:
- Fluorescence anisotropy studies to determine dimer-monomer dissociation constants.
- Circular dichroism and acrylamide quenching to assess protein structure.
- Sulfhydryl reactivity and urea denaturation to evaluate protein stability and energetics.
Main Results:
- The S228N mutant exhibits a dimer-monomer dissociation constant around 10(-5) M, similar to wild-type repressor when bound to a single operator.
- Structural analyses (CD, quenching, reactivity) at low protein concentrations (<10(-6) M) revealed no significant structural deviations from wild-type.
- Urea denaturation studies indicated comparable protein stability between the mutant and wild-type repressor.
Conclusions:
- The S228N mutation does not significantly alter the overall structure or stability of the lambda repressor.
- The mutant's retained cooperativity, despite impaired tetramer formation, highlights the importance of loop energetics in the cooperative binding process.
- These findings provide insights into the molecular mechanisms underlying cooperative DNA binding in regulatory proteins.