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Thermodynamics of the interactions of lac repressor with variants of the symmetric lac operator: effects of converting a consensus site to a non-specific site.
J Mol Biol. 1997 Apr 18; 267(5):1186-206.JM

Abstract

What are the thermodynamic consequences of the stepwise conversion of a highly specific (consensus) protein-DNA interface to one that is nonspecific? How do the magnitudes of key favorable contributions to complex stability (burial of hydrophobic surfaces and reduction of DNA phosphate charge density) change as the DNA sequence of the specific site is detuned? To address these questions we investigated the binding of lac repressor (LacI) to a series of 40 bp fragments carrying symmetric (consensus) and variant operator sequences over a range of temperatures and salt concentrations. Variant DNA sites contained symmetrical single and double base-pair substitutions at positions 4 and/or 5 [sequence: see text] in each 10 bp half site of the symmetric lac operator (Osym). Non-specific interactions were examined using a 40 bp non-operator DNA fragment. Disruption of the consensus interface by a single symmetrical substitution reduces the observed equilibrium association constant (K(obs)) for Osym by three to four orders of magnitude; double symmetrical substitutions approach the six orders in magnitude difference between specific and non-specific binding to a 40 bp fragment. At these adjacent positions in the consensus site, the free energy effects of multiple substitutions are non-additive: the first reduces /deltaG(obs)o/ by 3 to 5 kcal mol(-1), approximately halfway to the non-specific level, whereas the second is less deleterious, reducing /deltaG(obs)o/ by less than 3 kcal mol(-1). Variant-specific dependences of K(obs) on temperature and salt concentration characterize these LacI-operator interactions. In general, binding constants and standard free energies of binding both exhibit characteristic extrema near 290 K. As a consequence, both the enthalpic and entropic contributions to stability of Osym and variant complexes change from positive (i.e. entropy driven) at lower temperatures to negative (i.e. enthalpy driven) at higher temperatures, indicating that the heat capacity change upon binding, deltaC(obs)o, is large and negative. In general, /deltaC(obs)o/ decreases as the specificity and stability of the variant complex decreases. Stabilities of complexes of LacI with Osym and all variant operators are strongly [salt]-dependent. Binding constants for the variant complexes exhibit a power-dependence on [salt] that is larger in magnitude (i.e. more negative) than for Osym, but no obvious trend relates changes in contributions from the polyelectrolyte effect and the observed reductions in stability (delta deltaG(obs)o). These variant-specific thermodynamic signatures provide novel insights into the consequences of converting a consensus interface to a less specific one; such insights are not obtained from comparisons at the level of delta deltaG(obs)o. We propose that this variant-specific behavior arises from a strong effect of operator sequence on the extent of induced conformational changes in the protein (and possibly also in the DNA site) which accompany binding.

Authors+Show Affiliations

Department of Biochemistry, University of Wisconsin-Madison, 53706, USA.No affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info availableNo affiliation info available

Pub Type(s)

Journal Article
Research Support, U.S. Gov't, P.H.S.

Language

eng

PubMed ID

9150406

Citation

Frank, D E., et al. "Thermodynamics of the Interactions of Lac Repressor With Variants of the Symmetric Lac Operator: Effects of Converting a Consensus Site to a Non-specific Site." Journal of Molecular Biology, vol. 267, no. 5, 1997, pp. 1186-206.
Frank DE, Saecker RM, Bond JP, et al. Thermodynamics of the interactions of lac repressor with variants of the symmetric lac operator: effects of converting a consensus site to a non-specific site. J Mol Biol. 1997;267(5):1186-206.
Frank, D. E., Saecker, R. M., Bond, J. P., Capp, M. W., Tsodikov, O. V., Melcher, S. E., Levandoski, M. M., & Record, M. T. (1997). Thermodynamics of the interactions of lac repressor with variants of the symmetric lac operator: effects of converting a consensus site to a non-specific site. Journal of Molecular Biology, 267(5), 1186-206.
Frank DE, et al. Thermodynamics of the Interactions of Lac Repressor With Variants of the Symmetric Lac Operator: Effects of Converting a Consensus Site to a Non-specific Site. J Mol Biol. 1997 Apr 18;267(5):1186-206. PubMed PMID: 9150406.
* Article titles in AMA citation format should be in sentence-case
TY - JOUR T1 - Thermodynamics of the interactions of lac repressor with variants of the symmetric lac operator: effects of converting a consensus site to a non-specific site. AU - Frank,D E, AU - Saecker,R M, AU - Bond,J P, AU - Capp,M W, AU - Tsodikov,O V, AU - Melcher,S E, AU - Levandoski,M M, AU - Record,M T,Jr PY - 1997/4/18/pubmed PY - 2001/3/28/medline PY - 1997/4/18/entrez SP - 1186 EP - 206 JF - Journal of molecular biology JO - J Mol Biol VL - 267 IS - 5 N2 - What are the thermodynamic consequences of the stepwise conversion of a highly specific (consensus) protein-DNA interface to one that is nonspecific? How do the magnitudes of key favorable contributions to complex stability (burial of hydrophobic surfaces and reduction of DNA phosphate charge density) change as the DNA sequence of the specific site is detuned? To address these questions we investigated the binding of lac repressor (LacI) to a series of 40 bp fragments carrying symmetric (consensus) and variant operator sequences over a range of temperatures and salt concentrations. Variant DNA sites contained symmetrical single and double base-pair substitutions at positions 4 and/or 5 [sequence: see text] in each 10 bp half site of the symmetric lac operator (Osym). Non-specific interactions were examined using a 40 bp non-operator DNA fragment. Disruption of the consensus interface by a single symmetrical substitution reduces the observed equilibrium association constant (K(obs)) for Osym by three to four orders of magnitude; double symmetrical substitutions approach the six orders in magnitude difference between specific and non-specific binding to a 40 bp fragment. At these adjacent positions in the consensus site, the free energy effects of multiple substitutions are non-additive: the first reduces /deltaG(obs)o/ by 3 to 5 kcal mol(-1), approximately halfway to the non-specific level, whereas the second is less deleterious, reducing /deltaG(obs)o/ by less than 3 kcal mol(-1). Variant-specific dependences of K(obs) on temperature and salt concentration characterize these LacI-operator interactions. In general, binding constants and standard free energies of binding both exhibit characteristic extrema near 290 K. As a consequence, both the enthalpic and entropic contributions to stability of Osym and variant complexes change from positive (i.e. entropy driven) at lower temperatures to negative (i.e. enthalpy driven) at higher temperatures, indicating that the heat capacity change upon binding, deltaC(obs)o, is large and negative. In general, /deltaC(obs)o/ decreases as the specificity and stability of the variant complex decreases. Stabilities of complexes of LacI with Osym and all variant operators are strongly [salt]-dependent. Binding constants for the variant complexes exhibit a power-dependence on [salt] that is larger in magnitude (i.e. more negative) than for Osym, but no obvious trend relates changes in contributions from the polyelectrolyte effect and the observed reductions in stability (delta deltaG(obs)o). These variant-specific thermodynamic signatures provide novel insights into the consequences of converting a consensus interface to a less specific one; such insights are not obtained from comparisons at the level of delta deltaG(obs)o. We propose that this variant-specific behavior arises from a strong effect of operator sequence on the extent of induced conformational changes in the protein (and possibly also in the DNA site) which accompany binding. SN - 0022-2836 UR - https://www.unboundmedicine.com/medline/citation/9150406/Thermodynamics_of_the_interactions_of_lac_repressor_with_variants_of_the_symmetric_lac_operator:_effects_of_converting_a_consensus_site_to_a_non_specific_site_ DB - PRIME DP - Unbound Medicine ER -