Cooperativity Mutants of Bacteriophage λ cI Repressor: Temperature Dependence of Self-Assembly

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Abstract
Analytical ultracentrifugation was used to study higher order self-assembly of lambda cI repressors, including eight mutants whose monomer to dimer reactions were recently characterized [Burz et al. (1994) Biochemistry 33, 8399]. Six of the mutants were found to remain dimeric up to 50 microM total protein; the remaining mutants (EK102 and PT158) were found to undergo higher order oligomerization, as does wild-type cI. For these three repressors, we determined the stoichiometries and energetics of higher order assembly over the temperature range 5-40 degrees C. Weak dimerization exhibited by two other mutants, SN228 and SR228, was also evaluated by sedimentation equilibrium over this same temperature range. The end-state for higher order assembly of wild-type cI was determined to be octameric, in agreement with Senear et al. [(1993) Biochemistry 32, 6179-6189]. The assembly free energies resolved by the sedimentation analysis program NONLIN [Johnson, M. L., et al. (1981) Biophys. J. 36, 575-588] leads to the prediction that tetramers may contribute significantly to the intermediate populations during assembly. This analysis of the species populations is in accord with recent conclusions from fluorescence anisotropy studies [Banik et al. (1993) J. Biol. Chem. 268, 3938]. It was found that two of the mutant repressors (EK102 and PTI58) assemble into octamers, but with differing possible intermediates. PT158 satisfies the stoichiometry 8M1 <--> 2M4 <--> M8, while the EK102 data conforms to a 4M2 <--> 2M4 <--> M8 model, similar to WT (both the EK102 and WT data could also be described by a dimer-octamer model with no intermediates). Of the six repressors found in this study to remain dimeric, three exhibit non-cooperative DNA binding (GD147, KN192, YH210), two express intermediate cooperativity (EK188, SR228), and one is fully cooperative (SN228). The three octamerizing repressors are fully cooperative [Burz & Ackers (1994) Biochemistry 33, 8399], suggesting a correlation between their ability to form higher order assemblies and to engage in cooperative DNA binding. Linear van't Hoff plots were obtained for overall assembly of wild-type and EK102 dimers, while that of PT158 monomers was curved, indicating a negative heat capacity change. The van't Hoff analyses of dimerization constants for SN228 and SR228 were distinctly different from each other and also from that of wild type; such differences might be related to the disparate cooperative behavior found previously for these mutants (Burz & Ackers, 1994).