CALORIMETRIC STUDIES OF THE ENERGETICS OF PROTEIN-DNA INTERACTIONS INTHE ESCHERICHIA-COLI METHIONINE REPRESSOR (METJ) SYSTEM

Calorimetric measurements of binding of a specific DNA fragment and S- adenosyl methionine (SAM) co-repressor molecules to the E. coli methio nine repressor (MetJ) show significant differences in the energetics o f binary and ternary protein-DNA complexes. Formation of the MetJ:SAM: DNA ternary complex is significantly more exothermic (Delta H similar or equal to -99 kJ.mol(-1)) than either MetJ:DNA or MetJ:SAM binary co mplexes alone (Delta H similar or equal to -10 kJ.mol(-1) each). The p rotein is also significantly more stable to unfolding (Delta T-m simil ar or equal to 5.4 degrees C) when bound to DNA. These observations su ggest that binding of SAM to the protein-DNA complex leads to a signif icant reduction in dynamic flexibility of the ternary complex, with co nsiderable entropy-enthalpy compensation, not necessarily involving an y overall conformational change.