CALORIMETRIC STUDIES OF THE ENERGETICS OF PROTEIN-DNA INTERACTIONS IN THE 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 methionine repressor (MetJ) show significant differences in the energetics of 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 complexes alone (Delta H similar or equal to -10 kJ.mol(-1) each). The protein is also significantly more stable to unfolding (Delta T-m similar or equal to 5.4 degrees C) when bound to DNA. These observations suggest that binding of SAM to the protein-DNA complex leads to a significant reduction in dynamic flexibility of the ternary complex, with considerable entropy-enthalpy compensation, not necessarily involving any overall conformational change.