THERMOCHEMICAL AND KINETIC EVIDENCE FOR NUCLEOTIDE-SEQUENCE-DEPENDENTRECA-DNA INTERACTIONS

RecA catalyses homologous recombination in Escherichia coli by promoti ng pairing of homologous DNA molecules after formation of a helical nu cleoprotein filament with single-stranded DNA. The primary reaction of RecA with DNA is generally assumed to be unspecific. We show here, by direct measurement of the interaction enthalpy by means of isothermal titration calorimetry, that the polymerisation of RecA on single-stra nded DNA depends on the DNA sequence, with a high exothermic preferenc e for thymine bases. This enthalpic sequence preference of thymines by RecA correlates with faster binding kinetics of RecA to thymine DNA. Furthermore, the enthalpy of interaction between the RecA.DNA filament and a second DNA strand is large only when the added DNA is complemen tary to the bound DNA in RecA. This result suggests a possibility for a rapid search mechanism by RecA.DNA filaments for homologous DNA mole cules.