Kinetic Control of Mg²⁺-dependent Melting of Duplex DNA Ends by Escherichia coli RecBC

Escherichia coli RecBCD is a highly processive DNA helicase involved in double-strand break repair and recombination that possesses two helicase/translocase subunits with opposite translocation directionality (RecB (3′ to 5′) and RecD (5′ to 3′)). RecBCD has been shown to melt out ∼ 5-6 bp upon binding to a blunt-ended duplex DNA in a Mg²⁺-dependent, but ATP-independent reaction. Here, we examine the binding of E. coli RecBC helicase (minus RecD), also a processive helicase, to duplex DNA ends in the presence and in the absence of Mg²⁺ in order to determine if RecBC can also melt a duplex DNA end in the absence of ATP. Equilibrium binding of RecBC to DNA substrates with ends possessing pre-formed 3′ and/or 5′ single-stranded (ss)-(dT)_n flanking regions (tails) (n ranging from zero to 20 nt) was examined by competition with a fluorescently labeled reference DNA and by isothermal titration calorimetry. The presence of Mg²⁺ enhances the affinity of RecBC for DNA ends possessing 3′ or 5′-(dT)_n ssDNA tails with n < 6 nt, with the relative enhancement decreasing as n increases from zero to six nt. No effect of Mg²⁺ was observed for either the binding constant or the enthalpy of binding (ΔH_obs) for RecBC binding to DNA with ssDNA tail lengths, n ≥ 6 nucleotides. Upon RecBC binding to a blunt duplex DNA end in the presence of Mg²⁺, at least 4 bp at the duplex end become accessible to KMnO₄ attack, consistent with melting of the duplex end. Since Mg²⁺ has no effect on the affinity or binding enthalpy of RecBC for a DNA end that is fully pre-melted, this suggests that the role of Mg²⁺ is to overcome a kinetic barrier to melting of the DNA by RecBC and presumably also by RecBCD. These data also provide an accurate estimate (ΔH_obs = 8 ± 1 kcal/mol) for the average enthalpy change associated with the melting of a DNA base-pair by RecBC.