title: Energetics of DNA end binding by E.coli RecBC and RecBCD helicases indicate loop formation in the 3'-single-stranded DNA tail.
abstract: We examined the equilibrium binding of Escherichia coli RecBC and RecBCD helicases to duplex DNA ends possessing pre-existing single-stranded (ss) DNA ((dT)(n)) tails varying in length (n=0 to 20 nucleotides) in order to determine the contributions of both the 3' and 5' single strands to the energetics of complex formation. Protein binding was monitored by the fluorescence enhancement of a reference DNA labeled at its end with a Cy3 fluorophore. Binding to unlabeled DNA was examined by competition titrations with the Cy3-labeled reference DNA. The affinities of both RecBC and RecBCD increase as the 3'-(dT)(n) tail length increases from zero to six nucleotides, but then decrease dramatically as the 3'-(dT)(n) tail length increases from six to 20 nucleotides. Isothermal titration calorimetry experiments with RecBC show that the binding enthalpy is negative and increases in magnitude with increasing 3'-(dT)(n) tail length up to n=6 nucleotides, but remains constant for n > or =6. Hence, the decrease in binding affinity for 3'-(dT)(n) tail lengths with n > or =6 is due to an unfavorable entropic contribution. RecBC binds optimally to duplex DNA with (dT)6 tails on both the 3' and 5'-ends while RecBCD prefers duplex DNA with 3'-(dT)6 and 5'-(dT)10 tails. These data suggest that both RecBC and RecBCD helicases can destabilize or "melt out" six base-pairs upon binding to a blunt DNA duplex end in the absence of ATP. These results also provide the first evidence that a loop in the 3'-ssDNA tail can form upon binding of RecBC or RecBCD with DNA duplexes containing a pre-formed 3'-ssDNA tail with n > or =6 nucleotides. Such loops may be representative of those hypothesized to form upon interaction of a Chi site contained within the unwound 3' ss-DNA tail with the RecC subunit during DNA unwinding.
authors.affiliation: Department of Biochemistry and Molecular Biophysics, Washington University School of Medicine, 660 S. Euclid Avenue, Box 8231, Saint Louis, MO 63110-1093, USA.
authors.forename: C Jason
authors.initials: CJ
authors.lastname: Wong
authors.forename: Aaron L
authors.initials: AL
authors.lastname: Lucius
authors.forename: Timothy M
authors.initials: TM
authors.lastname: Lohman
mesh_terms: D001483:Base Sequence; D002232:Carbocyanines / Q000737:chemistry; D004277:DNA, Single-Stranded / Q000737:chemistry / Q000378:metabolism; D029968:Escherichia coli Proteins / Q000378:metabolism*; D043211:Exodeoxyribonuclease V / Q000378:metabolism*; D005456:Fluorescent Dyes / Q000737:chemistry; D046911:Macromolecular Substances; D008969:Molecular Sequence Data; D015394:Molecular Structure; D009690:Nucleic Acid Conformation; D011485:Protein Binding
chemical_list: D002232:Carbocyanines; D004277:DNA, Single-Stranded; D029968:Escherichia coli Proteins; D005456:Fluorescent Dyes; D046911:Macromolecular Substances; C089187:cyanine dye 3; D043211:Exodeoxyribonuclease V; C468322:exodeoxyribonuclease V, E coli