title: PicoNewton-millisecond force steps reveal the transition kinetics and mechanism of the double-stranded DNA elongation.
abstract: We study the kinetics of the overstretching transition in λ-phage double-stranded (ds) DNA from the basic conformation (B state) to the 1.7-times longer and partially unwound conformation (S state), using the dual-laser optical tweezers under force-clamp conditions at 25°C. The unprecedented resolution of our piezo servo-system, which can impose millisecond force steps of 0.5-2 pN, reveals the exponential character of the elongation kinetics and allows us to test the two-state nature of the B-S transition mechanism. By analyzing the load-dependence of the rate constant of the elongation, we find that the elementary elongation step is 5.85 nm, indicating a cooperativity of ~25 basepairs. This mechanism increases the free energy for the elementary reaction to ~94 k(B)T, accounting for the stability of the basic conformation of DNA, and explains why ds-DNA can remain in equilibrium as it overstretches.
authors.affiliation: Laboratorio di Fisiologia, Dipartimento di Biologia Evoluzionistica, Università degli Studi di Firenze, Sesto Fiorentino, Italy.
authors.forename: Pasquale
authors.initials: P
authors.lastname: Bianco
authors.forename: Lorenzo
authors.initials: L
authors.lastname: Bongini
authors.forename: Luca
authors.initials: L
authors.lastname: Melli
authors.forename: Mario
authors.initials: M
authors.lastname: Dolfi
authors.forename: Vincenzo
authors.initials: V
authors.lastname: Lombardi
mesh_terms: D010582:Bacteriophage lambda / Q000737:chemistry*; D001696:Biomechanical Phenomena; D003198:Computer Simulation; D004247:DNA / Q000737:chemistry*; D004279:DNA, Viral / Q000737:chemistry*; D007700:Kinetics; D013816:Thermodynamics; D013997:Time Factors
chemical_list: D004279:DNA, Viral; D004247:DNA