PicoNewton-Millisecond Force Steps Reveal the Transition Kinetics and Mechanism of the Double-Stranded DNA Elongation

We study the kinetics of the overstretching transition in lambda-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 degrees 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 similar to 25 basepairs. This mechanism increases the free energy for the elementary reaction to similar 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.