Stretching of single collapsed DNA molecules

The elastic response of single plasmid and lambda phage DNA molecules was p robed using optical tweezers at concentrations of trivalent cations that pr ovoked DNA condensation in bulk. For uncondensed plasmids, the persistence length, P, decreased with increasing spermidine concentration before reachi ng a limiting value 40 nm. When condensed plasmids were stretched, two type s of behavior were observed: a stick-release pattern and a plateau at simil ar to 20 pN. These behaviors are attributed to unpacking from a condensed s tructure, such as coiled DNA. Similarly, condensing concentrations of hexaa mmine cobalt(III) (CoHex) and spermidine induced extensive changes in the t ow and high force elasticity of lambda DNA. The high force (5-15 pN) entrop ic elasticity showed worm-like chain (WLC) behavior, with P two- to fivefol d lower than in low monovalent salt. At lower forces, a 14-pN plateau abrup tly appeared. This corresponds to an intramolecular attraction of 0.083-0.3 3 kT/bp, consistent with osmotic stress measurements in bulk condensed DNA. The intramolecular attractive force with CoHex is larger than with spermid ine, consistent with the greater efficiency with which CoHex condenses DNA in bulk. The transition from WLC behavior to condensation occurs at an exte nsion about 85% of the contour length, permitting looping and nucleation of condensation. Approximately half as many base pairs are required to nuclea te collapse in a stretched chain when CoHex is the condensing agent.