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Biophys J, April 2000, p. 1965-1978, Vol. 78, No. 4
*Department of Biochemistry, University of Minnesota, St. Paul, MN
55108; Department of Molecular and Cell Biology,
University of California, Berkeley, CA 94720; Department
of Physics, University of California, Berkeley, and Physical
Biosciences Division, Lawrence Berkeley Laboratory, Berkeley, CA 94720;
and §Department of Molecular Biology, Princeton
University, Princeton, NJ 08544
The elastic response of single plasmid and lambda phage
DNA molecules was probed using optical tweezers at concentrations of
trivalent cations that provoked DNA condensation in bulk. For uncondensed plasmids, the persistence length, P,
decreased with increasing spermidine concentration before reaching a
limiting value 40 nm. When condensed plasmids were stretched, two types of behavior were observed: a stick-release pattern and a plateau at
~20 pN. These behaviors are attributed to unpacking from a condensed
structure, such as coiled DNA. Similarly, condensing concentrations of
hexaammine cobalt(III) (CoHex) and spermidine induced extensive changes
in the low and high force elasticity of lambda DNA. The high force
(5-15 pN) entropic elasticity showed worm-like chain (WLC) behavior,
with P two- to fivefold lower than in low monovalent salt. At lower
forces, a 14-pN plateau abruptly appeared. This corresponds to an
intramolecular attraction of 0.083-0.33 kT/bp, consistent with osmotic
stress measurements in bulk condensed DNA. The intramolecular
attractive force with CoHex is larger than with spermidine, consistent
with the greater efficiency with which CoHex condenses DNA in bulk. The
transition from WLC behavior to condensation occurs at an extension
about 85% of the contour length, permitting looping and nucleation of condensation. Approximately half as many base pairs are required to
nucleate collapse in a stretched chain when CoHex is the condensing agent.
Biophys J, April 2000, p. 1965-1978, Vol. 78, No. 4
© 2000 by the Biophysical Society 0006-3495/00/04/1965/14 $2.00
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