Abstract:
:With a view to determine the configuration and regularity of plectonemically supercoiled DNA, we have measured the small angle neutron scattering from pUC18 plasmid in saline solutions. Furthermore, we have derived the mathematical expression for the single chain scattering function (form factor) of a superhelical structure, including the longitudinal and transverse interference over the plectonemic pitch and radius, respectively. It was found that an interwound configuration describes the data well, provided interactions among supercoils are accounted for in the second virial approximation. The opening angle was observed to be relatively constant and close to 58 degrees, but it was necessary to include a significant distribution in radius and pitch. For diluted supercoils with vanishing mutual interaction, the derived structural results agree with independent measurements, including the distribution in linking number deficit as determined by gel electrophoresis. With increasing plasmid concentration, prior and covering the transition to the liquid-crystalline phase, the radius and pitch are seen to decrease significantly. The latter observation shows that compaction of negatively supercoiled DNA by confinement results in a decrease in writhing number at the cost of a positive twist exerted on the DNA duplex. It is our conjecture that the free energy associated with this excess twist is of paramount importance in controlling the critical boundaries pertaining to the transition to the anisotropic, liquid-crystalline phase.
journal_name
Biophys Jjournal_title
Biophysical journalauthors
Zakharova SS,Jesse W,Backendorf C,Egelhaaf SU,Lapp A,van der Maarel JRdoi
10.1016/S0006-3495(02)75234-Xsubject
Has Abstractpub_date
2002-08-01 00:00:00pages
1106-18issue
2eissn
0006-3495issn
1542-0086pii
S0006-3495(02)75234-Xjournal_volume
83pub_type
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