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Science 3 June 1994:
Vol. 264. no. 5164, pp. 1460 - 1463
DOI: 10.1126/science.8197460
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Articles

Science, Vol 264, Issue 5164, 1460-1463
Copyright © 1994 by American Association for the Advancement of Science

articles

Liquid-crystalline mesophases of plasmid DNA in bacteria

Z Reich, EJ Wachtel, and A Minsky

Department of Organic Chemistry, Weizmann Institute of Science, Rehovot, Israel.

Bacterial plasmids may often reach a copy number larger than 1000 per cell, corresponding to a total amount of DNA that may exceed the amount of DNA within the bacterial chromosome. This observation highlights the problem of cellular accommodation of large amounts of closed-circular nucleic acids, whose interwound conformation offers negligible DNA compaction. As determined by x-ray scattering experiments conducted on intact bacteria, supercoiled plasmids segregate within the cells into dense clusters characterized by a long-range order. In vitro studies performed at physiological DNA concentrations indicated that interwound DNA spontaneously forms liquid crystalline phases whose macroscopic structural properties are determined by the features of the molecular supercoiling. Because these features respond to cellular factors, DNA supercoiling may provide a sensitive regulatory link between cellular parameters and the packaging modes of interwound DNA in vivo.


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