The addition of the trivalent or tetravalent cations spermidine or spermine to a solution of T7 DNA in aqueous solution causes an alteration of the DNA from its extended coil form to a condensed form. If performed at low DNA concentration and at low ionic strengths, this transformation results in a monomolecular collapse to form a particle with a hydrodynamic radius of about 500 A. We have monitored this change using quasielastic and total intensity light scattering. In a solution of 50% methanol in water, the divalent cations Mg2+ and putrescine also can cause the condensation of DNA. Using Manning's (1978) counterion condensation theory, we calculate a striking unity among these disparate ions: the collapse occurs in each case when from 89 to 90% of the DNA phosphate charges are neutralized by condensed counterions.

Counterion-induced condesation of deoxyribonucleic acid. a light-scattering study.

RECENT studies have revealed that the packaging of DNA into phage heads is an ordered sequence of structural and biochemical events rather than a simple, spontaneous self-assembly of component molecules. In the assembly of T7 for example1, heads containing the products of genes 8–10 and 14–16 are apparently formed first. Gene 9 protein is removed from the heads before or during the incorporation of DNA. At least two other gene products which play no direct structural role are required for the maturation of the heads. A somewhat similar situation exists for phage λ, which has been shown to require ATP for the in vitro packaging of DNA into preformed heads (petite λ)2.

Compact form of DNA induced by spermidine.

Condensed phases of DNA: Structures and phase transitions

DNA is stored in vivo in a highly compact, so-called condensed phase, where gene regulatory processes are governed by the intricate interplay between different states of DNA compaction. These systems often have surprising properties, which one would not predict from classical concepts of dilute solutions. The mechanistic details of DNA packing are essential for its functioning, as revealed by the recent developments coming from biochemistry, electrostatics, statistical mechanics, and molecular and cell biology. Different aspects of condensed DNA behavior are linked to each other, but the links are often hidden in the bulk of experimental and theoretical details. Here we try to condense some of these concepts and provide interconnections between the different fields. After a brief description of main experimental features of DNA condensation inside viruses, bacteria, eukaryotes and the test tube, main theoretical approaches for the description of these systems are presented. We end up with an extended discussion of the role of DNA condensation in the context of gene regulation and mention potential applications of DNA condensation in gene therapy and biotechnology.

Condensed DNA: condensing the concepts.

https://onlinelibrary.wiley.com/doi/pdf/10.1002/jps.22243

Drug delivery trends in clinical trials and translational medicine: challenges and opportunities in the delivery of nucleic acid-based therapeutics.

https://febs.onlinelibrary.wiley.com/doi/pdf/10.1016/0014-5793%2872%2980335-1

A.L. Platonov

Papers

A compact form of double-stranded DNA in solution