Nucleic Acids Research Advance Access published online on September 22, 2009
Nucleic Acids Research, doi:10.1093/nar/gkp683
Molecular Biology |
A universal description for the experimental behavior of salt-(in)dependent oligocation-induced DNA condensation
School of Biological Sciences, Nanyang Technological University, 60 Nanyang Drive, Singapore 637551
*To whom correspondence should be addressed. Tel: +65 6790 3737; Fax: +65 6896 80 32; Email: larsnor{at}ntu.edu.sg Correspondence may also be addressed to Nikolay Korolev. Tel: +65 6316 28 64; Fax: +65 6896 80 32; Email: korolev{at}ntu.edu.sg
Received June 15, 2009. Revised August 3, 2009. Accepted August 3, 2009.
We report a systematic study of the condensation of plasmid DNA by oligocations with variation of the charge, Z, from +3 to +31. The oligocations include a series of synthetic linear 
-oligo(L-lysines), (denoted Kn, n = 3–10, 31; n is the number of lysines equal to the ligand charge) and branched 
-substituted homologues of K10: YK10, LK10 (Z = +10); RK10, YRK10 and LYRK10 (Z = +20). Data were obtained by light scattering, UV absorption monitored precipitation assay and isothermal titration calorimetry in a wide range concentrations of DNA and monovalent salt (KCl, CKCl). The dependence of EC50 (ligand concentration at the midpoint of DNA condensation) on CKCl shows the existence of a salt-independent regime at low CKCl and a salt-dependent regime with a steep rise of EC50 with increase of CKCl. Increase of the ligand charge shifts the transition from the salt-independent to salt-dependent regime to higher CKCl. A novel and simple relationship describing the EC50 dependence on DNA concentration, charge of the ligand and the salt-dependent dissociation constant of the ligand–DNA complex is derived. For the -oligolysines K3–K10, the experimental dependencies of EC50 on CKCl and Z are well-described by an equation with a common set of parameters. Implications from our findings for understanding DNA condensation in chromatin are discussed.