DNA condensation by PAMAM dendrimers:: Self-assembly characteristics and effect on transcription

Electrostatic shielding and steric blocking by histones are two significant factors that participate in the control of the local rates of transcription in chromatin. As a simple model system to determine how the degree of DNA condensation affects enzyme accessibility and gene expression, we have used generation 5 polyamidoamine (G5 PAMAM) cationic dendrimer particles (size 5.4 nm) as a synthetic histone model together with an in vitro transcription assay. The degree of compaction, conformation, and binding availability of the dendrimer-DNA complexes is characterized by linear and circular dichroism, dynamic light scattering, and competitive binding of ethidium. Using ultracentrifugation we are able to show explicitly, for the first time, that dendrimer particles bind to DNA in a highly cooperative manner, and that the dendrimer-induced condensation of the DNA strongly attenuates transcription. Two fractions with different properties can be identified: a low-density fraction which behaves very similar to uncondensed DNA and a high-density fraction which is condensed to a high extent and where binding availability and transcription are strongly reduced. Circular dichroism gives clues to the structure of the condensed DNA indicating long-range order between the helices such as in polymer-salt-induced cholesteric liquid crystalline domains, one possible shape being a toroidal structure. On the basis of the experimental data, we propose a model for the self-assembly of the dendrimer-DNA system.