DNA Compaction in a Crowded Environment with Negatively Charged Proteins

We studied the conformational properties of DNA in a salt solution of the strongly charged protein bovine serum albumin. DNA is compacted when a suitable amount of bovine serum albumin is added to the solution due to a crowding effect and strong electrostatic repulsion between DNA and bovine serum albumin, both of which carry negative charges. However, DNA undergoes an unfolding transition with an increase in the salt concentration. This observation contradicts the current understanding of polymer- and salt-induced condensation, $\psi$ condensation. We propose a simple theoretical model by taking into account the competition between the translational entropy of ions and electrostatic interaction.

Figure 1

Conformational transition of DNA induced in a crowding environment with a negatively charged protein. Left: Distribution of the long-axis length of T4 DNA (165.6 kbp, where bp denotes base pairs) at different concentrations of BSA, deduced from single-molecule observation with FM. Right top: Schematic representation of the effect of salt on the folding-unfolding transition of DNA. The insets show pseudo-three-dimensional representations of the fluorescence intensity of DNA images obtained by FM observation.

Figure 2

Schematic representation of DNA in a BSA solution. The effective volume of DNA is shown by the dotted line.

Figure 3

Dependence of the swelling ratio $\alpha$ on the protein volume fraction ${\varphi }_P$ for different salt concentrations $n_s={10}^{-6}$ (a), $5\times {10}^{-6}$ (b), and ${10}^{-5}$ (c). ${\varphi }_i$ is the critical protein concentration for inducing the folding transition into a compact state.