Synergy of DNA-bending nucleoid proteins and macromolecular crowding in condensing DNA

Many prokaryotic nucleoid proteins bend DNA and form extended helical protein-DNA fibers rather than condensed structures. On the other hand, it is known that such proteins (such as bacterial HU) strongly promote DNA condensation by macromolecular crowding. Using theoretical arguments, we show that this synergy is a simple consequence of the larger diameter and lower net charge density of the protein-DNA filaments as compared to naked DNA, and hence, should be quite general. To illustrate this generality, we use light-scattering to show that the 7kDa basic archaeal nucleoid protein Sso7d from Sulfolobus solfataricus (known to sharply bend DNA) likewise does not significantly condense DNA by itself. However, the resulting protein-DNA fibers are again highly susceptible to crowding-induced condensation. Clearly, if DNA-bending nucleoid proteins fail to condense DNA in dilute solution, this does not mean that they do not contribute to DNA condensation in the context of the crowded living cell. © 2007 World Scientific Publishing Company.