Polymerization and mechanical properties of single RecA-DNA filaments.

The polymerization of individual RecA-DNA filaments, containing either single-stranded or double-stranded DNA, was followed in real time, and their mechanical properties were characterized with force-measuring laser tweezers. It was found that the stretch modulus of a filament is dominated by its (central) DNA component, while its bending rigidity is controlled by its (eccentric) protein component. The longitudinal stiffness of DNA increases 6- to 12-fold when the DNA is contained in the protein helix. Both the stretch modulus and the bending rigidity of a fiber change in the presence of various nucleotide cofactors-e.g., [gamma-thio]ATP, ATP, and ADP-indicating a substantial re-arrangement of spatial relationships between the nucleic acid and the protein scaffold. In particular, when complexed with ATP, a fiber becomes twice as extensible as a [gamma-thio]ATP fiber, suggesting that 32% of the DNA-binding sites have been released in its core. Such release may enable easy rotation of the DNA within the protein helix or slippage of the DNA through the center of the protein helix.