Salt dependent changes in structure and dynamics of circular single stranded DNA of filamentous phages of Escherichia coli.

We have analyzed the static and dynamic behaviour of the circular single stranded DNA of the filamentous Escherichia coli phages F1 and M13mp8 in solution as a function of salt concentration using static and dynamic light scattering and sedimentation analysis in the analytical ultracentrifuge. We show by static light scattering that native and denatured single stranded DNA behave like a randomly coiled macromolecule at all salt concentrations used. The size of the native single stranded DNA is governed by the formation of secondary structures. While the radius of gyration decreases with increasing salt concentration the translational diffusion of the center-of-mass of native single stranded DNA and the sedimentation coefficient increase with increasing salt concentration in a biphasic manner. Below 100 mM monovalent cation concentration there is a strong dependence of the hydrodynamic parameters upon salt which is reduced approx. 3-fold at higher salt concentrations. We attribute the compaction of single stranded DNA by salt to electrostatic shielding and, in case of native single stranded DNA, secondary structure formation. Internal motions of the native single stranded DNA are observable at all salt concentrations and can be interpreted with a model of segmental diffusion of the elements of the polymer chain. The observed segmental diffusion coefficient of the native single stranded polynucleotide increases with increasing salt under the conditions investigated.