SSB protein diffusion on single-stranded DNA stimulates RecA filament formation

Single-stranded DNA generated in the cell during DNA metabolism is stabilized and protected by binding of ssDNA-binding (SSB) proteins. Escherichia coli SSB, a representative homotetrameric SSB, binds to ssDNA by wrapping the DNA using its four subunits. However, such a tightly wrapped, high-affinity protein–DNA complex still needs to be removed or repositioned quickly for unhindered action of other proteins. Here we show, using single-molecule two- and three-colour fluorescence resonance energy transfer, that tetrameric SSB can spontaneously migrate along ssDNA. Diffusional migration of SSB helps in the local displacement of SSB by an elongating RecA filament. SSB diffusion also melts short DNA hairpins transiently and stimulates RecA filament elongation on DNA with secondary structure. This observation of diffusional movement of a protein on ssDNA introduces a new model for how an SSB protein can be redistributed, while remaining tightly bound to ssDNA during recombination and repair processes. Many transactions involving DNA generate a single-stranded intermediate that is protected from degradation by ssDNA-binding (SSB) protein. Whilst protecting the DNA, this protein needs to allow rapid access to other proteins, such as polymerases or repair factors, for subsequent DNA processing. A mechanism that allows SSB to stay firmly bound to the ssDNA whilst allowing access has been found in a single-molecule fluorescence resonance energy transfer study of the E. coli protein. This SSB is a tetramer that wraps ssDNA around its exterior. Surprisingly, it can migrate via a random walk along the ssDNA, so it can be repositioned although it remains tightly bound. This diffusional property appears to be physiologically relevant as, for example, it facilitates extension of the 3′ end of the filament formed by the DNA strand exchange protein, RecA. During DNA metabolism, single-stranded DNA intermediates are often generated that are protected from degradation by binding of ssDNA-binding (SSB) proteins. Bacterial SSB protein forms a tetramer that wraps ssDNA using its four subunits. Here it is shown that tetrameric SSB protein can spontaneously migrate along ssDNA; this diffusional movement introducing a new model for the redistribution of the SSB protein, while remaining bound to ssDNA during recombination and repair processes.