Visualization of SSB-ssDNA complexes active in the assembly of stable RecA-DNA filaments.

We have demonstrated that SSB binds to ssDNA in a complex manner, producing two fundamentally different structures: one that appears as a nucleosomal chain of beads and linkers, and the other as a smooth-contoured, extended nucleoprotein filament. Under physiologic salt conditions, only the beaded complexes were observed. Experiments indicate that the highly beaded forms are the most active in the assembly of the stable RecA-ssDNA filaments. These results further support our previous suggestion (Chrysogelos and Griffith 1982) that the protein-free linker regions are important in two ways. First, they provide access to the DNA template, and second, they provide a means for the two proteins to associate one with the other when bound to ssDNA. We suggest here that SSB should be considered as an assembly factor for RecA in its binding to ssDNA. Furthermore, our results argue that once RecA has associated with both the ssDNA template and SSB, some structural alteration in the (ATP-primed) RecA must occur that nucleates the formation of the very ordered, helical RecA filament along the ssDNA.