The direction of RecA protein assembly onto single strand DNA is the same as the direction of strand assimilation during strand exchange.

The RecA protein of Escherichia coli optimally promotes DNA strand exchange reactions in the presence of the single strand DNA-binding protein of E. coli (SSB protein). Under these conditions, assembly of RecA protein onto single-stranded DNA (ssDNA) occurs in three steps. First, the ssDNA is rapidly covered by SSB protein. The binding of RecA protein is then initiated by nucleation of a short tract of RecA protein onto the ssDNA. Finally, cooperative polymerization of additional RecA protein accompanied by displacement of SSB protein results in a ssDNA-RecA protein filament (Griffith, J. D., Harris, L. D., and Register, J. C. (1984) Cold Spring Harbor Symp. Quant. Biol. 49, 553-559). We report here that RecA protein assembly onto circular ssDNA yields RecA protein-covered circles in which greater than 85% are completely covered by RecA protein with no remaining SSB protein-covered segments (as detected by electron microscopy). However, when linear ssDNA is used, 90% of the filaments contain a short segment at one end complexed with SSB protein. This suggests that RecA protein assembly is unidirectional. Visualization of the assembly of RecA protein onto either long ssDNA tails (containing either 5' or 3' termini) or ssDNA gaps generated in double strand DNA allowed us to determine that the RecA protein polymerizes in the 5' to 3' direction on ssDNA and preferentially nucleates at ssDNA-double strand DNA junctions containing 5' termini.