Visualization of the paranemic joining of homologous DNA molecules catalyzed by the RecA protein of Escherichia coli.

In reactions catalyzed by the RecA protein of Escherichia coli, synapsis between two DNA molecules is believed to occur even in the absence of free homologous DNA ends and to involve a metastable interaction termed paranemic joining. We have used electron microscopic methods to visualize synapse formation between supertwisted M13 double-stranded DNA (dsDNA) and linear M13 mp7 single-stranded DNA (ssDNA) with non-M13 sequences at its ends. These non-M13 sequences block strand invasion and make this pairing equivalent to the joining of two fully circular molecules. We observed a high frequency of joining when the ssDNA was initially assembled into presynaptic filaments with RecA protein. Cleavage of the dsDNA in the joined complexes by Hpa I revealed that the joint was at a site of homology. In these joints, the dsDNA entered the presynaptic filament over a length of 360 +/- 80 base pairs, not visibly altering its ultrastructure, and then dissociated from the filament. Although the dsDNA in the complexes appeared topologically relaxed, deproteinization released supertwisted dsDNA, indicating that the dsDNA was unwound by 34 degrees per base pair in the paranemic joint. When supertwisted M13 dsDNA was paired with circular M13 ssDNA, similar joints were observed and both DNA circles appeared topologically relaxed.