Escherichia coli RecO protein anneals ssDNA complexed with its cognate ssDNA-binding protein: A common step in genetic recombination.

We present biochemical evidence for the functional similarity of Escherichia coli RecO protein and bacteriophage T4 UvsY protein to eukaryotic Rad52 protein. Although Rad52 protein is conserved in eukaryotes, no sequence homologue has been found in prokaryotes or archeabacteria. Rad52 protein has two unique activities: facilitation of replication protein-A (RPA) displacement by Rad51 protein and annealing of RPA-single-stranded DNA (ssDNA) complexes. Both activities require species-specific interaction between Rad52 protein and RPA. Both RecO and UvsY proteins also possess the former property with regard to their cognate ssDNA-binding protein. Here, we report that RecO protein anneals ssDNA that is complexed with only its cognate ssDNA-binding protein, suggesting the involvement of species-specific interactions. Optimal activity for RecO protein occurs after formation of a 1:1 complex with SSB protein. RecR protein, which is known to stimulate RecO protein to facilitate SSB protein displacement by RecA protein, inhibits annealing by RecO protein, suggesting that RecR protein may regulate the choice between the DNA strand invasion versus annealing pathways. In addition, we show that UvsY protein anneals ssDNA; furthermore, ssDNA, which is complexed only with its cognate ssDNA-binding protein, is annealed in the presence of UvsY protein. These results indicate that RecO and possibly UvsY proteins are functional counterparts of Rad52 protein. Based on the conservation of these functions, we propose a modified double-strand break repair model that includes DNA annealing as an important intermediate step.