RecA protein mediates homologous recognition via non-Watson-Crick bonds in base triplets

E. coli RecA protein, the prototype of a class, forms a helical nucleoprotein filament on single-stranded DNA that recognizes homology in duplex DNA, and initiates the exchange of strands in homologous recombination. The discovery of this reaction some years ago posed a quandary on how a third strand recognizes homology in duplex DNA, whose Watson-Crick bonds face inward in a hydrophobic core of stacked bases. Recent studies have shown that RecA protein promotes homologous recognition via non-Watson-Crick bonds in base triplets. The intermediates in the RecA reaction differ distinctly from triplex DNA that forms non-enzymically. The biological significance of the novel set of DNA interactions by which RecA protein effects homologous recognition is indicated by the importance of this protein in recombination, and the widespread distribution of homologous proteins in prokaryotes and eukaryotes.