POLARITY OF DNA STRAND EXCHANGE PROMOTED BY RECOMBINATION PROTEINS OFTHE RECA FAMILY

Homologs of Escherichia coli RecA recombination protein, which have be en found throughout the living kingdom, promote homologous pairing and strand exchange. The nucleoprotein filament, within which strand exch ange occurs, has been conserved through evolution, but conservation of the polarity of exchange and the significance of that directionality has not been settled. Using oligonucleotides as substrates, and assays based on fluorescence resonance energy transfer (FRET), we distinguis hed the biased formation of homologous joints at either end of duplex DNA from the subsequent directionality of strand exchange. As with E. coli RecA protein, the homologous Rad51 proteins from both Homo sapien s (HsRad51) and Saccharomyces cerevisiae (ScRad51) propagated DNA stra nd exchange preferentially in the 5' to 3' direction. The data suggest that 5' to 3' polarity is a conserved intrinsic property of recombina tion filaments.