DNA strand exchange mediated by a RAD51-ssDNA nucleoprotein filament with polarity opposite to that of RecA

Yeast RAD51 gene functions in genetic recombination and DNA double-strand break repair. In vitro, in the presence of ATP and replication protein A, RAD51 protein pairs single-stranded DNA (ssDNA) with homologous double-stranded DNA (dsDNA) and catalyzes strand exchange between the synapsed DNA partners. Electron microscopic analyses show that RAD51 forms helical filaments on both ssDNA and dsDNA, in which the DNA is highly extended. However, results presented here indicate that only the RAD51-ssDNA nucleoprotein filament is functionally relevant. Strand exchange is arrested when heterology is encountered in the duplex partner, and analysis of the configuration of the terminal joint thus formed reveals that pairing and strand exchange initiate at the 5' end of the complementary strand in the linear duplex, a reaction polarity opposite to that of the bacterial prototype RecA.