SnapShot: Homologous Recombination in DNA Double-Strand Break Repair

Homologous recombination (HR) provides an important mechanism to repair both accidental and programmed DNA double-strand breaks (DSBs) during mitosis and meiosis. Defects in HR are associated with mutagenesis and predispose to cancer, highlighting the importance of this pathway for preserving genome integrity (Moynahan and Jasin, 2010). HR is active in the S and G2 phases of the cell cycle where it promotes repair of a broken chromatid from an intact sister chromatid, ensuring error-free repair. The DNA transactions associated with HR are accompanied by modifications to histones, most notably phosphorylation of H2A/H2AX and chromatin remodeling. This SnapShot shows the yeast proteins directly involved in mitotic DSB repair; their mammalian counterparts are shown on the right. The central reaction in homologous recombination is the pairing and exchange of strands between two homologous DNA molecules. This step is catalyzed by the conserved Rad51/RecA family of proteins (Chen et al., 2008; San Filippo et al., 2008). To generate the substrate for binding of Rad51, DNA ends are processed to yield 3′ single-stranded DNA tails (5′–3′ resection). End resection occurs through a biphasic mechanism: first, the Mre11-Rad50-Xrs2 (MRX) complex and Sae2 clip 50–100 nucleotides from the 5′ ends of DNA, and then Exo1 or Sgs1-Top3-Rmi1 and Dna2 process the early intermediate to form extensive regions of single-stranded DNA (Mimitou and Symington, 2009). Replica-tion protein A (RPA) binds to the single-stranded DNA tails but is displaced by Rad51 aided by the mediator proteins Rad52 or BRCA2 (Rad51 filament formation) (San Filippo et al., 2008). The Rad51 paralogs in the budding yeast Saccharomyces cerevisiae, Rad55 and Rad57, are thought to extend or stabilize the Rad51 nucleoprotein filament (San Filippo et al., 2008). Once formed, the complex of Rad51 and single-stranded DNA searches for a homologous sequence in double-stranded DNA and then promotes invasion of the single-stranded DNA into donor double-stranded DNA to form a joint molecule with a displaced strand (D loop). Homologous pairing by Rad51 is stimulated by Rad54, a member of the Swi2/Snf2 family of chromatin remodeling proteins/ATPases (San Filippo et al., 2008). Rad54 displaces Rad51 from double-stranded DNA in vitro, and this could be important for turnover of Rad51, or for uncovering the 3′ end of paired intermediates to allow initiation of DNA synthesis (San Filippo et al., 2008). DNA polymerase δ extends the 3′ end from the broken chromosome using the donor strand as a template and replacing nucleotides lost by end …