The differentiated and conserved roles of Swi5-Sfr1 in homologous recombination

Abstract

Homologous recombination (HR) is the process whereby two DNA molecules that share high sequence similarity are able to recombine to generate hybrid DNA molecules. Throughout evolution, the ability of HR to identify highly similar DNA sequences has been adopted for numerous biological phenomena including DNA repair, meiosis, telomere maintenance, ribosomal DNA amplification and immunological diversity. Although Rad51 and Dmc1 are the key proteins that promote HR in mitotic and meiotic cells, respectively, accessory proteins that allow Rad51 and Dmc1 to effectively fulfil their functions have been identified in all examined model systems. In this Review, we discuss the roles of the highly conserved Swi5-Sfr1 accessory complex in yeast, mice and humans, and explore similarities and differences between these species.

Keywords: DNA repair; Dmc1; Rad51; Swi5-Sfr1; genome stability; homologous recombination.

Figure 1

Homologous recombination is a major DSB repair pathway. (1) A DSB forms and the dsDNA undergoes nucleolytic processing to reveal ssDNA. Loading of ssDNA‐binding protein RPA removes secondary structures which would otherwise impede HR. (2) With assistance from recombination accessory proteins, the eukaryotic recombinases Rad51/Dmc1 displace RPA and form a right‐handed helical filament around the ssDNA. This nucleoprotein filament, also known as the presynaptic filament, identifies homologous dsDNA and invades into it, displacing the noncomplementary strand to form a displacement loop (D‐loop). (3) The recombinases catalyse strand exchange as the invading 3′ end is elongated through DNA synthesis. The curved arrow from (2) denotes that at least some of those proteins also participate in promoting DNA strand exchange. (4) This invading strand is ejected, and having been extended using the intact duplex as a template, is able to anneal to the ssDNA on the other side of the DSB. This opposing ssDNA now uses the extended strand as a template for further DNA synthesis, and following the reformation of hydrogen bonds between the complementary strands, dsDNA containing single‐strand nicks is generated. (5) These nicks are repaired by a DNA ligase, resulting in the completion of HR‐mediated DSB repair. This model is a depiction of synthesis‐dependent strand annealing, a HR pathway that does not yield crossover products.

Figure 2

Stimulation of Rad51 activity by Swi5‐Sfr1. (1) Rad51 alone cannot displace RPA from ssDNA to form a filament. Mediators such as Rad52/BRCA2 and Rad51 paralogues facilitate the removal of RPA from ssDNA and subsequent formation of the Rad51 filament. This filament has relatively low strand exchange activity. (2) The Rad51 filament is restructured into a stabilised form, likely due to the insertion of Swi5‐Sfr1 into the grooves of the filament. This active filament is extended compared to the inactive filament. (3) Following strand invasion, Swi5‐Sfr1 stimulates ATP hydrolysis by Rad51 to enhance strand exchange, likely by making the filament more dynamic. Whether Swi5‐Sfr1 remains part of the groove or exerts an effect on strand exchange through a different mechanism is not known. Rad51 paralogues likely play a role here too.