Regulation Of Homologous Recombination: Robustness Through Reversibility

Abstract

Homologous recombination is a critical pathway to maintain genome stability. The signature reactions in recombination of homology search and DNA strand invasion are carefully regulated. The Rad51‐ssDNA filament, which conducts the homology search, exists in a balance between assembly and disassembly, where specific motor proteins, such as Srs2 in yeast, lead to disassembly of the filament. This process reverses Rad51 filament formation as a mechanism of anti‐recombination. A second mechanism of anti‐recombination is the disruption of the primary DNA strand invasion product, the nascent D‐loop. While it has been thought that the disruption of D‐loops is being conducted by motor proteins and helicases, we showed that nascent D‐loop dissolution by Top3‐Rmi1 represents a distinct mechanism of anti‐recombination. Another level of regulation affects the outcome of recombination, the formation of crossover or non‐crossover products. The Synthesis‐Dependent Strand Annealing (SDSA) pathway of DNA double‐strand break repair envisions the disruption of the extended D‐loop after DNA synthesis to enable the annealing of the extended invading strand with the second end of the double strand break. Hence, different enzymes are needed to target the nascent D‐loop and the extended D‐loop to achieve anti‐recombination or anti‐crossover (= pro‐SDSA), respectively.The model that homologous recombination proceeds as a pathway of successive metastable reversible intermediates has an interesting genetic implication that provides an additional mechanistic explanation for synthetic lethality between mutations in different genes. Eliminating forward and backwards steps in a single reversible pathway simultaneously may lead to trapping of a toxic intermediate or depletion of an essential compound. Such genetic interactions have been observed in the recombination pathway and are likely applicable to additional reversible pathways.Support or Funding InformationWe are grateful for the funding from the National Institutes of Health and the Department of Defense.