Homologous recombination enables the cell to access and copy intact DNA sequence information in , particularly to repair DNA damage affecting both strands of the double helix. Here, we discuss the DNA transactions and enzymatic activities required for this elegantly orchestrated process in the context of the repair of DNA double-strand breaks in somatic cells. This includes homology search, DNA strand invasion, repair DNA synthesis, and restoration of intact chromosomes. Aspects of DNA topology affecting individual steps are highlighted. Overall, recombination is a dynamic pathway with multiple metastable and reversible intermediates designed to achieve DNA repair with high fidelity.

Summary The displacement loop (D-loop) is the product of homology search and DNA strand invasion, constituting a central intermediate in homologous recombination (HR). In eukaryotes, Rad51 recombinase is assisted in D-loop formation by the Rad54 motor protein. Curiously, Rad54 also disrupts D-loops. How these opposing activities are coordinated toward productive recombination is unknown. Moreover, a seemingly disparate function Rad54 is removal of Rad51 from heteroduplex DNA (hDNA) to allow HR-associated DNA synthesis. Here, we uncover novel features of D-loop formation/dissociation dynamics, employing Rad51 filaments formed on ssDNAs that mimic physiological length and structure of in vivo substrates. The Rad54 motor is activated by Rad51 bound to synapsed DNAs and guided by a newly discovered single-stranded DNA binding domain. We present a unified model where Rad54 acts as an hDNA pump, which drives D-loop formation while simultaneously removing Rad51 from hDNA, consolidating both ATP-dependent activities of Rad54 into a single mechanistic step.

SUMMARY Inaccurate repair of broken chromosomes generates structural variants that can fuel evolution and inflict pathology. We describe a novel rearrangement mechanism in which translocation between intact chromosomes is induced by a lesion on a third chromosome. This multi-invasion-induced rearrangement (MIR) stems from a homologous recombination byproduct, where a broken DNA end simultaneously invades two intact donors. No homology is required between the donors, and the intervening sequence from the invading molecule is inserted at the translocation site. MIR is stimulated by increasing homology length and spatial proximity of the donors, and depends on the overlapping activities of the structure-selective endonucleases Mus81-Mms4, Slx1-Slx4, and Yen1. Conversely, the 3′-flap nuclease Rad1-Rad10 and enzymes known to disrupt recombination intermediates (Sgs1-Top3-Rmi1, Srs2 and Mph1) inhibit MIR. Resolution of MIR intermediates propagates secondary chromosome breaks that frequently cause additional rearrangements. MIR features have implications for the formation of simple and complex rearrangements underlying human pathologies.

SUMMARY Never-in-mitosis A-related kinase 1 (Nek1) has established roles in apoptosis and cell cycle regulation. We show that human Nek1 regulates homologous recombination (HR) by phosphorylating Rad54 at Ser572 in late G2 phase. Nek1 deficiency as well as expression of unphosphorylatable Rad54 (Rad54-S572A) cause unresolved Rad51 foci and confer a defect in HR. Phosphomimic Rad54 (Rad54-S572E), in contrast, promotes HR and rescues the HR defect associated with Nek1 loss. Although expression of phosphomimic Rad54 is beneficial for HR, it causes Rad51 removal from chromatin and degradation of stalled replication forks in S phase. Thus, G2-specific phosphorylation of Rad54 by Nek1 promotes Rad51 chromatin removal during HR in G2 phase, and its absence in S phase is required for replication fork stability. In summary, Nek1 regulates Rad51 removal to orchestrate HR and replication fork stability.

Highlights d Development of a physical assay for D-loop detection in cells d Srs2, Mph1, and Sgs1-Top3-Rmi1 (STR) regulate D-loop levels in vivo d Two distinct pathways (Srs2 and Mph1, STR) target different D-loop species d Rdh54 delineates the two D-loop reversal pathways