RAD54 N-terminal domain is a DNA sensor that couples ATP hydrolysis with branch migration of Holliday junctions

The translocation activity of Rad54 reduces crossover outcomes during homologous recombination

Homologous recombination (HR) is a template-based DNA double-strand break repair pathway that requires the selection of an appropriate DNA sequence to facilitate repair. Selection occurs during a homology search that must be executed rapidly and with high fidelity. Failure to efficiently perform the homology search can result in complex intermediates that generate genomic rearrangements, a hallmark of human cancers. Rad54 is an ATP dependent DNA motor protein that functions during the homology search by regulating the recombinase Rad51. How this regulation reduces genomic exchanges is currently unknown. To better understand how Rad54 can reduce these outcomes, we evaluated several amino acid mutations in Rad54 that were identified in the COSMIC database. COSMIC is a collection of amino acid mutations identified in human cancers. These substitutions led to reduced Rad54 function and the discovery of a conserved motif in Rad54. Through genetic, biochemical and single-molecule approaches, we show that disruption of this motif leads to failure in stabilizing early strand invasion intermediates, causing increased crossovers between homologous chromosomes. Our study also suggests that the translocation rate of Rad54 is a determinant in balancing genetic exchange. The latch domain's conservation implies an interaction likely fundamental to eukaryotic biology.

Disparate requirements for RAD54L in replication fork reversal

RAD54L is a DNA motor protein with multiple roles in homologous recombination DNA repair (HR). In vitro , RAD54L was shown to also catalyze the reversal and restoration of model replication forks. In cells, however, little is known about how RAD54L may regulate the dynamics of DNA replication. Here, we show that RAD54L restrains the progression of replication forks and functions as a fork remodeler in human cells. Analogous to HLTF, SMARCAL1, and FBH1, and consistent with a role in fork reversal, RAD54L decelerates fork progression in response to replication stress and suppresses the formation of replication-associated ssDNA gaps. Interestingly, loss of RAD54L prevents nascent strand DNA degradation in both BRCA1/2- and 53BP1-deficient cells, suggesting that RAD54L functions in both pathways of RAD51-mediated replication fork reversal. In the HLTF/SMARCAL1 pathway, RAD54L is critical, but its ability to catalyze branch migration is dispensable, indicative of its function downstream of HLTF/SMARCAL1. Conversely, in the FBH1 pathway, branch migration activity of RAD54L is essential, and FBH1 engagement is dependent on its concerted action with RAD54L. Collectively, our results reveal disparate requirements for RAD54L in two distinct RAD51-mediated fork reversal pathways, positing its potential as a future therapeutic target.

Untousling the Role of Tousled-like Kinase 1 in DNA Damage Repair

DNA damage repair lies at the core of all cells' survival strategy, including the survival strategy of cancerous cells. Therefore, targeting such repair mechanisms forms the major goal of cancer therapeutics. The mechanism of DNA repair has been tousled with the discovery of multiple kinases. Recent studies on tousled-like kinases have brought significant clarity on the effectors of these kinases which stand to regulate DSB repair. In addition to their well-established role in DDR and cell cycle checkpoint mediation after DNA damage or inhibitors of replication, evidence of their suspected involvement in the actual DSB repair process has more recently been strengthened by the important finding that TLK1 phosphorylates RAD54 and regulates some of its activities in HRR and localization in the cell. Earlier findings of its regulation of RAD9 during checkpoint deactivation, as well as defined steps during NHEJ end processing, were earlier hints of its broadly important involvement in DSB repair. All this has opened up new avenues to target cancer cells in combination therapy with genotoxins and TLK inhibitors.

Drug Discovery Targeting Post-Translational Modifications in Response to DNA Damages Induced by Space Radiation

DNA damage in astronauts induced by cosmic radiation poses a major barrier to human space exploration. Cellular responses and repair of the most lethal DNA double-strand breaks (DSBs) are crucial for genomic integrity and cell survival. Post-translational modifications (PTMs), including phosphorylation, ubiquitylation, and SUMOylation, are among the regulatory factors modulating a delicate balance and choice between predominant DSB repair pathways, such as non-homologous end joining (NHEJ) and homologous recombination (HR). In this review, we focused on the engagement of proteins in the DNA damage response (DDR) modulated by phosphorylation and ubiquitylation, including ATM, DNA-PKcs, CtIP, MDM2, and ubiquitin ligases. The involvement and function of acetylation, methylation, PARylation, and their essential proteins were also investigated, providing a repository of candidate targets for DDR regulators. However, there is a lack of radioprotectors in spite of their consideration in the discovery of radiosensitizers. We proposed new perspectives for the research and development of future agents against space radiation by the systematic integration and utilization of evolutionary strategies, including multi-omics analyses, rational computing methods, drug repositioning, and combinations of drugs and targets, which may facilitate the use of radioprotectors in practical applications in human space exploration to combat fatal radiation hazards.

Nanopore Analysis as a Tool for Studying Rapid Holliday Junction Dynamics and Analyte Binding

Holliday junctions (HJs) are an important class of nucleic acid structure utilized in DNA break repair processes. As such, these structures have great importance as therapeutic targets and for understanding the onset and development of various diseases. Single-molecule fluorescence resonance energy transfer (smFRET) has been used to study HJ structure-fluctuation kinetics, but given the rapid time scales associated with these kinetics (approximately sub-milliseconds) and the limited bandwidth of smFRET, these studies typically require one to slow down the structure fluctuations using divalent ions (e.g., Mg²⁺). This modification limits the ability to understand and model the underlying kinetics associated with HJ fluctuations. We address this here by utilizing nanopore sensing in a gating configuration to monitor DNA structure fluctuations without divalent ions. A nanopore analysis shows that HJ fluctuations occur on the order of 0.1-10 ms and that the HJ remains locked in a single conformation with short-lived transitions to a second conformation. It is not clear what role the nanopore plays in affecting these kinetics, but the time scales observed indicate that HJs are capable of undergoing rapid transitions that are not detectable with lower bandwidth measurement techniques. In addition to monitoring rapid HJ fluctuations, we also report on the use of nanopore sensing to develop a highly selective sensor capable of clear and rapid detection of short oligo DNA strands that bind to various HJ targets.

Evolutionary clustering of Lagrangian trajectories in turbulent Rayleigh-Bénard convection flows

We explore the transport mechanisms of heat in two- and three-dimensional turbulent convection flows by means of the long-term evolution of Lagrangian coherent sets. They are obtained from the spectral clustering of trajectories of massless fluid tracers that are advected in the flow. Coherent sets result from trajectories that stay closely together under the dynamics of the turbulent flow. For longer times, they are always destroyed by the intrinsic turbulent dispersion of material transport. Here, this constraint is overcome by the application of evolutionary clustering algorithms that add a time memory to the coherent set detection and allows individual trajectories to leak in or out of evolving clusters. Evolutionary clustering thus also opens the possibility to monitor the splits and mergers of coherent sets. These rare dynamic events leave clear footprints in the evolving eigenvalue spectrum of the Laplacian matrix of the trajectory network in both convection flows. The Lagrangian trajectories reveal the individual pathways of convective heat transfer across the fluid layer. We identify the long-term coherent sets as those fluid flow regions that contribute least to heat transfer. Thus, our evolutionary framework defines a complementary perspective on the slow dynamics of turbulent superstructure patterns in convection flows that were recently discussed in the Eulerian frame of reference. The presented framework might be well suited for studies in natural flows, which are typically based on sparse information from drifters and probes.

Studying the mechanism of sperm DNA damage caused by folate deficiency

Sperm DNA injury is one of the common causes of male infertility. Folic acid deficiency would increase the methylation level of the important genes, including those involved in DNA double-strand break (DSB) repair pathway. In the early stages, we analysed the correlation between seminal plasma folic acid concentration and semen parameters in 157 infertility patients and 91 sperm donor volunteers, and found that there was a significant negative correlation between seminal folic acid concentration and sperm DNA Fragmentation Index (DFI; r = -0.495, p < 0.01). Then through reduced representation bisulphite sequencing, global DNA methylation of sperm of patients in the low folic acid group and the high folic acid group was analysed, it was found that the methylation level in Rad54 promoter region increased in the folic acid deficiency group compared with the normal folic acid group. Meanwhile, the results of animal model and spermatocyte line (GC-2) also found that folic acid deficiency can increase the methylation level in Rad54 promoter region, increased sperm DFI in mice, increased the expression of γ-H2AX, that is, DNA injury marker protein, and increased sensitivity of GC-2 to external damage and stimulation. The study indicates that the expression of Rad54 is downregulated by folic acid deficiency via DNA methylation. This may be one of the mechanisms of sperm DNA damage caused by folate deficiency.

DNA Double Strand Break Repair Pathways in Response to Different Types of Ionizing Radiation

The superior dose distribution of particle radiation compared to photon radiation makes it a promising therapy for the treatment of tumors. However, the cellular responses to particle therapy and especially the DNA damage response (DDR) is not well characterized. Compared to photons, particles are thought to induce more closely spaced DNA lesions instead of isolated lesions. How this different spatial configuration of the DNA damage directs DNA repair pathway usage, is subject of current investigations. In this review, we describe recent insights into induction of DNA damage by particle radiation and how this shapes DNA end processing and subsequent DNA repair mechanisms. Additionally, we give an overview of promising DDR targets to improve particle therapy.

The fellowship of the RING: BRCA1, its partner BARD1 and their liaison in DNA repair and cancer

The breast cancer type 1 susceptibility protein (BRCA1) and its partner - the BRCA1-associated RING domain protein 1 (BARD1) - are key players in a plethora of fundamental biological functions including, among others, DNA repair, replication fork protection, cell cycle progression, telomere maintenance, chromatin remodeling, apoptosis and tumor suppression. However, mutations in their encoding genes transform them into dangerous threats, and substantially increase the risk of developing cancer and other malignancies during the lifetime of the affected individuals. Understanding how BRCA1 and BARD1 perform their biological activities therefore not only provides a powerful mean to prevent such fatal occurrences but can also pave the way to the development of new targeted therapeutics. Thus, through this review work we aim at presenting the major efforts focused on the functional characterization and structural insights of BRCA1 and BARD1, per se and in combination with all their principal mediators and regulators, and on the multifaceted roles these proteins play in the maintenance of human genome integrity.

Discrete roles for Rad54 and Rdh54 during homologous recombination

Rad54 and Rdh54 are Snf2 DNA motor proteins that function during maintenance of genomic integrity. Though highly related, Rad54 and Rdh54 have different biochemical and genetic functions during maintenance of genomic integrity. Rad54 functions primarily during the homology search and strand invasion steps of homologous recombination, while Rdh54 appears to play a minor role in these processes. More recently it has been shown that Rdh54 functions as a pathway branch point at HR intermediates, and as has a role in cell cycle recovery. Here we will explore recent advances that have improved our understanding of the role these two DNA motor proteins play during DNA repair.

Rad51 filament dynamics and its antagonistic modulators

Rad51 recombinase is the central player in homologous recombination, the faithful repair pathway for double-strand breaks and key event during meiosis. Rad51 forms nucleoprotein filaments on single-stranded DNA, exposed by a double-strand break. These filaments are responsible for homology search and strand invasion, which lead to homology-directed repair. Due to its central roles in DNA repair and genome stability, Rad51 is modulated by multiple factors and post-translational modifications. In this review, we summarize our current understanding of the dynamics of Rad51 filaments, the roles of other factors and their modes of action in modulating key stages of Rad51 filaments: formation, stability and disassembly.

Homologous recombination repair rathway and RAD54L in early-stage lung adenocarcinoma

OBJECTIVE

The current study aims to identify the dysregulated pathway involved in carcinogenesis and the essential survival-related dysregulated genes among this pathway in the early stage of lung adenocarcinoma (LUAD).

PATIENTS AND METHODS

Data from The Cancer Genome Atlas (TCGA) including 526 tumor tissues of LUAD and 59 healthy lung tissues were analyzed to gain differentially expressed genes (DEGs). Gene ontology (GO) analysis was conducted with DAVID, while the Kyoto Encyclopedia of Genes and Genomes (KEGG) pathway analysis of DEGs was performed, followed by gene set enrichment analysis (GSEA) methods. Survival analysis was implemented in TCGA dataset and validated in Gene Expression Omnibus (GEO) cohort GSE50081, which includes 127 patients with stage I LUAD.

RESULTS

GSEA enrichment analysis suggested that homologous recombination repair (HRR) pathway was significantly enriched. Subsequent KEGG pathway enrichment analysis indicated the significant up-regulation of HRR pathway in patients with T1 stage LUAD. Retrieved in Gene database, RAD54L is involved in HRR pathway and were recognized to be significantly differentially expressed in T1 stage LUAD in our study. The survival analysis indicated that high expression of RAD54L was significantly related to worse overall survival in patients with T1 stage LUAD (TCGA cohort: HR=2.10, 95% CI [1.47-2.98], P = 0.001; GSE50081 validation cohort: HR = 2.61, 95% CI [1.51-4.52], P = 0.002). Multivariate cox regression analysis indicated that RAD54L is an independent prognostic factor in the early-stage LUAD.

CONCLUSION

HRR pathway is up-regulated in LUAD, among which the expression of RAD54L was found to be significantly differentially expressed in T1 stage tumor tissue. Patients with high expression of RAD54L were associated with worse overall survival in the TCGA cohort and validation cohort. This study suggests a potential mechanism of lung cancer progression and provide a budding prognostic factor and treatment target in early-stage LUAD.

Rad54 and Rdh54 occupy spatially and functionally distinct sites within the Rad51-ssDNA presynaptic complex

Rad54 and Rdh54 are closely related ATP-dependent motor proteins that participate in homologous recombination (HR). During HR, these enzymes functionally interact with the Rad51 presynaptic complex (PSC). Despite their importance, we know little about how they are organized within the PSC, or how their organization affects PSC function. Here, we use single-molecule optical microscopy and genetic analysis of chimeric protein constructs to evaluate the binding distributions of Rad54 and Rdh54 within the PSC. We find that Rad54 and Rdh54 have distinct binding sites within the PSC, which allow these proteins to act cooperatively as DNA sequences are aligned during homology search. Our data also reveal that Rad54 must bind to a specific location within the PSC, whereas Rdh54 retains its function in the repair of MMS-induced DNA damage even when recruited to the incorrect location. These findings support a model in which the relative binding sites of Rad54 and Rdh54 help to define their functions during mitotic HR.

RAD54 promotes alternative lengthening of telomeres by mediating branch migration

Cancer cells can activate the alternative lengthening of telomeres (ALT) pathway to promote replicative immortality. The ALT pathway promotes telomere elongation through a homologous recombination pathway known as break‐induced replication (BIR), which is often engaged to repair single‐ended double‐stranded breaks (DSBs). Single‐ended DSBs are resected to promote strand invasion and facilitate the formation of a local displacement loop (D‐loop), which can trigger DNA synthesis, and ultimately promote telomere elongation. However, the exact proteins involved in the maturation, migration, and resolution of D‐loops at ALT telomeres are unclear. In vitro, the DNA translocase RAD54 both binds D‐loops and promotes branch migration suggesting that RAD54 may function to promote ALT activity. Here, we demonstrate that RAD54 is enriched at ALT telomeres and promotes telomeric DNA synthesis through its ATPase‐dependent branch migration activity. Loss of RAD54 leads to the formation of unresolved recombination intermediates at telomeres that form ultra‐fine anaphase bridges in mitosis. These data demonstrate an important role for RAD54 in promoting ALT‐mediated telomere synthesis.

A novel landscape of nuclear human CDK2 substrates revealed by in situ phosphorylation

Cyclin-dependent kinase 2 (CDK2) controls cell division and is central to oncogenic signaling. We used an "in situ" approach to identify CDK2 substrates within nuclei isolated from cells expressing CDK2 engineered to use adenosine 5'-triphosphate analogs. We identified 117 candidate substrates, ~40% of which are known CDK substrates. Previously unknown candidates were validated to be CDK2 substrates, including LSD1, DOT1L, and Rad54. The identification of many chromatin-associated proteins may have been facilitated by labeling conditions that preserved nuclear architecture and physiologic CDK2 regulation by endogenous cyclins. Candidate substrates include proteins that regulate histone modifications, chromatin, transcription, and RNA/DNA metabolism. Many of these proteins also coexist in multi-protein complexes, including epigenetic regulators, that may provide new links between cell division and other cellular processes mediated by CDK2. In situ phosphorylation thus revealed candidate substrates with a high validation rate and should be readily applicable to other nuclear kinases.

Ras-Mediated Activation of NF-κB and DNA Damage Response in Carcinogenesis

Cancer is a multi-step process during which cells acquire mutations that eventually lead to uncontrolled cell growth and division and evasion of programmed cell death. The oncogenes such as Ras and c-Myc may be responsible in all three major stages of cancer i.e., early, intermediate, and late. The NF-κB has been shown to control the expression of genes linked with tumor pathways such as chronic inflammation, tumor cell survival, anti-apoptosis, proliferation, invasion, and angiogenesis. In the last few decades, various biomarker pathways have been identified that play a critical role in carcinogenesis such as Ras, NF-κB and DNA damage.

Role of Rad51 and DNA repair in cancer: A molecular perspective

The maintenance of genome integrity is essential for any organism survival and for the inheritance of traits to offspring. To the purpose, cells have developed a complex DNA repair system to defend the genetic information against both endogenous and exogenous sources of damage. Accordingly, multiple repair pathways can be aroused from the diverse forms of DNA lesions, which can be effective per se or via crosstalk with others to complete the whole DNA repair process. Deficiencies in DNA healing resulting in faulty repair and/or prolonged DNA damage can lead to genes mutations, chromosome rearrangements, genomic instability, and finally carcinogenesis and/or cancer progression. Although it might seem paradoxical, at the same time such defects in DNA repair pathways may have therapeutic implications for potential clinical practice. Here we provide an overview of the main DNA repair pathways, with special focus on the role played by homologous repair and the RAD51 recombinase protein in the cellular DNA damage response. We next discuss the recombinase structure and function per se and in combination with all its principal mediators and regulators. Finally, we conclude with an analysis of the manifold roles that RAD51 plays in carcinogenesis, cancer progression and anticancer drug resistance, and conclude this work with a survey of the most promising therapeutic strategies aimed at targeting RAD51 in experimental oncology.

Homologous Recombination under the Single-Molecule Fluorescence Microscope

Homologous recombination (HR) is a complex biological process and is central to meiosis and for repair of DNA double-strand breaks. Although the HR process has been the subject of intensive study for more than three decades, the complex protein–protein and protein–DNA interactions during HR present a significant challenge for determining the molecular mechanism(s) of the process. This knowledge gap is largely because of the dynamic interactions between HR proteins and DNA which is difficult to capture by routine biochemical or structural biology methods. In recent years, single-molecule fluorescence microscopy has been a popular method in the field of HR to visualize these complex and dynamic interactions at high spatiotemporal resolution, revealing mechanistic insights of the process. In this review, we describe recent efforts that employ single-molecule fluorescence microscopy to investigate protein–protein and protein–DNA interactions operating on three key DNA-substrates: single-stranded DNA (ssDNA), double-stranded DNA (dsDNA), and four-way DNA called Holliday junction (HJ). We also outline the technological advances and several key insights revealed by these studies in terms of protein assembly on these DNA substrates and highlight the foreseeable promise of single-molecule fluorescence microscopy in advancing our understanding of homologous recombination.

Entrenching role of cell cycle checkpoints and autophagy for maintenance of genomic integrity

Genomic integrity of the cell is crucial for the successful transmission of genetic information to the offspring and its survival. Persistent DNA damage induced by endogenous and exogenous agents leads to various metabolic manifestations. To combat this, eukaryotes have developed complex DNA damage response (DDR) pathway which senses the DNA damage and activates an arsenal of enzymes for the repair of damaged DNA. The active pathways for DNA repair are nucleotide excision repair (NER), base excision repair (BER) and mismatch repair (MMR) for single-strand break repair whereas homologous recombination (HR) and non-homologous end-joining (NHEJ) for double-strand break repair. OGG1 is a DNA glycosylase which initiates BER while Mre11-Rad50-Nbs1 (MRN) protein complex is the primary responder to DSBs which gets localized to damage sites. DNA damage response is meticulously executed by three related kinases: ATM, ATR, and DNA-PK. ATM- and ATR-dependent phosphorylation of p53, Chk1, and Chk2 regulate the G1/S, intra-S, or G2/M checkpoints of the cell cycle, respectively. Autophagy is an evolutionarily conserved process that plays a pivotal role in the regulation of DNA repair and maintains the cellular homeostasis. Genotoxic stress-induced altered autophagy occurs in a P53 dependent manner which is also the master regulator of genotoxic stress. A plethora of proteins involved in autophagy is regulated by p53 which involve DRAM, DAPK, and AMPK. As evident, the mtDNA is more prone to damage than nuclear DNA because of its close proximity to the site of ROS generation. Depending on the extent of damage either the repair mechanism or mitophagy gets triggered. SIRT1 is the master regulator which directs the stress response to mitophagy. Nix, a LC3 adapter also participates in Parkin mediated mitophagy. This review highlights the intricate crosstalks between DNA damage and cell cycle checkpoints activation. The DNA damage mediated regulation of autophagy and mitophagy is also reviewed in detail.

DNA damage response and repair in perspective: Aedes aegypti, Drosophila melanogaster and Homo sapiens

Background

The maintenance of genomic integrity is the responsibility of a complex network, denominated the DNA damage response (DDR), which controls the lesion detection and DNA repair. The main repair pathways are base excision repair (BER), nucleotide excision repair (NER), mismatch repair (MMR), homologous recombination repair (HR) and non-homologous end joining repair (NHEJ). They correct double-strand breaks (DSB), single-strand breaks, mismatches and others, or when the damage is quite extensive and repair insufficient, apoptosis is activated.

Methods

In this study we used the BLAST reciprocal best-hit methodology to search for DDR orthologs proteins in Aedes aegypti. We also provided a comparison between Ae. aegypti, D. melanogaster and human DDR network.

Results

Our analysis revealed the presence of ATR and ATM signaling, including the H2AX ortholog, in Ae. aegypti. Key DDR proteins (orthologs to RAD51, Ku and MRN complexes, XP-components, MutS and MutL) were also identified in this insect. Other proteins were not identified in both Ae. aegypti and D. melanogaster, including BRCA1 and its partners from BRCA1-A complex, TP53BP1, PALB2, POLk, CSA, CSB and POLβ. In humans, their absence affects DSB signaling, HR and sub-pathways of NER and BER. Seven orthologs not known in D. melanogaster were found in Ae. aegypti (RNF168, RIF1, WRN, RAD54B, RMI1, DNAPKcs, ARTEMIS).

Conclusions

The presence of key DDR proteins in Ae. aegypti suggests that the main DDR pathways are functional in this insect, and the identification of proteins not known in D. melanogaster can help fill gaps in the DDR network. The mapping of the DDR network in Ae. aegypti can support mosquito biology studies and inform genetic manipulation approaches applied to this vector.

In vitro role of Rad54 in Rad51-ssDNA filament-dependent homology search and synaptic complexes formation

Homologous recombination (HR) uses a homologous template to accurately repair DNA double-strand breaks and stalled replication forks to maintain genome stability. During homology search, Rad51 nucleoprotein filaments probe and interact with dsDNA, forming the synaptic complex that is stabilized on a homologous sequence. Strand intertwining leads to the formation of a displacement-loop (D-loop). In yeast, Rad54 is essential for HR in vivo and required for D-loop formation in vitro, but its exact role remains to be fully elucidated. Using electron microscopy to visualize the DNA-protein complexes, here we find that Rad54 is crucial for Rad51-mediated synaptic complex formation and homology search. The Rad54−K341R ATPase-deficient mutant protein promotes formation of synaptic complexes but not D-loops and leads to the accumulation of stable heterologous associations, suggesting that the Rad54 ATPase is involved in preventing non-productive intermediates. We propose that Rad51/Rad54 form a functional unit operating in homology search, synaptic complex and D-loop formation.

Homologous recombination uses a template to accurately repair DNA double-strand breaks and stalled replication forks to maintain genome stability. Here authors use electron microscopy to investigate the role of Rad54 in homology search and synaptic complex formation.

Rad54 Phosphorylation Promotes Homologous Recombination by Balancing Rad54 Mobility and DNA Binding

The repair of DNA double-strand breaks by homologous recombination is of crucial importance for maintaining genomic stability. Two major players during this repair pathway are Rad51 and Rad54. Previously, it was shown that Rad54 exists as a monomer or oligomer when bound to DNA and drives the displacement of Rad51 by translocating along the DNA. Moreover, phosphorylation of Rad54 was reported to stimulate this clearance of Rad51 from DNA. However, it is currently unclear how phosphorylation of Rad54 modulates its molecular-structural function and how it affects the activity of monomeric or oligomeric Rad54 during the removal of Rad51. To examine the impact of Rad54 phosphorylation on a molecular-structural level, we applied molecular dynamics simulations of Rad54 monomers and hexamers in the absence or presence of DNA. Our results suggest that 1) phosphorylation of Rad54 stabilizes the monomeric form by reducing the interlobe movement of Rad54 monomers and might therefore facilitate multimer formation around DNA and 2) phosphorylation of Rad54 in a higher-order hexamer reduces its binding strength to DNA, which is a requirement for efficient mobility on DNA. To further address the relationship between the mobility of Rad54 and its phosphorylation state, we performed fluorescence recovery after photobleaching experiments in living cells, which expressed different versions of the Rad54 protein. Here, we could measure that the phosphomimetic version of Rad54 was highly mobile on DNA, whereas a nonphosphorylatable mutant displayed a mobility defect. Taken together, these data show that the phosphorylation of Rad54 is a critical event in balancing the DNA binding strength and mobility of Rad54 and might therefore provide optimal conditions for DNA translocation and subsequent removal of Rad51 during homologous recombination repair.

Single-strand annealing mediates the conservative repair of double-strand DNA breaks in homologous recombination-defective germ cells of Caenorhabditis elegans

A missense mutation in C. elegans RAD-54, a homolog of RAD54 that operates in the homologous recombination (HR) pathway, was found to decrease ATPase activity in vitro. The hypomorphic mutation caused hypersensitivity of C. elegans germ cells to double-strand DNA breaks (DSBs). Although the formation of RAD-51 foci at DSBs was normal in both the mutant and knockdown worms, their subsequent dissipation was slow. The rad-54-deficient phenotypes were greatly aggravated when combined with an xpf-1 mutation, suggesting a conservative role of single-strand annealing (SSA) for DSB repair in HR-defective worms. The phenotypes of doubly-deficient rad-54;xpf-1 worms were partially suppressed by a mutation of lig-4, a nonhomologous end-joining (NHEJ) factor. In summary, RAD-54 is required for the dissociation of RAD-51 from DSB sites in C. elegans germ cells. Also, NHEJ and SSA exert negative and positive effects, respectively, on genome stability when HR is defective.