Rad54 Drives ATP Hydrolysis-Dependent DNA Sequence Alignment during Homologous Recombination.

Homologous recombination (HR) helps maintain genome integrity, and HR defects give rise to disease, especially cancer. During HR, damaged DNA must be aligned with an undamaged template through a process referred to as the homology search. Despite decades of study, key aspects of this search remain undefined. Here, we use single-molecule imaging to demonstrate that Rad54, a conserved Snf2-like protein found in all eukaryotes, switches the search from the diffusion-based pathways characteristic of the basal HR machinery to an active process in which DNA sequences are aligned via an ATP-dependent molecular motor-driven mechanism. We further demonstrate that Rad54 disrupts the donor template strands, enabling the search to take place within a migrating DNA bubble-like structure that is bound by replication protein A (RPA). Our results reveal that Rad54, working together with RPA, fundamentally alters how DNA sequences are aligned during HR.

Rad52 Restrains Resection at DNA Double-Strand Break Ends in Yeast.

Rad52 is a key factor for homologous recombination (HR) in yeast. Rad52 helps assemble Rad51-ssDNA nucleoprotein filaments that catalyze DNA strand exchange, and it mediates single-strand DNA annealing. We find that Rad52 has an even earlier function in HR in restricting DNA double-stranded break ends resection that generates 3' single-stranded DNA (ssDNA) tails. In fission yeast, Exo1 is the primary resection nuclease, with the helicase Rqh1 playing a minor role. We demonstrate that the choice of two extensive resection pathways is regulated by Rad52. In rad52 cells, the resection rate increases from ∼3-5 kb/h up to ∼10-20 kb/h in an Rqh1-dependent manner, while Exo1 becomes dispensable. Budding yeast Rad52 similarly inhibits Sgs1-dependent resection. Single-molecule analysis with purified budding yeast proteins shows that Rad52 competes with Sgs1 for DNA end binding and inhibits Sgs1 translocation along DNA. These results identify a role for Rad52 in limiting ssDNA generated by end resection.

Defining the influence of Rad51 and Dmc1 lineage-specific amino acids on genetic recombination.

The vast majority of eukaryotes possess two DNA recombinases: Rad51, which is ubiquitously expressed, and Dmc1, which is meiosis-specific. The evolutionary origins of this two-recombinase system remain poorly understood. Interestingly, Dmc1 can stabilize mismatch-containing base triplets, whereas Rad51 cannot. Here, we demonstrate that this difference can be attributed to three amino acids conserved only within the Dmc1 lineage of the Rad51/RecA family. Chimeric Rad51 mutants harboring Dmc1-specific amino acids gain the ability to stabilize heteroduplex DNA joints with mismatch-containing base triplets, whereas Dmc1 mutants with Rad51-specific amino acids lose this ability. Remarkably, RAD-51 from Caenorhabditis elegans, an organism without Dmc1, has acquired "Dmc1-like" amino acids. Chimeric C. elegans RAD-51 harboring "canonical" Rad51 amino acids gives rise to toxic recombination intermediates, which must be actively dismantled to permit normal meiotic progression. We propose that Dmc1 lineage-specific amino acids involved in the stabilization of heteroduplex DNA joints with mismatch-containing base triplets may contribute to normal meiotic recombination.

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.

Rad53 regulates the lifetime of Rdh54 at homologous recombination intermediates.

Rdh54 is a conserved DNA translocase that participates in homologous recombination (HR), DNA checkpoint adaptation, and chromosome segregation. Saccharomyces cerevisiae Rdh54 is a known target of the Mec1/Rad53 signaling axis, which globally protects genome integrity during DNA metabolism. While phosphorylation of DNA repair proteins by Mec1/Rad53 is critical for HR progression little is known about how specific post translational modifications alter HR reactions. Phosphorylation of Rdh54 is linked to protection of genomic integrity but the consequences of modification remain poorly understood. Here, we demonstrate that phosphorylation of the Rdh54 C-terminus by the effector kinase Rad53 regulates Rdh54 clustering activity as revealed by single molecule imaging. This stems from phosphorylation dependent and independent interactions between Rdh54 and Rad53. Genetic assays reveal that loss of phosphorylation leads to phenotypic changes resulting in loss-of-heterozygosity (LOH) outcomes. Our data highlight Rad53 as a key regulator of HR intermediates through activation and attenuation of Rdh54 motor function.

Rdh54 stabilizes Rad51 at displacement loop intermediates to regulate genetic exchange between chromosomes.

Homologous recombination (HR) is a double-strand break DNA repair pathway that preserves chromosome structure. To repair damaged DNA, HR uses an intact donor DNA sequence located elsewhere in the genome. After the double-strand break is repaired, DNA sequence information can be transferred between donor and recipient DNA molecules through different mechanisms, including DNA crossovers that form between homologous chromosomes. Regulation of DNA sequence transfer is an important step in effectively completing HR and maintaining genome integrity. For example, mitotic exchange of information between homologous chromosomes can result in loss-of-heterozygosity (LOH), and in higher eukaryotes, the development of cancer. The DNA motor protein Rdh54 is a highly conserved DNA translocase that functions during HR. Several existing phenotypes in rdh54Δ strains suggest that Rdh54 may regulate effective exchange of DNA during HR. In our current study, we used a combination of biochemical and genetic techniques to dissect the role of Rdh54 on the exchange of genetic information during DNA repair. Our data indicate that RDH54 regulates DNA strand exchange by stabilizing Rad51 at an early HR intermediate called the displacement loop (D-loop). Rdh54 acts in opposition to Rad51 removal by the DNA motor protein Rad54. Furthermore, we find that expression of a catalytically inactivate allele of Rdh54, rdh54K318R, favors non-crossover outcomes. From these results, we propose a model for how Rdh54 may kinetically regulate strand exchange during homologous recombination.

Rad54 and Rdh54 prevent Srs2-mediated disruption of Rad51 presynaptic filaments.

Srs2 is a superfamily 1 (SF1) helicase that participates in several pathways necessary for the repair of damaged DNA. Srs2 regulates formation of early homologous recombination (HR) intermediates by actively removing the recombinase Rad51 from single-stranded DNA (ssDNA). It is not known whether and how Srs2 itself is down-regulated to allow for timely HR progression. Rad54 and Rdh54 are two closely related superfamily 2 (SF2) motor proteins that promote the formation of Rad51-dependent recombination intermediates. Rad54 and Rdh54 bind tightly to Rad51-ssDNA and act downstream of Srs2, suggesting that they may affect the ability of Srs2 to dismantle Rad51 filaments. Here, we used DNA curtains to determine whether Rad54 and Rdh54 alter the ability of Srs2 to disrupt Rad51 filaments. We show that Rad54 and Rdh54 act synergistically to greatly restrict the antirecombinase activity of Srs2. Our findings suggest that Srs2 may be accorded only a limited time window to act and that Rad54 and Rdh54 fulfill a role of prorecombinogenic licensing factors.

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.

All who wander are not lost: the search for homology during homologous recombination.

Homologous recombination (HR) is a template-based DNA double-strand break repair pathway that functions to maintain genomic integrity. A vital component of the HR reaction is the identification of template DNA to be used during repair. This occurs through a mechanism known as the homology search. The homology search occurs in two steps: a collision step in which two pieces of DNA are forced to collide and a selection step that results in homologous pairing between matching DNA sequences. Selection of a homologous template is facilitated by recombinases of the RecA/Rad51 family of proteins in cooperation with helicases, translocases, and topoisomerases that determine the overall fidelity of the match. This menagerie of molecular machines acts to regulate critical intermediates during the homology search. These intermediates include recombinase filaments that probe for short stretches of homology and early strand invasion intermediates in the form of displacement loops (D-loops) that stabilize paired DNA. Here, we will discuss recent advances in understanding how these specific intermediates are regulated on the molecular level during the HR reaction. We will also discuss how the stability of these intermediates influences the ultimate outcomes of the HR reaction. Finally, we will discuss recent physiological models developed to explain how the homology search protects the genome.

Regulation of Hed1 and Rad54 binding during maturation of the meiosis-specific presynaptic complex.

Most eukaryotes have two Rad51/RecA family recombinases, Rad51, which promotes recombination during mitotic double-strand break (DSB) repair, and the meiosis-specific recombinase Dmc1. During meiosis, the strand exchange activity of Rad51 is downregulated through interactions with the meiosis-specific protein Hed1, which helps ensure that strand exchange is driven by Dmc1 instead of Rad51. Hed1 acts by preventing Rad51 from interacting with Rad54, a cofactor required for promoting strand exchange during homologous recombination. However, we have a poor quantitative understanding of the regulatory interplay between these proteins. Here, we use real-time single-molecule imaging to probe how the Hed1- and Rad54-mediated regulatory network contributes to the identity of mitotic and meiotic presynaptic complexes. Based on our findings, we define a model in which kinetic competition between Hed1 and Rad54 helps define the functional identity of the presynaptic complex as cells undergo the transition from mitotic to meiotic repair.

Regulatory control of Sgs1 and Dna2 during eukaryotic DNA end resection.

In the repair of DNA double-strand breaks by homologous recombination, the DNA break ends must first be processed into 3' single-strand DNA overhangs. In budding yeast, end processing requires the helicase Sgs1 (BLM in humans), the nuclease/helicase Dna2, Top3-Rmi1, and replication protein A (RPA). Here, we use single-molecule imaging to visualize Sgs1-dependent end processing in real-time. We show that Sgs1 is recruited to DNA ends through Top3-Rmi1-dependent or -independent means, and in both cases Sgs1 is maintained in an immoble state at the DNA ends. Importantly, the addition of Dna2 triggers processive Sgs1 translocation, but DNA resection only occurs when RPA is also present. We also demonstrate that the Sgs1-Dna2-Top3-Rmi1-RPA ensemble can efficiently disrupt nucleosomes, and that Sgs1 itself possesses nucleosome remodeling activity. Together, these results shed light on the regulatory interplay among conserved protein factors that mediate the nucleolytic processing of DNA ends in preparation for homologous recombination-mediated chromosome damage repair.

The RecQ helicase Sgs1 drives ATP-dependent disruption of Rad51 filaments.

DNA helicases of the RecQ family are conserved among the three domains of life and play essential roles in genome maintenance. Mutations in several human RecQ helicases lead to diseases that are marked by cancer predisposition. The Saccharomyces cerevisiae RecQ helicase Sgs1 is orthologous to human BLM, defects in which cause the cancer-prone Bloom's Syndrome. Here, we use single-molecule imaging to provide a quantitative mechanistic understanding of Sgs1 activities on single stranded DNA (ssDNA), which is a central intermediate in all aspects of DNA metabolism. We show that Sgs1 acts upon ssDNA bound by either replication protein A (RPA) or the recombinase Rad51. Surprisingly, we find that Sgs1 utilizes a novel motor mechanism for disrupting ssDNA intermediates bound by the recombinase protein Rad51. The ability of Sgs1 to disrupt Rad51-ssDNA filaments may explain some of the defects engendered by RECQ helicase deficiencies in human cells.

Meiosis-specific recombinase Dmc1 is a potent inhibitor of the Srs2 antirecombinase.

Cross-over recombination products are a hallmark of meiosis because they are necessary for accurate chromosome segregation and they also allow for increased genetic diversity during sexual reproduction. However, cross-overs can also cause gross chromosomal rearrangements and are therefore normally down-regulated during mitotic growth. The mechanisms that enhance cross-over product formation upon entry into meiosis remain poorly understood. In Saccharomyces cerevisiae, the Superfamily 1 (Sf1) helicase Srs2, which is an ATP hydrolysis-dependent motor protein that actively dismantles recombination intermediates, promotes synthesis-dependent strand annealing, the result of which is a reduction in cross-over recombination products. Here, we show that the meiosis-specific recombinase Dmc1 is a potent inhibitor of Srs2. Biochemical and single-molecule assays demonstrate that Dmc1 acts by inhibiting Srs2 ATP hydrolysis activity, which prevents the motor protein from undergoing ATP hydrolysis-dependent translocation on Dmc1-bound recombination intermediates. We propose a model in which Dmc1 helps contribute to cross-over formation during meiosis by antagonizing the antirecombinase activity of Srs2.

Dynamic interactions of the homologous pairing 2 (Hop2)-meiotic nuclear divisions 1 (Mnd1) protein complex with meiotic presynaptic filaments in budding yeast.

Homologous recombination (HR) is a universally conserved DNA repair pathway that can result in the exchange of genetic material. In eukaryotes, HR has evolved into an essential step in meiosis. During meiosis many eukaryotes utilize a two-recombinase pathway. This system consists of Rad51 and the meiosis-specific recombinase Dmc1. Both recombinases have distinct activities during meiotic HR, despite being highly similar in sequence and having closely related biochemical activities, raising the question of how these two proteins can perform separate functions. A likely explanation for their differential regulation involves the meiosis-specific recombination proteins Hop2 and Mnd1, which are part of a highly conserved eukaryotic protein complex that participates in HR, albeit through poorly understood mechanisms. To better understand how Hop2-Mnd1 functions during HR, here we used DNA curtains in conjunction with single-molecule imaging to measure and quantify the binding of the Hop2-Mnd1 complex from Saccharomyces cerevisiae to recombination intermediates comprising Rad51- and Dmc1-ssDNA in real time. We found that yeast Hop2-Mnd1 bound rapidly to Dmc1-ssDNA filaments with high affinity and remained bound for ∼1.3 min before dissociating. We also observed that this binding interaction was highly specific for Dmc1 and found no evidence for an association of Hop2-Mnd1 with Rad51-ssDNA or RPA-ssDNA. Our findings provide new quantitative insights into the binding dynamics of Hop2-Mnd1 with the meiotic presynaptic complex. On the basis of these findings, we propose a model in which recombinase specificities for meiotic accessory proteins enhance separation of the recombinases' functions during meiotic HR.

Single-molecule FRET method to investigate the dynamics of transcription elongation through the nucleosome by RNA polymerase II.

Transcription elongation through the nucleosome is a precisely coordinated activity to ensure timely production of RNA and accurate regulation of co-transcriptional histone modifications. Nucleosomes actively participate in transcription regulation at various levels and impose physical barriers to RNA polymerase II (RNAPII) during transcription elongation. Despite its high significance, the detailed dynamics of how RNAPII translocates along nucleosomal DNA during transcription elongation and how the nucleosome structure dynamically conforms to the changes necessary for RNAPII progression remain poorly understood. Transcription elongation through the nucleosome is a complex process and investigating the changes of the nucleosome structure during this process by ensemble measurements is daunting. This is because it is nearly impossible to synchronize elongation complexes within a nucleosome or a sub-nucleosome to a designated location at a high enough efficiency for desired sample homogeneity. Here we review our recently developed single-molecule FRET experimental system and method that has fulfilled this deficiency. With our method, one can follow the changes in the structure of individual nucleosomes during transcription elongation. We demonstrated that this method enables the detailed measurements of the kinetics of transcription elongation through the nucleosome and its regulation by a transcription factor, which can be easily extended to investigations of the roles of environmental variables and histone post-translational modifications in regulating transcription elongation.

Spontaneous self-segregation of Rad51 and Dmc1 DNA recombinases within mixed recombinase filaments.

During meiosis, the two DNA recombinases Rad51 and Dmc1 form specialized presynaptic filaments that are adapted for performing recombination between homologous chromosomes. There is currently a limited understanding of how these two recombinases are organized within the meiotic presynaptic filament. Here, we used single molecule imaging to examine the properties of presynaptic complexes composed of both Rad51 and Dmc1. We demonstrate that Rad51 and Dmc1 have an intrinsic ability to self-segregate, even in the absence of any other recombination accessory proteins. Moreover, we found that the presence of Dmc1 stabilizes the adjacent Rad51 filaments, suggesting that cross-talk between these two recombinases may affect their biochemical properties. Based upon these findings, we describe a model for the organization of Rad51 and Dmc1 within the meiotic presynaptic complex, which is also consistent with in vivo observations, genetic findings, and biochemical expectations. This model argues against the existence of extensively intermixed filaments, and we propose that Rad51 and Dmc1 have intrinsic capacities to form spatially distinct filaments, suggesting that additional recombination cofactors are not required to segregate the Rad51 and Dmc1 filaments.

Biochemical Analysis of Yeast Suppressor of Ty 4/5 (Spt4/5) Reveals the Importance of Nucleic Acid Interactions in the Prevention of RNA Polymerase II Arrest.

RNA polymerase II (RNAPII) undergoes structural changes during the transitions from initiation, elongation, and termination, which are aided by a collection of proteins called elongation factors. NusG/Spt5 is the only elongation factor conserved in all domains of life. Although much information exists about the interactions between NusG/Spt5 and RNA polymerase in prokaryotes, little is known about how the binding of eukaryotic Spt4/5 affects the biochemical activities of RNAPII. We characterized the activities of Spt4/5 and interrogated the structural features of Spt5 required for it to interact with elongation complexes, bind nucleic acids, and promote transcription elongation. The eukaryotic specific regions of Spt5 containing the Kyrpides, Ouzounis, Woese domains are involved in stabilizing the association with the RNAPII elongation complex, which also requires the presence of the nascent transcript. Interestingly, we identify a region within the conserved NusG N-terminal (NGN) domain of Spt5 that contacts the non-template strand of DNA both upstream of RNAPII and in the transcription bubble. Mutating charged residues in this region of Spt5 did not prevent Spt4/5 binding to elongation complexes, but abrogated the cross-linking of Spt5 to DNA and the anti-arrest properties of Spt4/5, thus suggesting that contact between Spt5 (NGN) and DNA is required for Spt4/5 to promote elongation. We propose that the mechanism of how Spt5/NGN promotes elongation is fundamentally conserved; however, the eukaryotic specific regions of the protein evolved so that it can serve as a platform for other elongation factors and maintain its association with RNAPII as it navigates genomes packaged into chromatin.