Competition between the DNA unwinding and strand pairing activities of the Werner and Bloom syndrome proteins

Response to Replication Stress and Maintenance of Genome Stability by WRN, the Werner Syndrome Protein

Werner syndrome (WS) is an autosomal recessive disease caused by loss of function of WRN. WS is a segmental progeroid disease and shows early onset or increased frequency of many characteristics of normal aging. WRN possesses helicase, annealing, strand exchange, and exonuclease activities and acts on a variety of DNA substrates, even complex replication and recombination intermediates. Here, we review the genetics, biochemistry, and probably physiological functions of the WRN protein. Although its precise role is unclear, evidence suggests WRN plays a role in pathways that respond to replication stress and maintain genome stability particularly in telomeric regions.

Single-cell RNA-seq reveals alterations in peripheral CX3CR1 and nonclassical monocytes in familial tauopathy

BACKGROUND

Emerging evidence from mouse models is beginning to elucidate the brain's immune response to tau pathology, but little is known about the nature of this response in humans. In addition, it remains unclear to what extent tau pathology and the local inflammatory response within the brain influence the broader immune system.

METHODS

To address these questions, we performed single-cell RNA sequencing (scRNA-seq) of peripheral blood mononuclear cells (PBMCs) from carriers of pathogenic variants in MAPT, the gene encoding tau (n = 8), and healthy non-carrier controls (n = 8). Primary findings from our scRNA-seq analyses were confirmed and extended via flow cytometry, droplet digital (dd)PCR, and secondary analyses of publicly available transcriptomics datasets.

RESULTS

Analysis of ~ 181,000 individual PBMC transcriptomes demonstrated striking differential expression in monocytes and natural killer (NK) cells in MAPT pathogenic variant carriers. In particular, we observed a marked reduction in the expression of CX3CR1-the gene encoding the fractalkine receptor that is known to modulate tau pathology in mouse models-in monocytes and NK cells. We also observed a significant reduction in the abundance of nonclassical monocytes and dysregulated expression of nonclassical monocyte marker genes, including FCGR3A. Finally, we identified reductions in TMEM176A and TMEM176B, genes thought to be involved in the inflammatory response in human microglia but with unclear function in peripheral monocytes. We confirmed the reduction in nonclassical monocytes by flow cytometry and the differential expression of select biologically relevant genes dysregulated in our scRNA-seq data using ddPCR.

CONCLUSIONS

Our results suggest that human peripheral immune cell expression and abundance are modulated by tau-associated pathophysiologic changes. CX3CR1 and nonclassical monocytes in particular will be a focus of future work exploring the role of these peripheral signals in additional tau-associated neurodegenerative diseases.

Werner syndrome protein works as a dimer for unwinding and replication fork regression

The determination of the oligomeric state of functional enzymes is essential for the mechanistic understanding of their catalytic activities. RecQ helicases have diverse biochemical activities, but it is still unclear how their activities are related to their oligomeric states. We use single-molecule multi-color fluorescence imaging to determine the oligomeric states of Werner syndrome protein (WRN) during its unwinding and replication fork regression activities. We reveal that WRN binds to a forked DNA as a dimer, and unwinds it without any change of its oligomeric state. In contrast, WRN binds to a replication fork as a tetramer, and is dimerized during activation of replication fork regression. By selectively inhibiting the helicase activity of WRN on specific strands, we reveal how the active dimers of WRN distinctly use the energy of ATP hydrolysis for repetitive unwinding and replication fork regression.

Genome Protection by DNA Polymerase θ

DNA polymerase θ (Pol θ) is a DNA repair enzyme widely conserved in animals and plants. Pol θ uses short DNA sequence homologies to initiate repair of double-strand breaks by theta-mediated end joining. The DNA polymerase domain of Pol θ is at the C terminus and is connected to an N-terminal DNA helicase-like domain by a central linker. Pol θ is crucial for maintenance of damaged genomes during development, protects DNA against extensive deletions, and limits loss of heterozygosity. The cost of using Pol θ for genome protection is that a few nucleotides are usually deleted or added at the repair site. Inactivation of Pol θ often enhances the sensitivity of cells to DNA strand-breaking chemicals and radiation. Since some homologous recombination-defective cancers depend on Pol θ for growth, inhibitors of Pol θ may be useful in treating such tumors.

Assessing spermatozoal small ribonucleic acids and their relationship to blastocyst development in idiopathic infertile males

Clinical testing strategies for diagnosing male factor infertility are limited. A deeper analysis of spermatozoa-derived factors could potentially diagnose some cases of 'unexplained infertility'. Spermatozoa carry a rich and dynamic profile of small RNAs, which have demonstrated potential developmental importance and association with fertility status. We used next-generation sequencing to correlate sperm small RNA profiles of normozoospermic males (n = 54) with differing blastocyst development rates, when using young donor oocytes. While ribosomal RNAs accounted for the highest number of sequencing reads, transfer RNA fragments of tRNA^Gly/GCC and tRNA^Val-CAC were the most abundant sequences across all sperm samples. A total of 324 small RNAs were differentially expressed between samples with high (n = 18) and low (n = 14) blastocyst rates (p-adj < 0.05). Ninety three miRNAs were differentially expressed between these groups (p-adj < 0.05). Differentially expressed transfer RNA fragments included: 5'-tRF-Asp-GTC; 5'-tRF-Phe-GAA; and 3'-tRF-Ser-GCA. Differentially expressed miRNAs included: let-7f-2-5p; miR-4755-3p; and miR-92a-3p. This study provides the foundation on which to validate a clinical panel of fertility-related sperm small RNAs, as well as to pursue potential mechanisms through which they alter blastocyst development.

Probing the structure and function of polymerase θ helicase-like domain

DNA Polymerase θ is the key actuator of the recently identified double-strand break repair pathway, theta-mediated end joining (TMEJ). It is the only known polymerase to have a 3-domain architecture containing an independently functional family A DNA polymerase tethered by a long central region to an N-terminal helicase-like domain (HLD). Full-length polymerase θ and the isolated HLD hydrolyze ATP in the presence of DNA, but no processive DNA duplex unwinding has been observed. Based on sequence and structure conservation, the HLD is classified as a member of helicase superfamily II and, more specifically, the Ski2-like family. The specific subdomain composition and organization most closely resemble that of archaeal DNA repair helicases Hel308 and Hjm. The underlying structural basis as to why the HLD is not able to processively unwind duplex DNA, despite its similarity to bona fide helicases, remains elusive. Activities of the HLD include ATP hydrolysis, protein displacement, and annealing of complementary DNA. These observations have led to speculation about the role of the HLD within the context of double-strand break repair via TMEJ, such as removal of single-stranded DNA binding proteins like RPA and RAD51 and microhomology alignment. This review summarizes the structural classification and organization of the polymerase θ HLD and its homologs and explores emerging data on its biochemical activities. We conclude with a simple, speculative model for the HLD's role in TMEJ.

The catalytic core of Leishmania donovani RECQ helicase unwinds a wide spectrum of DNA substrates and is stimulated by replication protein A

RecQ helicases are superfamily 2 (SF2) DNA helicases that unwind a wide spectrum of complex DNA structures in a 3' to 5' direction and are involved in maintaining genome stability. RecQ helicases from protozoan parasites have gained significant interest in recent times because of their involvement in cellular DNA repair pathways, making them important targets for drug development. In this study, we report biophysical and biochemical characterization of the catalytic core of a RecQ helicase from hemoflagellate protozoan parasite Leishmania donovani. Among the two putative RecQ helicases identified in L. donovani, we cloned, overexpressed and purified the catalytic core of LdRECQb. The catalytic core was found to be very efficient in unwinding a wide variety of DNA substrates like forked duplex, 3' tailed duplex and Holliday junction DNA. Interestingly, the helicase core also unwound blunt duplex with slightly less efficiency. The enzyme exhibited high level of DNA-stimulated ATPase activity with preferential stimulation by forked duplex, Holliday junction and 3' tailed duplex. Walker A motif lysine mutation severely affected the ATPase activity and significantly affected unwinding activity. Like many other RecQ helicases, L. donovani RECQb also possesses strand annealing activity. Unwinding of longer DNA substrates by LdRECQb catalytic core was found to be stimulated in the presence of replication protein A (LdRPA-1) from L. donovani. Detailed biochemical characterization and comparison of kinetic parameters indicate that L. donovani RECQb shares considerable functional similarity with human Bloom syndrome helicase.

Maintenance of Yeast Genome Integrity by RecQ Family DNA Helicases

With roles in DNA repair, recombination, replication and transcription, members of the RecQ DNA helicase family maintain genome integrity from bacteria to mammals. Mutations in human RecQ helicases BLM, WRN and RecQL4 cause incurable disorders characterized by genome instability, increased cancer predisposition and premature adult-onset aging. Yeast cells lacking the RecQ helicase Sgs1 share many of the cellular defects of human cells lacking BLM, including hypersensitivity to DNA damaging agents and replication stress, shortened lifespan, genome instability and mitotic hyper-recombination, making them invaluable model systems for elucidating eukaryotic RecQ helicase function. Yeast and human RecQ helicases have common DNA substrates and domain structures and share similar physical interaction partners. Here, we review the major cellular functions of the yeast RecQ helicases Sgs1 of Saccharomyces cerevisiae and Rqh1 of Schizosaccharomyces pombe and provide an outlook on some of the outstanding questions in the field.

Replication fork collapse at a protein-DNA roadblock leads to fork reversal, promoted by the RecQ helicase

Proteins that bind DNA are the cause of the majority of impediments to replication fork progression and can lead to subsequent collapse of the replication fork. Failure to deal with fork collapse efficiently leads to mutation or cell death. Several models have been proposed for how a cell processes a stalled or collapsed replication fork; eukaryotes and bacteria are not dissimilar in terms of the general pathways undertaken to deal with these events. This study shows that replication fork regression, the combination of replication fork reversal leading to formation of a Holliday Junction along with exonuclease digestion, is the preferred pathway for dealing with a collapsed fork in Escherichia coli. Direct endo-nuclease activity at the replication fork was not observed. The protein that had the greatest effect on these fork processing events was the RecQ helicase, while RecG and RuvABC, which have previously been implicated in this process, were found to play a lesser role. Eukaryotic RecQ homologues, BLM and WRN, have also been implicated in processing events following replication fork collapse and may reflect a conserved mechanism. Finally, the SOS response was not induced by the protein-DNA roadblock under these conditions, so did not affect fork processing.

Transcriptome analysis demonstrates that long noncoding RNA is involved in the hypoxic response in Larimichthys crocea

The large yellow croaker (Larimichthys crocea) has low hypoxia tolerance compared with other fish species, and the mRNA levels of hypoxia-inducible factor (HIF)-1α in its brain do not change markedly under hypoxic conditions. In this study, we investigated noncoding transcription in the hypoxic response mechanism of L. crocea. We generated a catalog of long noncoding RNAs (lncRNAs) from the brain of L. crocea individuals under hypoxic stress, investigated lncRNA expression patterns, and analyzed the HIF signaling pathway by RNA sequencing. Prolyl hydroxylase domain 2 (PHD2) expression significantly increased after 6 and 12 h of hypoxia, and a lncRNA (Linc_06633.1) was found in the upstream, antisense region of PHD2. Linc_06633.1 may be an important regulator that promotes PDH2 expression under hypoxia in L. crocea, and we constructed a regulatory profile of L. crocea under hypoxic conditions. To the best of our knowledge, it is the first study that has been conducted on hypoxia signaling pathway regulation by lncRNAs in L. crocea and elucidates the role played by lncRNAs in the regulation of the hypoxia stress response in teleost fish.

Structural and biochemical analysis of the dual-specificity Trm10 enzyme from Thermococcus kodakaraensis prompts reconsideration of its catalytic mechanism

tRNA molecules get heavily modified post-transcriptionally. The N-1 methylation of purines at position 9 of eukaryal and archaeal tRNA is catalyzed by the SPOUT methyltranferase Trm10. Remarkably, while certain Trm10 orthologs are specific for either guanosine or adenosine, others show a dual specificity. Structural and functional studies have been performed on guanosine- and adenosine-specific enzymes. Here we report the structure and biochemical analysis of the dual-specificity enzyme from Thermococcus kodakaraensis (TkTrm10). We report the first crystal structure of a construct of this enzyme, consisting of the N-terminal domain and the catalytic SPOUT domain. Moreover, crystal structures of the SPOUT domain, either in the apo form or bound to S-adenosyl-l-methionine or S-adenosyl-l-homocysteine reveal the conformational plasticity of two active site loops upon substrate binding. Kinetic analysis shows that TkTrm10 has a high affinity for its tRNA substrates, while the enzyme on its own has a very low methyltransferase activity. Mutation of either of two active site aspartate residues (Asp206 and Asp245) to Asn or Ala results in only modest effects on the N-1 methylation reaction, with a small shift toward a preference for m1G formation over m1A formation. Only a double D206A/D245A mutation severely impairs activity. These results are in line with the recent finding that the single active-site aspartate was dispensable for activity in the guanosine-specific Trm10 from yeast, and suggest that also dual-specificity Trm10 orthologs use a noncanonical tRNA methyltransferase mechanism without residues acting as general base catalysts.

Interactive Roles of DNA Helicases and Translocases with the Single-Stranded DNA Binding Protein RPA in Nucleic Acid Metabolism

Helicases and translocases use the energy of nucleoside triphosphate binding and hydrolysis to unwind/resolve structured nucleic acids or move along a single-stranded or double-stranded polynucleotide chain, respectively. These molecular motors facilitate a variety of transactions including replication, DNA repair, recombination, and transcription. A key partner of eukaryotic DNA helicases/translocases is the single-stranded DNA binding protein Replication Protein A (RPA). Biochemical, genetic, and cell biological assays have demonstrated that RPA interacts with these human molecular motors physically and functionally, and their association is enriched in cells undergoing replication stress. The roles of DNA helicases/translocases are orchestrated with RPA in pathways of nucleic acid metabolism. RPA stimulates helicase-catalyzed DNA unwinding, enlists translocases to sites of action, and modulates their activities in DNA repair, fork remodeling, checkpoint activation, and telomere maintenance. The dynamic interplay between DNA helicases/translocases and RPA is just beginning to be understood at the molecular and cellular levels, and there is still much to be learned, which may inform potential therapeutic strategies.

Molecular characterization of mudskipper (Boleophthalmus pectinirostris) hypoxia-inducible factor-1α (HIF-1α) and analysis of its function in monocytes/macrophages

Hypoxia-inducible factor-1α (HIF-1α) plays a critical role in immune and inflammatory responses and is important in controlling a variety of processes in monocytes and macrophages. However, very little information is available about the functions of HIF-1α in fish monocytes/macrophages (MO/MФ). In this study, the cDNA sequence of the mudskipper (Boleophthalmus pectinirostris) HIF-1α gene (BpHIF-1α) was determined. Sequence comparison and phylogenetic tree analysis showed that BpHIF-1α is clustered in the fish HIF-1α tree. Constitutive expression of BpHIF-1α mRNA was detected by real-time quantitative PCR in all tested tissues, and the expression was found to be dramatically increased in the skin, liver, spleen, and kidney after Edwardsiella tarda infection. In addition, hypoxia and infection induced the expression of the BpHIF-1α transcript and protein in MO/MФ, respectively. Hypoxia caused an increase in phagocytic and bactericidal capacity of mudskipper MO/MФ in a BpHIF-1α-dependent manner. BpHIF-1α induced an anti-inflammatory status in MO/MФ upon E. tarda infection and hypoxia. Therefore, BpHIF-1α may play a predominant role in the modulation of mudskipper MO/MФ function in the innate immune system.

A two-step integrated approach to detect differentially expressed genes in RNA-Seq data

One of the primary objectives of ribonucleic acid (RNA) sequencing or RNA-Seq experiment is to identify differentially expressed (DE) genes in two or more treatment conditions. It is a common practice to assume that all read counts from RNA-Seq data follow overdispersed (OD) Poisson or negative binomial (NB) distribution, which is sometimes misleading because within each condition, some genes may have unvarying transcription levels with no overdispersion. In such a case, it is more appropriate and logical to consider two sets of genes: OD and non-overdispersed (NOD). We propose a new two-step integrated approach to distinguish DE genes in RNA-Seq data using standard Poisson and NB models for NOD and OD genes, respectively. This is an integrated approach because this method can be merged with any other NB-based methods for detecting DE genes. We design a simulation study and analyze two real RNA-Seq data to evaluate the proposed strategy. We compare the performance of this new method combined with the three [Formula: see text]-software packages namely edgeR, DESeq2, and DSS with their default settings. For both the simulated and real data sets, integrated approaches perform better or at least equally well compared to the regular methods embedded in these [Formula: see text]-packages.

RECQL5 has unique strand annealing properties relative to the other human RecQ helicase proteins

The RecQ helicases play important roles in genome maintenance and DNA metabolism (replication, recombination, repair, and transcription). Five different homologs are present in humans, three of which are implicated in accelerated aging genetic disorders: Rothmund Thomson (RECQL4), Werner (WRN), and Bloom (BLM) syndromes. While the DNA helicase activities of the 5 human RecQ helicases have been extensively characterized, much less is known about their DNA double strand annealing activities. Strand annealing is an important integral enzymatic activity in DNA metabolism, including DNA repair. Here, we have characterized the strand annealing activities of all five human RecQ helicase proteins and compared them. Interestingly, the relative strand annealing activities of the five RecQ proteins are not directly (inversely) related to their helicase activities. RECQL5 possesses relatively strong annealing activity on long or small duplexed substrates compared to the other RecQs. Additionally, the strand annealing activity of RECQL5 is not inhibited by the presence of ATP, unlike the other RecQs. We also show that RECQL5 efficiently catalyzes annealing of RNA to DNA in vitro in the presence or absence of ATP, revealing a possible new function for RECQL5. Additionally, we investigate how different known RecQ interacting proteins, RPA, Ku, FEN1 and RAD51, regulate their strand annealing activity. Collectively, we find that the human RecQ proteins possess differential DNA double strand annealing activities and we speculate on their individual roles in DNA repair. This insight is important in view of the many cellular DNA metabolic actions of the RecQ proteins and elucidates their unique functions in the cell.

Yeast Pif1 accelerates annealing of complementary DNA strands

Pif1 is a helicase involved in the maintenance of nuclear and mitochondrial genomes in eukaryotes. Here we report a new activity of Saccharomyces cerevisiae Pif1, annealing of complementary DNA strands. We identified preferred substrates for annealing as those that generate a duplex product with a single-stranded overhang relative to a blunt end duplex. Importantly, we show that Pif1 can anneal DNA in the presence of ATP and Mg(2+). Pif1-mediated annealing also occurs in the presence of single-stranded DNA binding proteins. Additionally, we show that partial duplex substrates with 3'-single-stranded overhangs such as those generated during double-strand break repair can be annealed by Pif1.

Multimerization domains are associated with apparent strand exchange activity in BLM and WRN DNA helicases

BLM and WRN are members of the RecQ family of DNA helicases that act to suppress genome instability and cancer predisposition. In addition to a RecQ helicase domain, each of these proteins contains an N-terminal domain of approximately 500 amino acids (aa) that is incompletely characterized. Previously, we showed that the N-terminus of Sgs1, the yeast ortholog of BLM, contains a physiologically important 200 aa domain (Sgs1103-322) that displays single-stranded DNA (ssDNA) binding, strand annealing (SA), and apparent strand-exchange (SE) activities in vitro. Here we used a genetic assay to search for heterologous proteins that could functionally replace this domain of Sgs1 in vivo. In contrast to Rad59, the oligomeric Rad52 protein provided in vivo complementation, suggesting that multimerization is functionally important. An N-terminal domain of WRN was also identified that could replace Sgs1103-322 in yeast. This domain, WRN235-526, contains a known coiled coil and displays the same SA and SE activities as Sgs1103-322. The coiled coil domain of WRN235-526 is required for both its in vivo activity and its in vitro SE activity. Based on this result, a potential coiled coil was identified within Sgs1103-322. This 25 amino acid region was similarly essential for wt Sgs1 activity in vivo and was replaceable by a heterologous coiled coil. Taken together, the results indicate that a coiled coil and a closely linked apparent SE activity are conserved features of the BLM and WRN DNA helicases.

Hypoxia turns genotypic female medaka fish into phenotypic males

Hypoxia caused by eutrophication is amongst the most pressing global problems in aquatic systems. Notably, more than 400 "dead zones" have been identified worldwide, resulting in large scale collapse of fisheries and major changes in the structure and trophodynamics. Recent studies further discovered that hypoxia can also disrupt sex hormone metabolism and alter the sexual differentiation of fish, resulting in male biased F1 generations and therefore posing a threat to the sustainability of natural populations. However, it is not known whether, and if so how, hypoxia can also change the sex ratio in vertebrates that have sex-determining XX/XY chromosomes. Using the Japanese medaka (Oryzias latipes) as a model, we demonstrate, for the first time, that hypoxia can turn genotypic female fish with XX chromosomes into phenotypic males. Over half of the XX females exposed to hypoxia exhibit male secondary sexual characteristics and develop testis instead of ovary. We further revealed that hypoxia can: (a) down-regulate the vasa gene, which controls proliferation of primordial germ cells and gonadal sex differentiation into ovary, and (b) up-regulate the DMY gene which resides at the sex-determining locus of the Y chromosome, and direct testis differentiation. This is the first report that hypoxia can directly act on genes that regulate sex determination and differentiation, thereby turning genotypic females into phenotypic males and leading to a male-dominant F1 population.

Strand exchange of telomeric DNA catalyzed by the Werner syndrome protein (WRN) is specifically stimulated by TRF2

Werner syndrome (WS), caused by loss of function of the RecQ helicase WRN, is a hereditary disease characterized by premature aging and elevated cancer incidence. WRN has DNA binding, exonuclease, ATPase, helicase and strand annealing activities, suggesting possible roles in recombination-related processes. Evidence indicates that WRN deficiency causes telomeric abnormalities that likely underlie early onset of aging phenotypes in WS. Furthermore, TRF2, a protein essential for telomere protection, interacts with WRN and influences its basic helicase and exonuclease activities. However, these studies provided little insight into WRN's specific function at telomeres. Here, we explored the possibility that WRN and TRF2 cooperate during telomeric recombination processes. Our results indicate that TRF2, through its interactions with both WRN and telomeric DNA, stimulates WRN-mediated strand exchange specifically between telomeric substrates; TRF2's basic domain is particularly important for this stimulation. Although TRF1 binds telomeric DNA with similar affinity, it has minimal effects on WRN-mediated strand exchange of telomeric DNA. Moreover, TRF2 is displaced from telomeric DNA by WRN, independent of its ATPase and helicase activities. Together, these results suggest that TRF2 and WRN act coordinately during telomeric recombination processes, consistent with certain telomeric abnormalities associated with alteration of WRN function.

Human RecQ helicases in DNA repair, recombination, and replication

RecQ helicases are an important family of genome surveillance proteins conserved from bacteria to humans. Each of the five human RecQ helicases plays critical roles in genome maintenance and stability, and the RecQ protein family members are often referred to as guardians of the genome. The importance of these proteins in cellular homeostasis is underscored by the fact that defects in BLM, WRN, and RECQL4 are linked to distinct heritable human disease syndromes. Each human RecQ helicase has a unique set of protein-interacting partners, and these interactions dictate its specialized functions in genome maintenance, including DNA repair, recombination, replication, and transcription. Human RecQ helicases also interact with each other, and these interactions have significant impact on enzyme function. Future research goals in this field include a better understanding of the division of labor among the human RecQ helicases and learning how human RecQ helicases collaborate and cooperate to enhance genome stability.

Replication stress induces specific enrichment of RECQ1 at common fragile sites FRA3B and FRA16D

BACKGROUND

Stalled replication forks at common fragile sites are a major cause of genomic instability. RecQ helicases, a highly conserved family of DNA-unwinding enzymes, are believed to ease 'roadblocks' that pose challenge to replication fork progression. Among the five known RecQ homologs in humans, functions of RECQ1, the most abundant of all, are poorly understood. We previously determined that RECQ1 helicase preferentially binds and unwinds substrates that mimic DNA replication/repair intermediates, and interacts with proteins involved in DNA replication restart mechanisms.

METHOD

We have utilized chromatin immunoprecipitation followed by quantitative real-time PCR to investigate chromatin interactions of RECQ1 at defined genetic loci in the presence or absence of replication stress. We have also tested the sensitivity of RECQ1-depleted cells to aphidicolin induced replication stress.

RESULTS

RECQ1 binds to the origins of replication in unperturbed cells. We now show that conditions of replication stress induce increased accumulation of RECQ1 at the lamin B2 origin in HeLa cells. Consistent with a role in promoting fork recovery or repair, RECQ1 is specifically enriched at two major fragile sites FRA3B and FRA16D where replication forks have stalled following aphidicolin treatment. RECQ1-depletion results in attenuated checkpoint activation in response to replication stress, increased sensitivity to aphidicolin and chromosomal instability.

CONCLUSIONS

Given a recent biochemical observation that RECQ1 catalyzes strand exchange on stalled replication fork structures in vitro, our results indicate that RECQ1 facilitates repair of stalled or collapsed replication forks and preserves genome integrity. Our findings provide the first evidence of a crucial role for RECQ1 at naturally occurring fork stalling sites and implicate RECQ1 in mechanisms underlying common fragile site instability in cancer.

Functional deficit associated with a missense Werner syndrome mutation

Werner syndrome (WS) is a rare autosomal recessive disorder caused by mutations in the WRN gene. WRN helicase, a member of the RecQ helicase family, is involved in various DNA metabolic pathways including DNA replication, recombination, DNA repair and telomere maintenance. In this study, we have characterized the G574R missense mutation, which was recently identified in a WS patient. Our biochemical experiments with purified mutant recombinant WRN protein showed that the G574R mutation inhibits ATP binding, and thereby leads to significant decrease in helicase activity. Exonuclease activity of the mutant protein was not significantly affected, whereas its single strand DNA annealing activity was higher than that of wild type. Deficiency in the helicase activity of the mutant may cause defects in replication and other DNA metabolic processes, which in turn could be responsible for the Werner syndrome phenotype in the patient. In contrast to the usual appearance of WS, the G574R patient has normal stature. Thus the short stature normally associated with WS may not be due to helicase deficiency.

An N-terminal acidic region of Sgs1 interacts with Rpa70 and recruits Rad53 kinase to stalled forks

DNA replication fork stalling poses a major threat to genome stability. This is counteracted in part by the intra-S phase checkpoint, which stabilizes arrested replication machinery, prevents cell-cycle progression and promotes DNA repair. The checkpoint kinase Mec1/ATR and RecQ helicase Sgs1/BLM contribute synergistically to fork maintenance on hydroxyurea (HU). Both enzymes interact with replication protein A (RPA). We identified and deleted the major interaction sites on Sgs1 for Rpa70, generating a mutant called sgs1-r1. In contrast to a helicase-dead mutant of Sgs1, sgs1-r1 did not significantly reduce recovery of DNA polymerase α at HU-arrested replication forks. However, the Sgs1 R1 domain is a target of Mec1 kinase, deletion of which compromises Rad53 activation on HU. Full activation of Rad53 is achieved through phosphorylation of the Sgs1 R1 domain by Mec1, which promotes Sgs1 binding to the FHA1 domain of Rad53 with high affinity. We propose that the recruitment of Rad53 by phosphorylated Sgs1 promotes the replication checkpoint response on HU. Loss of the R1 domain increases lethality selectively in cells lacking Mus81, Slx4, Slx5 or Slx8.

BLM and RMI1 alleviate RPA inhibition of TopoIIIα decatenase activity

RPA is a single-stranded DNA binding protein that physically associates with the BLM complex. RPA stimulates BLM helicase activity as well as the double Holliday junction dissolution activity of the BLM-topoisomerase IIIα complex. We investigated the effect of RPA on the ssDNA decatenase activity of topoisomerase IIIα. We found that RPA and other ssDNA binding proteins inhibit decatenation by topoisomerase IIIα. Complex formation between BLM, TopoIIIα, and RMI1 ablates inhibition of decatenation by ssDNA binding proteins. Together, these data indicate that inhibition by RPA does not involve species-specific interactions between RPA and BLM-TopoIIIα-RMI1, which contrasts with RPA modulation of double Holliday junction dissolution. We propose that topoisomerase IIIα and RPA compete to bind to single-stranded regions of catenanes. Interactions with BLM and RMI1 enhance toposiomerase IIIα activity, promoting decatenation in the presence of RPA.

HIF-1α mRNA levels in Eurasian perch (Perca fluviatilis) exposed to acute and chronic hypoxia

The big advantage of using molecular biomarkers to monitor oxygen levels in aquatic systems is that responses at the molecular level tend to be more sensitive, and usually occur earlier than those at higher levels of biological organization Aquatic hypoxia is a frequent event, which can occur naturally in a variety of marine, estuarine and freshwater habitats. More often, however, hypoxia arises as a result of euthrophication of aquatic ecosystem and can lead to changes in community structure by eliminating hypoxia-sensitive species. Consequently fish have develop various physiological and biochemical mechanisms to cope with this environmental stress. Many of these adjustments depend to changes in expression of a wide range of genes. The transcriptional responses to hypoxia are primarily mediated by hypoxia-inducible factor-1 (HIF-1), a heterodimer composed of an α and β subunit. This study investigated if HIF-1α mRNA levels were regulated by hypoxia in Eurasian perch (Perca fluviatilis), a hypoxia-sensitive fresh water species. The real-time PCR was utilized to monitor dynamic changes in levels of HIF-1α mRNA in response to acute (DO 0.4 ± 0.1 mg/l for 1 h) and chronic (DO 2.8 ± 0.3 mg/l for 15 days) hypoxia. Our results indicated an up-regulation of HIF-1α in brain and liver, but not in muscle tissue after acute hypoxic treatment, whereas significant changes of HIF-1α mRNA levels were detected in muscle, but not in brain and liver after chronic hypoxia exposure. This study suggests that HIF-1α mRNA level in selected perch tissues could be an useful indicator of acute exposure to hypoxia.

Cooperation of DNA-PKcs and WRN helicase in the maintenance of telomeric D-loops

Werner syndrome is an inherited human progeriod syndrome caused by mutations in the gene encoding the Werner Syndrome protein, WRN. It has both 3'-5' DNA helicase and exonuclease activities, and is suggested to have roles in many aspects of DNA metabolism, including DNA repair and telomere maintenance. The DNA-PK complex also functions in both DNA double strand break repair and telomere maintenance. Interaction between WRN and the DNA-PK complex has been reported in DNA double strand break repair, but their possible cooperation at telomeres has not been reported. This study analyzes thein vitro and in vivo interaction at the telomere between WRN and DNA-PKcs, the catalytic subunit of DNA-PK. The results show that DNA-PKcs selectively stimulates WRN helicase but not WRN exonuclease in vitro, affecting that WRN helicase unwinds and promotes the release of the full-length invading strand of a telomere D-loop model substrate. In addition, the length of telomeric G-tails decreases in DNA-PKcs knockdown cells, and this phenotype is reversed by overexpression of WRN helicase. These results suggest that WRN and DNA-PKcs may cooperatively prevent G-tail shortening in vivo.

LNAzyme targeting the HCV internal ribosome entry site inhibits HCV RNA expression in HepG2.9706 cells

AIM: To investigate the inhibitory effects of LNAzyme targeting the hepatitis C virus (HCV) internal ribosome entry site (IRES) on HCV RNA expression in HepG2.9706 cells.

METHODS: The sequences encoding DNAzyme, thiolmodificated DNAzyme and LNAzyme directing against the HBV IRES (located in the 5' non-coding region) were designed and synthesized. The following experimental groups were set up: Lipofectamine-DNAzyme group, Lipofectamine-S-DNAzyme group, Lipofectamine-LNAzyme group, blank control group, empty Lipofectamine group, DNAzyme group, and random DNAzyme group. On days 1, 3, 5 and 7 after transfection, the expression of HCV RNA and luciferase gene in HepG2.9706 cells was detected.

RESULTS: Significant down-regulation of HCV RNA and luciferase gene expression was noted in the Lipofectamine-DNAzyme, Lipofectamine-S-DNAzyme and Lipofectamine-LNAzyme groups when compared with other groups (all P < 0.05). The reduced rates of HCV RNA expression in the above three groups were 28.10%, 35.25% and 44.58%, respectively. The reduced rates of luciferase gene expression were 31.18%, 40.69% and 52.33%, respectively. LNAzyme did not exert cytotoxicity in HepG2.9706 cells.

CONCLUSION: LNAzyme has a significant inhibitory effect on HCV RNA expression in HepG2.9706 cells. The inhibitory effect of LNAzyme on HCV RNA expression is stronger than that of thiolmodificated DNAzyme.

An essential DNA strand-exchange activity is conserved in the divergent N-termini of BLM orthologs

The gene mutated in Bloom's syndrome, BLM, encodes a member of the RecQ family of DNA helicases that is needed to suppress genome instability and cancer predisposition. BLM is highly conserved and all BLM orthologs, including budding yeast Sgs1, have a large N-terminus that binds Top3-Rmi1 but has no known catalytic activity. In this study, we describe a sub-domain of the Sgs1 N-terminus that shows in vitro single-strand DNA (ssDNA) binding, ssDNA annealing and strand-exchange (SE) activities. These activities are conserved in the human and Drosophila orthologs. SE between duplex DNA and homologous ssDNA requires no cofactors and is inhibited by a single mismatched base pair. The SE domain of Sgs1 is required in vivo for the suppression of hyper-recombination, suppression of synthetic lethality and heteroduplex rejection. The top3Delta slow-growth phenotype is also SE dependent. Surprisingly, the highly divergent human SE domain functions in yeast. This work identifies SE as a new molecular function of BLM/Sgs1, and we propose that at least one role of SE is to mediate the strand-passage events catalysed by Top3-Rmi1.

A SRS2 homolog from Arabidopsis thaliana disrupts recombinogenic DNA intermediates and facilitates single strand annealing

Genetic and biochemical analyses of SRS2 homologs in fungi indicate a function in the processing of homologous recombination (HR) intermediates. To date, no SRS2 homologs have been described and analyzed in higher eukaryotes. Here, we report the first biochemical characterization of an SRS2 homolog from a multicellular eukaryote, the plant Arabidopsis thaliana. We studied the basic properties of AtSRS2 and were able to show that it is a functional 3'- to 5'-helicase. Furthermore, we characterized its biochemical function on recombinogenic intermediates and were able to show the unwinding of nicked Holliday junctions (HJs) and partial HJs (PX junctions). For the first time, we demonstrated strand annealing activity for an SRS2 homolog and characterized its strand pairing activity in detail. Our results indicate that AtSRS2 has properties that enable it to be involved in different steps during the processing of recombination intermediates. On the one hand, it could be involved in the unwinding of an elongating invading strand from a donor strand, while on the other hand, it could be involved in the annealing of the elongated strand at a later step.

Nucleic acid binding activity of human Cockayne syndrome B protein and identification of Ca(2+) as a novel metal cofactor

The Cockayne syndrome group B protein (CSB) is a member of the SWI/SNF2 subgroup of Superfamily 2 ATPases/nucleic acid translocases/helicases and is defective in the autosomal recessive segmental progeroid disorder Cockayne syndrome. This study examines the ATP-dependent and the ATP-independent biochemical functions of human CSB. We show that Ca(2+) is a novel metal cofactor of CSB for ATP hydrolysis, mainly through the enhancement of k(cat), and that a variety of biologically relevant model nucleic acid substrates can function to activate CSB ATPase activity with either Mg(2+) or Ca(2+) present. However, CSB lacked detectable ATP-dependent helicase and single- or double-stranded nucleic acid translocase activities in the presence of either divalent metal. CSB was found to support ATP-independent complementary strand annealing of DNA/DNA, DNA/RNA, and RNA/RNA duplexes, with Ca(2+) again promoting optimal activity. CSB formed a stable protein:DNA complex with a 34mer double-stranded DNA in electrophoretic mobility-shift assays, independent of divalent metal or nucleotide (e.g. ATP). Moreover, CSB was able to form a stable complex with a range of nucleic acid substrates, including bubble and "pseudo-triplex" double-stranded DNAs that resemble replication and transcription intermediates, as well as forked duplexes of DNA/DNA, DNA/RNA, and RNA/RNA composition, the latter two of which do not promote CSB ATPase activity. Association of CSB with DNA, independent of ATP binding or hydrolysis, was seemingly sufficient to displace or rearrange a stable pre-bound protein:DNA complex, a property potentially important for its roles in transcription and DNA repair.

Structure of the human RECQ1 helicase reveals a putative strand-separation pin

RecQ-like helicases, which include 5 members in the human genome, are important in maintaining genome integrity. We present a crystal structure of a truncated form of the human RECQ1 protein with Mg-ADP. The truncated protein is active in DNA fork unwinding but lacks other activities of the full-length enzyme: disruption of Holliday junctions and DNA strand annealing. The structure of human RECQ1 resembles that of Escherichia coli RecQ, with some important differences. All structural domains are conserved, including the 2 RecA-like domains and the RecQ-specific zinc-binding and winged-helix (WH) domains. However, the WH domain is positioned at a different orientation from that of the E. coli enzyme. We identify a prominent beta-hairpin of the WH domain as essential for DNA strand separation, which may be analogous to DNA strand-separation features of other DNA helicases. This hairpin is significantly shorter in the E. coli enzyme and is not required for its helicase activity, suggesting that there are significant differences between the modes of action of RecQ family members.

Model of human aging: recent findings on Werner's and Hutchinson-Gilford progeria syndromes

The molecular mechanisms involved in human aging are complicated. Two progeria syndromes, Werner's syndrome (WS) and Hutchinson-Gilford progeria syndrome (HGPS), characterized by clinical features mimicking physiological aging at an early age, provide insights into the mechanisms of natural aging. Based on recent findings on WS and HGPS, we suggest a model of human aging. Human aging can be triggered by two main mechanisms, telomere shortening and DNA damage. In telomere-dependent aging, telomere shortening and dysfunction may lead to DNA damage responses which induce cellular senescence. In DNA damage-initiated aging, DNA damage accumulates, along with DNA repair deficiencies, resulting in genomic instability and accelerated cellular senescence. In addition, aging due to both mechanisms (DNA damage and telomere shortening) is strongly dependent on p53 status. These two mechanisms can also act cooperatively to increase the overall level ofgenomic instability, triggering the onset of human aging phenotypes.

Biochemical analysis of human PIF1 helicase and functions of its N-terminal domain

The evolutionary conserved PIF1 DNA helicase family appears to have largely nonoverlapping cellular functions. To better understand the functions of human PIF1, we investigated biochemical properties of this protein. Analysis of single-stranded (ss) DNA-dependent ATPase activity revealed nonstructural ssDNA to greatly stimulate ATPase activity due to a high affinity for PIF1, even though PIF1 preferentially unwinds forked substrates. This suggests that PIF1 needs a ssDNA region for loading and a forked structure for translocation entrance into a double strand region. Deletion analysis demonstrated novel functions of a unique N-terminal portion, named the PIF1 N-terminal (PINT) domain. When the PINT domain was truncated, apparent affinity for ssDNA and unwinding activity were much reduced, even though the maximum velocity of ATPase activity and K(m) value for ATP were not affected. We suggest that the PINT domain contributes to enhancing the interaction with ssDNA through intrinsic binding activity. In addition, we found DNA strand-annealing activity, also residing in the PINT domain. Notably, the unwinding and annealing activities were inhibited by replication protein A. These results suggest that the functions of PIF1 might be restricted with particular situations and DNA structures.

Intrinsic ssDNA annealing activity in the C-terminal region of WRN

Werner syndrome (WS) is a rare autosomal recessive disorder in humans characterized by premature aging and genetic instability. WS is caused by mutations in the WRN gene, which encodes a member of the RecQ family of DNA helicases. Cellular and biochemical studies suggest that WRN plays roles in DNA replication, DNA repair, telomere maintenance, and homologous recombination and that WRN has multiple enzymatic activities including 3' to 5' exonuclease, 3' to 5' helicase, and ssDNA annealing. The goal of this study was to map and further characterize the ssDNA annealing activity of WRN. Enzymatic studies using truncated forms of WRN identified a C-terminal 79 amino acid region between the RQC and the HRDC domains (aa1072-1150) that is required for ssDNA annealing activity. Deletion of the region reduced or eliminated ssDNA annealing activity of the WRN protein. Furthermore, the activity appears to correlate with DNA binding and oligomerization status of the protein.

Catalysis of strand annealing by replication protein A derives from its strand melting properties

Eukaryotic DNA-binding protein replication protein A (RPA) has a strand melting property that assists polymerases and helicases in resolving DNA secondary structures. Curiously, previous results suggested that human RPA (hRPA) promotes undesirable recombination by facilitating annealing of flaps produced transiently during DNA replication; however, the mechanism was not understood. We designed a series of substrates, representing displaced DNA flaps generated during maturation of Okazaki fragments, to investigate the strand annealing properties of RPA. Until cleaved by FEN1 (flap endonuclease 1), such flaps can initiate homologous recombination. hRPA inhibited annealing of strands lacking secondary structure but promoted annealing of structured strands. Apparently, both processes primarily derive from the strand melting properties of hRPA. These properties slowed the spontaneous annealing of unstructured single strands, which occurred efficiently without hRPA. However, structured strands without hRPA displayed very slow spontaneous annealing because of stable intramolecular hydrogen bonding. hRPA appeared to transiently melt the single strands so that they could bind to form double strands. In this way, melting ironically promoted annealing. Time course measurements in the presence of hRPA suggest that structured single strands achieve an equilibrium with double strands, a consequence of RPA driving both annealing and melting. Promotion of annealing reached a maximum at a specific hRPA concentration, presumably when all structured single-stranded DNA was melted. Results suggest that displaced flaps with secondary structure formed during Okazaki fragment maturation can be melted by hRPA and subsequently annealed to a complementary ectopic DNA site, forming recombination intermediates that can lead to genomic instability.

Replication fork regression in vitro by the Werner syndrome protein (WRN): holliday junction formation, the effect of leading arm structure and a potential role for WRN exonuclease activity

The premature aging and cancer-prone disease Werner syndrome stems from loss of WRN protein function. WRN deficiency causes replication abnormalities, sensitivity to certain genotoxic agents, genomic instability and early replicative senescence in primary fibroblasts. As a RecQ helicase family member, WRN is a DNA-dependent ATPase and unwinding enzyme, but also possesses strand annealing and exonuclease activities. RecQ helicases are postulated to participate in pathways responding to replication blockage, pathways possibly initiated by fork regression. In this study, a series of model replication fork substrates were used to examine the fork regression capability of WRN. Our results demonstrate that WRN catalyzes fork regression and Holliday junction formation. This process is an ATP-dependent reaction that is particularly efficient on forks containing single-stranded gaps of at least 11-13 nt on the leading arm at the fork junction. Importantly, WRN exonuclease activity, by digesting the leading daughter strand, enhances regression of forks with smaller gaps on the leading arm, thus creating an optimal structure for regression. Our results suggest that the multiple activities of WRN cooperate to promote replication fork regression. These findings, along with the established cellular consequences of WRN deficiency, strongly support a role for WRN in regression of blocked replication forks.

DNA strand displacement, strand annealing and strand swapping by the Drosophila Bloom's syndrome helicase

Genetic analysis of the Drosophila Bloom's syndrome helicase homolog (mus309/DmBLM) indicates that DmBLM is required for the synthesis-dependent strand annealing (SDSA) pathway of homologous recombination. Here we report the first biochemical study of DmBLM. Recombinant, epitope-tagged DmBLM was expressed in Drosophila cell culture and highly purified protein was prepared from nuclear extracts. Purified DmBLM exists exclusively as a high molecular weight (∼1.17 MDa) species, is a DNA-dependent ATPase, has 3′→5′ DNA helicase activity, prefers forked substrate DNAs and anneals complementary DNAs. High-affinity DNA binding is ATP-dependent and low-affinity ATP-independent interactions contribute to forked substrate DNA binding and drive strand annealing. DmBLM combines DNA strand displacement with DNA strand annealing to catalyze the displacement of one DNA strand while annealing a second complementary DNA strand.

The role of WRN in DNA repair is affected by post-translational modifications

Werner syndrome (WS) is an autosomal recessive progeroid disease characterized by genomic instability. WRN gene encodes one of the RecQ helicase family proteins, WRN, which has ATPase, helicase, exonuclease and single stranded DNA annealing activities. There is accumulating evidence suggesting that WRN contributes to the maintenance of genomic integrity through its involvement in DNA repair, replication and recombination. The role of WRN in these pathways can be modulated by its post-translational modifications in response to DNA damage. Here, we review the functional consequences of post-translational modifications on WRN as well as specific DNA repair pathways where WRN is involved and discuss how these modifications affect DNA repair pathways.

Host cell DNA repair pathways in adeno-associated viral genome processing

Recentstudies have shown that wild-type and recombinant adeno-associated virus (AAV and rAAV) genomes persist in human tissue predominantly as double-stranded (ds) circular episomes derived from input linear single-stranded virion DNA. Using self-complementary recombinant AAV (scAAV) vectors, we generated intermediates that directly transition to ds circular episomes. The scAAV genome ends are palindromic hairpin-structured terminal repeats, resembling a double-stranded break repair intermediate. Utilizing this substrate, we found cellular DNA recombination and repair factors to be essential for generating circular episomal products. To identify the specific cellular proteins involved, the scAAV circularization-dependent vector was used as a reporter in 19 mammalian DNA repair-deficient cell lines. The results show that RecQ helicase family members (BLM and WRN), Mre11 and NBS1 of the Mre11-Rad50-Nbs1 (MRN) complex, and ATM are required for efficient scAAV genome circularization. We further demonstrated that the scAAV genome requires ATM and DNA-PK(CS), but not NBS1, to efficiently convert to a circular form in nondividing cells in vivo using transgenic mice. These studies identify specific pathways involved for further elucidating viral and cellular mechanisms of DNA maintenance important to the viral life cycle and vector utilizations.

The T4 phage UvsW protein contains both DNA unwinding and strand annealing activities

UvsW protein belongs to the SF2 helicase family and is one of three helicases found in T4 phage. UvsW governs the transition from origin-dependent to origin-independent replication through the dissociation of R-loops located at the T4 origins of replication. Additionally, in vivo evidence indicates that UvsW plays a role in recombination-dependent replication and/or DNA repair. Here, the biochemical properties of UvsW helicase are described. UvsW is a 3' to 5' helicase that unwinds a wide variety of substrates, including those resembling stalled replication forks and recombination intermediates. UvsW also contains a potent single-strand DNA annealing activity that is enhanced by ATP hydrolysis but does not require it. The annealing activity is inhibited by the non-hydrolysable ATP analog (adenosine 5'-O-(thiotriphosphate)), T4 single-stranded DNA-binding protein (gp32), or a small 8.8-kDa polypeptide (UvsW.1). Fluorescence resonance energy transfer experiments indicate that UvsW and UvsW.1 form a complex, suggesting that the UvsW helicase may exist as a heterodimer in vivo. Fusion of UvsW and UvsW.1 results in a 68-kDa protein having nearly identical properties as the UvsW-UvsW.1 complex, indicating that the binding locus of UvsW.1 is close to the C terminus of UvsW. The biochemical properties of UvsW are similar to the RecQ protein family and suggest that the annealing activity of these helicases may also be modulated by protein-protein interactions. The dual activities of UvsW are well suited for the DNA repair pathways described for leading strand lesion bypass and synthesis-dependent strand annealing.

Single strand annealing and ATP-independent strand exchange activities of yeast and human DNA2: possible role in Okazaki fragment maturation

The Dna2 protein is a multifunctional enzyme with 5'-3' DNA helicase, DNA-dependent ATPase, 3' exo/endonuclease, and 5' exo/endonuclease. The enzyme is highly specific for structures containing single-stranded flaps adjacent to duplex regions. We report here two novel activities of both the yeast and human Dna2 helicase/nuclease protein: single strand annealing and ATP-independent strand exchange on short duplexes. These activities are independent of ATPase/helicase and nuclease activities in that mutations eliminating either nuclease or ATPase/helicase do not inhibit strand annealing or strand exchange. ATP inhibits strand exchange. A model rationalizing the multiple catalytic functions of Dna2 and leading to its coordination with other enzymes in processing single-stranded flaps during DNA replication and repair is presented.

Mechanisms of RecQ helicases in pathways of DNA metabolism and maintenance of genomic stability

Helicases are molecular motor proteins that couple the hydrolysis of NTP to nucleic acid unwinding. The growing number of DNA helicases implicated in human disease suggests that their vital specialized roles in cellular pathways are important for the maintenance of genome stability. In particular, mutations in genes of the RecQ family of DNA helicases result in chromosomal instability diseases of premature aging and/or cancer predisposition. We will discuss the mechanisms of RecQ helicases in pathways of DNA metabolism. A review of RecQ helicases from bacteria to human reveals their importance in genomic stability by their participation with other proteins to resolve DNA replication and recombination intermediates. In the light of their known catalytic activities and protein interactions, proposed models for RecQ function will be summarized with an emphasis on how this distinct class of enzymes functions in chromosomal stability maintenance and prevention of human disease and cancer.

Mechanisms by which Bloom protein can disrupt recombination intermediates of Okazaki fragment maturation

Bloom syndrome is a familial genetic disorder associated with sunlight sensitivity and a high predisposition to cancers. The mutated gene, Bloom protein (BLM), encodes a DNA helicase that functions in genome maintenance via roles in recombination repair and resolution of recombination structures. We designed substrates representing illegitimate recombination intermediates formed when a displaced DNA flap generated during maturation of Okazaki fragments escapes cleavage by flap endonuclease-1 and anneals to a complementary ectopic DNA site. Results show that displaced, replication protein A (RPA)-coated flaps could readily bind and ligate at the complementary site to initiate recombination. RPA also displayed a strand-annealing activity that hastens the rate of recombination intermediate formation. BLM helicase activity could directly disrupt annealing at the ectopic site and promote flap endonuclease-1 cleavage. Additionally, BLM has its own strand-annealing and strand-exchange activities. RPA inhibited the BLM strand-annealing activity, thereby promoting helicase activity and complex dissolution. BLM strand exchange could readily dissociate invading flaps, e.g. in a D-loop, if the exchange step did not involve annealing of RPA-coated strands. Use of ATP to activate the helicase function did not aid flap displacement by exchange, suggesting that this is a helicase-independent mechanism of complex dissociation. When RPA could bind, it displayed its own strand-exchange activity. We interpret these results to explain how BLM is well equipped to deal with alternative recombination intermediate structures.

Sit down, relax and unwind: structural insights into RecQ helicase mechanisms

Helicases are specialized molecular motors that separate duplex nucleic acids into single strands. The RecQ family of helicases functions at the interface of DNA replication, recombination and repair in bacterial and eukaryotic cells. They are key, multifunctional enzymes that have been linked to three human diseases: Bloom's, Werner's and Rothmund-Thomson's syndromes. This review summarizes recent studies that relate the structures of RecQ proteins to their biochemical activities.