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

BRCA1/BRC-1 and SMC-5/6 regulate DNA repair pathway engagement during Caenorhabditis elegans meiosis

The preservation of genome integrity during sperm and egg development is vital for reproductive success. During meiosis, the tumor suppressor BRCA1/BRC-1 and structural maintenance of chromosomes 5/6 (SMC-5/6) complex genetically interact to promote high fidelity DNA double strand break (DSB) repair, but the specific DSB repair outcomes these proteins regulate remain unknown. Using genetic and cytological methods to monitor resolution of DSBs with different repair partners in Caenorhabditis elegans, we demonstrate that both BRC-1 and SMC-5 repress intersister crossover recombination events. Sequencing analysis of conversion tracts from homolog-independent DSB repair events further indicates that BRC-1 regulates intersister/intrachromatid noncrossover conversion tract length. Moreover, we find that BRC-1 specifically inhibits error prone repair of DSBs induced at mid-pachytene. Finally, we reveal functional interactions of BRC-1 and SMC-5/6 in regulating repair pathway engagement: BRC-1 is required for localization of recombinase proteins to DSBs in smc-5 mutants and enhances DSB repair defects in smc-5 mutants by repressing theta-mediated end joining (TMEJ). These results are consistent with a model in which some functions of BRC-1 act upstream of SMC-5/6 to promote recombination and inhibit error-prone DSB repair, while SMC-5/6 acts downstream of BRC-1 to regulate the formation or resolution of recombination intermediates. Taken together, our study illuminates the coordinated interplay of BRC-1 and SMC-5/6 to regulate DSB repair outcomes in the germline.

Division of Labor by the HELQ, BLM, and FANCM Helicases during Homologous Recombination Repair in Drosophila melanogaster

Repair of DNA double-strand breaks by homologous recombination (HR) requires a carefully orchestrated sequence of events involving many proteins. One type of HR, synthesis-dependent strand annealing (SDSA), proceeds via the formation of a displacement loop (D-loop) when RAD51-coated single-stranded DNA invades a homologous template. The 3' end of the single-stranded DNA is extended by DNA synthesis. In SDSA, the D-loop is then disassembled prior to strand annealing. While many helicases can unwind D-loops in vitro, how their action is choreographed in vivo remains to be determined. To clarify the roles of various DNA helicases during SDSA, we used a double-strand gap repair assay to study the outcomes of homologous recombination repair in Drosophila melanogaster lacking the BLM, HELQ, and FANCM helicases. We found that the absence of any of these three helicases impairs gap repair. In addition, flies lacking both BLM and HELQ or HELQ and FANCM had more severe SDSA defects than the corresponding single mutants. In the absence of BLM, a large percentage of repair events were accompanied by flanking deletions. Strikingly, these deletions were mostly abolished in the blm helq and blm fancm double mutants. Our results suggest that the BLM, HELQ, and FANCM helicases play distinct roles during SDSA, with HELQ and FANCM acting early to promote the formation of recombination intermediates that are then processed by BLM to prevent repair by deletion-prone mechanisms.

CopyCatchers are versatile active genetic elements that detect and quantify inter-homolog somatic gene conversion

CRISPR-based active genetic elements, or gene-drives, copied via homology-directed repair (HDR) in the germline, are transmitted to progeny at super-Mendelian frequencies. Active genetic elements also can generate widespread somatic mutations, but the genetic basis for such phenotypes remains uncertain. It is generally assumed that such somatic mutations are generated by non-homologous end-joining (NHEJ), the predominant double stranded break repair pathway active in somatic cells. Here, we develop CopyCatcher systems in Drosophila to detect and quantify somatic gene conversion (SGC) events. CopyCatchers inserted into two independent genetic loci reveal unexpectedly high rates of SGC in the Drosophila eye and thoracic epidermis. Focused RNAi-based genetic screens identify several unanticipated loci altering SGC efficiency, one of which (c-MYC), when downregulated, promotes SGC mediated by both plasmid and homologous chromosome-templates in human HEK293T cells. Collectively, these studies suggest that CopyCatchers can serve as effective discovery platforms to inform potential gene therapy strategies.

The role of DNA repair genes in radiation-induced adaptive response in Drosophila melanogaster is differential and conditional

Studies in human and mammalian cell cultures have shown that induction of DNA repair mechanisms is required for the formation of stimulation effects of low doses of ionizing radiation, named "hormesis". Nevertheless, the role of cellular defense mechanisms in the formation of radiation-induced hormesis at the level of whole organism remains poorly studied. The aim of this work was to investigate the role of genes involved in different mechanisms and stages of DNA repair in radioadaptive response and radiation hormesis by lifespan parameters in Drosophila melanogaster. We studied genes that control DNA damage sensing (D-Gadd45, Hus1, mnk), nucleotide excision repair (mei-9, mus210, Mus209), base excision repair (Rrp1), DNA double-stranded break repair by homologous recombination (Brca2, spn-B, okr) and non-homologous end joining (Ku80, WRNexo), and the Mus309 gene that participates in several mechanisms of DNA repair. The obtained results demonstrate that in flies with mutations in studied genes radioadaptive response and radiation hormesis are absent or appear to a lesser extent than in wild-type Canton-S flies. Chronic exposure of γ-radiation in a low dose during pre-imaginal stages of development leads to an increase in expression of the studied DNA repair genes, which is maintained throughout the lifespan of flies. However, the activation of conditional ubiquitous overexpression of DNA repair genes does not induce resistance to an acute exposure to γ-radiation and reinforces its negative impact.

Purification and enzymatic characterization of Gallus gallus BLM helicase

Mutations in human BLM helicase give rise to the autosomal recessive Bloom syndrome, which shows high predisposition to types of malignant tumours. Though lots of biochemical and structural investigations have shed lights on the helicase core, structural investigations of the whole BLM protein are still limited due to its low stability and production. Here by comparing with the expression systems and functions of other BLM homologues, we developed the heterologous high-level expression and high-yield purification systems for Gallus gallus BLM (gBLM) in Escherichia coli. Subsequent DNA binding and unwinding determinations demonstrated that gBLM was a vigorous atypical DNA structure specific helicase, which not only showed high preference for the 3'-tailed DNA structures but also could efficiently unwind bubble DNA structures with blunt-ends, indicating its biological roles in processing DNA metabolism intermediates. Further comparative analysis between gBLM and gBLM Core revealed that the long N-terminal domain facilitated the binding affinity of forked and bubble DNA structures and it was also required for the DNA unwinding activities of gBLM. Thus, we present the first enzymatic characterization of gBLM and its N-terminal domain, providing a new model for probing the mechanism and structure of human BLM.

The Role of Blm Helicase in Homologous Recombination, Gene Conversion Tract Length, and Recombination Between Diverged Sequences in Drosophilamelanogaster

DNA double-strand breaks (DSBs) are a particularly deleterious class of DNA damage that threatens genome integrity. DSBs are repaired by three pathways: nonhomologous-end joining (NHEJ), homologous recombination (HR), and single-strand annealing (SSA). Drosophila melanogaster Blm (DmBlm) is the ortholog of Saccharomyces cerevisiae SGS1 and human BLM, and has been shown to suppress crossovers in mitotic cells and repair mitotic DNA gaps via HR. To further elucidate the role of DmBlm in repair of a simple DSB, and in particular recombination mechanisms, we utilized the Direct Repeat of white (DR-white) and Direct Repeat of whitewith mutations (DR-white.mu) repair assays in multiple mutant allele backgrounds. DmBlm null and helicase-dead mutants both demonstrated a decrease in repair by noncrossover HR, and a concurrent increase in non-HR events, possibly including SSA, crossovers, deletions, and NHEJ, although detectable processing of the ends was not significantly impacted. Interestingly, gene conversion tract lengths of HR repair events were substantially shorter in DmBlm null but not helicase-dead mutants, compared to heterozygote controls. Using DR-white.mu, we found that, in contrast to Sgs1, DmBlm is not required for suppression of recombination between diverged sequences. Taken together, our data suggest that DmBlm helicase function plays a role in HR, and the steps that contribute to determining gene conversion tract length are helicase-independent.

RECQ helicases are deregulated in hematological malignancies in association with a prognostic value

BACKGROUND

RECQ helicase family members act as guardians of the genome to assure proper DNA metabolism in response to genotoxic stress. Hematological malignancies are characterized by genomic instability that is possibly related to underlying defects in DNA repair of genomic stability maintenance.

METHODS

We have investigated the expression of RECQ helicases in different hematological malignancies and in their normal counterparts using publicly available gene expression data. Furthermore, we explored whether RECQ helicases expression could be associated with tumor progression and prognosis.

RESULTS

Expression of at least one RECQ helicase family member was found significantly deregulated in all hematological malignancies investigated when compared to their normal counterparts. In addition, RECQ helicase expression was associated with a prognostic value in acute myeloid leukemia, chronic lymphocytic leukemia, lymphoma and multiple myeloma.

CONCLUSION

RECQ helicase expression is deregulated in hematological malignancies compared to their normal counterparts in association with a prognostic value. Deregulation of RECQ helicases appears to play a role in tumorigenesis and represent potent therapeutic targets for synthetic lethal approaches in hematological malignancies.

Characterization of the Caenorhabditis elegans HIM-6/BLM helicase: unwinding recombination intermediates

Mutations in three human RecQ genes are implicated in heritable human syndromes. Mutations in BLM, a RecQ gene, cause Bloom syndrome (BS), which is characterized by short stature, cancer predisposition, and sensitivity to sunlight. BLM is a RecQ DNA helicase that, with interacting proteins, is able to dissolve various DNA structures including double Holliday junctions. A BLM ortholog, him-6, has been identified in Caenorhabditis elegans, but little is known about its enzymatic activities or its in vivo roles. By purifying recombinant HIM-6 and performing biochemical assays, we determined that the HIM-6 has DNA-dependent ATPase activity HIM-6 and helicase activity that proceeds in the 3'-5' direction and needs at least five 3' overhanging nucleotides. HIM-6 is also able to unwind DNA structures including D-loops and Holliday junctions. Worms with him-6 mutations were defective in recovering the cell cycle arrest after HU treatment. These activities strongly support in vivo roles for HIM-6 in processing recombination intermediates.

Essential functions of C terminus of Drosophila Topoisomerase IIIα in double holliday junction dissolution

Topoisomerase IIIα (Top3α) is an essential component of the double Holliday junction (dHJ) dissolvasome complex in metazoans, along with Blm and Rmi1/2. This important anti-recombinogenic function cannot be performed by Top3β, the other type IA topoisomerase present in metazoans. The two share a catalytic core but diverge in their tail regions. To understand this difference in function, we investigated the role of the unique C terminus of Top3α. The Drosophila C terminus contains an insert region not conserved among metazoans. This insert contributes an independent interaction with Blm, which may account for the absence of Rmi1 in Drosophila. Mutant Top3α lacking this insert maintains the ability to perform dHJ dissolution but only partially rescues a top3α null fly line, indicating an in vivo role for the insert. Truncation of the C terminus has a minimal effect on the type IA relaxation activity of Top3α; however, dHJ dissolution is greatly reduced. The Top3α C terminus was found to strongly interact with both Blm and DNA, which are critical to the dissolution reaction; these interactions are greatly reduced in the truncated enzyme. The truncation mutant also cannot rescue the viability of top3α null flies, indicating an essential in vivo role. Our data therefore suggest that the Top3α C terminus has an important role in dHJ dissolution (by providing an interaction interface for Blm and DNA) and an essential function in vivo.

Unwinding and rewinding: double faces of helicase?

Helicases are enzymes that use ATP-driven motor force to unwind double-stranded DNA or RNA. Recently, increasing evidence demonstrates that some helicases also possess rewinding activity—in other words, they can anneal two complementary single-stranded nucleic acids. All five members of the human RecQ helicase family, helicase PIF1, mitochondrial helicase TWINKLE, and helicase/nuclease Dna2 have been shown to possess strand-annealing activity. Moreover, two recently identified helicases—HARP and AH2 have only ATP-dependent rewinding activity. These findings not only enhance our understanding of helicase enzymes but also establish the presence of a new type of protein: annealing helicases. This paper discusses what is known about these helicases, focusing on their biochemical activity to zip and unzip double-stranded DNA and/or RNA, their possible regulation mechanisms, and biological functions.

Enzymatic activities and DNA substrate specificity of Mycobacterium tuberculosis DNA helicase XPB

XPB, also known as ERCC3 and RAD25, is a 3′→5′ DNA repair helicase belonging to the superfamily 2 of helicases. XPB is an essential core subunit of the eukaryotic basal transcription factor complex TFIIH. It has two well-established functions: in the context of damaged DNA, XPB facilitates nucleotide excision repair by unwinding double stranded DNA (dsDNA) surrounding a DNA lesion; while in the context of actively transcribing genes, XPB facilitates initiation of RNA polymerase II transcription at gene promoters. Human and other eukaryotic XPB homologs are relatively well characterized compared to conserved homologs found in mycobacteria and archaea. However, more insight into the function of bacterial helicases is central to understanding the mechanism of DNA metabolism and pathogenesis in general. Here, we characterized Mycobacterium tuberculosis XPB (Mtb XPB), a 3′→5′ DNA helicase with DNA-dependent ATPase activity. Mtb XPB efficiently catalyzed DNA unwinding in the presence of significant excess of enzyme. The unwinding activity was fueled by ATP or dATP in the presence of Mg²⁺/Mn²⁺. Consistent with the 3′→5′ polarity of this bacterial XPB helicase, the enzyme required a DNA substrate with a 3′ overhang of 15 nucleotides or more. Although Mtb XPB efficiently unwound DNA model substrates with a 3′ DNA tail, it was not active on substrates containing a 3′ RNA tail. We also found that Mtb XPB efficiently catalyzed ATP-independent annealing of complementary DNA strands. These observations significantly enhance our understanding of the biological roles of Mtb XPB.

Rmi1 stimulates decatenation of double Holliday junctions during dissolution by Sgs1-Top3

A double Holliday junction (dHJ) is a central intermediate of homologous recombination which can be processed to yield crossover or non-crossover recombination products. To preserve genomic integrity, cells possess mechanisms to avoid crossing-over. Here we show that Saccharomyces cerevisiae Sgs1 and Top3 proteins are sufficient to migrate and disentangle a dHJ to produce exclusively non-crossover recombination products, in a reaction termed “dissolution”. Furthermore, we show that Rmi1 stimulates dHJ dissolution at low Sgs1–Top3 protein concentrations, although it has no effect on the initial rate of Holliday junction (HJ) migration. Rmi1 serves to stimulate DNA decatenation, thereby removing the last linkages between the repaired and template DNA molecules. Dissolution of a dHJ is a highly efficient and concerted alternative to nucleolytic resolution that prevents crossing over of chromosomes during recombinational DNA repair in mitotic cells, and thereby contributes to genomic integrity.

Evidence based on studies of the mus309 mutant, deficient in DNA double-strand break repair, that meiotic crossing over in Drosophila melanogaster is a two-phase process

The mus309 gene in Drosophila melanogaster encodes a RecQ helicase which is involved in DNA double-strand break (DSB) repair and specifically in the choice between the different pathways of the repair. In a brood pattern analysis of mus309 and wild type females which either had or had not experienced a temperature shock, different parameters of meiotic crossing over including map distances and crossover interference in the X chromosome were measured. The results suggest that, like in other eukaryotes studied, the control of meiotic crossover formation also in D. melanogaster is a two-phase process. The first phase seems to be temperature shock sensitive, independent of the mus309 gene and coincidental with the premeiotic DNA synthesis, thus most likely representing the formation of DSBs. The second phase seems to be temperature shock tolerant, dependent on the mus309 gene, occurring during the meiotic prophase and most likely representing the choice made by the oocyte between the different pathways of the DSB repair. A hypothesis of the localization of chiasmata is also presented, combining the mechanisms of interference and the so-called centromere effect, and based on the balance between the SDSA and DSBR pathways of DSB repair.

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.

Promotion and regulation of homologous recombination by DNA helicases

In eukaryotes, homologous recombination (HR) provides an important means to eliminate DNA double-stranded breaks and other chromosomal lesions. Accordingly, failure in HR leads to genomic instability and a predisposition to various cancer types. While HR is clearly beneficial for genome maintenance, inappropriate or untimely events can be harmful. For this reason, HR must be tightly regulated. Several DNA helicases contribute to HR regulation, by way of mechanisms that are conserved from yeast to humans. Mutations in several HR-specific helicases e.g. BLM and RECQ5, are either associated with cancer-prone human syndromes or engender the cancer phenotype in animal models. Therefore, delineating the role of DNA helicases in HR regulation has direct relevance to cancer etiology. Genetic, cytological, biochemical, and other analyses have shown that DNA helicases participate in early or late stages of HR, to disrupt nucleoprotein filaments that harbor the Rad51 recombinase or dissociate the D-loop intermediate made by Rad51, or to prevent undesirable events and/or minimize potentially deleterious crossover products. Moreover, the ensemble that harbors BLM and topoisomerase IIIalpha can dissolve the double-Holliday junction, a complex DNA intermediate generated during HR, to produce non-crossover products. These regulatory pathways function in parallel to promote the usage of the genome-preserving synthesis-dependent strand annealing HR pathway or otherwise suppress crossover formation.

Distinct roles of RECQ1 in the maintenance of genomic stability

Five human RecQ helicases (WRN, BLM, RECQ4, RECQ5, RECQ1) exist in humans. Of these, three are genetically linked to diseases of premature aging and/or cancer. Neither RECQ1 nor RECQ5 has yet been implicated in a human disease. However, cellular studies and genetic analyses of model organisms indicate that RECQ1 (and RECQ5) play an important role in the maintenance of genomic stability. Biochemical studies of purified RECQ1 protein demonstrate that the enzyme catalyzes DNA unwinding and strand annealing, and these activities are likely to be important for its role in DNA repair. RECQ1 also physically and functionally interacts with proteins involved in genetic recombination. In this review, we will summarize our current knowledge of RECQ1 roles in cellular nucleic acid metabolism and propose avenues of investigation for future studies.

The effect of the mus309 mutation, defective in DNA double-strand break repair, on crossing over in Drosophila melanogaster suggests a mechanism for the centromere effect of crossing over

The mus309 gene in Drosophila melanogaster encodes a RecQ helicase which is involved in DNA double-strand break (DSB) repair. In a brood pattern analysis, it was observed that in mus309 mutant females, the frequency of single crossovers in the central cv-v interval of the X chromosome was reduced in young females but returned to the level of the wild type control as the females aged. In the proximal v-f interval, the frequency of single crossovers was increased during the entire experimental period. In particular, it was observed that the frequency of double crossovers, as well as the coefficient of coincidence first increased but then gradually decreased, finally reaching the level of the control flies, as the females aged. Map distances increased due to the mus309 mutation in both gene interval studies, but they did not change as the females aged, a result suggesting that the mus309 gene controls the distribution of DSBs to be repaired as crossovers instead of non-crossovers. The results suggest a mechanism for the centromere effect of crossing over in Drosophila, viz the fact the frequency of meiotic crossing over reduces with the age of the female, and that the reduction is more pronounced the closer the interval is to the proximal heterochromatin of the chromosome arm. According to the model suggested, the centromere effect is simply a matter of the balance between different pathways of the repair of the DSBs of DNA.

The effect of the mus309 mutation, defective in DNA double-strand break repair, on crossing over in Drosophila melanogaster suggests a mechanism for interference

The mus309 gene in Drosophila melanogaster encodes a RecQ helicase which is involved in DNA double-strand break (DSB) repair. In a brood pattern analysis, it was observed that in mus309 mutant females, the frequency of single crossovers in the central cv - v interval of the X chromosome was decreased in young females but returned to the level of the wild type control as the females aged. In the proximal v - f interval, the frequency of single crossovers was increased during the whole experimental period. In particular, it was observed that the frequency of double crossovers, as well as the coefficient of coincidence first increased but then gradually decreased, finally reaching the level of the control flies, as the females aged. Map distances increased due to the mus309 mutation in both gene interval studies, but they did not change as the females aged, a result suggesting that the mus309 gene controls the distribution of DSBs to be repaired as crossovers instead of non-crossovers. The results are consistent with the hypothesis that in general the DSBs are initially independently distributed on the chromosome but non-randomly repaired so that the distribution of crossovers in the mutant flies becomes uniform, but uneven in wild-type flies. The results are consistent with the counting number model of crossover interference, based on genetic distance. On the other hand, the data are not consistent with the reaction-diffusion model based on physical distance. Consequently, the view that crossover interference in Drosophila is tightly tied to genetic distance is supported.

Removal of the bloom syndrome DNA helicase extends the utility of imprecise transposon excision for making null mutations in Drosophila

Transposable elements are frequently used in Drosophila melanogaster for imprecise excision screens to delete genes of interest. However, these screens are highly variable in the number and size of deletions that are recovered. Here, we show that conducting excision screens in mus309 mutant flies that lack DmBlm, the Drosophila ortholog of the Bloom syndrome protein, increases the percentage and overall size of flanking deletions recovered after excision of either P or Minos elements.

Bloom DNA helicase facilitates homologous recombination between diverged homologous sequences

Bloom syndrome caused by inactivation of the Bloom DNA helicase (Blm) is characterized by increases in the level of sister chromatid exchange, homologous recombination (HR) associated with cross-over. It is therefore believed that Blm works as an anti-recombinase. Meanwhile, in Drosophila, DmBlm is required specifically to promote the synthesis-dependent strand anneal (SDSA), a type of HR not associating with cross-over. However, conservation of Blm function in SDSA through higher eukaryotes has been a matter of debate. Here, we demonstrate the function of Blm in SDSA type HR in chicken DT40 B lymphocyte line, where Ig gene conversion diversifies the immunoglobulin V gene through intragenic HR between diverged homologous segments. This reaction is initiated by the activation-induced cytidine deaminase enzyme-mediated uracil formation at the V gene, which in turn converts into abasic site, presumably leading to a single strand gap. Ig gene conversion frequency was drastically reduced in BLM(-/-) cells. In addition, BLM(-/-) cells used limited donor segments harboring higher identity compared with other segments in Ig gene conversion event, suggesting that Blm can promote HR between diverged sequences. To further understand the role of Blm in HR between diverged homologous sequences, we measured the frequency of gene targeting induced by an I-SceI-endonuclease-mediated double-strand break. BLM(-/-) cells showed a severer defect in the gene targeting frequency as the number of heterologous sequences increased at the double-strand break site. Conversely, the overexpression of Blm, even an ATPase-defective mutant, strongly stimulated gene targeting. In summary, Blm promotes HR between diverged sequences through a novel ATPase-independent mechanism.

Initiation of DNA replication by a third parallel DNA strand bound in a triple-helix manner leads to strand invasion

According to current knowledge, DNA polymerases accommodate only two polynucleotide strands in their catalytic site: the template and the primer to be elongated. Here we show that in addition to these two polynucleotide strands, HIV-1 and AMV reverse transcriptases, human DNA polymerases beta, gamma, and lambda, and the archaebacterial Dpo4 can elongate 10-nucleotide primers bound in a triple-helix manner to hairpin duplex DNA tethered by a few thymidine residues. The elongation occurs when the primer is parallel to the homologous strand. This feature was confirmed by using complementary single-stranded DNA with restricted nucleotide composition which bound polypurine and polypyrimidine primers at an asymmetric site. The results unambiguously confirmed the previous experiments, showing binding of the primer strand parallel to the homologous sequence. The common feature of these DNA polymerases is that they all elongated dG-rich primers, whereas they behaved differently when other polynucleotide sequences were used. Interestingly, only five to seven dG residues at similar positions between the primer and its binding site can allow elongation, which may even be facilitated by a single C/C mismatch. We suggest that DNA polymerases displace the primer form Hoogsteen bonds to from Watson-Crick pairings, enabling subsequent priming of replication. These experiments indicate that DNA polymerases may bind three DNA strands, as RNA polymerases do, and provide a molecular basis for 3'-OH invasion at short similar sequences in the DNA double helix, yielding potential DNA rearrangements upon single-strand breakage.

Two closely related RecQ helicases have antagonistic roles in homologous recombination and DNA repair in Arabidopsis thaliana

RecQ helicases are involved in the processing of DNA structures arising during replication, recombination, and repair throughout all kingdoms of life. Mutations of different RecQ homologues are responsible for severe human diseases, such as Blooms (BLM) or Werner (WRN) syndrome. The loss of RecQ function is often accompanied by hyperrecombination caused by a lack of crossover suppression. In the Arabidopsis genome seven different RecQ genes are present. Two of them (AtRECQ4A and 4B) arose because of a recent duplication and are still nearly 70% identical on a protein level. Knockout of these genes leads to antagonistic phenotypes: the RECQ4A mutant shows sensitivity to DNA-damaging agents, enhanced homologous recombination (HR) and lethality in a mus81 background. Moreover, mutation of RECQ4A partially suppresses the lethal phenotype of an AtTOP3alpha mutant, a phenomenon that had previously been demonstrated for RecQ homologues of unicellular eukaryotes only. Together, these facts strongly suggest that in plants RECQ4A is functionally equivalent to SGS1 of Saccharomyces cerevisiae and the mammalian BLM protein. In stark contrast, mutants of the closely related RECQ4B are not mutagen-sensitive, not viable in a mus81 background, and unable to suppress the induced lethality caused by loss of TOP3alpha. Moreover, they are strongly impaired in HR. Thus, AtRECQ4B is specifically required to promote but not to suppress crossovers, a role in which it differs from all eukaryotic RecQ homologues known.

Gene conversion: mechanisms, evolution and human disease

Gene conversion, one of the two mechanisms of homologous recombination, involves the unidirectional transfer of genetic material from a 'donor' sequence to a highly homologous 'acceptor'. Considerable progress has been made in understanding the molecular mechanisms that underlie gene conversion, its formative role in human genome evolution and its implications for human inherited disease. Here we assess current thinking about how gene conversion occurs, explore the key part it has played in fashioning extant human genes, and carry out a meta-analysis of gene-conversion events that are known to have caused human genetic disease.

Multiple functions of Drosophila BLM helicase in maintenance of genome stability

Bloom Syndrome, a rare human disorder characterized by genomic instability and predisposition to cancer, is caused by mutation of BLM, which encodes a RecQ-family DNA helicase. The Drosophila melanogaster ortholog of BLM, DmBlm, is encoded by mus309. Mutations in mus309 cause hypersensitivity to DNA-damaging agents, female sterility, and defects in repairing double-strand breaks (DSBs). To better understand these phenotypes, we isolated novel mus309 alleles. Mutations that delete the N terminus of DmBlm, but not the helicase domain, have DSB repair defects as severe as those caused by null mutations. We found that female sterility is due to a requirement for DmBlm in early embryonic cell cycles; embryos lacking maternally derived DmBlm have anaphase bridges and other mitotic defects. These defects were less severe for the N-terminal deletion alleles, so we used one of these mutations to assay meiotic recombination. Crossovers were decreased to about half the normal rate, and the remaining crossovers were evenly distributed along the chromosome. We also found that spontaneous mitotic crossovers are increased by several orders of magnitude in mus309 mutants. These results demonstrate that DmBlm functions in multiple cellular contexts to promote genome stability.