recQ Gene Research Articles

Insights into the presence of multiple RecQ helicases in the ancient cyanobacterium, Nostoc sp. strain PCC7120: bioinformatics and expression analysis approach.

Owing to their crucial role in genome maintenance, RecQ helicases are ubiquitous and present across organisms. Though the multiplicity of RecQ helicases is well known in higher organisms, it is rare among bacteria. The ancient cyanobacterium Nostoc sp. strain PCC7120 was found to have three annotated RecQ helicases. This study aims at understanding its structural differences and evolution through bioinformatics approach and functionality through expression analysis studies. Nostoc RecQ helicases were found to be transcriptionally regulated by LexA and DNA damage inducing stresses. Bioinformatic analysis revealed that all three RecQ helicases of Nostoc possess helicases_C and Zn+2-binding domains. Two of the helicases (AnRecQ and AnRecQ2) lacked the complete RQC and HRDC domains, and AnRecQ2 had an additional Phosphoribosyl transferase domain (Pribosyltran), also seen in RecQ-like helicase (RqlH) protein of Mycobacterium smegmatis. AnRecQ1, which was similar to most bacterial RecQ helicases, differed in having a long C-terminal tail. STRING analysis revealed that the proteins also differed in their predicted protein interactome. Phylogenetic analysis suggested that the multiple recQ genes may have been acquired through duplication and acquisition of additional domains from the smallest of the RecQ helicases (AnRecQ) to cater multiple functions required to deal with the harsh environmental conditions. In course of evolution, however, the multiplicity was lost with the modern-day bacteria and lower eukaryotes which retained fewer RecQ helicases, while further duplication of the acquired RECQ occurred in higher animals and plants to deal with cellular complexity.

Features of DNA Helicase Encoded by the uvrD Gene of Deinococcus radiodurans R1 in Escherichia coli K-12 Cells

Reparative helicase II of Deinococcus radiodurans performs an unexpected critical function in repair of double-strand DNA breaks through the mechanism of extended synthesis-dependent strand annealing (ESDSA), while it is considered as an optional participant in the RecF pathway of recombinational repair in Escherichia coli. A fragment of genomic DNA of the radioresistant bacterium Deinococcus radiodurans with the uvrD gene encoding DNA helicase II, which is involved in excision repair of nucleotides, mismatch repair, and recombinational repair and replication, was cloned in the cells of the model object, Escherichia coli K-12. The pCR 2.1-uvrD+ plasmid restores resistance to ultraviolet light of mutant cells of Escherichia coliuvrD–, helD–, and rep– defective in reparative helicase II, helicase IV, and replicative helicase Rep, respectively, almost to the level of wild-type AB1157 and uvrD+, and, to a lesser extent, the strain with a mutation in the recQ gene encoding the key helicase of recombinational repair RecQ. The protective effect is also noticeable when strains with the plasmid are irradiated with γ-rays. It is established that Deinococcus radiodurans UvrD helicase possesses broad possibilities.

Genetic marker gene, recQ, differentiating Bacillus subtilis and the closely related Bacillus species.

RecQ, which encodes a DNA helicase, was selected in searching for a marker gene of Bacillus subtilis and related species via genome mining. RecQ gene sequence similarity of type strains among Bacillus species used in this study ranged from 66.2% to 96.6%, whereas orthologous average nucleotide identity ranged from 72.6% to 95.8%. According to the phylogenetic tree based on recQ sequences, each type strain of all Bacillus species or subspecies used in this study was placed in a unique taxonomic position. Four B. subtilis subspecies, Bacillus tequilensis and Bacillus vallismortis were grouped in one cluster (cluster A). Strains of B. subtilis subsp. subtilis were classified into A1 cluster, and divided into subgroups. Isolates from Natto, Japanese fermented bean food, were classified into one subgroup, whereas those from Cheonggukjang, Korean fermented bean food, were divided into several subgroups within A1. Type strains of Bacillus halotolerans and Bacillus mojavensis were grouped into another cluster (cluster B), related to cluster A. Bacillus siamensis, Bacillus velezensis and Bacillus amyloliquefaciens were grouped into an independent cluster (cluster E). Sequencing of recQ was useful for the classification or differentiation of B. subtilis and closely related species. Therefore, recQ gene can be applied to the classification of these taxa.

RecBCD- RecFOR-independent pathway of homologous recombination in Escherichia coli

The RecA protein is a key bacterial recombination enzyme that catalyzes pairing and strand exchange between homologous DNA duplexes. In Escherichia coli, RecA protein assembly on DNA is mediated either by the RecBCD or RecFOR protein complexes. Correspondingly, two recombination pathways, RecBCD and RecF (or RecFOR), are distinguished in E. coli. Inactivation of both pathways in recB(CD) recF(OR) mutants results in severe recombination deficiency. Here we describe a novel, RecBCD- RecFOR-independent (RecBFI) recombination pathway that is active in ΔrecBCD sbcB15 sbcC(D) ΔrecF(OR) mutants of E. coli. In transductional crosses, these mutants show only four-fold decrease of recombination frequency relative to the wild-type strain. At the same time they recombine 40- to 90-fold better than their sbcB+ sbcC+ and ΔsbcB sbcC counterparts. The RecBFI pathway strongly depends on recA, recJ and recQ gene functions, and moderately depends on recG and lexA functions. Inactivation of dinI, helD, recX, recN, radA, ruvABC and uvrD genes has a slight effect on RecBFI recombination. After exposure to UV and gamma irradiation, the ΔrecBCD sbcB15 sbcC ΔrecF mutants show moderately increased DNA repair proficiency relative to their sbcB+ sbcC+ and ΔsbcB sbcC counterparts. However, introduction of recA730 allele (encoding RecA protein with enhanced DNA binding properties) completely restores repair proficiency to ΔrecBCD sbcB15 sbcC ΔrecF mutants, but not to their sbcB+ sbcC+ and ΔsbcB sbcC derivatives. Fluorescence microscopy with UV-irradiated recA-gfp fusion mutants suggests that the kinetics of RecA filament formation might be slowed down in the RecBFI pathway. Inactivation of 3′-5′ exonucleases ExoVII, ExoIX and ExoX cannot activate the RecBFI pathway in ΔrecBCD ΔsbcB sbcC ΔrecF mutants. Taken together, our results show that the product of the sbcB15 allele is crucial for RecBFI pathway. Besides protecting 3′ overhangs, SbcB15 protein might play an additional, more active role in formation of the RecA filament.

PO-020 Functional characterisation of variant of unknown significate in familial breast cancer

IntroductionGlioblastoma (GBM), the common and aggressive human primary brain tumour (WHO grade IV), is highly resistant to standard radio- and chemotherapy. This is partly due to numerous genetic alterations in oncogenes and DNA damage repair systems. Despite progress in understanding the molecular background of GBM and advances in treatment modalities, survival of GMB patients is only 14 months post-diagnosis. RECQL4 belongs to RecQ family of ATP-dependent DNA helicases and plays an important role in genomic integrity and stability maintenance via involvement in DNA replication, repair, recombination, transcription and telomere maintenance. Mutations in the human RecQ genes are linked with cancer predisposition and/or premature ageing. Of all five human helicases, only RECQL4 is found in mitochondria. We explored if alterations of RECQL4 expression or functions contribute to pathogenesis of human GBM.Material and methodsWe determined the RECQL4 expression in various tumour specimens (tumour samples, human primary and established glioma cell cultures) by qPCR and Western Blotting. We determined the effect of RECQL4 depletion on cell viability, proliferation and GBM sphere formation using MTT metabolism, BrdU incorporation and tumour sphere forming assays, respectively.Results and discussionsWe found the upregulated expression of RECQL4 in GBM at mRNA and protein levels when compared to non-transformed human astrocytes. This finding was corroborated by TCGA data analysis. Fractionation of mitochondrial and cytosolic fractions from human glioma cells revealed the presence of RECQL4 in mitochondria. Downregulation (by siRNA) or genetic depletion of RECQL4 (by CRISPRCas9 knockout) in human glioma LN18 and U87-MG cells impaired cell viability and proliferation. We found upregulation of RECQL4 expression in GBM sphere cultures, enriched in glioma stem cells. Transient knock-down of RECQL4 significantly affected tumour sphere formation as evidenced by decreased numbers and sizes of cultured spheres.ConclusionThese data indicate that deregulation of RECQL4 expression or function may play an important role in GBM pathobiology. Our results provide a rationale for further studies of RECQL4 role in gliomagenesis.Supported by National Science Centre grants 2015/19/N/NZ3/02374 (SKK) and 2016/22/M/NZ3/00679 (BK) and the Foundation for Polish Science TEAM-TECH Core Facility project „NGS platform for comprehensive diagnostics and personalised therapy in neuro-oncology’ (AK).

RecQ Helicases Function in Development, DNA Repair, and Gene Targeting in Physcomitrella patens.

RecQ DNA helicases are genome surveillance proteins found in all kingdoms of life. They are characterized best in humans, as mutations in RecQ genes lead to developmental abnormalities and diseases. To better understand RecQ functions in plants we concentrated on Arabidopsis thaliana and Physcomitrella patens, the model species predominantly used for studies on DNA repair and gene targeting. Phylogenetic analysis of the six P. patens RecQ genes revealed their orthologs in humans and plants. Because Arabidopsis and P. patens differ in their RecQ4 and RecQ6 genes, reporter and deletion moss mutants were generated and gene functions studied in reciprocal cross-species and cross-kingdom approaches. Both proteins can be found in meristematic moss tissues, although at low levels and with distinct expression patterns. PpRecQ4 is involved in embryogenesis and in subsequent development as demonstrated by sterility of ΔPpRecQ4 mutants and by morphological aberrations. Additionally, ΔPpRecQ4 displays an increased sensitivity to DNA damages and an increased rate of gene targeting. Therefore, we conclude that PpRecQ4 acts as a repressor of recombination. In contrast, PpRecQ6 is not obviously important for moss development or DNA repair but does function as a potent enhancer of gene targeting.

The role of post-translational modifications in fine-tuning BLM helicase function during DNA repair.

RecQ-like helicases are a highly conserved family of proteins which are critical for preserving genome integrity. Genome instability is considered a hallmark of cancer and mutations within three of the five human RECQ genes cause hereditary syndromes that are associated with cancer predisposition. The human RecQ-like helicase BLM has a central role in DNA damage signaling, repair, replication, and telomere maintenance. BLM and its budding yeast orthologue Sgs1 unwind double-stranded DNA intermediates. Intriguingly, BLM functions in both a pro- and anti-recombinogenic manner upon replicative damage, acting on similar substrates. Thus, BLM activity must be intricately controlled to prevent illegitimate recombination events that could have detrimental effects on genome integrity. In recent years it has become evident that post-translational modifications (PTMs) of BLM allow a fine-tuning of its function. To date, BLM phosphorylation, ubiquitination, and SUMOylation have been identified, in turn regulating its subcellular localization, protein-protein interactions, and protein stability. In this review, we will discuss the cellular context of when and how these different modifications of BLM occur. We will reflect on the current model of how PTMs control BLM function during DNA damage repair and compare this to what is known about post-translational regulation of the budding yeast orthologue Sgs1. Finally, we will provide an outlook toward future research, in particular to dissect the cross-talk between the individual PTMs on BLM.

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.

The BLM dissolvasome in DNA replication and repair

RecQ DNA helicases are critical for proper maintenance of genomic stability, and mutations in multiple human RecQ genes are linked with genetic disorders characterized by a predisposition to cancer. RecQ proteins are conserved from prokaryotes to humans and in all cases form higher-order complexes with other proteins to efficiently execute their cellular functions. The focus of this review is a conserved complex that is formed between RecQ helicases and type-I topoisomerases. In humans, this complex is referred to as the BLM dissolvasome or BTR complex, and is comprised of the RecQ helicase BLM, topoisomerase IIIα, and the RMI proteins. The BLM dissolvasome functions to resolve linked DNA intermediates without exchange of genetic material, which is critical in somatic cells. We will review the history of this complex and highlight its roles in DNA replication, recombination, and repair. Additionally, we will review recently established interactions between BLM dissolvasome and a second set of genome maintenance factors (the Fanconi anemia proteins) that appear to allow coordinated genome maintenance efforts between the two systems.

Genome-wide study predicts promoter-G4 DNA motifs regulate selective functions in bacteria: radioresistance of D. radiodurans involves G4 DNA-mediated regulation

A remarkable number of guanine-rich sequences with potential to adopt non-canonical secondary structures called G-quadruplexes (or G4 DNA) are found within gene promoters. Despite growing interest, regulatory role of quadruplex DNA motifs in intrinsic cellular function remains poorly understood. Herein, we asked whether occurrence of potential G4 (PG4) DNA in promoters is associated with specific function(s) in bacteria. Using a normalized promoter-PG4-content (PG4P) index we analysed >60 000 promoters in 19 well-annotated species for (a) function class(es) and (b) gene(s) with enriched PG4P. Unexpectedly, PG4-associated functional classes were organism specific, suggesting that PG4 motifs may impart specific function to organisms. As a case study, we analysed radioresistance. Interestingly, unsupervised clustering using PG4P of 21 genes, crucial for radioresistance, grouped three radioresistant microorganisms including Deinococcus radiodurans. Based on these predictions we tested and found that in presence of nanomolar amounts of the intracellular quadruplex-binding ligand N-methyl mesoporphyrin (NMM), radioresistance of D. radiodurans was attenuated by ∼60%. In addition, important components of the RecF recombinational repair pathway recA, recF, recO, recR and recQ genes were found to harbour promoter-PG4 motifs and were also down-regulated in presence of NMM. Together these results provide first evidence that radioresistance may involve G4 DNA-mediated regulation and support the rationale that promoter-PG4s influence selective functions.

The RecQ DNA Helicases in DNA Repair

The RecQ helicases are conserved from bacteria to humans and play a critical role in genome stability. In humans, loss of RecQ gene function is associated with cancer predisposition and/or premature aging. Recent experiments have shown that the RecQ helicases function during distinct steps during DNA repair; DNA end resection, displacement-loop (D-loop) processing, branch migration, and resolution of double Holliday junctions (dHJs). RecQ function in these different processing steps has important implications for its role in repair of double-strand breaks (DSBs) that occur during DNA replication and meiosis, as well as at specific genomic loci such as telomeres.

Mismatch-induced lethality due to a defect in Escherichia coli RecQ helicase in exonuclease-deficient background: Dependence on MutS and UvrD functions

Escherichia coli DNA-unwinding protein RecQ has roles in the regulation of general recombination and the processing of stalled replication forks. In this study, we found that knockout of the recQ gene in combination with xonA xseA recJ mutations, which inhibit methyl-directed mismatch repair (MMR), caused about 100-fold increase in sensitivity to a purine analog 2-aminopurine (2AP). Intriguingly, inactivation of a MMR initiator due to the either mutation mutS or uvrD completely suppressed the 2AP sensitivity caused by recQ xonA xseA recJ mutations, suggesting that RecQ helicase might act on the DNA structures that are generated by the processing of DNA by the MutSLH complex and UvrD helicase. Moreover, the recQ gene knockout in combination with xonA xseA recJ mutations enhanced 2AP-induced filament formation, and increased by twofold the rate of spontaneous forward mutations in the thyA locus but did not increase the rate of rifampicin-resistant mutations. We discuss about the possible interplay between E. coli RecQ helicase and mismatch recognition factors.

When transcription meets recombination: a lesson from the human RECQ protein complexes

Since the cloning of the first human RECQ gene, RECQ1, more than 15 years ago, RECQ helicases have been a major focus in cancer research. Recent studies of human RECQ protein complexes are providing insight into their roles in various DNA metabolic pathways that protect the integrity of our genome.

AtRECQ2, a RecQ helicase homologue from Arabidopsis thaliana, is able to disrupt various recombinogenic DNA structures in vitro

SummaryRecQ helicases play an important role in the maintenance of genomic stability in pro‐ and eukaryotes. This is highlighted by the human genetic diseases Werner, Bloom’s and Rothmund–Thomson syndrome, caused by respective mutations in three of the five human RECQ genes. The highest numbers of RECQ homologous genes are found in plants, e.g. seven in Arabidopsis thaliana. However, only limited information is available on the functions of plant RecQ helicases, and no biochemical characterization has been performed. Here, we demonstrate that AtRECQ2 is a (d)NTP‐dependent 3′→5′ DNA helicase. We further characterized its basal properties and its action on various partial DNA duplexes. Importantly, we demonstrate that AtRECQ2 is able to disrupt recombinogenic structures: by disrupting various D‐loop structures, AtRECQ2 may prevent non‐productive recombination events on the one hand, and may channel repair processes into non‐recombinogenic pathways on the other hand, thus facilitating genomic stability. We show that a synthetic partially mobile Holliday junction is processed towards splayed‐arm products, possibly indicating a branch migration function for AtRECQ2. The biochemical properties defined in this work support the hypothesis that AtRECQ2 might be functionally orthologous to the helicase part of the human RecQ homologue HsWRN.

The Rb/E2F pathway and Ras activation regulate RecQ helicase gene expression

Disruption of the Rb (retinoblastoma protein)/E2F cell-cycle pathway and Ras activation are two of the most frequent events in cancer, and both of these mutations place oncogenic stress on cells to increase DNA replication. In the present study, we demonstrate that these mutations have an additive effect on induction of members of the RecQ DNA helicase family. RecQ activity is important for genomic stability, initiation of DNA replication and telomere maintenance, and mutation of the BLM (Bloom's syndrome gene), WRN (Werner's syndrome gene) or RECQL4 (Rothmund-Thomson syndrome gene) family members leads to premature aging syndromes characterized by genetic instability and telomere loss. RecQ family members are frequently overexpressed in cancers, and overexpression of BLM has been shown to cause telomere elongation. Concomitant with induction of RecQ genes in response to Rb family mutation and Ras activation, we show an increase in the number of telomeric repeats. We suggest that this induction of RecQ genes in response to common oncogenic mutations may explain the up-regulation of the genes seen in cancers, and it may provide a means for transformed cells to respond to an increased demand for DNA replication.

AtRECQ2, a RecQ-helicase homologue from Arabidopsis thaliana, is able to disrupt different recombinogenic DNA-structures in vitro

RecQ helicases play an important role in the maintenance of genomic stability in pro- and eukaryotes. This is highlighted by the human genetic diseases Werner, Bloom's and Rothmund-Thomson syndrome, caused by respective mutations in three of the five human RECQ genes. The highest numbers of RECQ homologous genes are found in plants, e.g. seven in Arabidopsis thaliana. However, only limited information is available on the functions of plant RecQ helicases, and no biochemical characterization has been performed. Here, we demonstrate that AtRECQ2 is a (d)NTP-dependent 3'-->5' DNA helicase. We further characterized its basal properties and its action on various partial DNA duplexes. Importantly, we demonstrate that AtRECQ2 is able to disrupt recombinogenic structures: by disrupting various D-loop structures, AtRECQ2 may prevent non-productive recombination events on the one hand, and may channel repair processes into non-recombinogenic pathways on the other hand, thus facilitating genomic stability. We show that a synthetic partially mobile Holliday junction is processed towards splayed-arm products, possibly indicating a branch migration function for AtRECQ2. The biochemical properties defined in this work support the hypothesis that AtRECQ2 might be functionally orthologous to the helicase part of the human RecQ homologue HsWRN.

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.

Manipulating Mitotic Recombination in the Zebrafish Embryo Through RecQ Helicases

RecQ DNA helicases resolve Rad-51-mediated recombination and suppress aberrant homologous recombination. RecQ gene loss is associated with cancer susceptibility and increased mitotic recombination. We have developed an in vivo assay based on a zebrafish pigment mutant for suppression of RecQ activity, and demonstrate that zebrafish RecQ genes have conserved function in suppressing mitotic recombination.

Crystal Structure of the HRDC Domain of Human Werner Syndrome Protein, WRN

Werner syndrome is a human premature aging disorder characterized by chromosomal instability. The disease is caused by the functional loss of WRN, a member of the RecQ-helicase family that plays an important role in DNA metabolic pathways. WRN contains four structurally folded domains comprising an exonuclease, a helicase, a winged-helix, and a helicase-and-ribonuclease D/C-terminal (HRDC) domain. In contrast to the accumulated knowledge pertaining to the biochemical functions of the three N-terminal domains, the function of C-terminal HRDC remains unknown. In this study, the crystal structure of the human WRN HRDC domain has been determined. The domain forms a bundle of alpha-helices similar to those of Saccharomyces cerevisiae Sgs1 and Escherichia coli RecQ. Surprisingly, the extra ten residues at each of the N and C termini of the domain were found to participate in the domain architecture by forming an extended portion of the first helix alpha1, and a novel looping motif that traverses straight along the domain surface, respectively. The motifs combine to increase the domain surface of WRN HRDC, which is larger than that of Sgs1 and E. coli. In WRN HRDC, neither of the proposed DNA-binding surfaces in Sgs1 or E. coli is conserved, and the domain was shown to lack DNA-binding ability in vitro. Moreover, the domain was shown to be thermostable and resistant to protease digestion, implying independent domain evolution in WRN. Coupled with the unique long linker region in WRN, the WRN HRDC may be adapted to play a distinct function in WRN that involves protein-protein interactions.

The RecQ gene family in plants

RecQ helicases are conserved throughout all kingdoms of life regarding their overall structure and function. They are 3'-5' DNA helicases resolving different recombinogenic DNA structures. The RecQ helicases are key factors in a number of DNA repair and recombination pathways involved in the maintenance of genome integrity. In eukaryotes the number of RecQ genes and the structure of RecQ proteins vary strongly between organisms. Therefore, they have been named RecQ-like genes. Knockouts of several RecQ-like genes cause severe diseases in animals or harmful cellular phenotypes in yeast. Until now the largest number of RecQ-like genes per organism has been found in plants. Arabidopsis and rice possess seven different RecQ-like genes each. In the almost completely sequenced genome of the moss Physcomitrella patens at least five RecQ-like genes are present. One of the major present and future research aims is to define putative plant-specific functions and to assign their roles in DNA repair and recombination pathways in relation to RecQ genes from other eukaryotes. Regarding their intron positions, the structures of six RecQ-like genes of dicots and monocots are virtually identical indicating a conservation over a time scale of 150 million years. In contrast to other eukaryotes one gene (RecQsim) exists exclusively in plants. It possesses an interrupted helicase domain but nevertheless seems to have maintained the RecQ function. Owing to a recent gene duplication besides the AtRecQl4A gene an additional RecQ-like gene (AtRecQl4B) exists in the Brassicaceae only. Genetic studies indicate that a AtRecQl4A knockout results in sensitivity to mutagens as well as an hyper-recombination phenotype. Since AtRecQl4B was still present, both genes must have non-redundant roles. Analysis of plant RecQ-like genes will not only increase the knowledge on DNA repair and recombination, but also on the evolution and radiation of protein families.