Early postnatal ataxia and abnormal cerebellar development in mice lacking Xeroderma pigmentosum Group A and Cockayne syndrome Group B DNA repair genes

Xeroderma pigmentosum (XP) and Cockayne syndrome (CS) are rare autosomal recessive disorders associated with a defect in the nucleotide excision repair (NER) pathway required for the removal of DNA damage induced by UV light and distorting chemical adducts. Although progressive neurological dysfunction is one of the hallmarks of CS and of some groups of XP patients, the causative mechanisms are largely unknown. Here we show that mice lacking both the XPA (XP-group A) and CSB (CS-group B) genes in contrast to the single mutants display severe growth retardation, ataxia, and motor dysfunction during early postnatal development. Their cerebella are hypoplastic and showed impaired foliation and stunted Purkinje cell dendrites. Reduced neurogenesis and increased apoptotic cell death occur in the cerebellar external granular layer. These findings suggest that XPA and CSB have additive roles in the mouse nervous system and support a crucial role for these genes in normal brain development.

Sequential assembly of the nucleotide excision repair factors in vivo

Here, we describe the assembly of the nucleotide excision repair (NER) complex in normal and repair-deficient (xeroderma pigmentosum) human cells, employing a novel technique of local UV irradiation combined with fluorescent antibody labeling. The damage recognition complex XPC-hHR23B appears to be essential for the recruitment of all subsequent NER factors in the preincision complex, including transcription repair factor TFIIH. XPA associates relatively late, is required for anchoring of ERCC1-XPF, and may be essential for activation of the endonuclease activity of XPG. These findings identify XPC as the earliest known NER factor in the reaction mechanism, give insight into the order of subsequent NER components, provide evidence for a dual role of XPA, and support a concept of sequential assembly of repair proteins at the site of the damage rather than a preassembled repairosome.

Differential ultraviolet-B-induced immunomodulation in XPA, XPC, and CSB DNA repair-deficient mice

Ultraviolet B irradiation has serious consequences for cellular immunity and can suppress the rejection of skin tumors and the resistance to infectious diseases. DNA damage plays a crucial role in these immunomodulatory effects of ultraviolet B, as impaired repair of ultraviolet-B-induced DNA damage has been shown to cause suppression of cellular immunity. Ultraviolet-B-induced DNA damage is repaired by the nucleotide excision repair mechanism very efficiently. Nucleotide excision repair comprises two subpathways: transcription-coupled and global genome repair. In this study the immunologic consequences of specific nucleotide excision repair defects in three mouse models, XPA, XPC, and CSB mutant mice, were investigated. XPA mice carry a total nucleotide excision repair defect, whereas XPC and CSB mice only lack global genome and transcription-coupled nucleotide excision repair, respectively. Our data demonstrate that cellular immune parameters in XPA, XPC, and CSB mice are normal compared with their wild-type (control) littermates. This may indicate that the reported altered cellular responses in xeroderma pigmentosum patients are not constitutive but could be due to external factors, such as ultraviolet B. Upon exposure to ultraviolet B, only XPA mice are very sensitive to ultraviolet-B-induced inhibition of Th1-mediated contact hypersensitivity responses and interferon-gamma production in skin draining lymph nodes. Lipopolysaccharide-stimulated tumor necrosis factor alpha and interleukin-10 production are significantly augmented in both XPA and CSB mice after ultraviolet B exposure. Lymph node cell numbers were increased very significantly in XPA, mildly increased in CSB, and not in XPC mice. In general XPC mice do not exhibit any indication of enhanced ultraviolet B susceptibility with regard to the immune parameters analyzed. These data suggest that both global genome repair and transcription-coupled repair are needed to prevent immunomodulation by ultraviolet B, whereas transcription-coupled repair is the major DNA repair subpathway of nucleotide excision repair that prevents the acute ultraviolet-B-induced effects such as erythema.

Genome maintenance mechanisms for preventing cancer

The early notion that cancer is caused by mutations in genes critical for the control of cell growth implied that genome stability is important for preventing oncogenesis. During the past decade, knowledge about the mechanisms by which genes erode and the molecular machinery designed to counteract this time-dependent genetic degeneration has increased markedly. At the same time, it has become apparent that inherited or acquired deficiencies in genome maintenance systems contribute significantly to the onset of cancer. This review summarizes the main DNA caretaking systems and their impact on genome stability and carcinogenesis.

Chromosomal stability and the DNA double-stranded break connection

Genome stability is of primary importance for the survival and proper functioning of all organisms. Double-stranded breaks in DNA are important threats to genome integrity because they can result in chromosomal aberrations that can affect, simultaneously, many genes, and lead to cell malfunctioning and cell death. These detrimental consequences are counteracted by two mechanistically distinct pathways of double-stranded break repair: homologous recombination and non-homologous end-joining. Recently, unexpected links between these double-stranded break-repair systems, and several human genome instability and cancer predisposition syndromes, have emerged. Now, interactions between both double-stranded break-repair pathways and other cellular processes, such as cell-cycle regulation and replication, are being unveiled.

A temperature-sensitive disorder in basal transcription and DNA repair in humans

The xeroderma pigmentosum group D (XPD) helicase subunit of TFIIH functions in DNA repair and transcription initiation. Different mutations in XPD give rise to three ultraviolet-sensitive syndromes: the skin cancer-prone disorder xeroderma pigmentosum (XP), in which repair of ultraviolet damage is affected; and the severe neurodevelopmental conditions Cockayne syndrome (CS) and trichothiodystrophy (TTD). In the latter two, the basal transcription function of TFIIH is also presumed to be affected. Here we report four unusual TTD patients with fever-dependent reversible deterioration of TTD features such as brittle hair. Cells from these patients show an in vivo temperature-sensitive defect of transcription and DNA repair due to thermo-instability of TFIIH. Our findings reveal the clinical consequences of impaired basal transcription and mutations in very fundamental processes in humans, which previously were only known in lower organisms.

From xeroderma pigmentosum to the biological clock contributions of Dirk Bootsma to human genetics

This paper commemorates the multiple contributions of Dirk Bootsma to human genetics. During a scientific 'Bootsma' cruise on his sailing-boat 'de Losbol', we visit a variety of scenery locations along the lakes and canals in Friesland, passing the highlights of Dirk Bootsma's scientific oeuvre. Departing from 'de Fluessen', his homeport, with his PhD work on the effect of X-rays and UV on cell cycle progression, we head for the pioneering endeavours of his team on mapping genes on human chromosomes by cell hybridization. Next we explore the use of cell hybrids by the Bootsma team culminating in the molecular cloning of one of the first chromosomal breakpoints involved in oncogenesis: the bcr-abl fusion gene responsible for chronic myelocytic leukemia. This seminal achievement enabled later development of new methods for early detection and very promising therapeutic intervention. A series of highlights at the horizon constitute the contributions of his team to the field of DNA repair, beginning with the discovery of genetic heterogeneity in the repair syndrome xeroderma pigmentosum (XP) followed later by the cloning of a large number of human repair genes. This led to the discovery that DNA repair is strongly conserved in evolution rendering knowledge from yeast relevant for mammals and vice versa. In addition, it resolved the molecular basis of several repair syndromes and permitted functional analysis of the encoded proteins. Another milestone is the discovery of the surprising connection between DNA repair and transcription initiation via the dual functional TFIIH complex in collaboration with Jean-Marc Egly et al. in Strasbourg. This provided an explanation for many puzzling clinical features and triggered a novel concept in human genetics: the existence of repair/transcription syndromes. The generation of many mouse mutants carrying defects in repair pathways yielded valuable models for assessing the clinical relevance of DNA repair including carcinogenesis and the identification of a link between DNA damage and premature aging. His team also opened a fascinating area of cell biology with the analysis of repair and transcription in living cells. A final surprising evolutionary twist was the discovery that photolyases designed for the light-dependent repair of UV-induced DNA lesions appeared to be adopted for driving the mammalian biological clock. The latter indicates that it is time to return to 'de Fluessen', where we will consider briefly the merits of Dirk Bootsma for Dutch science in general.

Novel functional interactions between nucleotide excision DNA repair proteins influencing the enzymatic activities of TFIIH, XPG, and ERCC1-XPF

The multisubunit basal transcription factor IIH (TFIIH) has a dual involvement in nucleotide excision repair (NER) of a variety of DNA lesions, including UV-induced photoproducts, and RNA polymerase II transcription. In both processes, TFIIH is implicated with local DNA unwinding, which is attributed to its helicase subunits XPB and XPD. To further define the role of TFIIH in NER, functional interactions between TFIIH and other DNA repair proteins were analyzed. We show that the TFIIH-associated ATPase activity is stimulated by both XPA and the XPC-HR23B complex. However, while XPA promotes the ATPase activity specifically in the presence of damaged DNA, stimulation by XPC-HR23B is lesion independent. Furthermore, we reveal that TFIIH inhibits the structure-specific endonuclease activities of both XPG and ERCC1-XPF, responsible for the 3' and 5' incision in NER, respectively. The inhibition occurs in the absence of ATP and is reversed upon addition of ATP. These results point toward additional roles for TFIIH and ATP during NER distinct from a requirement for DNA unwinding in the regulation of the endonuclease activities of XPG and ERCC1-XPF.

Cloning of a human homolog of the yeast nucleotide excision repair gene MMS19 and interaction with transcription repair factor TFIIH via the XPB and XPD helicases

Nucleotide excision repair (NER) removes UV-induced photoproducts and numerous other DNA lesions in a highly conserved 'cut-and-paste' reaction that involves approximately 25 core components. In addition, several other proteins have been identified which are dispensable for NER in vitro but have an undefined role in vivo and may act at the interface of NER and other cellular processes. An intriguing example is the Saccharomyces cerevisiae Mms19 protein that has an unknown dual function in NER and RNA polymerase II transcription. Here we report the cloning and characterization of a human homolog, designated hMMS19, that encodes a 1030 amino acid protein with 26% identity and 51% similarity to S.cerevisiae Mms19p and with a strikingly similar size. The expression profile and nuclear location are consistent with a repair function. Co-immunoprecipitation experiments revealed that hMMS19 directly interacts with the XPB and XPD subunits of NER-transcription factor TFIIH. These findings extend the conservation of the NER apparatus and the link between NER and basal transcription and suggest that hMMS19 exerts its function in repair and transcription by interacting with the XPB and XPD helicases.

XAB2, a novel tetratricopeptide repeat protein involved in transcription-coupled DNA repair and transcription

Nucleotide excision repair is a highly versatile DNA repair system responsible for elimination of a wide variety of lesions from the genome. It is comprised of two subpathways: transcription-coupled repair that accomplishes efficient removal of damage blocking transcription and global genome repair. Recently, the basic mechanism of global genome repair has emerged from biochemical studies. However, little is known about transcription-coupled repair in eukaryotes. Here we report the identification of a novel protein designated XAB2 (XPA-binding protein 2) that was identified by virtue of its ability to interact with XPA, a factor central to both nucleotide excision repair subpathways. The XAB2 protein of 855 amino acids consists mainly of 15 tetratricopeptide repeats. In addition to interacting with XPA, immunoprecipitation experiments demonstrated that a fraction of XAB2 is able to interact with the transcription-coupled repair-specific proteins CSA and CSB as well as RNA polymerase II. Furthermore, antibodies against XAB2 inhibited both transcription-coupled repair and transcription in vivo but not global genome repair when microinjected into living fibroblasts. These results indicate that XAB2 is a novel component involved in transcription-coupled repair and transcription.

Sublimiting concentration of TFIIH transcription/DNA repair factor causes TTD-A trichothiodystrophy disorder

The repair-deficient form of trichothiodystrophy (TTD) most often results from mutations in the genes XPB or XPD, encoding helicases of the transcription/repair factor TFIIH. The genetic defect in a third group, TTD-A, is unknown, but is also caused by dysfunctioning TFIIH. None of the TFIIH subunits carry a mutation and TFIIH from TTD-A cells is active in both transcription and repair. Instead, immunoblot and immunofluorescence analyses reveal a strong reduction in the TFIIH concentration. Thus, the phenotype of TTD-A appears to result from sublimiting amounts of TFIIH, probably due to a mutation in a gene determining the complex stability. The reduction of TFIIH mainly affects its repair function and hardly influences transcription.

Characterization of mRAD18Sc, a mouse homolog of the yeast postreplication repair gene RAD18

The RAD18 gene of the yeast Saccharomyces cerevisiae encodes a protein with ssDNA binding activity that interacts with the ubiquitin-conjugating enzyme RAD6 and plays an important role in postreplication repair. We identified and characterized the putative mouse homolog of RAD18, designated mRAD18Sc. The mRAD18Sc open reading frame encodes a 509-amino-acid polypeptide that is strongly conserved in size and sequence between yeast and mammals, with specific conservation of the RING-zinc-finger and the classic zinc-finger domain. The degree of sequence conservation between mRAD18Sc, RAD18, and homologous sequences identified in other species (NuvA from Aspergillus nidulans and Uvs-2 from Neurospora crassa) is entirely consistent with the evolutionary relationship of these organisms, strongly arguing that these genes are one another's homologs. Consistent with the presence of a nuclear translocation signal in the amino acid sequence, we observed the nuclear localization of GFP-tagged mRAD18Sc after stable transfection to HeLa cells. mRNA expression of mRAD18Sc in the mouse was observed in thymus, spleen, brain, and ovary, but was most pronounced in testis, with the highest level of expression in pachytene-stage primary spermatocytes, suggesting that mRAD18Sc plays a role in meiosis of spermatogenesis. Finally, we mapped the mRAD18Sc gene on mouse chromosome 6F.

ATP-dependent chromatin remodeling by the Cockayne syndrome B DNA repair-transcription-coupling factor

The Cockayne syndrome B protein (CSB) is required for coupling DNA excision repair to transcription in a process known as transcription-coupled repair (TCR). Cockayne syndrome patients show UV sensitivity and severe neurodevelopmental abnormalities. CSB is a DNA-dependent ATPase of the SWI2/SNF2 family. SWI2/SNF2-like proteins are implicated in chromatin remodeling during transcription. Since chromatin structure also affects DNA repair efficiency, chromatin remodeling activities within repair are expected. Here we used purified recombinant CSB protein to investigate whether it can remodel chromatin in vitro. We show that binding of CSB to DNA results in an alteration of the DNA double-helix conformation. In addition, we find that CSB is able to remodel chromatin structure at the expense of ATP hydrolysis. Specifically, CSB can alter DNase I accessibility to reconstituted mononucleosome cores and disarrange an array of nucleosomes regularly spaced on plasmid DNA. In addition, we show that CSB interacts not only with double-stranded DNA but also directly with core histones. Finally, intact histone tails play an important role in CSB remodeling. CSB is the first repair protein found to play a direct role in modulating nucleosome structure. The relevance of this finding to the interplay between transcription and repair is discussed.

Disruption of mouse SNM1 causes increased sensitivity to the DNA interstrand cross-linking agent mitomycin C

DNA interstrand cross-links (ICLs) represent lethal DNA damage, because they block transcription, replication, and segregation of DNA. Because of their genotoxicity, agents inducing ICLs are often used in antitumor therapy. The repair of ICLs is complex and involves proteins belonging to nucleotide excision, recombination, and translesion DNA repair pathways in Escherichia coli, Saccharomyces cerevisiae, and mammals. We cloned and analyzed mammalian homologs of the S. cerevisiae gene SNM1 (PSO2), which is specifically involved in ICL repair. Human Snm1, a nuclear protein, was ubiquitously expressed at a very low level. We generated mouse SNM1(-/-) embryonic stem cells and showed that these cells were sensitive to mitomycin C. In contrast to S. cerevisiae snm1 mutants, they were not significantly sensitive to other ICL agents, probably due to redundancy in mammalian ICL repair and the existence of other SNM1 homologs. The sensitivity to mitomycin C was complemented by transfection of the human SNM1 cDNA and by targeting of a genomic cDNA-murine SNM1 fusion construct to the disrupted locus. We also generated mice deficient for murine SNM1. They were viable and fertile and showed no major abnormalities. However, they were sensitive to mitomycin C. The ICL sensitivity of the mammalian SNM1 mutant suggests that SNM1 function and, by implication, ICL repair are at least partially conserved between S. cerevisiae and mammals.

Transcription-coupled and global genome repair differentially influence UV-B-induced acute skin effects and systemic immunosuppression

Exposure to UV-B radiation impairs immune responses in mammals by inhibiting especially Th1-mediated contact hypersensitivity and delayed-type hypersensitivity. Immunomodulation is not restricted to the exposed skin, but is also observed at distant sites, indicating the existence of mediating factors such as products from exposed skin cells or photoactivated factors present in the superficial layers. DNA damage appears to play a key role, because enhanced nucleotide excision repair (NER) strongly counteracts immunosuppression. To determine the effects of the type and genomic location of UV-induced DNA damage on immunosuppression and acute skin reactions (edema and erythema) four congenic mouse strains carrying different defects in NER were compared: CSB and XPC mice lacking transcription-coupled or global genome NER, respectively, as well as XPA and TTD/XPD mice carrying complete or partial defects in both NER subpathways, respectively. The major conclusions are that 1) transcription-coupled DNA repair is the dominant determinant in protection against acute skin effects; 2) systemic immunomodulation is only affected when both NER subpathways are compromised; and 3) sunburn is not related to UV-B-induced immunosuppression.

Dimerization and nuclear entry of mPER proteins in mammalian cells

Nuclear entry of circadian oscillatory gene products is a key step for the generation of a 24-hr cycle of the biological clock. We have examined nuclear import of clock proteins of the mammalian period gene family and the effect of serum shock, which induces a synchronous clock in cultured cells. Previously, mCRY1 and mCRY2 have been found to complex with PER proteins leading to nuclear import. Here we report that nuclear translocation of mPER1 and mPER2 (1) involves physical interactions with mPER3, (2) is accelerated by serum treatment, and (3) still occurs in mCry1/mCry2 double-deficient cells lacking a functional biological clock. Moreover, nuclear localization of endogenous mPER1 was observed in cultured mCry1/mCry2 double-deficient cells as well as in the liver and the suprachiasmatic nuclei (SCN) of mCry1/mCry2 double-mutant mice. This indicates that nuclear translocation of at least mPER1 also can occur under physiological conditions (i.e., in the intact mouse) in the absence of any CRY protein. The mPER3 amino acid sequence predicts the presence of a cytoplasmic localization domain (CLD) and a nuclear localization signal (NLS). Deletion analysis suggests that the interplay of the CLD and NLS proposed to regulate nuclear entry of PER in Drosophila is conserved in mammals, but with the novel twist that mPER3 can act as the dimerizing partner.

Mouse RAD54 affects DNA double-strand break repair and sister chromatid exchange

Cells can achieve error-free repair of DNA double-strand breaks (DSBs) by homologous recombination through gene conversion with or without crossover. In contrast, an alternative homology-dependent DSB repair pathway, single-strand annealing (SSA), results in deletions. In this study, we analyzed the effect of mRAD54, a gene involved in homologous recombination, on the repair of a site-specific I-SceI-induced DSB located in a repeated DNA sequence in the genome of mouse embryonic stem cells. We used six isogenic cell lines differing solely in the orientation of the repeats. The combination of the three recombination-test substrates used discriminated among SSA, intrachromatid gene conversion, and sister chromatid gene conversion. DSB repair was most efficient for the substrate that allowed recovery of SSA events. Gene conversion with crossover, indistinguishable from long tract gene conversion, preferentially involved the sister chromatid rather than the repeat on the same chromatid. Comparing DSB repair in mRAD54 wild-type and knockout cells revealed direct evidence for a role of mRAD54 in DSB repair. The substrate measuring SSA showed an increased efficiency of DSB repair in the absence of mRAD54. The substrate measuring sister chromatid gene conversion showed a decrease in gene conversion with and without crossover. Consistent with this observation, DNA damage-induced sister chromatid exchange was reduced in mRAD54-deficient cells. Our results suggest that mRAD54 promotes gene conversion with predominant use of the sister chromatid as the repair template at the expense of error-prone SSA.

Targeted disruption of the cell-cycle checkpoint gene ATR leads to early embryonic lethality in mice

Checkpoints of DNA integrity are conserved throughout evolution, as are the kinases ATM (Ataxia Telangiectasia mutated) and ATR (Ataxia- and Rad-related), which are related to phosphatidylinositol (PI) 3-kinase [1] [2] [3]. The ATM gene is not essential, but mutations lead to ataxia telangiectasia (AT), a pleiotropic disorder characterised by radiation sensitivity and cellular checkpoint defects in response to ionising radiation [4] [5] [6]. The ATR gene has not been associated with human syndromes and, structurally, is more closely related to the canonical yeast checkpoint genes rad3(Sp) and MEC1(Sc) [7] [8]. ATR has been implicated in the response to ultraviolet (UV) radiation and blocks to DNA synthesis [8] [9] [10] [11], and may phosphorylate p53 [12] [13], suggesting that ATM and ATR may have similar and, perhaps, complementary roles in cell-cycle control after DNA damage. Here, we report that targeted inactivation of ATR in mice by disruption of the kinase domain leads to early embryonic lethality before embryonic day 8.5 (E8.5). Heterozygous mice were fertile and had no aberrant phenotype, despite a lower ATR mRNA level. No increase was observed in the sensitivity of ATR(+/-) embryonic stem (ES) cells to a variety of DNA-damaging agents. Attempts to target the remaining wild-type ATR allele in heterozygous ATR(+/-) ES cells failed, supporting the idea that loss of both alleles of the ATR gene, even at the ES-cell level, is lethal. Thus, in contrast to the closely related checkpoint gene ATM, ATR has an essential function in early mammalian development.

Homologous and non-homologous recombination differentially affect DNA damage repair in mice

Ionizing radiation and interstrand DNA crosslinking compounds provide important treatments against cancer due to their extreme genotoxicity for proliferating cells. Both the efficacies of such treatments and the mutagenic potential of these agents are modulated by the ability of cells to repair the inflicted DNA damage. Here we demonstrate that homologous recombination-deficient mRAD54(-/-) mice are hypersensitive to ionizing radiation at the embryonic but, unexpectedly, not at the adult stage. However, at the adult stage mRAD54 deficiency dramatically aggravates the ionizing radiation sensitivity of severe combined immune deficiency (scid) mice that are impaired in DNA double-strand break repair through DNA end-joining. In contrast, regardless of developmental stage, mRAD54(-/-) mice are hypersensitive to the interstrand DNA crosslinking compound mitomycin C. These results demonstrate that the two major DNA double-strand break repair pathways in mammals have overlapping as well as specialized roles, and that the relative contribution of these pathways towards repair of ionizing radiation-induced DNA damage changes during development of the animal.

Nucleotide excision repair and human syndromes

DNA damage is implicated in cancer and aging, and several DNA repair mechanisms exist that safeguard the genome from these deleterious consequences. Nucleotide excision repair (NER) removes a wide diversity of lesions, the main of which include UV-induced lesions, bulky chemical adducts and some forms of oxidative damage. The NER process involves the action of at least 30 proteins in a 'cut-and-paste'-like mechanism. The consequences of a defect in one of the NER proteins are apparent from three rare recessive syndromes: xeroderma pigmentosum (XP), Cockayne syndrome (CS) and the photosensitive form of the brittle hair disorder trichothiodystrophy (TTD). Sun-sensitive skin is associated with skin cancer predisposition in the case of XP, but remarkably not in CS and TTD. Moreover, the spectrum of clinical symptoms differs considerably between the three syndromes. CS and TTD patients exhibit a spectrum of neurodevelopmental abnormalities and, in addition, TTD is associated with ichthyosis and brittle hair. These typical CS and TTD abnormalities are difficult to comprehend as a consequence of defective NER. This review briefly describes the biochemistry of the NER process, summarizes the clinical features of the NER disorders and speculates on the molecular basis underlying these pleitropic syndromes.

TFIIH with inactive XPD helicase functions in transcription initiation but is defective in DNA repair

TFIIH is a multisubunit protein complex involved in RNA polymerase II transcription and nucleotide excision repair, which removes a wide variety of DNA lesions including UV-induced photoproducts. Mutations in the DNA-dependent ATPase/helicase subunits of TFIIH, XPB and XPD, are associated with three inherited syndromes as follows: xeroderma pigmentosum with or without Cockayne syndrome and trichothiodystrophy. By using epitope-tagged XPD we purified mammalian TFIIH carrying a wild type or an active-site mutant XPD subunit. Contrary to XPB, XPD helicase activity was dispensable for in vitro transcription, catalytic formation of trinucleotide transcripts, and promoter opening. Moreover, in contrast to XPB, microinjection of mutant XPD cDNA did not interfere with in vivo transcription. These data show directly that XPD activity is not required for transcription. However, during DNA repair, neither 5' nor 3' incisions in defined positions around a DNA adduct were detected in the presence of TFIIH containing inactive XPD, although substantial damage-dependent DNA synthesis was induced by the presence of mutant XPD both in cells and cell extracts. The aberrant damage-dependent DNA synthesis caused by the mutant XPD does not lead to effective repair, consistent with the discrepancy between repair synthesis and survival in cells from a number of XP-D patients.

Photic induction of mPer1 and mPer2 in cry-deficient mice lacking a biological clock

Mice lacking mCry1 and mCry2 are behaviorally arrhythmic. As shown here, cyclic expression of the clock genes mPer1 and mPer2 (mammalian Period genes 1 and 2) in the suprachiasmatic nucleus and peripheral tissues is abolished and mPer1 and mPer2 mRNA levels are constitutively high. These findings indicate that the biological clock is eliminated in the absence of both mCRY1 and mCRY2 (mammalian cryptochromes 1 and 2) and support the idea that mammalian CRY proteins act in the negative limb of the circadian feedback loop. The mCry double-mutant mice retain the ability to have mPer1 and mPer2 expression induced by a brief light stimulus known to phase-shift the biological clock in wild-type animals. Thus, mCRY1 and mCRY2 are dispensable for light-induced phase shifting of the biological clock.

Interaction of hHR23 with S5a. The ubiquitin-like domain of hHR23 mediates interaction with S5a subunit of 26 S proteasome

hHR23B is one of two human homologs of the Saccharomyces cerevisiae nucleotide excision repair (NER) gene product RAD23 and a component of a protein complex that specifically complements the NER defect of xeroderma pigmentosum group C (XP-C) cell extracts in vitro. Although a small proportion of hHR23B is tightly complexed with the XP-C responsible gene product, XPC protein, a vast majority exists as an XPC-free form, indicating that hHR23B has additional functions other than NER in vivo. Here we demonstrate that the human NER factor hHR23B as well as another human homolog of RAD23, hHR23A, interact specifically with S5a, a subunit of the human 26 S proteasome using the yeast two-hybrid system. Furthermore, hHR23 proteins were detected with S5a at the position where 26 S proteasome sediments in glycerol gradient centrifugation of HeLa S100 extracts. Intriguingly, hHR23B showed the inhibitory effect on the degradation of (125)I-lysozyme in the rabbit reticulocyte lysate. hHR23 proteins thus appear to associate with 26 S proteasome in vivo. From co-precipitation experiments using several series of deletion mutants, we defined the domains in hHR23B and S5a that mediate this interaction. From these results, we propose that part of hHR23 proteins are involved in the proteolytic pathway in cells.

[Mutant mice as a model of human diseases of aging. Defective transcription and premature aging]

Mouse mutants as model for human ageing disorders. Defective transcription and premature ageing. DNA damage has been implicated in carcinogenesis and ageing. Our group examines the nucleotide excision DNA repair (NER) process, which can detect and repair various UV- and chemically-induced DNA damages, in addition to some types of oxidative DNA damage. Recently, two mouse models were generated with a defect in one of the NER genes (XPD and CSB), displaying distinctive symptoms of premature ageing. Remarkably, in both cases the NER gene plays an additional role in transcription. Research using these mouse models aims to investigate the link between premature ageing and molecular defects in NER and transcription.

Mouse model for the DNA repair/basal transcription disorder trichothiodystrophy reveals cancer predisposition

Patients with the nucleotide excision repair (NER) disorder xeroderma pigmentosum (XP) are highly predisposed to develop sunlight-induced skin cancer, in remarkable contrast to photosensitive NER-deficient trichothiodystrophy (TTD) patients carrying mutations in the same XPD gene. XPD encodes a helicase subunit of the dually functional DNA repair/basal transcription complex TFIIH. The pleiotropic disease phenotype is hypothesized to be, in part, derived from a repair defect causing UV sensitivity and, in part, from a subtle, viable basal transcription deficiency accounting for the cutaneous, developmental, and the typical brittle hair features of TTD. To understand the relationship between deficient NER and tumor susceptibility, we used a mouse model for TTD that mimics an XPD point mutation of a TTD patient in the mouse germline. Like the fibroblasts from the patient, mouse cells exhibit a partial NER defect, evident from the reduced UV-induced DNA repair synthesis (residual repair capacity approximately 25%), limited recovery of RNA synthesis after UV exposure, and a relatively mild hypersensitivity to cell killing by UV or 7,12-dimethylbenz[a]anthracene. In accordance with the cellular studies, TTD mice exhibit a modestly increased sensitivity to UV-induced inflammation and hyperplasia of the skin. In striking contrast to the human syndrome, TTD mice manifest a dear susceptibility to UV- and 7,12-dimethylbenz[a]anthracene-induced skin carcinogenesis, albeit not as pronounced as the totally NER-deficient XPA mice. These findings open up the possibility that TTD is associated with a so far unnoticed cancer predisposition and support the notion that a NER deficiency enhances cancer susceptibility. These findings have important implications for the etiology of the human disorder and for the impact of NER on carcinogenesis.

Action of DNA repair endonuclease ERCC1/XPF in living cells

To study the nuclear organization and dynamics of nucleotide excision repair (NER), the endonuclease ERCC1/XPF (for excision repair cross complementation group 1/xeroderma pigmentosum group F) was tagged with green fluorescent protein and its mobility was monitored in living Chinese hamster ovary cells. In the absence of DNA damage, the complex moved freely through the nucleus, with a diffusion coefficient (15 +/- 5 square micrometers per second) consistent with its molecular size. Ultraviolet light-induced DNA damage caused a transient dose-dependent immobilization of ERCC1/XPF, likely due to engagement of the complex in a single repair event. After 4 minutes, the complex regained mobility. These results suggest (i) that NER operates by assembly of individual NER factors at sites of DNA damage rather than by preassembly of holocomplexes and (ii) that ERCC1/XPF participates in repair of DNA damage in a distributive fashion rather than by processive scanning of large genome segments.

DNA repair methyltransferase (Mgmt) knockout mice are sensitive to the lethal effects of chemotherapeutic alkylating agents

We have generated mice deficient in O6-methylguanine DNA methyltransferase activity encoded by the murine Mgmt gene using homologous recombination to delete the region encoding the Mgmt active site cysteine. Tissues from Mgmt null mice displayed very low O6-methylguanine DNA methyltransferase activity, suggesting that Mgmt constitutes the major, if not the only, O6-methylguanine DNA methyltransferase. Primary mouse embryo fibroblasts and bone marrow cells from Mgmt -/- mice were significantly more sensitive to the toxic effects of the chemotherapeutic alkylating agents 1,3-bis(2-chloroethyl)-1-nitrosourea, streptozotocin and temozolomide than those from Mgmt wild-type mice. As expected, Mgmt-deficient fibroblasts and bone marrow cells were not sensitive to UV light or to the crosslinking agent mitomycin C. In addition, the 50% lethal doses for Mgmt -/- mice were 2- to 10-fold lower than those for Mgmt +/+ mice for 1,3-bis(2chloroethyl)-1-nitrosourea, N-methyl-N-nitrosourea and streptozotocin; similar 50% lethal doses were observed for mitomycin C. Necropsies of both wild-type and Mgmt -/mice following drug treatment revealed histological evidence of significant ablation of hematopoietic tissues, but such ablation occurred at much lower doses for the Mgmt -/- mice. These results demonstrate the critical importance of O6-methylguanine DNA methyltransferase in protecting cells and animals against the toxic effects of alkylating agents used for cancer chemotherapy.

Rec8p, a meiotic recombination and sister chromatid cohesion phosphoprotein of the Rad21p family conserved from fission yeast to humans

Our work and that of others defined mitosis-specific (Rad21 subfamily) and meiosis-specific (Rec8 subfamily) proteins involved in sister chromatid cohesion in several eukaryotes, including humans. Mutation of the fission yeast Schizosaccharomyces pombe rec8 gene was previously shown to confer a number of meiotic phenotypes, including strong reduction of recombination frequencies in the central region of chromosome III, absence of linear element polymerization, reduced pairing of homologous chromosomes, reduced sister chromatid cohesion, aberrant chromosome segregation, defects in spore formation, and reduced spore viability. Here we extend the description of recombination reduction to the central regions of chromosomes I and II. We show at the protein level that expression of rec8 is meiosis specific and that Rec8p localizes to approximately 100 foci per prophase nucleus. Rec8p was present in an unphosphorylated form early in meiotic prophase but was phosphorylated prior to meiosis I, as demonstrated by analysis of the mei4 mutant blocked before meiosis I. Evidence for the persistence of Rec8p beyond meiosis I was obtained by analysis of the mutant mes1 blocked before meiosis II. A human gene, which we designate hrec8, showed significant primary sequence similarity to rec8 and was mapped to chromosome 14. High mRNA expression of mouse and human rec8 genes was found only in germ line cells, specifically in testes and, interestingly, in spermatids. hrec8 was also expressed at a low level in the thymus. Sequence similarity and testis-specific expression indicate evolutionarily conserved functions of Rec8p in meiosis. Possible roles of Rec8p in the integration of different meiotic events are discussed.

Mammalian Cry1 and Cry2 are essential for maintenance of circadian rhythms

Many biochemical, physiological and behavioural processes show circadian rhythms which are generated by an internal time-keeping mechanism referred to as the biological clock. According to rapidly developing models, the core oscillator driving this clock is composed of an autoregulatory transcription-(post) translation-based feedback loop involving a set of 'dock' genes. Molecular clocks do not oscillate with an exact 24-hour rhythmicity but are entrained to solar day/night rhythms by light. The mammalian proteins Cryl and Cry2, which are members of the family of plant blue-light receptors (cryptochromes) and photolyases, have been proposed as candidate light receptors for photoentrainment of the biological clock. Here we show that mice lacking the Cryl or Cry2 protein display accelerated and delayed free-running periodicity of locomotor activity, respectively. Strikingly, in the absence of both proteins, an instantaneous and complete loss of free-running rhythmicity is observed. This suggests that, in addition to a possible photoreceptor and antagonistic clock-adjusting function, both proteins are essential for the maintenance of circadian rhythmicity.

Mouse Rad54 affects DNA conformation and DNA-damage-induced Rad51 foci formation

Error-free repair by homologous recombination of DNA double-strand breaks induced by ionizing radiation (IR) requires the Rad52 group proteins, including Rad51 and Rad54, in the yeast Saccharomyces cerevisiae [1]. The formation of a 'joint' molecule between the damaged DNA and the homologous repair template is a key step in recombination mediated by Rad51 and stimulated by Rad54 [2] [3] [4] [5]. Mammalian homologs of Rad51 and Rad54 have been identified [2] [3] [6]. Here, we demonstrate that mouse Rad54 (mRad54) formed IR-induced nuclear foci that colocalized with mRad51. Interaction between mRad51 and mRad54 was induced by genotoxic stress, but only when lesions that required mRad54 for their repair were formed. Interestingly, mRad54 was essential for the formation of IR-induced mRad51 foci. Rad54 belongs to the SWI2/SNF2 protein family, members of which modulate protein-DNA interactions in an ATP-driven manner [7]. Results of a topological assay suggested that purified human Rad54 (hRad54) protein can unwind double-stranded (ds) DNA at the expense of ATP hydrolysis. Unwinding of the homologous repair template could promote the formation or stabilization of hRad51-mediated joint molecules. Rad54 appears to be required downstream of other Rad52 group proteins, such as Rad52 and the Rad55-Rad57 heterodimer, that assist Rad51 in interacting with the broken DNA [2] [3] [4].

Histone ubiquitination and chromatin remodeling in mouse spermatogenesis

Male infertility in HR6B knockout mice is associated with impairment of spermatogenesis. The HR6B gene is a mammalian, autosomal homolog of the Saccharomyces cerevisiae gene Rad6 encoding a ubiquitin-conjugating enzyme. In addition, X-chromosomal HR6A has been identified, in human and mouse. RAD6 in yeast is required for a variety of cellular functions, including sporulation, DNA repair, and mutagenesis. Since RAD6 and its mammalian homologs can ubiquitinate histones in vitro, we have investigated the pattern of histone ubiquitination in mouse testis. By immunoblot and immunohistochemical analysis of wild-type mouse testis, a high amount of ubiquitinated H2A (uH2A) was detected in pachytene spermatocytes. This signal became undetectable in round spermatids, but then increased again during a relatively short developmental period, in elongating spermatids. No other ubiquitinated histones were observed. In the HR6B knockout mice, we failed to detect an overt defect in the overall pattern of histone ubiquitination. For somatic cell types, it has been shown that histone ubiquitination is associated with destabilization of nucleosomes, in relation to active gene transcription. Unexpectedly, the most intense uH2A signal in pachytene spermatocytes was detected in the sex body, an inactive nuclear structure that contains the heterochromatic X and Y chromosomes. The postmeiotic uH2A immunoexpression in elongating spermatids indicates that nucleosome destabilization induced by histone ubiquitination may play a facilitating role during histone-to-protamine replacement.

Human and mouse homologs of the Schizosaccharomyces pombe rad17+ cell cycle checkpoint control gene

The Schizosaccharomyces pombe rad17+ cell cycle checkpoint control gene is required for S-phase and G2/M arrest in response to both DNA damage and incomplete DNA replication. We isolated and characterized the putative human (RAD17Sp) and mouse (mRAD17Sp) homologs of the S. pombe Rad17 (Rad17Sp) protein. The human RAD17Sp open reading frame (ORF) encodes a protein of 681 amino acids; the mRAD17Sp ORF codes for a protein of 688 amino acids. The mRAD17Sp messenger is highly expressed in the testis as a single 3-kb mRNA species. The human RAD17Sp and mRAD17Sp proteins are 24% identical and 46% similar to the S.pombe Rad17Sp protein. Sequence homology was also noted with the Saccharomyces cerevisiae Rad24Sc (which is the structural counterpart of S.pombe Rad17Sp) and structurally related polypeptides from Caenorhabditis elegans, Arabidopsis thaliana, Pyrococcus horikoshii, and Drosophila melanogaster. The degree of conservation between the mammalian RAD17Sp proteins and those of the other species is consistent with the evolutionary distance between the species, indicating that these proteins are most likely true counterparts. In addition, homology was found between the Rad17Sp homologs and proteins identified as components of mammalian replication factor C (RF-C)/activator 1, especially in several highly conserved RF-C-like domains including a "Walker A" motif. Using FISH and analysis of a panel of rodent-human cell hybrids, the human RAD17Sp gene (HGMW-approved symbol RAD17 could be localized on human chromosome 5q13-q14, a region implicated in the etiology of small cell lung carcinoma, non-small-cell lung carcinoma, duodenal adenocarcinoma, and head and neck squamous cell carcinoma. Our results suggest that the structure and function of the checkpoint "rad" genes in the G2/M checkpoint pathway are evolutionary conserved between yeast and higher eukaryotes.

Cancer from the outside, aging from the inside: mouse models to study the consequences of defective nucleotide excision repair

In recent years, mouse models have been generated to study the syndromes associated with a defect in nucleotide excision repair (NER). Thus, via conventional knockout gene targeting or by mimicking patient-specific alleles, mouse models for xeroderma pigmentosum (XP), Cockayne syndrome (CS) and photosensitive trichothiodystrophy (TTD) have been obtained. The generation of this series of mouse mutants allows in vivo investigation of some intriguing questions that have puzzled the field, such as the paradoxical absence of cancer development in TTD and CS despite their NER deficiencies, and the role of the ERCC1 gene in mitotic recombination and cross-link repair. Other interesting issues include the pathophysiology of the non-NER related clinical symptoms in TTD and CS patients and the proposed involvement of NER and transcription in the process of aging. This review will focus on data obtained thus far and discuss further utilization of the mouse mutants for unraveling some of the fascinating and medically relevant aspects associated with defects in NER and related processes.

Molecular mechanisms of DNA double strand break repair

DNA double-strand breaks (DSBs) are major threats to the genomic integrity of cells. If not taken care of properly, they can cause chromosome fragmentation, loss and translocation, possibly resulting in carcinogenesis. Upon DSB formation, cell-cycle checkpoints are triggered and multiple DSB repair pathways can be activated. Recent research on the Nijmegen breakage syndrome, which predisposes patients to cancer, suggests a direct link between activation of cell-cycle checkpoints and DSB repair. Furthermore, the biochemical activities of proteins involved in the two major DSB repair pathways, homologous recombination and DNA end-joining, are now beginning to emerge. This review discusses these new findings and their implications for the mechanisms of DSB repair.

A human and mouse homolog of the Schizosaccharomyces pombe rad1+ cell cycle checkpoint control gene

The Schizosaccharomyces pombe rad1+ cell cycle checkpoint control gene is required for S-phase and G2/M arrest in response to both DNA damage and incomplete DNA replication. We isolated and characterized the putative human RAD1 (hRAD1) and mouse RAD1 (mRAD1) homologs of the S. pombe Rad1 (Rad1) protein. The human RAD1 open reading frame (ORF) encodes a protein of 282 amino acids; the mRAD1 ORF codes for a protein of 280 amino acids. The human RAD1 and mRAD1 messengers are highly expressed in the testis as different mRNA species (varying from 1.0, 1.4, 1.5, to 3.0 kb). The hRAD1 and mRAD1 proteins are 30% identical and 56% similar to the S. pombe Rad1 protein. Sequence homology was also noted with the Saccharomyces cerevisiae Rad17p, the putative 3'-5' exonuclease Rec1 from Ustilago maydis, and the structurally related polypeptides from Arabidopsis thaliana and Caenorhabditis elegans. The degree of conservation between the mammalian RAD1 proteins and those of the other species is consistent with the evolutionary distance between the species, implicating that these proteins are most likely true counterparts. Together, this suggests that the structure and function of the checkpoint "rad" genes in the G2/M checkpoint pathway are evolutionarily conserved between yeasts and higher eukaryotes. The human RAD1 gene could be localized on human chromosome 5p13, a region that has been implicated in the etiology of small cell lung carcinomas, squamous cell carcinomas, adenocarcinomas, and bladder cancer.

Knockout mouse model and gametogenic failure

To evaluate the function of a defined gene in gametogenesis, exciting opportunities are offered by the introduction of techniques to generate knockout mice. In this short article, we briefly describe a few gene knockout mouse models, which show a phenotype that involves impairment of gametogenesis and/or fertility. The focus will be on the mHR6B gene knockout mouse, which shows male infertility. The mHR6B gene encodes an ubiquitin-conjugating enzyme, and the data point to an important role of the ubiquitin pathway in gametogenesis.

The human RAD54 recombinational DNA repair protein is a double-stranded DNA-dependent ATPase

DNA double-strand break repair through the RAD52 homologous recombination pathway in the yeast Saccharomyces cerevisiae requires, among others, the RAD51, RAD52, and RAD54 genes. The biological importance of homologous recombination is underscored by the conservation of the RAD52 pathway from fungi to humans. The critical roles of the RAD52 group proteins in the early steps of recombination, the search for DNA homology and strand exchange, are now becoming apparent. Here, we report the purification of the human Rad54 protein. We showed that human Rad54 has ATPase activity that is absolutely dependent on double-stranded DNA. Unexpectedly, the ATPase activity appeared not absolutely required for the DNA repair function of human Rad54 in vivo. Despite the presence of amino acid sequence motifs that are conserved in a large family of DNA helicases, no helicase activity of human Rad54 was observed on a variety of different DNA substrates. Possible functions of human Rad54 in homologous recombination that couple the energy gained from ATP hydrolysis to translocation along DNA, rather than disruption of base pairing, are discussed.

Mapping of interaction domains between human repair proteins ERCC1 and XPF

ERCC1-XPF is a heterodimeric protein complexinvolved in nucleotide excision repair and recombinational processes. Like its homologous complex in Saccharomyces cerevisiae , Rad10-Rad1, it acts as a structure-specific DNA endonuclease, cleaving at duplex-single-stranded DNA junctions. In repair, ERCC1-XPF and Rad10-Rad1 make an incision on the the 5'-side of the lesion. No humans with a defect in the ERCC1 subunit of this protein complex have been identified and ERCC1-deficient mice suffer from severe developmental problems and signs of premature aging on top of a repair-deficient phenotype. Xeroderma pigmentosum group F patients carry mutations in the XPF subunit and generally show the clinical symptoms of mild DNA repair deficiency. All XP-F patients examined demonstrate reduced levels of XPF and ERCC1 protein, suggesting that proper complex formation is required for stability of the two proteins. To better understand the molecular and clinical consequences of mutations in the ERCC1-XPF complex, we decided to map the interaction domains between the two subunits. The XPF-binding domain comprises C-terminal residues 224-297 of ERCC1. Intriguingly, this domain resides outside the region of homology with its yeast Rad10 counterpart. The ERCC1-binding domain in XPF maps to C-terminal residues 814-905. ERCC1-XPF complex formation is established by a direct interaction between these two binding domains. A mutation from an XP-F patient that alters the ERCC1-binding domain in XPF indeed affects complex formation with ERCC1.

DNA-binding polarity of human replication protein A positions nucleases in nucleotide excision repair

The human single-stranded DNA-binding replication A protein (RPA) is involved in various DNA-processing events. By comparing the affinity of hRPA for artificial DNA hairpin structures with 3'- or 5'-protruding single-stranded arms, we found that hRPA binds ssDNA with a defined polarity; a strong ssDNA interaction domain of hRPA is positioned at the 5' side of its binding region, a weak ssDNA-binding domain resides at the 3' side. Polarity appears crucial for positioning of the excision repair nucleases XPG and ERCC1-XPF on the DNA. With the 3'-oriented side of hRPA facing a duplex ssDNA junction, hRPA interacts with and stimulates ERCC1-XPF, whereas the 5'-oriented side of hRPA at a DNA junction allows stable binding of XPG to hRPA. Our data pinpoint hRPA to the undamaged strand during nucleotide excision repair. Polarity of hRPA on ssDNA is likely to contribute to the directionality of other hRPA-dependent processes as well.

Xeroderma pigmentosum group C protein complex is the initiator of global genome nucleotide excision repair

The XPC-HR23B complex is specifically involved in global genome but not transcription-coupled nucleotide excision repair (NER). Its function is unknown. Using a novel DNA damage recognition-competition assay, we identified XPC-HR23B as the earliest damage detector to initiate NER: it acts before the known damage-binding protein XPA. Coimmunoprecipitation and DNase I footprinting show that XPC-HR23B binds to a variety of NER lesions. These results resolve the function of XPC-HR23B, define the first NER stages, and suggest a two-step mechanism of damage recognition involving damage detection by XPC-HR23B followed by damage verification by XPA. This provides a plausible explanation for the extreme damage specificity exhibited by global genome repair. In analogy, in the transcription-coupled NER subpathway, RNA polymerase II may take the role of XPC. After this subpathway-specific initial lesion detection, XPA may function as a common damage verifier and adaptor to the core of the NER apparatus.

A mouse model for the basal transcription/DNA repair syndrome trichothiodystrophy

The sun-sensitive form of the severe neurodevelopmental, brittle hair disorder trichothiodystrophy (TTD) is caused by point mutations in the essential XPB and XPD helicase subunits of the dual functional DNA repair/basal transcription factor TFIIH. The phenotype is hypothesized to be in part derived from a nucleotide excision repair defect and in part from a subtle basal transcription deficiency accounting for the nonrepair TTD features. Using a novel gene-targeting strategy, we have mimicked the causative XPD point mutation of a TTD patient in the mouse. TTD mice reflect to a remarkable extent the human disorder, including brittle hair, developmental abnormalities, reduced life span, UV sensitivity, and skin abnormalities. The cutaneous symptoms are associated with reduced transcription of a skin-specific gene strongly supporting the concept of TTD as a human disease due to inborn defects in basal transcription and DNA repair.

Biochemical and biological characterization of wild-type and ATPase-deficient Cockayne syndrome B repair protein

Cockayne syndrome (CS) is a nucleotide excision repair disorder characterized by sun (UV) sensitivity and severe developmental problems. Two genes have been shown to be involved: CSA and CSB. Both proteins play an essential role in preferential repair of transcription-blocking lesions from active genes. In this study we report the purification and characterization of baculovirus-produced HA-His6-tagged CSB protein (dtCSB), using a highly efficient three-step purification protocol. Microinjection of dtCSB protein in CS-B fibroblasts shows that it is biologically functional in vivo. dtCSB exhibits DNA-dependent ATPase activity, stimulated by naked as well as nucleosomal DNA. Using structurally defined DNA oligonucleotides, we show that double-stranded DNA and double-stranded DNA with partial single-stranded character but not true single-stranded DNA act as efficient cofactors for CSB ATPase activity. Using a variety of substrates, no overt DNA unwinding by dtCSB could be detected, as found with other SNF2/SWI2 family proteins. By site-directed mutagenesis the invariant lysine residue in the NTP-binding motif of CSB was substituted with a physicochemically related arginine. As expected, this mutation abolished ATPase activity. Surprisingly, the mutant protein was nevertheless able to partially rescue the defect in recovery of RNA synthesis after UV upon microinjection in CS-B fibroblasts. These results indicate that integrity of the conserved nucleotide-binding domain is important for the in vivo function of CSB but that also other properties independent from ATP hydrolysis may contribute to CSB biological functions.

DNA structural elements required for ERCC1-XPF endonuclease activity

The heterodimeric complex ERCC1-XPF is a structure-specific endonuclease responsible for the 5' incision during mammalian nucleotide excision repair (NER). Additionally, ERCC1-XPF is thought to function in the repair of interstrand DNA cross-links and, by analogy to the homologous Rad1-Rad10 complex in Saccharomyces cerevisiae, in recombination between direct repeated DNA sequences. To gain insight into the role of ERCC1-XPF in such recombinational processes and in the NER reaction, we studied in detail the DNA structural elements required for ERCC1-XPF endonucleolytic activity. Recombinant ERCC1-XPF, purified from insect cells, was found to cleave stem-loop substrates at the DNA junction in the absence of other proteins like replication protein A, showing that the structure-specific endonuclease activity is intrinsic to the complex. Cleavage depended on the presence of divalent cations and was optimal in low Mn2+ concentrations (0.2 mM). A minimum of 4-8 unpaired nucleotides was required for incisions by ERCC1-XPF. Splayed arm and flap substrates were also cut by ERCC1-XPF, resulting in the removal of 3' protruding single-stranded arms. All incisions occurred in one strand of duplex DNA at the 5' side of a junction with single-stranded DNA. The exact cleavage position varied from 2 to 8 nucleotides away from the junction. One single-stranded arm, protruding either in the 3' or 5' direction, was necessary and sufficient for correct positioning of incisions by ERCC1-XPF. Our data specify the engagement of ERCC1-XPF in NER and allow a more direct search for its specific role in recombination.

Plant homologue of human excision repair gene ERCC1 points to conservation of DNA repair mechanisms

Nucleotide excision repair (NER), a highly versatile DNA repair mechanism, is capable of removing various types of DNA damage including those induced by UV radiation and chemical mutagens. NER has been well characterized in yeast and mammalian systems but its presence in plants has not been reported. Here it is reported that a plant gene isolated from male germline cells of lily (Lilium longiflorum) shows a striking amino acid sequence similarity to the DNA excision repair proteins human ERCC1 and yeast RAD10. Homologous genes are also shown to be present in a number of taxonomically diverse plant genera tested, suggesting that this gene may have a conserved function in plants. The protein encoded by this gene is able to correct significantly the sensitivity to the cross-linking agent mitomycin C in ERCC1-deficient Chinese hamster ovary (CHO) cells. These findings suggest that the NER mechanism is conserved in yeast, animals and higher plants.