Cloning and characterization of the Drosophila homolog of the xeroderma pigmentosum complementation-group B correcting gene, ERCC3

Structural basis of the XPB helicase-Bax1 nuclease complex interacting with the repair bubble DNA

Nucleotide excision repair (NER) removes various DNA lesions caused by UV light and chemical carcinogens. The DNA helicase XPB plays a key role in DNA opening and coordinating damage incision by nucleases during NER, but the underlying mechanisms remain unclear. Here, we report crystal structures of XPB from Sulfurisphaera tokodaii (St) bound to the nuclease Bax1 and their complex with a bubble DNA having one arm unwound in the crystal. StXPB and Bax1 together spirally encircle 10 base pairs of duplex DNA at the double-/single-stranded (ds–ss) junction. Furthermore, StXPB has its ThM motif intruding between the two DNA strands and gripping the 3′-overhang while Bax1 interacts with the 5′-overhang. This ternary complex likely reflects the state of repair bubble extension by the XPB and nuclease machine. ATP binding and hydrolysis by StXPB could lead to a spiral translocation along dsDNA and DNA strand separation by the ThM motif, revealing an unconventional DNA unwinding mechanism. Interestingly, the DNA is kept away from the nuclease domain of Bax1, potentially preventing DNA incision by Bax1 during repair bubble extension.

Structural basis of the XPB-Bax1 complex as a dynamic helicase-nuclease machinery for DNA repair

Nucleotide excision repair (NER) is a major DNA repair pathway for a variety of DNA lesions. XPB plays a key role in DNA opening at damage sites and coordinating damage incision by nucleases. XPB is conserved from archaea to human. In archaea, XPB is associated with a nuclease Bax1. Here we report crystal structures of XPB in complex with Bax1 from Archaeoglobus fulgidus (Af) and Sulfolobus tokodaii (St). These structures reveal for the first time four domains in Bax1, which interacts with XPB mainly through its N-terminal domain. A Cas2-like domain likely helps to position Bax1 at the forked DNA allowing the nuclease domain to incise one arm of the fork. Bax1 exists in monomer or homodimer but forms a heterodimer exclusively with XPB. StBax1 keeps StXPB in a closed conformation and stimulates ATP hydrolysis by XPB while AfBax1 maintains AfXPB in the open conformation and reduces its ATPase activity. Bax1 contains two distinguished nuclease active sites to presumably incise DNA damage. Our results demonstrate that protein-protein interactions regulate the activities of XPB ATPase and Bax1 nuclease. These structures provide a platform to understand the XPB-nuclease interactions important for the coordination of DNA unwinding and damage incision in eukaryotic NER.

Natural Genetic Variation Screen in Drosophila Identifies Wnt Signaling, Mitochondrial Metabolism, and Redox Homeostasis Genes as Modifiers of Apoptosis

Apoptosis is the primary cause of degeneration in a number of neuronal, muscular, and metabolic disorders. These diseases are subject to a great deal of phenotypic heterogeneity in patient populations, primarily due to differences in genetic variation between individuals. This creates a barrier to effective diagnosis and treatment. Understanding how genetic variation influences apoptosis could lead to the development of new therapeutics and better personalized treatment approaches. In this study, we examine the impact of the natural genetic variation in the Drosophila Genetic Reference Panel (DGRP) on two models of apoptosis-induced retinal degeneration: overexpression of p53 or reaper (rpr). We identify a number of known apoptotic, neural, and developmental genes as candidate modifiers of degeneration. We also use Gene Set Enrichment Analysis (GSEA) to identify pathways that harbor genetic variation that impact these apoptosis models, including Wnt signaling, mitochondrial metabolism, and redox homeostasis. Finally, we demonstrate that many of these candidates have a functional effect on apoptosis and degeneration. These studies provide a number of avenues for modifying genes and pathways of apoptosis-related disease.

Mechanisms of interstrand DNA crosslink repair and human disorders

Interstrand DNA crosslinks (ICLs) are the link between Watson-Crick strands of DNAs with the covalent bond and prevent separation of DNA strands. Since the ICL lesion affects both strands of the DNA, the ICL repair is not simple. So far, nucleotide excision repair (NER), structure-specific endonucleases, translesion DNA synthesis (TLS), homologous recombination (HR), and factors responsible for Fanconi anemia (FA) are identified to be involved in ICL repair. Since the presence of ICL lesions causes severe defects in transcription and DNA replication, mutations in these DNA repair pathways give rise to a various hereditary disorders. NER plays an important role for the ICL recognition and removal in quiescent cells, and defects of NER causes congential progeria syndrome, such as xeroderma pigmentosum, Cockayne syndrome, and trichothiodystrophy. On the other hand, the ICL repair in S phase requires more complicated orchestration of multiple factors, including structure-specific endonucleases, and TLS, and HR. Disturbed this ICL repair orchestration in S phase causes genome instability resulting a cancer prone disease, Fanconi anemia. So far more than 30 factors in ICL repair have already identified. Recently, a new factor, UHRF1, was discovered as a sensor of ICLs. In addition to this, numbers of nucleases that are involved in the first incision, also called unhooking, of ICL lesions have also been identified. Here we summarize the recent studies of ICL associated disorders and repair mechanism, with emphasis in the first incision of ICLs.

DNA repair and transcriptional deficiencies caused by mutations in the Drosophila p52 subunit of TFIIH generate developmental defects and chromosome fragility

The transcription and DNA repair factor TFIIH is composed of 10 subunits. Mutations in the XPB, XPD, and p8 subunits are genetically linked to human diseases, including cancer. However, no reports of mutations in other TFIIH subunits have been reported in higher eukaryotes. Here, we analyze at genetic, molecular, and biochemical levels the Drosophila melanogaster p52 (DMP52) subunit of TFIIH. We found that DMP52 is encoded by the gene marionette in Drosophila and that a defective DMP52 produces UV light-sensitive flies and specific phenotypes during development: organisms are smaller than their wild-type siblings and present tumors and chromosomal instability. The human homologue of DMP52 partially rescues some of these phenotypes. Some of the defects observed in the fly caused by mutations in DMP52 generate trichothiodystrophy and cancer-like phenotypes. Biochemical analysis of DMP52 point mutations introduced in human p52 at positions homologous to those of defects in DMP52 destabilize the interaction between p52 and XPB, another TFIIH subunit, thus compromising the assembly of the complex. This study significantly extends the role of p52 in regulating XPB ATPase activity and, consequently, both its transcriptional and nucleotide excision repair functions.

Conserved XPB core structure and motifs for DNA unwinding: implications for pathway selection of transcription or excision repair

The human xeroderma pigmentosum group B (XPB) helicase is essential for transcription, nucleotide excision repair, and TFIIH functional assembly. Here, we determined crystal structures of an Archaeoglobus fulgidus XPB homolog (AfXPB) that characterize two RecA-like XPB helicase domains and discover a DNA damage recognition domain (DRD), a unique RED motif, a flexible thumb motif (ThM), and implied conformational changes within a conserved functional core. RED motif mutations dramatically reduce helicase activity, and the DRD and ThM, which flank the RED motif, appear structurally as well as functionally analogous to the MutS mismatch recognition and DNA polymerase thumb domains. Substrate specificity is altered by DNA damage, such that AfXPB unwinds dsDNA with 3' extensions, but not blunt-ended dsDNA, unless it contains a lesion, as shown for CPD or (6-4) photoproducts. Together, these results provide an unexpected mechanism of DNA unwinding with implications for XPB damage verification in nucleotide excision repair.

DNA repair and transcriptional effects of mutations in TFIIH in Drosophila development

Mutations in XPB and XPD TFIIH helicases have been related with three hereditary human disorders: xeroderma pigmentosum, Cockayne syndrome, and trichothiodystrophy. The dual role of TFIIH in DNA repair and transcription makes it difficult to discern which of the mutant TFIIH phenotypes is due to defects in any of these different processes. We used haywire (hay), the Drosophila XPB homolog, to dissect this problem. Our results show that when hay dosage is affected, the fly shows defects in structures that require high levels of transcription. We found a genetic interaction between hay and cdk7, and we propose that some of these phenotypes are due to transcriptional deficiencies. We also found more apoptotic cells in imaginal discs and in the CNS of hay mutant flies than in wild-type flies. Because this abnormal level of apoptosis was not detected in cdk7 flies, this phenotype could be related to defects in DNA repair. In addition the apoptosis induced by p53 Drosophila homolog (Dmp53) is suppressed in heterozygous hay flies.

Isolation and genetic characterisation of the Drosophila homologue of (SCE)REV3, encoding the catalytic subunit of DNA polymerase zeta

In Drosophila, about 30 mutants are known that show hypersensitivity to the methylating agent methyl methane sulfonate (MMS). Addition of this agent to the medium results in an increased larval mortality of the mutants. Using a P-insertion mutagenesis screen, three MMS-sensitive mutants on chromosome II were isolated. One of these is allelic to the known EMS-induced mus205 (mutagen sensitive) mutant. In the newly isolated mutant, a P-element is detected in region 43E by in situ hybridisation. The localisation of mus205 to this region was confirmed by deficiency mapping. The gene was cloned and shows strong homology to the Saccharomyces cerevisiae REV3 gene. The REV3 gene encodes the catalytic subunit of DNA polymerase zeta, involved in translesion synthesis. The P-element is inserted in the first exon of the mus205 gene resulting in an aberrant mRNA, encoding a putative truncated protein containing only the first 13 of the 2130 aa native Drosophila protein. The mus205 mutant is hypersensitive to alkylating agents and UV, but not to ionising radiation. In contrast to reported data, in germ cells, the mutant has no effect on mutability by X-rays, NQO and alkylating agents. In somatic cells, the mutant shows no effect on MMS-induced mutations and recombinations. This phenotype of the Drosophila mus205 mutant is strikingly different from the phenotype of the yeast rev3 mutant, which is hypomutable after UV, X-rays, NQO and alkylating agents.

DNA repair defects and other (mus)takes in Drosophila melanogaster

Preservation of the structural integrity of DNA in any organism is crucial to its health and survival. Such preservation is achieved by an extraordinary cellular arsenal of damage surveillance and repair functions, many of which are now being defined at the gene and protein levels. Mutants hypersensitive to the killing effects of DNA-damaging agents have been instrumental in helping to identify DNA repair-related genes and to elucidate repair mechanisms. In Drosophila melanogaster, such strains are generally referred to as mutagen-sensitive (mus) mutants and currently define more than 30 genetic loci. Whereas most mus mutants have been recovered on the basis of hypersensitivity to the monofunctional alkylating agent methyl methanesulfonate, they nevertheless constitute a phenotypically diverse group, with many mutants having effects beyond mutagen sensitivity. These phenotypes include meiotic dysfunctions, somatic chromosome instabilities, chromatin abnormalities, and cell proliferation defects. Within the last few years numerous mus and other DNA repair-related genes of Drosophila have been molecularly cloned, providing new insights into the functions of these genes. This article outlines strategies for isolating mus mutations and reviews recent advances in the Drosophila DNA repair field, emphasizing mutant analysis and gene cloning.

A mutation in repB, the dictyostelium homolog of the human xeroderma pigmentosum B gene, has increased sensitivity to UV-light but normal morphogenesis

Nucleotide excision repair (NER) is an important cellular defense mechanism which protects the integrity of the genome by removing DNA damage caused by UV-light or chemical agents. In humans, defects in the NER pathway result in the disease xeroderma pigmentosum (XP) which is characterized by increased UV-sensitivity, with increased propensity for skin cancer, and an array of developmental abnormalities. Some XP patients exhibit, in addition, symptoms of Cockayne's syndrome (CS) and trichothiodystrophy (TTD), which are characterized by increased UV-sensitivity, without increased cancer incidence, and an array of developmental abnormalities. Some NER genes, including the DNA helicases XPB and XPD, have been shown to function in transcription as well as repair, by virtue of being an integral part of the transcription initiation factor TFIIH. This dual function may account for the above-mentioned wide pleiotropy of phenotypes associated with defects in NER genes, and may explain why some XP patients exhibit developmental abnormalities in addition to XP symptoms. To date, only five XPB patients with three different mutations in the XPB gene have been reported. One of these mutations is a C to A transversion at the splice site at the beginning of the last exon, which resulted in a frameshift throughout the last exon. This patient shows combined clinical symptoms of XP and CS. The recent cloning of the repB gene, the Dictyostelium discoideum homolog of XPB, allowed us to generate a similar C-terminal mutation in the Dictyostelium, in order to test whether the defect in this NER gene has an effect on growth or development. To this end, we have constructed a C-terminal deletion repB mutant in Dictyostelium. To avoid the possibility that a null mutant would be lethal, we used direct homologous recombination to create a 46 amino acid C-terminal deletion mutant. Indeed, we were unable to obtain mutants with a longer 95 amino acid deletion. The repB delta C46 mutants showed an increased sensitivity to UV-light, but a normal pattern of UV-induced expression of repair genes, and no immediately obvious defect in either growth rate or development. The results suggest that the associated developmental defects in the human XPB patients may be due to mutations in another gene.

Cloning of a cDNA from Arabidopsis thaliana homologous to the human XPB gene

The human gene XPB, defective in xeroderma pigmentosum patients complementation group B, encodes a DNA helicase involved in several DNA metabolic pathways, including DNA repair and transcription. The high conservation of this gene has allowed the cloning of homologs in various species, such as mouse, yeast and Drosophila. Not much information on the molecular basis of nucleotide excision repair in plants is available, but these organisms may have similar mechanisms to other eukaryotes. A homolog of XPB was isolated in Arabidopsis thaliana by using polymerase chain reaction (PCR) with degenerate oligonucleotides based on protein domains which are conserved among several species. Screening of an Arabidopsis cDNA library led to the identification and isolation of a cDNA clone with 2670 bp encoding a predicted protein of 767 amino acids, denoted araXPB. Genomic analysis indicated that this is a nuclear single copy gene in plant cells. Northern blot with the cDNA probe revealed a major transcript which migrated at approx. 2,800 b, in agreement with the size of the cDNA isolated. The araXPB protein shares approximately 50% identical and 70% conserved amino acids with the yeast and human homologs. The plant protein maintains all the functional domains found in the other proteins, including nuclear localization signal, DNA-binding domain and helicase motifs, suggesting that it might also act as part of the RNA transcription apparatus, as well as nucleotide excision repair in plant cells.

Search for DNA repair pathways in Drosophila melanogaster

The knowledge about the existence of different pathways for the repairing of DNA lesions has made possible a better understanding of mutation processes. The double mutant method has been shown to be useful for grouping rad mutants in yeast. Through this method, three different groups of repair mechanisms were found: (a) RAD3 group corresponding to the excision repair of UV lesions, (b) RAD6 group corresponding to the translesion type of post-replication repair and (c) RAD52 group corresponding to the recombination type of post-replication repair. In this work, a search for a classification of Drosophila mus mutants in groups analogous to yeast RAD groups is done. Information obtained by double mutant studies was integrated with that obtained by biochemical, recombination, DNA damaging agent sensitivity and mutation studies. The following groups were found: (a) group of mei9 and mus201, analogous to RAD3, (b) group of mei41 and mus302 analogous to RAD52 and, (c) group of mus104 and mus101 analogous to RAD6. In addition, there are mutants that belong to a group corresponding to pre-replication repair of MMS lesions such as mus103, mus306 and mus207. As a peculiarity of Drosophila, it was found that interaction between pre- and post-replication repair mechanisms is indifferent and not synergistic as was found in yeast. A possible explanation could be a weaker control of post-replication repair mechanisms in Drosophila than in yeast. It is expected that this research could help for a better understanding of repair mechanisms in complex organisms.

Differential developmental expression of the rep B and rep D xeroderma pigmentosum related DNA helicase genes from Dictyostelium discoideum

DNA helicases are essential to many cellular processes including recombination, replication and transcription, and some helicases function in multiple processes. The helicases encoded by the Xeroderma pigmentosum (XP) B and D genes function in both nucleotide excision repair and transcription initiation. Mutations that affect the repair function of these proteins result in XP while mutations affecting transcription result in neurological and developmental abnormalities, although the underlying molecular and cellular basis for these phenotypes is not well understood. To better understand the developmental roles of these genes, we have now identified and characterized the rep B and rep D genes from the cellular slime mold Dictyostelium discoideum . Both genes encode DNA helicases of the SF2 superfamily of helicases. The rep D gene contains no introns and the rep B gene contains only one intron, which makes their genomic structures dramatically different from the corresponding genes in mammals and fish. However the predicted Dictyostelium proteins share high homology with the human XPB and XPD proteins. The single copy of the rep B and D genes map to chromosomes 3 and 1, respectively. The expression of rep B and D (and the previously isolated rep E) genes during multicellular development was examined, and it was determined that each rep gene has a unique pattern of expression, consistent with the idea that they have specific roles in development. The pattern and extent of expression of these genes was not affected by the growth history of the cells, implying that the expression of these genes is tightly regulated by the developmental program. The expression of the rep genes is a very early step in development and may well represent a key event in the initiation of development in this organism.

Cloning of Schizosaccharomyces pombe rph16+, a gene homologous to the Saccharomyces cerevisiae RAD16 gene

The RAD16 gene is involved in the nucleotide excision repair of UV damage in the transcriptional silenced mating type loci (Terleth et al., 1990 and Bang et al., 1992) and in non-transcribed stands of active genes in Saccharomyces cerevisiae (Verhage et al., 1994). Using touchdown-PCR with primers derived from various domains of the S. cerevisiae Rad 16 protein, a specific Schizosaccharomyces pombe probe was isolated. This probe was used to obtain the complete RAD16 homologous gene from a S. pombe chromosomal bank. DNA sequence analysis of the rph16+ gene revealed an open reading frame of 854 amino acids. Comparison of the amino acid sequences of the Rhp16 and Rad16 proteins showed a high level of conservation: 68% similarity. The Rhp16 protein sequence contains the two Zn-finger motifs and the putative helicase domains as found in the Rad16 protein. Like the RAD16, the rph16+ gene is UV-inducible (Bang et al., 1995). In analogy with the rad16 mutant, the rhp16 disruption mutant is viable and grows normally, indicating that the gene does not have an essential function. The rhp16 disruption mutant is not sensitive for UV but is sensitive for cisplatin. The rhp16+ gene cloned behind the GAI 1 promoter partially complements the UV sensitivity and the defect in the non-transcribed strand DNA repair of a S. cerevisiae rad16 mutant, indicating functional homology between the rhp16+ and RAD16 genes. The structural and functional homology between the two genes suggests that the RAD16 dependent subpathway of NER for the repair of non-transcribed DNA is evolutionary conserved.

Isolation of the Schizosaccharomyces pombe RAD54 homologue, rhp54+, a gene involved in the repair of radiation damage and replication fidelity

The RAD54 gene of Saccharomyces cerevisiae encodes a putative helicase, which is involved in the recombinational repair of DNA damage. The RAD54 homologue of the fission yeast Schizosaccharomyces pombe, rhp54+, was isolated by using the RAD54 gene as a heterologous probe. The gene is predicted to encode a protein of 852 amino acids. The overall homology between the mutual proteins of the two species is 67% with 51% identical amino acids and 16% similar amino acids. A rhp54 deletion mutant is very sensitive to both ionizing radiation and UV. Fluorescence microscopy of the rhp54 mutant cells revealed that a large portion of the cells are elongated and occasionally contain aberrant nuclei. In addition, FACS analysis showed an increased DNA content in comparison with wild-type cells. Through a minichromosome-loss assay it was shown that the rhp54 deletion mutant has a very high level of chromosome loss. Furthermore, the rhp54 mutation in either a rad17 or a cdc2.3w mutant background (where the S-phase/mitosis checkpoint is absent) shows a significant reduction in viability. It is hypothesized that the rhp54+ gene is involved in the recombinational repair of UV and X-ray damage and plays a role in the processing of replication-specific lesions.

Purification and characterisation of a DNA helicase, dheI I, from Drosophila melanogaster embryos

We have purified a DNA helicase (dhel l) from early Drosophila embryos. dhel l co-purifies with the single-stranded DNA binding protein dRP-A over two purification steps, however, the proteins can be separated by their different native molecular weight, with dhel l activity co-sedimenting with a polypeptide of approximately 200 kDa and a sedimentation coefficient of 8.6 S. The enzyme needs ATP hydrolysis and divalent cations for displacement activity. It is very salt sensitive, having a Mg2+ optimum of 0.5 mM and being inhibited by NaCl concentration > 10 mM. Dhel l moves 5'-->3' on the DNA strand to which it is bound. Unwinding activity decreases with increasing length of the double-stranded region suggesting a distributive mode of action. However, addition of dRP-A to the displacement reaction stimulates the activity on substrates with >300 nucleotides double-stranded region suggesting a specific interaction between these two proteins.

Expression and functional analyses of the Dxpa gene, the Drosophila homolog of the human excision repair gene XPA

Xeroderma pigmentosum (XP) is a human hereditary disease characterized by a defect in DNA repair after exposure to ultraviolet light. Among the seven groups of XP, group A (XP-A) patients show the most severe deficiency in excision repair and a wide variety of cutaneous and neurological disorders. We have cloned homologs of the human XPA gene from chicken, Xenopus, and Drosophila, and sequence analysis revealed that these genes are highly conserved throughout evolution. Here, we report characterization of the Drosophila homolog of the human XPA gene (Dxpa). The Dxpa gene product shows DNA repair activities in an in vitro repair system, and Dxpa cDNA has been shown to complement a mutant allele of human XP-A cells by transfection. Polytene chromosome in situ hybridization mapped Dxpa to 3F6-8 on the X chromosome, where no mutant defective in excision repair was reported. Northern blot analysis showed that the gene is continuously expressed in all stages of fly development. Interestingly, the Dxpa protein is strongly expressed in the central nervous system and muscles as revealed by immunohistochemical analysis using anti-Dxpa antibodies, consistent with the results obtained in transgenic flies expressing a Dxpa-beta-galactosidase fusion gene driven by the Dxpa promoter.

Expression of the excision repair gene, ERCC3 (excision repair cross-complementing), during mouse development

Expression of the human ERCC3 (excision repair cross-complementing) gene in cells from patients with xeroderma pigmentosum (XP) group B (XP-B) corrects the defect in repair of UV light-induced DNA damage. XP-B is one of three groups of XP which exhibit the clinical symptoms of both XP and Cockayne's Syndrome (CS). CS and XP-B/CS patients develop severe neurological dysfunction during development. In order to explore the link between the defective gene and the neurological deficits in XP/CS, we have studied the expression of ERCC3 mRNA in developing mice by in situ hybridisation. ERCC3 was found to be ubiquitously expressed in cells from all regions and all developmental stages, from 9 day post-coitum embryo, to 15 day post-natal brain. In post-natal brain, regional differences in expression correlated with cell density and there was no evidence of cell specific or developmental alterations in levels of expression. These results indicate that the constitutively expressed gene does not perform a discrete developmental function. The neurological defects apparent in XP-B are likely to arise pleiotypically from the participation of ERCC3 in interactions with other elements involved in particular aspects of neurodevelopmental control. These results emphasise the developmental importance of genes whose primary functions are apparently unconnected with development.

Mutational analysis of ERCC3, which is involved in DNA repair and transcription initiation: identification of domains essential for the DNA repair function

The human ERCC3 gene, which corrects specifically the nucleotide excision repair defect in human xeroderma pigmentosum group B and cross-complements the repair deficiency in rodent UV-sensitive mutants of group 3, encodes a presumed DNA helicase that is identical to the p89 subunit of the general transcription factor TFIIH/BTF2. To examine the significance of the postulated functional domains in ERCC3, we have introduced mutations in the ERCC3 cDNA by means of site-specific mutagenesis and have determined the repair capacity of each mutant to complement the UV-sensitive phenotype of rodent group 3 cells. A conservative substitution of arginine for the invariant lysine residue in the ATPase motif (helicase domain I), six deletion mutations in the other helicase domains, and a deletion in the potential helix-turn-helix DNA-binding motif fail to complement the ERCC3 excision repair defect of rodent group 3 mutants, which implies that the helicase domains as well as the potential DNA-binding motif are required for the repair function of ERCC3. Analysis of carboxy-terminal deletions suggests that the carboxy-terminal exon may comprise a distinct determinant for the DNA repair function. In addition, we show that a functional epitope-tagged version of ERCC3 accumulates in the nucleus. Deletion of the putative nuclear location signal impairs neither the nuclear location nor the repair function, indicating that other sequences may (also) be involved in translocation of ERCC3 to the nucleus.

Mutational analysis of ERCC3, which is involved in DNA repair and transcription initiation: identification of domains essential for the DNA repair function

The human ERCC3 gene, which corrects specifically the nucleotide excision repair defect in human xeroderma pigmentosum group B and cross-complements the repair deficiency in rodent UV-sensitive mutants of group 3, encodes a presumed DNA helicase that is identical to the p89 subunit of the general transcription factor TFIIH/BTF2. To examine the significance of the postulated functional domains in ERCC3, we have introduced mutations in the ERCC3 cDNA by means of site-specific mutagenesis and have determined the repair capacity of each mutant to complement the UV-sensitive phenotype of rodent group 3 cells. A conservative substitution of arginine for the invariant lysine residue in the ATPase motif (helicase domain I), six deletion mutations in the other helicase domains, and a deletion in the potential helix-turn-helix DNA-binding motif fail to complement the ERCC3 excision repair defect of rodent group 3 mutants, which implies that the helicase domains as well as the potential DNA-binding motif are required for the repair function of ERCC3. Analysis of carboxy-terminal deletions suggests that the carboxy-terminal exon may comprise a distinct determinant for the DNA repair function. In addition, we show that a functional epitope-tagged version of ERCC3 accumulates in the nucleus. Deletion of the putative nuclear location signal impairs neither the nuclear location nor the repair function, indicating that other sequences may (also) be involved in translocation of ERCC3 to the nucleus.

Cloning the Drosophila homolog of the xeroderma pigmentosum complementation group C gene reveals homology between the predicted human and Drosophila polypeptides and that encoded by the yeast RAD4 gene

A human xeroderma pigmentosum group C (XPC) cDNA has been previously isolated by functional complementation (Legerski and Peterson, Nature, 359, 70-73, 1992). Sequence analysis did not reveal protein motifs which might suggest a possible biochemical function for the putative XPC protein. In order to identify functional domains in the translated XPC sequence the homologous gene from Drosophila melanogaster, designated XPCDM, was cloned by DNA hybridization. Sequence analysis of an apparently full-length cDNA revealed an open reading frame which can encode a predicted polypeptide of 1293 amino acids. Significant homology of the C-terminal 346 amino acids with both the human XPC and Saccharomyces cerevisiae Rad4 protein sequences is observed, suggesting that these proteins are functional homologs.

A new nucleotide-excision-repair gene associated with the disorder trichothiodystrophy

The sun-sensitive, cancer-prone genetic disorder xeroderma pigmentosum (XP) is associated in most cases with a defect in the ability to carry out excision repair of UV damage. Seven genetically distinct complementation groups (i.e., A-G) have been identified. A large proportion of patients with the unrelated disorder trichothiodystrophy (TTD), which is characterized by hair-shaft abnormalities, as well as by physical and mental retardation, are also deficient in excision repair of UV damage. In most of these cases the repair deficiency is in the same complementation group as is XP group D. We report here on cells from a patient, TTD1BR, in which the repair defect complements all known XP groups (including XP-D). Furthermore, microinjection of various cloned human repair genes fails to correct the repair defect in this cell strain. The defect in TTD1BR cells is therefore in a new gene involved in excision repair in human cells. The finding of a second DNA repair gene that is associated with the clinical features of TTD argues strongly for an involvement of repair proteins in hair-shaft development.

Cloning the RAD51 homologue of Schizosaccharomyces pombe

The RAD51 gene of Saccharomyces cerevisiae encodes a RecA like protein, which is involved in the recombinational repair of double strand breaks. We have isolated the RAD51 homologue, rhp51+, of the distantly related yeast strain Schizosaccharomyces pombe by heterologous hybridization. DNA sequence analysis of the rhp51+ gene revealed an open reading frame of 365 amino acids. Comparison of the amino acid sequences of RAD51 and rhp51+ showed a high level of conservation: 69% identical amino acids. There are two Mlul sites in the upstream region which may be associated with cell cycle regulation of the rhp51+ gene. The rhp51+ null allele, constructed by disruption of the coding region, is extremely sensitive to X-rays, indicating that the rhp51+ gene, like RAD51, is also involved in the repair of X-ray damage. The structural and functional homology between rhp51+ and RAD51 suggests evolutionary conservation of certain steps in the recombinational repair pathway.

Mammalian DNA-repair genes

The sequence and functional homology of certain genes between mammalian and non-mammalian eukaryotes has facilitated significant advances in our understanding of mammalian DNA repair. Several novel DNA damage and repair genes have been identified by using a variety of approaches. Study of these genes will lead to an increased understanding of the biological consequences of aberrant DNA maintenance in humans and other species.