Cloning of the cDNA and genomic DNA that correspond to the recA-like gene of Drosophila melanogaster

The consequences of Rad51 overexpression for normal and tumor cells

The Rad51 recombinase is an essential factor for homologous recombination and the repair of DNA double strand breaks, binding transiently to both single stranded and double stranded DNA during the recombination reaction. The use of a homologous recombination mechanism to repair DNA damage is controlled at several levels, including the binding of Rad51 to single stranded DNA to form the Rad51 nucleofilament, which is controlled through the action of DNA helicases that can counteract nucleofilament formation. Overexpression of Rad51 in different organisms and cell types has a wide assortment of consequences, ranging from increased homologous recombination and increased resistance to DNA damaging agents to disruption of the cell cycle and apoptotic cell death. Rad51 expression is increased in p53-negative cells, and since p53 is often mutated in tumor cells, there is a tendency for Rad51 to be overexpressed in tumor cells, leading to increased resistance to DNA damage and drugs used in chemotherapies. As cells with increased Rad51 levels are more resistant to DNA damage, there is a selection for tumor cells to have higher Rad51 levels. While increased Rad51 can provide drug resistance, it also leads to increased genomic instability and may contribute to carcinogenesis.

Characterization of Drosophila Rad51/SpnA protein in DNA binding and embryonic development

The Rad51 is a highly conserved protein throughout the eukaryotic kingdom and an essential enzyme in DNA repair and recombination. It possesses DNA binding activity and ATPase activity, and interacts with meiotic chromosomes during prophase I of meiosis. Drosophila Rad51, Spindle-A (SpnA) protein has been shown to be involved in repair of DNA damage in somatic cells and meiotic recombination in female germ cells. In this study, DNA binding activity of SpnA is demonstrated by both agarose gel mobility shift assay and restriction enzyme protection assay. SpnA is also shown to interact with meiotic chromosomes during prophase I in the primary spermatocytes of hsp26-spnA transgenic flies. In addition, SpnA is highly expressed in embryos, and the depletion of SpnA by RNA interference (RNAi) leads to embryonic lethality implying that SpnA is involved in early embryonic development. Therefore, these results suggest that Drosophila SpnA protein possesses properties similar to mammalian Rad51 homologs.

Functional analysis of the Drosophila Rad51 gene (spn-A) in repair of DNA damage and meiotic chromosome segregation

Rad51 is a crucial enzyme in DNA repair, mediating the strand invasion and strand exchange steps of homologous recombination (HR). Mutations in the Drosophila Rad51 gene (spn-A) disrupt somatic as well as meiotic double-strand break (DSB) repair, similar to fungal Rad51 genes. However, the sterility of spn-A mutant females prevented a thorough analysis of the role of Rad51 in meiosis. In this study, we generated transgenic animals that express spn-A dsRNA under control of an inducible promoter, and examined the effects of inhibiting expression of spn-A on DNA repair, meiotic recombination and meiotic chromosome pairing and segregation. We found that depletion of spn-A mRNA had no effect on the viability of non-mutagen-treated transgenic animals but greatly reduced the survival of larvae that were exposed to the radiomimetic drug MMS, in agreement with the MMS and X-ray sensitivity of spn-A mutant animals. We also found that increases in dose of spn-A gene enhanced larval resistance to MMS exposure, suggesting that at high damage levels, Rad51 protein levels may be limiting for DNA repair. spn-A RNAi strongly stimulated X-X nondisjunction and decreased recombination along the X in female meiosis, consistent with a requirement of Rad51 in meiotic recombination. However, neither RNAi directed against the spn-A mRNA nor homozygosity for a spn-A null mutation had any effect on male fertility or on X-Y segregation in male meiosis, indicating that Rad51 likely plays no role in male meiotic chromosome pairing. Our results support a central role for Rad51 in HR in both somatic and meiotic DSB repair, but indicate that Rad51 in Drosophila is dispensable for meiotic chromosome pairing. Our results also provide the first demonstration that RNAi can be used to inhibit the functions of meiotic genes in Drosophila.

Genetic and cytological characterization of the RecA-homologous proteins Rad51 and Dmc1 of Schizosaccharomyces pombe

The Schizosaccharomyces pombe rad51(+) and dmc1(+) genes code for homologues of the Escherichia coli recombination protein RecA. Deletion of rad51(+) causes slow growth, retardation of cell division and a decrease in viability. rad51Delta cells have a defect in mating-type switching. The DNA modification at the mating-type locus required for mating-type switching contributes to slow growth in the rad51 mutant. Cell mating is reduced in crosses homozygous for rad51Delta. Ectopic expression of the dmc1(+) gene allowed us to demonstrate that the reduction in meiotic recombination in dmc1 mutants is not caused by a disturbance of rad24 expression from the dmc1- rad24 bicistronic RNA. We describe the functional defects of terminally epitope-tagged Dmc1 and Rad51 and discuss it in terms of protein interaction. Presumptive Rad51 and Dmc1 foci were detected on spreads of meiotic chromatin.

Genomic integrity and the repair of double-strand DNA breaks

The induction of double-strand breaks (DSBs) in DNA by exposure to DNA damaging agents or as intermediates in normal cellular processes, creates a severe threat for the integrity of the genome. Unrepaired or incorrectly repaired DSBs lead to broken chromosomes and/or gross chromosomal rearrangements which are frequently associated with tumor formation in mammals. To maintain the integrity of the genome and to prevent the formation of chromosomal aberrations, several pathways exist in eukaryotes: homologous recombination (HR), non-homologous end joining (NHEJ) and single-strand annealing (SSA). These mechanisms are conserved in evolution, but the relative contribution depends on the organism, cell type and stage of the cell cycle. In yeast, DSBs are primarily repaired via HR while in higher eukaryotes, both HR and NHEJ are important. In mammals, defects in both HR or NHEJ lead to a predisposition to cancer and at the cellular level, the frequency of chromosomal aberrations is increased. This review summarizes our current knowledge about DSB-repair with emphasis on recent progress in understanding the precise biochemical activities of individual proteins involved.

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.

RAD51 is required for propagation of the germinal nucleus in Tetrahymena thermophila

RAD51, the eukaryote homolog of the Escherichia coli recA recombinase, participates in homologous recombination during mitosis, meiosis, and in the repair of double-stranded DNA breaks. The Tetrahymena thermophila RAD51 gene was recently cloned, and the in vitro activities and induction of Rad51p following DNA damage were shown to be similar to that of RAD51 from other species. This study describes the pattern of Tetrahymena RAD51 expression during both the cell cycle and conjugation. Tetrahymena RAD51 mRNA abundance is elevated during macronuclear S phase during vegetative cell growth and with both meiotic prophase and new macronuclear development during conjugation. Gene disruption of the macronuclear RAD51 locus leads to severe abnormalities during both vegetative growth and conjugation. rad51 nulls divide slowly and incur rapid deterioration of their micronuclear chromosomes. Conjugation of two rad51 nulls leads to an arrest early during prezygotic development (meiosis I). We discuss the potential usefulness of the ciliates' characteristic nuclear duality for further analyses of the potentially unique roles of Tetrahymena RAD51.

Newt RAD51: cloning of cDNA and analysis of gene expression during spermatogenesis

A cDNA encoding a newt homolog of Escherichia coli RecA and yeast RAD51 from a testis cDNA library was isolated. The newt RAD51 (nRAD51) cDNA predicted a 337 amino acid protein with a 95-96% amino acid identity to Xenopus and mammalian RAD51. Northern blot analysis showed that nRAD51 mRNA, 1.7 kb in length, was expressed strongly in the testis and ovary, but weakly in the liver, kidney and brain. In situ hybridization revealed that expression of nRAD51 mRNA was barely observed in primary spermatogonia (one cell in a cyst) and early secondary spermatogonia (two to four cells in a cyst), but increased in late secondary spermatogonia (> or =eight cells in a cyst), reaching a maximum level in leptotene-zygotene spermatocytes, and thereafter declined. These results suggest that nRAD51 is involved in mitotic recombination in spermatogonia as well as in meiotic recombination in spermatocytes.

The RAD51 protein supports homologous recombination by an exchange mechanism in mammalian cells

Information concerning the function of recombination proteins in mammalian cells has been obtained from biochemical studies, but little is known about their mechanisms of action in growing cells. The eukaryotic recombination protein RAD51, a homologue of the Escherichia coli RecA protein, has been shown to interact with various proteins, including the p53 protein, the guardian of genomic stability maintenance. Here, the hamster RAD51 protein, CgRAD51, has been overexpressed in the SPD8 cell line, derived from Chinese hamster V79 cells. This cell line offers unique possibilities for studying different mechanisms for homologous recombination on endogenous substrates. We report that the SPD8 cell line contains a mutated p53 gene, which provides new insights into the recombination process in these cells. The present study demonstrates that overexpression of CgRAD51 in these cells results in a two- to threefold increase in endogenous recombination. In addition, sequence analysis indicated that RAD51 promotes homologous recombination by a chromatid exchange mechanism.

Targeted inactivation of mouse RAD52 reduces homologous recombination but not resistance to ionizing radiation

The RAD52 epistasis group is required for recombinational repair of double-strand breaks (DSBs) and shows strong evolutionary conservation. In Saccharomyces cerevisiae, RAD52 is one of the key members in this pathway. Strains with mutations in this gene show strong hypersensitivity to DNA-damaging agents and defects in recombination. Inactivation of the mouse homologue of RAD52 in embryonic stem (ES) cells resulted in a reduced frequency of homologous recombination. Unlike the yeast Scrad52 mutant, MmRAD52(-/-) ES cells were not hypersensitive to agents that induce DSBs. MmRAD52 null mutant mice showed no abnormalities in viability, fertility, and the immune system. These results show that, as in S. cerevisiae, MmRAD52 is involved in recombination, although the repair of DNA damage is not affected upon inactivation, indicating that MmRAD52 may be involved in certain types of DSB repair processes and not in others. The effect of inactivating MmRAD52 suggests the presence of genes functionally related to MmRAD52, which can partly compensate for the absence of MmRad52 protein.

In vivo gap repair in Drosophila: a one-way street with many destinations

While it has long been possible to study the process of recombination in yeast and other single-celled organisms, it has been difficult to distinguish between pathways of meiotic and mitotic recombination in multicellular eukaryotes. The experimental system described here bridges the historically separated fields of Genetic Recombination and DNA Repair in Drosophila. It is now feasible to study the repair of unique double-strand breaks induced in the Drosophila genome by the excision of a P-transposable element or by cleavage at an introduced endonuclease recognition sequence. This repair can be studied in both somatic cells and mitotically dividing germ cells. The repair of these breaks occurs mainly by copying sequence from a template located anywhere in the karyoplasm, and occurs in both male and female flies. This system, which was the first of its kind in metazoan organisms, is now being used for gene targeting in Drosophila. This review summarizes results that provide new insights into the process of gap repair in Drosophila and outline some recent experiments that demonstrate the power of the gene targeting technique.

Mammalian Rad51 protein: a RecA homologue with pleiotropic functions

During the last years, homologues of E coli RecA have been cloned in numerous species including man. These Rad51 proteins share sequence as well as functional homologies with the bacterial protein. Human Rad51 (HsRad51) is able to catalyze strand exchange in vitro between homologous DNAs, but with a lower efficiency compared to that of RecA. This suggests the requirement of additional factors. A very interesting feature of Rad51 is its essential role in mouse which could mean that it has gained an essential function in cell growth. The interaction of HsRad51 with several tumor suppressor genes namely p53, BRCA1 and BRCA2 implies possible role(s) of this protein in tumorigenesis. Thus, the continued study of Rad51 should bring important insights not only into homologous recombination mechanisms but also into cell proliferation regulation.

The Drosophila melanogaster RAD54 homolog, DmRAD54, is involved in the repair of radiation damage and recombination

The RAD54 gene of Saccharomyces cerevisiae plays a crucial role in recombinational repair of double-strand breaks in DNA. Here the isolation and functional characterization of the RAD54 homolog of the fruit fly Drosophila melanogaster, DmRAD54, are described. The putative Dmrad54 protein displays 46 to 57% identity to its homologs from yeast and mammals. DmRAD54 RNA was detected at all stages of fly development, but an increased level was observed in early embryos and ovarian tissue. To determine the function of DmRAD54, a null mutant was isolated by random mutagenesis. DmRADS4-deficient flies develop normally, but the females are sterile. Early development appears normal, but the eggs do not hatch, indicating an essential role for DmRAD54 in development. The larvae of mutant flies are highly sensitive to X rays and methyl methanesulfonate. Moreover, this mutant is defective in X-ray-induced mitotic recombination as measured by a somatic mutation and recombination test. These phenotypes are consistent with a defect in the repair of double-strand breaks and imply that the RAD54 gene is crucial in repair and recombination in a multicellular organism. The results also indicate that the recombinational repair pathway is functionally conserved in evolution.

Genomic characterization of the mouse homolog of the Saccharomyces cerevisiae recombination and double-strand break repair gene RAD52

The yeast Saccharomyces cerevisiae RAD52 gene is involved in recombination and DNA double-strand break repair. Recently, mouse and human homologs of the yeast RAD52 gene have been identified. Here we present the genomic organization of the mouse RAD52 gene. It consists of 12 exons ranging in size from 67 to 374 bp spread over a region of approximately 18 kb. The first ATG is located in exon 2. Analysis of the promoter region revealed no classical promoter elements such as CCAAT or TATA boxes. Transcriptional mapping analysis revealed one major transcription start point. Analogous to the situation in yeast, transcription of the RAD52 gene in human skin fibroblasts and mouse Ltk- cells was not induced by methyl methanesulfonate treatment. Furthermore, no specific alteration in human RAD52 expression levels throughout the cell cycle was observed.

recA-like genes from three archaean species with putative protein products similar to Rad51 and Dmc1 proteins of the yeast Saccharomyces cerevisiae

The process of homologous recombination has been documented in bacterial and eucaryotic organisms. The Escherichia coli RecA and Saccharomyces cerevisiae Rad51 proteins are the archetypal members of two related families of proteins that play a central role in this process. Using the PCR process primed by degenerate oligonucleotides designed to encode regions of the proteins showing the greatest degree of identity, we examined DNA from three organisms of a third phylogenetically divergent group, Archaea, for sequences encoding proteins similar to RecA and Rad51. The archaeans examined were a hyperthermophilic acidophile, Sulfolobus sofataricus (Sso); a halophile, Haloferax volcanii (Hvo); and a hyperthermophilic piezophilic methanogen, Methanococcus jannaschii (Mja). The PCR generated DNA was used to clone a larger genomic DNA fragment containing an open reading frame (orf), that we refer to as the radA gene, for each of the three archaeans. As shown by amino acid sequence alignments, percent amino acid identities and phylogenetic analysis, the putative proteins encoded by all three are related to each other and to both the RecA and Rad51 families of proteins. The putative RadA proteins are more similar to the Rad51 family (approximately 40% identity at the amino acid level) than to the RecA family (approximately 20%). Conserved sequence motifs, putative tertiary structures and phylogenetic analysis implied by the alignment are discussed. The 5' ends of mRNA transcripts to the Sso radA were mapped. The levels of radA mRNA do not increase after treatment with UV irradiation as do recA and RAD51 transcripts in E.coli and S.cerevisiae. Hence it is likely that radA in this organism is a constitutively expressed gene and we discuss possible implications of the lack of UV-inducibility.

A recA-like gene in Drosophila melanogaster that is expressed at high levels in female but not male meiotic tissues

The RecA protein is the central enzyme in prokaryotic recombination. It catalyzes pairing and strand exchange between homologous DNA molecules, and functions in both DNA repair and genetic recombination. The RecA-like proteins Rad51 and Dmc1 of yeast are both required for meiotic recombination and the former is also necessary for repair of double-strand breaks in vegetative cells. Genes encoding Rad51 homologs have been isolated recently from several higher eukaryotes. This paper describes the isolation and molecular characterization of a genomic DNA fragment from Drosophila melanogaster containing the coding sequence for a RecA-like protein. This protein exhibits strong sequence homology with the Rad51 proteins of budding yeast, fission yeast, chickens, mouse and humans, and slightly less (but still strong) homology with yeast Dmc1. Both in situ hybridization and Southern analysis indicate that the Rad51 gene is present only once per genome in Drosophila (at 99D on chromosome arm 3R). However, there are at least three other fragments that cross-hybridize strongly at low stringency. RNA blotting analysis detects a single transcript of about 1.35 kb that is present throughout development at low levels. Transcript levels are induced at least tenfold in ovaries, as measured by RNase protection analysis, suggestive of a role in female meiosis. Transcript levels are significantly lower in testes than in bulk RNA of adult males, however, indicating that Rad51 may be repressed in meiosis of Drosophila males.

Identification and expression of the Neurospora crassa mei-3 gene which encodes a protein homologous to Rad51 of Saccharomyces cerevisiae

The mei-3 gene of Neurospora crassa encodes a homolog of the Escherichia coli RecA and Saccharomyces cerevisiae Rad51 proteins, which are required for recombination and repair of DNA double-strand breaks. To determine the molecular function of MEI3 protein, anti-MEI3 antibody was prepared and used in Western blot analysis. The antibody cross-reacted only with crude extracts prepared from perithecia, the fruiting bodies of Neurospora. The molecular weight of the MEI3 protein was estimated to be 38 kDa. Transformation experiments showed that a DNA fragment longer than previously reported was needed to complement the mei-3 mutation. On sequencing cDNA and genomic DNA, one open reading frame (ORF) was found, which consists of three exons interrupted by two small introns. This ORF encoded a MEI3 protein of 353 amino acids, and the inferred MW of 38 kDa is in good agreement with the results from Western blot analysis. Comparisons of MEI3 with other Rad51 homologs indicated that MEI3 protein contains the two conserved core domains (I and II) generally observed in Rad51 homologs in eukaryotes. Northern blot analysis showed that expression of mei-3 was raised remarkably after UV-irradiation or methyl methanesulfonate (MMS)-treatment. The transcript size was 1.6 kb and this was also larger than was reported previously.

Localization of RecA-like recombination proteins on chromosomes of the lily at various meiotic stages

The Rad51 and Lim15 proteins of lily, which are homologs of the bacterial RecA protein, were found on chromosomes in various stages of meiotic prophase 1. The presence of both Rad51 and Lim15 proteins as discrete foci on leptotene and zygotene chromosomes and their colocalization suggest that meiotic recombination begins at the leptotene stage with the cooperation of these proteins and continues in zygotene. Localization of the foci on or adjacent to the chromosomes suggests that these proteins bind to the chromatin loops that extend from the axial cores. The proteins in these foci may participate in the searching and pairing of homologous DNA sequences, as the RecA protein does. The different pattern of localization of the Rad51 protein between the leptotene and pachytene stages and the absence of the Lim15 protein in the pachytene stage suggest that the Rad51 protein plays different roles in these stages.