Recombination of Ty elements in yeast can be induced by a double-strand break

Shuffling the yeast genome using CRISPR/Cas9-generated DSBs that target the transposable Ty1 elements

Although homologous recombination between transposable elements can drive genomic evolution in yeast by facilitating chromosomal rearrangements, the details of the underlying mechanisms are not fully clarified. In the genome of the yeast Saccharomyces cerevisiae, the most common class of transposon is the retrotransposon Ty1. Here, we explored how Cas9-induced double-strand breaks (DSBs) directed to Ty1 elements produce genomic alterations in this yeast species. Following Cas9 induction, we observed a significant elevation of chromosome rearrangements such as deletions, duplications and translocations. In addition, we found elevated rates of mitotic recombination, resulting in loss of heterozygosity. Using Southern analysis coupled with short- and long-read DNA sequencing, we revealed important features of recombination induced in retrotransposons. Almost all of the chromosomal rearrangements reflect the repair of DSBs at Ty1 elements by non-allelic homologous recombination; clustered Ty elements were hotspots for chromosome rearrangements. In contrast, a large proportion (about three-fourths) of the allelic mitotic recombination events have breakpoints in unique sequences. Our analysis suggests that some of the latter events reflect extensive processing of the broken ends produced in the Ty element that extend into unique sequences resulting in break-induced replication. Finally, we found that haploid and diploid strain have different preferences for the pathways used to repair double-stranded DNA breaks. Our findings demonstrate the importance of DNA lesions in retrotransposons in driving genome evolution.

The effect of hybridization on transposable element accumulation in an undomesticated fungal species

Transposable elements (TEs) are mobile genetic elements that can profoundly impact the evolution of genomes and species. A long-standing hypothesis suggests that hybridization could deregulate TEs and trigger their accumulation, although it received mixed support from studies mostly in plants and animals. Here, we tested this hypothesis in fungi using incipient species of the undomesticated yeast Saccharomyces paradoxus. Population genomic data revealed no signature of higher transposition in natural hybrids. As we could not rule out the elimination of past transposition increase signatures by natural selection, we performed a laboratory evolution experiment on a panel of artificial hybrids to measure TE accumulation in the near absence of selection. Changes in TE copy numbers were not predicted by the level of evolutionary divergence between the parents of a hybrid genotype. Rather, they were highly dependent on the individual hybrid genotypes, showing that strong genotype-specific deterministic factors govern TE accumulation in yeast hybrids.

Meiotic Chromosome Association 1 Interacts with TOP3α and Regulates Meiotic Recombination in Rice

Homologous recombination plays a central role in guaranteeing chromosome segregation during meiosis. The precise regulation of the resolution of recombination intermediates is critical for the success of meiosis. Many proteins, including the RECQ DNA helicases (Sgs1/BLM) and Topoisomerase 3α (TOP3α), have essential functions in managing recombination intermediates. However, many other factors involved in this process remain to be defined. Here, we report the isolation of meiotic chromosome association 1 (MEICA1), a novel protein participating in meiotic recombination in rice (Oryza sativa). Loss of MEICA1 leads to nonhomologous chromosome association, the formation of massive chromosome bridges, and fragmentation. MEICA1 interacts with MSH7, suggesting its role in preventing nonallelic recombination. In addition, MEICA1 has an anticrossover activity revealed by suppressing the defects of crossover formation in msh5 meica1 compared with that in msh5, showing the similar function with its interacted protein TOP3α. Thus, our data establish two pivotal roles for MEICA1 in meiosis: preventing aberrant meiotic recombination and regulating crossover formation.

Repetitive DNA loci and their modulation by the non-canonical nucleic acid structures R-loops and G-quadruplexes

Cells have evolved intricate mechanisms to maintain genome stability despite allowing mutational changes to drive evolutionary adaptation. Repetitive DNA sequences, which represent the bulk of most genomes, are a major threat to genome stability often driving chromosome rearrangements and disease. The major source of repetitive DNA sequences and thus the most vulnerable constituents of the genome are the rDNA (rDNA) repeats, telomeres, and transposable elements. Maintaining the stability of these loci is critical to overall cellular fitness and lifespan. Therefore, cells have evolved mechanisms to regulate rDNA copy number, telomere length and transposon activity, as well as DNA repair at these loci. In addition, non-canonical structure-forming DNA motifs can also modulate the function of these repetitive DNA loci by impacting their transcription, replication, and stability. Here, we discuss key mechanisms that maintain rDNA repeats, telomeres, and transposons in yeast and human before highlighting emerging roles for non-canonical DNA structures at these repetitive loci.

DNA repair by RNA: Templated, or not templated, that is the question

Cells are continuously exposed to both endogenous and exogenous sources of genomic stress. To maintain chromosome stability, a variety of mechanisms have evolved to cope with the multitude of genetic abnormalities that can arise over the life of a cell. Still, failures to repair these lesions are the driving force of cancers and other degenerative disorders. DNA double-strand breaks (DSBs) are among the most toxic genetic lesions, inhibiting cell ability to replicate, and are sites of mutations and chromosomal rearrangements. DSB repair is known to proceed via two major mechanisms: homologous recombination (HR) and non-homologous end joining (NHEJ). HR reliance on the exchange of genetic information between two identical or nearly identical DNA molecules offers increased accuracy. While the preferred substrate for HR in mitotic cells is the sister chromatid, this is limited to the S and G2 phases of the cell cycle. However, abundant amounts of homologous genetic substrate may exist throughout the cell cycle in the form of RNA. Considered an uncommon occurrence, the direct transfer of information from RNA to DNA is thought to be limited to special circumstances. Studies have shown that RNA molecules reverse transcribed into cDNA can be incorporated into DNA at DSB sites via a non-templated mechanism by NHEJ or a templated mechanism by HR. In addition, synthetic RNA molecules can directly template the repair of DSBs in yeast and human cells via an HR mechanism. New work suggests that even endogenous transcript RNA can serve as a homologous template to repair a DSB in chromosomal DNA. In this perspective, we will review and discuss the recent advancements in DSB repair by RNA via non-templated and templated mechanisms. We will provide current findings, models and future challenges investigating RNA and its role in DSB repair.

Competitive repair by naturally dispersed repetitive DNA during non-allelic homologous recombination

Genome rearrangements often result from non-allelic homologous recombination (NAHR) between repetitive DNA elements dispersed throughout the genome. Here we systematically analyze NAHR between Ty retrotransposons using a genome-wide approach that exploits unique features of Saccharomyces cerevisiae purebred and Saccharomyces cerevisiae/Saccharomyces bayanus hybrid diploids. We find that DNA double-strand breaks (DSBs) induce NAHR–dependent rearrangements using Ty elements located 12 to 48 kilobases distal to the break site. This break-distal recombination (BDR) occurs frequently, even when allelic recombination can repair the break using the homolog. Robust BDR–dependent NAHR demonstrates that sequences very distal to DSBs can effectively compete with proximal sequences for repair of the break. In addition, our analysis of NAHR partner choice between Ty repeats shows that intrachromosomal Ty partners are preferred despite the abundance of potential interchromosomal Ty partners that share higher sequence identity. This competitive advantage of intrachromosomal Tys results from the relative efficiencies of different NAHR repair pathways. Finally, NAHR generates deleterious rearrangements more frequently when DSBs occur outside rather than within a Ty repeat. These findings yield insights into mechanisms of repeat-mediated genome rearrangements associated with evolution and cancer.

The human genome is structurally dynamic, frequently undergoing loss, duplication, and rearrangement of large chromosome segments. These structural changes occur both in normal and in cancerous cells and are thought to cause both benign and deleterious changes in cell function. Many of these structural alterations are generated when two dispersed repeated DNA sequences at non-allelic sites recombine during non-allelic homologous recombination (NAHR). Here we study NAHR on a genome-wide scale using the experimentally tractable budding yeast as a eukaryotic model genome with its fully sequenced family of repeated DNA elements, the Ty retrotransposons. With our novel system, we simultaneously measure the effects of known recombination parameters on the frequency of NAHR to understand which parameters most influence the occurrence of rearrangements between repetitive sequences. These findings provide a basic framework for interpreting how structural changes observed in the human genome may have arisen.

Double-strand breaks associated with repetitive DNA can reshape the genome

Ionizing radiation is an established source of chromosome aberrations (CAs). Although double-strand breaks (DSBs) are implicated in radiation-induced and other CAs, the underlying mechanisms are poorly understood. Here, we show that, although the vast majority of randomly induced DSBs in G(2) diploid yeast cells are repaired efficiently through homologous recombination (HR) between sister chromatids or homologous chromosomes, approximately 2% of all DSBs give rise to CAs. Complete molecular analysis of the genome revealed that nearly all of the CAs resulted from HR between nonallelic repetitive elements, primarily Ty retrotransposons. Nonhomologous end-joining (NHEJ) accounted for few, if any, of the CAs. We conclude that only those DSBs that fall at the 3-5% of the genome composed of repetitive DNA elements are efficient at generating rearrangements with dispersed small repeats across the genome, whereas DSBs in unique sequences are confined to recombinational repair between the large regions of homology contained in sister chromatids or homologous chromosomes. Because repeat-associated DSBs can efficiently lead to CAs and reshape the genome, they could be a rich source of evolutionary change.

Finding a match: how do homologous sequences get together for recombination?

Decades of research into homologous recombination have unravelled many of the details concerning the transfer of information between two homologous sequences. By contrast, the processes by which the interacting molecules initially colocalize are largely unknown. How can two homologous needles find each other in the genomic haystack? Is homologous pairing the result of a damage-induced homology search, or is it an enduring and general feature of the genomic architecture that facilitates homologous recombination whenever and wherever damage occurs? This Review presents the homologous-pairing enigma, delineates our current understanding of the process and offers guidelines for future research.

Ty1-copiaretrotransposon-based SSAP marker development and its potential in the genetic study of cucurbits

Three long terminal repeat (LTR) sequences of Ty1-copia retrotransposons were identified in cucumber ( Cucumis sativus L.) and named Tcs 1, Tcs 2, and Tcs 3. A sequence-specific amplification polymorphism (SSAP) marker system based on these LTR sequences displayed a higher level of polymorphism than AFLPs in cucumber. This marker system could also detect loci in other Cucumis species for genetic diversity analysis. The three Tcs LTRs existed within the exons of genes because of the effective amplification band patterns from the cDNA templates. The potential usefulness of the SSAP marker system in studies of the evolution of genes or genomes was verified after exploring loci changes in first and second generations of a synthetic allotetraploid in Cucumis. This study is the first report of the development of a retrotransposon-based marker system and the SSAP technique in cucurbits.

Mutator phenotype of Caenorhabditis elegans DNA damage checkpoint mutants

DNA damage response proteins identify sites of DNA damage and signal to downstream effectors that orchestrate either apoptosis or arrest of the cell cycle and DNA repair. The C. elegans DNA damage response mutants mrt-2, hus-1, and clk-2(mn159) displayed 8- to 15-fold increases in the frequency of spontaneous mutation in their germlines. Many of these mutations were small- to medium-sized deletions, some of which had unusual sequences at their breakpoints such as purine-rich tracts or direct or inverted repeats. Although DNA-damage-induced apoptosis is abrogated in the mrt-2, hus-1, and clk-2 mutant backgrounds, lack of the apoptotic branch of the DNA damage response pathway in cep-1/p53, ced-3, and ced-4 mutants did not result in a Mutator phenotype. Thus, DNA damage checkpoint proteins suppress the frequency of mutation by ensuring that spontaneous DNA damage is accurately repaired in C. elegans germ cells. Although DNA damage response defects that predispose humans to cancer are known to result in large-scale chromosome aberrations, our results suggest that small- to medium-sized deletions may also play roles in the development of cancer.

Transpositions and translocations induced by site-specific double-strand breaks in budding yeast

Much of what we know about the molecular mechanisms of repairing a broken chromosome has come from the analysis of site-specific double-strand breaks (DSBs). Such DSBs can be generated by conditional expression of meganucleases such as HO or I-SceI or by the excision of a DNA transposable element. The synchronous creation of DSBs in nearly all cells of the population has made it possible to observe the progress of recombination by monitoring both the DNA itself and proteins that become associated with the recombining DNA. Both homologous recombination mechanisms and non-homologous end-joining (NHEJ) mechanisms of recombination have been defined by using these approaches. Here I focus on recombination events that lead to alterations of chromosome structure: transpositions, translocations, deletions, DNA fragment capture and other small insertions. These rearrangements can occur from ectopic gene conversions accompanied by crossing-over, break-induced replication, single-strand annealing or non-homologous end-joining.

Recombination between retrotransposons as a source of chromosome rearrangements in the yeast Saccharomyces cerevisiae

Homologous recombination between dispersed repeated genetic elements is an important source of genetic variation. In this review, we discuss chromosome rearrangements that are a consequence of homologous recombination between transposable elements in the yeast Saccharomyces cerevisiae. The review will be divided into five sections: (1) Introduction (mechanisms of homologous recombination involving ectopic repeats), (2) Spontaneous chromosome rearrangements in wild-type yeast cells, (3) Chromosome rearrangements induced by low DNA polymerase, mutagenic agents or mutations in genes affecting genome stability, (4) Recombination between retrotransposons as a mechanism of genome evolution, and (5) Important unanswered questions about homologous recombination between retrotransposons. This review complements several others [S. Liebman, S. Picologlou, Recombination associated with yeast retrotransposons, in: Y. Koltin, M.J. Leibowitz (Eds.), Viruses of Fungi and Simple Eukaryotes, Marcel Dekker Inc., New York, 1988, pp. 63-89; P. Lesage, A.L. Todeschini, Happy together: the life and times of Ty retrotransposons and their hosts, Cytogenet. Genome Res. 110 (2005) 70-90; D.J. Garfinkel, Genome evolution mediated by Ty elements in Saccharomyces, Cytogenet. Genome Res. 110 (2005) 63-69] that discuss genomic rearrangements involving Ty elements.

Happy together: the life and times of Ty retrotransposons and their hosts

The aim of this review is to describe the level of intimacy between Ty retrotransposons (Ty1-Ty5) and their host the yeast Saccharomyces cerevisiae. The effects of Ty location in the genome and of host proteins on the expression and mobility of Ty elements are highlighted. After a brief overview of Ty diversity and evolution, we describe the factors that dictate Ty target-site preference and the impact of targeting on Ty and adjacent gene expression. Studies on Ty3 and Ty5 have been especially informative in unraveling the role of host factors (Pol III machinery and silencing proteins, respectively) and integrase in controlling the specificity of integration. In contrast, not much is known regarding Ty1, Ty2 and Ty4, except that their insertion depends on the transcriptional competence of the adjacent Pol III gene and might be influenced by some chromatin components. This review also brings together recent findings on the regulation of Ty1 retrotransposition. A large number of host proteins (over 30) involved in a wide range of cellular processes controls either directly or indirectly Ty1 mobility, primarily at post-transcriptional steps. We focus on several genes for which more detailed analyses have permitted the elaboration of regulatory models. In addition, this review describes new data revealing that repression of Ty1 mobility also involves two forms of copy number control that act at both the trancriptional and post-transcriptional levels. Since S. cerevisiae lacks the conserved pathways for copy number control via transcriptional and post-transcriptional gene silencing found in other eukaryotes, Ty1 copy number control must be via another mechanism whose features are outlined. Ty1 response to stress also implicates activation at both transcriptional and postranscriptional steps of Ty1. Finally, we provide several insights in the role of Ty elements in chromosome evolution and yeast adaptation and discuss the factors that might limit Ty ectopic recombination.

Compartmentalization of the yeast meiotic nucleus revealed by analysis of ectopic recombination

As yeast cells enter meiosis, chromosomes move from a centromere-clustered (Rabl) to a telomere-clustered (bouquet) configuration and then to states of progressive homolog pairing where telomeres are more dispersed. It is uncertain at which stage of this process sequences commit to recombine with each other. Previous analyses using recombination between dispersed homologous sequences (ectopic recombination) support the view that, on average, homologs are aligned end to end by the time of commitment to recombination. We have undertaken further analyses incorporating new inserts, chromosome rearrangements, an alternate mode of recombination initiation, and mutants that disrupt nuclear structure or telomere metabolism. Our findings support previous conclusions and reveal that distance from the nearest telomere is an important parameter influencing recombination between dispersed sequences. In general, the farther dispersed sequences are from their nearest telomere, the less likely they are to engage in ectopic recombination. Neither the mode of initiating recombination nor the formation of the bouquet appears to affect this relationship. We suggest that aspects of telomere localization and behavior influence the organization and mobility of chromosomes along their entire length, during a critical period of meiosis I prophase that encompasses the homology search.

The Mre11 nuclease is not required for 5' to 3' resection at multiple HO-induced double-strand breaks

Current hypotheses suggest the Mre11 nuclease activity could be directly involved in double-strand break (DSB) resection in the presence of a large number of DSBs or limited to processing abnormal DNA ends. To distinguish between these possibilities, we used two methods to create large numbers of DSBs in Saccharomyces cerevisiae chromosomes, without introducing other substrates for the Mre11 nuclease. Multiple DSBs were created either by expressing the HO endonuclease in strains containing several HO cut sites embedded within randomly dispersed Ty1 elements or by phleomycin treatment. Analysis of resection by single-strand DNA formation in these systems showed no difference between strains containing MRE11 or the mre11-D56N nuclease defective allele, suggesting that the Mre11 nuclease is not involved in the extensive 5' to 3' resection of DSBs. We postulate that the ionizing radiation (IR) sensitivity of mre11 nuclease-defective mutants results from the accumulation of IR-induced DNA damage that is normally processed by the Mre11 nuclease. We also report that the processivity of 5' to 3' DSB resection and the yield of repaired products are affected by the number of DSBs in a dose-dependent manner. Finally, we show that the exonuclease Exo1 is involved in the processivity of 5' to 3' resection of an HO-induced DSB at the MAT locus.

DSB repair: the yeast paradigm

Genome stability is of primary importance for the survival and proper functioning of all organisms. Double-strand breaks (DSBs) arise spontaneously during growth, or can be created by external insults. In response to even a single DSB, organisms must trigger a series of events to promote repair of the DNA damage in order to survive and restore chromosomal integrity. In doing so, cells must regulate a fine balance between potentially competing DSB repair pathways. These are generally classified as either homologous recombination (HR) or non-homologous end joining (NHEJ). The yeast Saccharomyces cerevisiae is an ideal model organism for studying these repair processes. Indeed, much of what we know today on the mechanisms of repair in eukaryotes come from studies carried out in budding yeast. Many of the proteins involved in the various repair pathways have been isolated and the details of their mode of action are currently being unraveled at the molecular level. In this review, we focus on exciting new work eminating from yeast research that provides fresh insights into the DSB repair process. This recent work supplements and complements the wealth of classical genetic research that has been performed in yeast systems over the years. Given the conservation of the repair mechanisms and genes throughout evolution, these studies have profound implications for other eukaryotic organisms.

New insights into the mechanism of homologous recombination in yeast

Genome stability is of primary importance for the survival and proper functioning of all organisms. Double-strand breaks (DSBs) arise spontaneously during growth, or can be created by external insults. Repair of DSBs by homologous recombination provides an efficient and fruitful pathway to restore chromosomal integrity. Exciting new work in yeast has lately provided insights into this complex process. Many of the proteins involved in recombination have been isolated and the details of the repair mechanism are now being unraveled at the molecular level. In this review, we focus on recent studies which dissect the recombinational repair of a single broken chromosome. After DSB formation, a decision is made regarding the mechanism of repair (recombination or non-homologous end-joining). This decision is under genetic control. Once committed to the recombination pathway, the broken chromosomal ends are resected by a still unclear mechanism in which the DNA damage checkpoint protein Rad24 participates. At this stage several proteins are recruited to the broken ends, including Rad51p, Rad52p, Rad55p, Rad57p, and possibly Rad54p. A genomic search for homology ensues, followed by strand invasion, promoted by the Rad51 filament with the participation of Rad55p, Rad57p and Rad54p. DNA synthesis then takes place, restoring the resected ends. Crossing-over formation depends on the length of the homologous recombining sequences, and is usually counteracted by the activity of the mismatch repair system. Given the conservation of the repair mechanisms and genes throughout evolution, these studies have profound implications for other eukaryotic organisms.

ELG1, a yeast gene required for genome stability, forms a complex related to replication factor C

Many overlapping surveillance and repair mechanisms operate in eukaryotic cells to ensure the stability of the genome. We have screened to isolate yeast mutants exhibiting increased levels of recombination between repeated sequences. Here we characterize one of these mutants, elg1. Strains lacking Elg1p exhibit elevated levels of recombination between homologous and nonhomologous chromosomes, as well as between sister chromatids and direct repeats. These strains also exhibit increased levels of chromosome loss. The Elg1 protein shares sequence homology with the large subunit of the clamp loader replication factor C (RFC) and with the product of two additional genes involved in checkpoint functions and genome maintenance: RAD24 and CTF18. Elg1p forms a complex with the Rfc2-5 subunits of RFC that is distinct from the previously described RFC-like complexes containing Rad24 and Ctf18. Genetic data indicate that the Elg1, Ctf18, and Rad24 RFC-like complexes work in three separate pathways important for maintaining the integrity of the genome and for coping with various genomic stresses.

Molecular dissection of mitotic recombination in the yeast Saccharomyces cerevisiae

Recombination plays a central role in the repair of broken chromosomes in all eukaryotes. We carried out a systematic study of mitotic recombination. Using several assays, we established the chronological sequence of events necessary to repair a single double-strand break. Once a chromosome is broken, yeast cells become immediately committed to recombinational repair. Recombination is completed within an hour and exhibits two kinetic gaps. By using this kinetic framework we also characterized the role played by several proteins in the recombinational process. In the absence of Rad52, the broken chromosome ends, both 5' and 3', are rapidly degraded. This is not due to the inability to recombine, since the 3' single-stranded DNA ends are stable in a strain lacking donor sequences. Rad57 is required for two consecutive strand exchange reactions. Surprisingly, we found that the Srs2 helicase also plays an early positive role in the recombination process.

Dasheng: a recently amplified nonautonomous long terminal repeat element that is a major component of pericentromeric regions in rice

A new and unusual family of LTR elements, Dasheng, has been discovered in the genome of Oryza sativa following database searches of approximately 100 Mb of rice genomic sequence and 78 Mb of BAC-end sequence information. With all of the cis-elements but none of the coding domains normally associated with retrotransposons (e.g., gag, pol), Dasheng is a novel nonautonomous LTR element with high copy number. Over half of the approximately 1000 Dasheng elements in the rice genome are full length (5.6-8.6 kb), and 60% are estimated to have amplified in the past 500,000 years. Using a modified AFLP technique called transposon display, 215 elements were mapped to all 12 rice chromosomes. Interestingly, more than half of the mapped elements are clustered in the heterochromatic regions around centromeres. The distribution pattern was further confirmed by FISH analysis. Despite clustering in heterochromatin, Dasheng elements are not nested, suggesting their potential value as molecular markers for these marker-poor regions. Taken together, Dasheng is one of the highest-copy-number LTR elements and one of the most recent elements to amplify in the rice genome.

Retrovirus insertion and transcriptional activation of the multidrug-resistance gene in leukemias treated by a chemotherapeutic agent in vivo

To understand the molecular basis for multidrug-resistant (MDR) cancer cells in vivo, this study analyzed molecular changes of the mdr1a gene region in leukemia cells in mice during continuous treatment with vincristine. An inverse insertion of murine leukemia retrovirus (MuLV) into the 5'-flanking region of the mdr1a gene was found. This insertion was concomitantly accompanied by up-regulation of the mdr1a gene and the loss of chemosensitivity. Deletion of long-terminal repeat (LTR) sequences dramatically decreased the mdr1a promoter-driven reporter activity. The MuLV LTR insertion appears to exert its enhancer activity on mdr1a transcription during the appearance of MDR leukemia cells. Two mechanisms were postulated to explain the mdr1a gene activation by retrovirus insertion during in vivo chemotreatment: de novo insertion of MuLV induced by vincristine treatment and selection of a small fraction of pre-existing cells carrying MuLV insertion during vincristine treatment. No rearranged sequence was detected by polymerase chain reaction in parental cells. This result argued for the first mechanism. The randomly altered distribution of MuLV during repetitive chemotreatment might also be consistent with this hypothesis. On the other hand, the retrovirus insertion was detected at the same site of the mdr1a promoter region in 2 independent experiments, which suggests the second mechanism. It should be noted that in vivo chemotreatment using vincristine could generate the mdr1a-overexpressing cells through retrovirus insertion and the enhancer effect of the LTR.

Damage-induced recombination in the yeast Saccharomyces cerevisiae

Prokaryotic and eukaryotic cells have developed a network of DNA repair systems that restore genomic integrity following DNA damage from endogenous and exogenous genotoxic sources. One of the mechanisms used to repair damaged chromosomes is genetic recombination, in which information present as a second chromosomal copy is used to repair a damaged region of the genome. In this review, I summarized what is known about the molecular and cellular mechanisms by which various DNA-damaging agents induce recombination in yeast. The yeast Saccharomyces cerevisiae has served as an excellent model organism to study the induction of recombination. It has helped to define the basic phenomenology and to isolate the genes involved in the process. Given the evolutionary conservation of the various DNA repair systems in eukaryotes, it is likely that the knowledge gathered about induced recombination in yeast is applicable to mammalian cells and thus to humans. Many carcinogens are known to induce recombination and to cause chromosomal rearrangements. An understanding of the mechanisms, by which genotoxic agents cause increased levels of recombination will have important consequences for the treatment of cancer, and for the assessment of risks arising from exposure to genotoxic agents in humans.

A double-strand break in a chromosomal LINE element can be repaired by gene conversion with various endogenous LINE elements in mouse cells

A double-strand break (DSB) in the mammalian genome has been shown to be a very potent signal for the cell to activate repair processes. Two different types of repair have been identified in mammalian cells. Broken ends can be rejoined with or without loss or addition of DNA or, alternatively, a homologous template can be used to repair the break. For most genomic sequences the latter event would involve allelic sequences present on the sister chromatid or homologous chromosome. However, since more than 30% of our genome consists of repetitive sequences, these would have the option of using nonallelic sequences for homologous repair. This could have an impact on the evolution of these sequences and of the genome itself. We have designed an assay to look at the repair of DSBs in LINE-1 (L1) elements which number 10(5) copies distributed throughout the genome of all mammals. We introduced into the genome of mouse epithelial cells an L1 element with an I-SceI endonuclease site. We induced DSBs at the I-SceI site and determined their mechanism of repair. We found that in over 95% of cases, the DSBs were repaired by an end-joining process. However, in almost 1% of cases, we found strong evidence for repair involving gene conversion with various endogenous L1 elements, with some being used preferentially. In particular, the T(F) family and the L1Md-A2 subfamily, which are the most active in retrotransposition, appeared to be contributing the most in this process. The degree of homology did not seem to be a determining factor in the selection of the endogenous elements used for repair but may be based instead on accessibility. Considering their abundance and dispersion, gene conversion between repetitive elements may be occurring frequently enough to be playing a role in their evolution.

Retrotransposon BARE-1 and Its Role in Genome Evolution in the Genus Hordeum

The replicative retrotransposon life cycle offers the potential for explosive increases in copy number and consequent inflation of genome size. The BARE-1 retrotransposon family of barley is conserved, disperse, and transcriptionally active. To assess the role of BARE-1 in genome evolution, we determined the copy number of its integrase, its reverse transcriptase, and its long terminal repeat (LTR) domains throughout the genus Hordeum. On average, BARE-1 contributes 13.7 x 10(3) full-length copies, amounting to 2.9% of the genome. The number increases with genome size. Two LTRs are associated with each internal domain in intact retrotransposons, but surprisingly, BARE-1 LTRs were considerably more prevalent than would be expected from the numbers of intact elements. The excess in LTRs increases as both genome size and BARE-1 genomic fraction decrease. Intrachromosomal homologous recombination between LTRs could explain the excess, removing BARE-1 elements and leaving behind solo LTRs, thereby reducing the complement of functional retrotransposons in the genome and providing at least a partial "return ticket from genomic obesity."

Retrotransposon BARE-1 and Its Role in Genome Evolution in the Genus Hordeum

The replicative retrotransposon life cycle offers the potential for explosive increases in copy number and consequent inflation of genome size. The BARE-1 retrotransposon family of barley is conserved, disperse, and transcriptionally active. To assess the role of BARE-1 in genome evolution, we determined the copy number of its integrase, its reverse transcriptase, and its long terminal repeat (LTR) domains throughout the genus Hordeum. On average, BARE-1 contributes 13.7 x 10(3) full-length copies, amounting to 2.9% of the genome. The number increases with genome size. Two LTRs are associated with each internal domain in intact retrotransposons, but surprisingly, BARE-1 LTRs were considerably more prevalent than would be expected from the numbers of intact elements. The excess in LTRs increases as both genome size and BARE-1 genomic fraction decrease. Intrachromosomal homologous recombination between LTRs could explain the excess, removing BARE-1 elements and leaving behind solo LTRs, thereby reducing the complement of functional retrotransposons in the genome and providing at least a partial "return ticket from genomic obesity."

Multiple pathways of recombination induced by double-strand breaks in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been the principal organism used in experiments to examine genetic recombination in eukaryotes. Studies over the past decade have shown that meiotic recombination and probably most mitotic recombination arise from the repair of double-strand breaks (DSBs). There are multiple pathways by which such DSBs can be repaired, including several homologous recombination pathways and still other nonhomologous mechanisms. Our understanding has also been greatly enriched by the characterization of many proteins involved in recombination and by insights that link aspects of DNA repair to chromosome replication. New molecular models of DSB-induced gene conversion are presented. This review encompasses these different aspects of DSB-induced recombination in Saccharomyces and attempts to relate genetic, molecular biological, and biochemical studies of the processes of DNA repair and recombination.

Multiple pathways of recombination induced by double-strand breaks in Saccharomyces cerevisiae

The budding yeast Saccharomyces cerevisiae has been the principal organism used in experiments to examine genetic recombination in eukaryotes. Studies over the past decade have shown that meiotic recombination and probably most mitotic recombination arise from the repair of double-strand breaks (DSBs). There are multiple pathways by which such DSBs can be repaired, including several homologous recombination pathways and still other nonhomologous mechanisms. Our understanding has also been greatly enriched by the characterization of many proteins involved in recombination and by insights that link aspects of DNA repair to chromosome replication. New molecular models of DSB-induced gene conversion are presented. This review encompasses these different aspects of DSB-induced recombination in Saccharomyces and attempts to relate genetic, molecular biological, and biochemical studies of the processes of DNA repair and recombination.

Mating-type gene switching in Saccharomyces cerevisiae

Saccharomyces cerevisiae can change its mating type as often as every generation by a highly choreographed, site-specific recombination event that replaces one MAT allele with different DNA sequences encoding the opposite allele. The study of this process has yielded important insights into the control of cell lineage, the silencing of gene expression, and the formation of heterochromatin, as well as the molecular events of double-strand break-induced recombination. In addition, MAT switching provides a remarkable example of a small locus control region--the Recombination Enhancer--that controls recombination along an entire chromosome arm.

Analysis of repair and mutagenesis of chromium-induced DNA damage in yeast, mammalian cells, and transgenic mice

Chromium (Cr) is a widespread environmental contaminant and a known human carcinogen. We have used shuttle vector systems in yeast, mammalian cells, and transgenic mice to characterize the mutational specificity and premutational DNA damage induced by Cr(VI) and its reduction intermediates in order to elucidate the mechanism by which Cr induces mutations. In the yeast system, treatment of vector-containing cells with Cr(VI) results in a dose-dependent increase in mutations in the SUP4-o target gene of the vector; mutagenesis is enhanced in an apn-1 yeast mutant, deficient in the capacity to repair oxidative-type DNA damage. In vector-containing mammalian cells, treatment with Cr(VI) also results in a dose-dependent increase in mutations in the vector target gene supF. The Cr-induced mutations in supF occurred mostly at G:C base pairs and were widely distributed across the gene, a pattern similar to those observed with ionizing radiation or hydrogen peroxide. These results support the hypothesis that Cr(VI)-induced oxidative-type DNA damage is responsible for Cr mutagenesis in the cell. Recently these studies were extended into the Big Blue transgenic mouse system in which Cr-induced mutagenesis was observed in the lung, the target organ for Cr carcinogenesis in humans. Analysis of the spectrum of these mutations will test whether Cr mutagenesis occurs by similar mechanisms in the intact animal as in cell culture systems and yeast.

Effects of DNA double-strand and single-strand breaks on intrachromosomal recombination events in cell-cycle-arrested yeast cells

Intrachromosomal recombination between repeated elements can result in deletion (DEL recombination) events. We investigated the inducibility of such intrachromosomal recombination events at different stages of the cell cycle and the nature of the primary DNA lesions capable of initiating these events. Two genetic systems were constructed in Saccharomyces cerevisiae that select for DEL recombination events between duplicated alleles of CDC28 and TUB2. We determined effects of double-strand breaks (DSBs) and single-strand breaks (SSBs) between the duplicated alleles on DEL recombination when induced in dividing cells or cells arrested in G1 or G2. Site-specific DSBs and SSBs were produced by overexpression of the I-Sce I endonuclease and the gene II protein (gIIp), respectively. I-Sce I-induced DSBs caused an increase in DEL recombination frequencies in both dividing and cell-cycle-arrested cells, indicating that G1- and G2-arrested cells are capable of completing DSB repair. In contrast, gIIp-induced SSBs caused an increase in DEL recombination frequency only in dividing cells. To further examine these phenomena we used both gamma-irradiation, inducing DSBs as its most relevant lesion, and UV, inducing other forms of DNA damage. UV irradiation did not increase DEL recombination frequencies in G1 or G2, whereas gamma-rays increased DEL recombination frequencies in both phases. Both forms of radiation, however, induced DEL recombination in dividing cells. The results suggest that DSBs but not SSBs induce DEL recombination, probably via the single-strand annealing pathway. Further, DSBs in dividing cells may result from the replication of a UV or SSB-damaged template. Alternatively, UV induced events may occur by replication slippage after DNA polymerase pausing in front of the damage.

The Saccharomyces cerevisiae RAD9 checkpoint reduces the DNA damage-associated stimulation of directed translocations

Genetic instability in the Saccharomyces cerevisiae rad9 mutant correlates with failure to arrest the cell cycle in response to DNA damage. We quantitated the DNA damage-associated stimulation of directed translocations in RAD9+ and rad9 mutants. Directed translocations were generated by selecting for His+ prototrophs that result from homologous, mitotic recombination between two truncated his3 genes, GAL1::his3-delta5' and trp1::his3-delta3'::HOcs. Compared to RAD9+ strains, the rad9 mutant exhibits a 5-fold higher rate of spontaneous, mitotic recombination and a greater than 10-fold increase in the number of UV- and X-ray-stimulated His+ recombinants that contain translocations. The higher level of recombination in rad9 mutants correlated with the appearance of nonreciprocal translocations and additional karyotypic changes, indicating that genomic instability also occurred among non-his3 sequences. Both enhanced spontaneous recombination and DNA damage-associated recombination are dependent on RAD1, a gene involved in DNA excision repair. The hyperrecombinational phenotype of the rad9 mutant was correlated with a deficiency in cell cycle arrest at the G2-M checkpoint by demonstrating that if rad9 mutants were arrested in G2 before irradiation, the numbers both of UV- and gamma-ray-stimulated recombinants were reduced. The importance of G2 arrest in DNA damage-induced sister chromatid exchange (SCE) was evident by a 10-fold reduction in HO endonuclease-induced SCE and no detectable X-ray stimulation of SCE in a rad9 mutant. We suggest that one mechanism by which the RAD9-mediated G2-M checkpoint may reduce the frequency of DNA damage-induced translocations is by channeling the repair of double-strand breaks into SCE.

Damage-induced ectopic recombination in the yeast Saccharomyces cerevisiae

Mitotic recombination in the yeast Saccharomyces cerevisiae is induced when cells are irradiated with UV or X-rays, reflecting the efficient repair of damage by recombinational repair mechanisms. We have used multiply marked haploid strains that allow the simultaneous detection of several types of ectopic recombination events. We show that inter-chromosomal ectopic conversion of lys2 heteroalleles and, to a lesser extent, direct repeat recombination (DRR) between non-tandem repeats, are increased by DNA-damaging agents; in contrast, ectopic recombination of the naturally occurring Ty element is not induced. We have tested several hypotheses that could explain the preferential lack of induction of Ty recombination by DNA-damaging agents. We have found that the lack of induction cannot be explained by a cell cycle control or by an effect of the mating-type genes. We also found no role for the flanking long terminal repeats (LTRs) of the Ty in preventing the induction. Ectopic conversion, DRR, and forward mutation of artificial repeats show different kinetics of induction at various positions of the cell cycle, reflecting different mechanisms of recombination. We discuss the mechanistic and evolutionary aspects of these results.

Karyotype variability in yeast caused by nonallelic recombination in haploid meiosis

Chromosomes of altered size were found in the meiotic products of a haploid Saccharomyces cerevisiae strain by pulsed field gel electrophoretic separation of whole chromosomes. About 7% of haploid meioses produced chromosomes that differed by > or = 10 kb from their wild-type counterparts. Chromosomes most often became enlarged or shortened due to recombination events between sister chromatids at nonallelic sequences. By this mechanism chromosome III acquired tandem arrays of up to eight extra copies of the approximately kb MAT-HMR segment during repeated rounds of haploid meioses. Enlarged chromosomes III were unstable and changed their size during meiosis more often than remaining unchanged. Altered chromosomes appeared also as the products of intrachromatid recombination and of reciprocal translocations caused by ectopic recombination between nonhomologous chromosomes. In diploid meiosis, chromosomes of altered size occurred at least 10 times less frequently, whereas in mitotic cultures cells with altered karyotypes were virtually absent. The results show that various forms of ectopic recombination are promoted by the absence of allelic homologies.

Induction of Ty recombination in yeast by cDNA and transcription: role of the RAD1 and RAD52 genes

In the yeast Saccharomyces cerevisiae ectopic recombination has been shown to occur at high frequencies for artificially created repeats, but at relatively low frequencies for a natural family of repeated sequences, the Ty family. Little is known about the mechanism(s) that prevent recombination between repeated sequences. We have previously shown that nonreciprocal recombination (gene conversion) of a genetically marked Ty can be induced either by the presence of high levels of Ty cDNA or by transcription of the marked Ty from a GAL1 promoter. These two kinds of induction act in a synergistic manner. To further characterize these two kinds of Ty recombination, we have investigated the role played by the RAD52 and RAD1 genes. We have found that the RAD52 and RAD1 gene products are essential to carry out transcription-induced Ty conversion whereas cDNA-mediated conversion can take place in their absence.

The role of DNA repair genes in recombination between repeated sequences in yeast

The presence of repeated sequences in the genome represents a potential source of karyotypic instability. Genetic control of recombination is thus important to preserve the integrity of the genome. To investigate the genetic control of recombination between repeated sequences, we have created a series of isogenic strains in which we could assess the role of genes involved in DNA repair in two types of recombination: direct repeat recombination and ectopic gene conversion. Naturally occurring (Ty elements) and artificially constructed repeats could be compared in the same cell population. We have found that direct repeat recombination and gene conversion have different genetic requirements. The role of the RAD51, RAD52, RAD54, RAD55, and RAD57 genes, which are involved in recombinational repair, was investigated. Based on the phenotypes of single and double mutants, these genes can be divided into three functional subgroups: one composed of RAD52, a second one composed of RAD51 and RAD54, and a third one that includes the RAD55 and RAD57 genes. Among seven genes involved in excision repair tested, only RAD1 and RAD10 played a role in the types of recombination studied. We did not detect a differential effect of any rad mutation on Ty elements as compared to artificially constructed repeats.