Chromatin-mediated regulators of meiotic recombination revealed by proteomics of a recombination hotspot

Role of ATP-dependent chromatin remodelers in meiosis

In eukaryotic cells, DNA is wrapped around histone octamers to compact the genome. Although such compaction is required for the precise segregation of the genome during cell division, it restricts the DNA-protein interactions essential for several cellular processes. During meiosis, a specialized cell division process that produces gametes, several DNA-protein interactions are crucial for assembling meiosis-specific chromosome structures, meiotic recombination, chromosome segregation, and transcriptional regulation. The role of chromatin remodelers (CRs) in facilitating DNA-protein transactions during mitosis is well appreciated, whereas how they facilitate meiosis-specific processes is poorly understood. In this review, we summarize experimental evidence supporting the role of CRs in meiosis in various model systems and suggest future perspectives to advance the field.

Meiosis as a mechanism for epigenetic reprogramming and cellular rejuvenation

Meiosis is a hallmark of sexual reproduction because it represents the transition from one life cycle to the next and, in animals, meiosis produces gametes. Why meiosis evolved has been debated and most studies have focused on recombination of the parental alleles as the main function of meiosis. However, 40 years ago, Robin Holliday proposed that an essential function of meiosis is to oppose the consequence of successive mitoses that cause cellular aging. Cellular aging results from accumulated defective organelles and proteins and modifications of chromatin in the form of DNA methylation and histone modifications referred to collectively as epigenetic marks. Here, recent findings supporting the hypothesis that meiosis opposes cellular aging are reviewed and placed in the context of the diversity of the life cycles of eukaryotes, including animals, yeast, flowering plants and the bryophyte Marchantia.

Agar lot-specific inhibition in the plating efficiency of yeast spores and cells

The fission yeast Schizosaccharomyces pombe and the budding yeast Saccharomyces cerevisiae are highly diverged (530 mya), single-celled, model eukaryotic organisms. Scientists employ mating, meiosis, and the plating of ascospores and cells to generate strains with novel genotypes and to discover biological processes. Our three laboratories encountered independently sudden-onset, major impediments to such research. Spore suspensions and vegetative cells no longer plated effectively on minimal media. By systematically analyzing multiple different media components from multiple different suppliers, we identified the source of the problem. Specific lots of agar were toxic. We report that this sporadic toxicity affects independently the agar stocks of multiple vendors, has occurred repeatedly over at least three decades, and extends to species in highly diverged taxa. Interestingly, the inhibitory effects displayed variable penetrance and were attenuated on rich media. Consequently, quality control checks that use only rich media can provide false assurances on the quality of the agar. Lastly, we describe likely sources of the toxicity and we provide specific guidance for quality control measures that should be applied by all vendors as preconditions for their sale of agar.

Creating Meiotic Recombination-Regulating DNA Sites by SpEDIT in Fission Yeast Reveals Inefficiencies, Target-Site Duplications, and Ectopic Insertions

Recombination hotspot-activating DNA sites (e.g., M26, CCAAT, Oligo-C) and their binding proteins (e.g., Atf1-Pcr1 heterodimer; Php2-Php3-Php5 complex, Rst2, Prdm9) regulate the distribution of Spo11 (Rec12)-initiated meiotic recombination. We sought to create 14 different candidate regulatory DNA sites via bp substitutions in the ade6 gene of Schizosaccharomyces pombe. We used a fission yeast-optimized CRISPR-Cas9 system (SpEDIT) and 196 bp-long dsDNA templates with centrally located bp substitutions designed to ablate the genomic PAM site, create specific 15 bp-long DNA sequences, and introduce a stop codon. After co-transformation with a plasmid that encoded both the guide RNA and Cas9 enzyme, about one-third of colonies had a phenotype diagnostic for DNA sequence changes at ade6. PCR diagnostics and DNA sequencing revealed a diverse collection of alterations at the target locus, including: (A) complete or (B) partial template-directed substitutions; (C) non-homologous end joinings; (D) duplications; (E) bp mutations, and (F) insertions of ectopic DNA. We concluded that SpEDIT can be used successfully to generate a diverse collection of DNA sequence elements within a reporter gene of interest. However, its utility is complicated by low efficiency, incomplete template-directed repair events, and undesired alterations to the target locus.

Distance-dependent effects on CRISPR/Cas9-mediated genome editing in Schizosaccharomyces pombe compromise efficiency and create unsought alleles

Discrete DNA sites position meiotic recombination at hotspots. We sought to create four different, 15 bp long, candidate regulatory DNA sites within the ura4 reporter gene. Each effort employed a fission yeast-optimized CRISPR system (SpEDIT), optimal guide RNA, and one of four homologous recombination templates with 10 to 15 bp substitutions. Remarkably, every Ura - transformant analyzed had template-directed, PAM-disabling bp substitutions near (5-6 bp away from) the DSB but no DNA site-generating substitutions at distance (42-56 bp). An unsought novel allele, ura4-P127* , has two substitutions (C379T, C380A) that create a stop codon, rendering strains unable to grow without uracil.

Laboratory horror stories: Poison in the agars

The fission yeast Schizosaccharomyces pombe is a single-celled eukaryote that can be cultured as a haploid or as a diploid. Scientists employ mating, meiosis, and the plating of ascospores and cells to generate strains with novel genotypes and to discover biological processes. Our two laboratories encountered independently sudden-onset, major impediments to such research. Spore suspensions and vegetative cells no longer plated effectively on minimal media. By systematically analyzing multiple different media components from multiple different suppliers, we identified the source of the problem. Specific lots of agar, from different suppliers, were toxic. Interestingly, the inhibitory effect was attenuated on rich media. Consequently, quality control checks that use only rich media can provide false assurances on the quality of the agar. Lastly, we describe likely sources of the toxicity and we provide specific guidance for quality control measures that should be applied by all vendors as preconditions for their sale of agar.

Metabolic stress-induced long ncRNA transcription governs the formation of meiotic DNA breaks in the fission yeast fbp1 gene

Meiotic recombination is a pivotal process that ensures faithful chromosome segregation and contributes to the generation of genetic diversity in offspring, which is initiated by the formation of double-strand breaks (DSBs). The distribution of meiotic DSBs is not uniform and is clustered at hotspots, which can be affected by environmental conditions. Here, we show that non-coding RNA (ncRNA) transcription creates meiotic DSBs through local chromatin remodeling in the fission yeast fbp1 gene. The fbp1 gene is activated upon glucose starvation stress, in which a cascade of ncRNA-transcription in the fbp1 upstream region converts the chromatin configuration into an open structure, leading to the subsequent binding of transcription factors. We examined the distribution of meiotic DSBs around the fbp1 upstream region in the presence and absence of glucose and observed several new DSBs after chromatin conversion under glucose starvation conditions. Moreover, these DSBs disappeared when cis-elements required for ncRNA transcription were mutated. These results indicate that ncRNA transcription creates meiotic DSBs in response to stress conditions in the fbp1 upstream region. This study addressed part of a long-standing unresolved mechanism underlying meiotic recombination plasticity in response to environmental fluctuation.

Nascent evolution of recombination rate differences as a consequence of chromosomal rearrangements

Reshuffling of genetic variation occurs both by independent assortment of chromosomes and by homologous recombination. Such reshuffling can generate novel allele combinations and break linkage between advantageous and deleterious variants which increases both the potential and the efficacy of natural selection. Here we used high-density linkage maps to characterize global and regional recombination rate variation in two populations of the wood white butterfly (Leptidea sinapis) that differ considerably in their karyotype as a consequence of at least 27 chromosome fissions and fusions. The recombination data were compared to estimates of genetic diversity and measures of selection to assess the relationship between chromosomal rearrangements, crossing over, maintenance of genetic diversity and adaptation. Our data show that the recombination rate is influenced by both chromosome size and number, but that the difference in the number of crossovers between karyotypes is reduced as a consequence of a higher frequency of double crossovers in larger chromosomes. As expected from effects of selection on linked sites, we observed an overall positive association between recombination rate and genetic diversity in both populations. Our results also revealed a significant effect of chromosomal rearrangements on the rate of intergenic diversity change between populations, but limited effects on polymorphisms in coding sequence. We conclude that chromosomal rearrangements can have considerable effects on the recombination landscape and consequently influence both maintenance of genetic diversity and efficiency of selection in natural populations.

Actin-related protein ACTL7B ablation leads to OAT with multiple morphological abnormalities of the flagellum and male infertility in mice†

The formation of fertilisation-competent sperm requires spermatid morphogenesis (spermiogenesis), a poorly understood program that involves complex coordinated restructuring and specialised cytoskeletal structures. A major class of cytoskeletal regulators are the actin-related proteins (ARPs), which include conventional actin variants, and related proteins that play essential roles in complexes regulating actin dynamics, intracellular transport, and chromatin remodeling. Multiple testis-specific ARPs are well conserved among mammals, but their functional roles are unknown. One of these is actin-like 7b (Actl7b) that encodes an orphan ARP highly similar to the ubiquitously expressed beta actin (ACTB). Here we report ACTL7B is expressed in human and mouse spermatids through the elongation phase of spermatid development. In mice, ACTL7B specifically localises to the developing acrosome, within the nucleus of early spermatids, and to the flagellum connecting region. Based on this localisation pattern and high level of sequence conservation in mice, humans, and other mammals, we examined the requirement for ACTL7B in spermiogenesis by generating and characterising the reproductive phenotype of male Actl7b KO mice. KO mice were infertile, with severe and variable oligoteratozoospermia (OAT) and multiple morphological abnormalities of the flagellum (MMAF) and sperm head. These defects phenocopy human OAT and MMAF, which are leading causes of idiopathic male infertility. In conclusion, this work identifies ACTL7B as a key regulator of spermiogenesis that is required for male fertility.

Regulation Mechanisms of Meiotic Recombination Revealed from the Analysis of a Fission Yeast Recombination Hotspot ade6-M26

Meiotic recombination is a pivotal event that ensures faithful chromosome segregation and creates genetic diversity in gametes. Meiotic recombination is initiated by programmed double-strand breaks (DSBs), which are catalyzed by the conserved Spo11 protein. Spo11 is an enzyme with structural similarity to topoisomerase II and induces DSBs through the nucleophilic attack of the phosphodiester bond by the hydroxy group of its tyrosine (Tyr) catalytic residue. DSBs caused by Spo11 are repaired by homologous recombination using homologous chromosomes as donors, resulting in crossovers/chiasmata, which ensure physical contact between homologous chromosomes. Thus, the site of meiotic recombination is determined by the site of the induced DSB on the chromosome. Meiotic recombination is not uniformly induced, and sites showing high recombination rates are referred to as recombination hotspots. In fission yeast, ade6-M26, a nonsense point mutation of ade6 is a well-characterized meiotic recombination hotspot caused by the heptanucleotide sequence 5'-ATGACGT-3' at the M26 mutation point. In this review, we summarize the meiotic recombination mechanisms revealed by the analysis of the fission ade6-M26 gene as a model system.

Adaptive Control of the Meiotic Recombination Landscape by DNA Site-dependent Hotspots With Implications for Evolution

Meiosis is an essential component of the sexual life cycle in eukaryotes. The independent assortment of chromosomes in meiosis increases genetic diversity at the level of whole chromosomes and meiotic recombination increases genetic diversity within chromosomes. The resulting variability fuels evolution. Interestingly, global mapping of recombination in diverse taxa revealed dramatic changes in its frequency distribution between closely related species, subspecies, and even isolated populations of the same species. New insight into mechanisms for these evolutionarily rapid changes has come from analyses of environmentally induced plasticity of recombination in fission yeast. Many different DNA sites, and where identified their binding/activator proteins, control the positioning of recombination at hotspots. Each different class of hotspots functions as an independently controlled rheostat that modulates rates of recombination over a broad dynamic range in response to changing conditions. Together, this independent modulation can rapidly and dramatically alter the global frequency distribution of recombination. This process likely contributes substantially to (i.e., can largely explain) evolutionarily rapid, Prdm9-independent changes in the recombination landscape. Moreover, the precise control mechanisms allow cells to dynamically favor or disfavor newly arising combinations of linked alleles in response to changing extracellular and intracellular conditions, which has striking implications for the impacts of meiotic recombination on evolution.

Histone Chaperones as Cardinal Players in Development

Dynamicity and flexibility of the chromatin landscape are critical for most of the DNA-dependent processes to occur. This higher-order packaging of the eukaryotic genome into the chromatin is mediated by histones and associated non-histone proteins that determine the states of chromatin. Histone chaperones- “the guardian of genome stability and epigenetic information” controls the chromatin accessibility by escorting the nucleosomal and non-nucleosomal histones as well as their variants. This distinct group of molecules is involved in all facets of histone metabolism. The selectivity and specificity of histone chaperones to the histones determine the maintenance of the chromatin in an open or closed state. This review highlights the functional implication of the network of histone chaperones in shaping the chromatin function in the development of an organism. Seminal studies have reported embryonic lethality at different stages of embryogenesis upon perturbation of some of the chaperones, suggesting their essentiality in development. We hereby epitomize facts and functions that emphasize the relevance of histone chaperones in orchestrating different embryonic developmental stages starting from gametogenesis to organogenesis in multicellular organisms.

Molecular mechanisms for environmentally induced and evolutionarily rapid redistribution (plasticity) of meiotic recombination

It has long been known (circa 1917) that environmental conditions, as well as speciation, can affect dramatically the frequency distribution of Spo11/Rec12-dependent meiotic recombination. Here, by analyzing DNA sequence-dependent meiotic recombination hotspots in the fission yeast Schizosaccharomyces pombe, we reveal a molecular basis for these phenomena. The impacts of changing environmental conditions (temperature, nutrients, and osmolarity) on local rates of recombination are mediated directly by DNA site-dependent hotspots (M26, CCAAT, and Oligo-C). This control is exerted through environmental condition-responsive signal transduction networks (involving Atf1, Pcr1, Php2, Php3, Php5, and Rst2). Strikingly, individual hotspots modulate rates of recombination over a very broad dynamic range in response to changing conditions. They can range from being quiescent to being highly proficient at promoting activity of the basal recombination machinery (Spo11/Rec12 complex). Moreover, each different class of hotspot functions as an independently controlled rheostat; a condition that increases the activity of one class can decrease the activity of another class. Together, the independent modulation of recombination rates by each different class of DNA site-dependent hotspots (of which there are many) provides a molecular mechanism for highly dynamic, large-scale changes in the global frequency distribution of meiotic recombination. Because hotspot-activating DNA sites discovered in fission yeast are conserved functionally in other species, this process can also explain the previously enigmatic, Prdm9-independent, evolutionarily rapid changes in hotspot usage between closely related species, subspecies, and isolated populations of the same species.

Evolution and Plasticity of Genome-Wide Meiotic Recombination Rates

Sex, as well as meiotic recombination between homologous chromosomes, is nearly ubiquitous among eukaryotes. In those species that use it, recombination is important for chromosome segregation during gamete production, and thus for fertility. Strikingly, although in most species only one crossover event per chromosome is required to ensure proper segregation, recombination rates vary considerably above this minimum and show variation within and among species. However, whether this variation in recombination is adaptive or neutral and what might shape it remain unclear. Empirical studies and theory support the idea that recombination is generally beneficial but can also have costs. Here, we review variation in genome-wide recombination rates, explore what might cause this, and discuss what is known about its mechanistic basis. We end by discussing the environmental sensitivity of meiosis and recombination rates, how these features may relate to adaptation, and their implications for a broader understanding of recombination rate evolution. Expected final online publication date for the Annual Review of Genetics, Volume 55 is November 2021. Please see http://www.annualreviews.org/page/journal/pubdates for revised estimates.

The epigenetic and morphogenetic effects of molecular oxygen and its derived reactive species in development

The development of multicellular organisms involves the unpacking of a complex genetic program. Extensive characterization of discrete developmental steps has revealed the genetic program is controlled by an epigenetic state. Shifting the epigenome is a group of epigenetic enzymes that modify DNA and proteins to regulate cell type specific gene expression. While the role of these modifications in development has been established, the input(s) responsible for electing changes in the epigenetic state remains unknown. Development is also associated with dynamic changes in cellular metabolism, redox, free radical production, and oxygen availability. It has previously been postulated that these changes are causal in development by affecting gene expression. This suggests that oxygen is a morphogenic compound that impacts the removal of epigenetic marks. Likewise, metabolism and reactive oxygen species influence redox signaling through iron and glutathione to limit the availability of key epigenetic cofactors such as α-ketoglutarate, ascorbate, NAD⁺ and S-adenosylmethionine. Given the close relationship between these cofactors and epigenetic marks it seems likely that the two are linked. Here we describe how changing these inputs might affect the epigenetic state during development to drive gene expression. Combined, these cofactors and reactive oxygen species constitute the epigenetic landscape guiding cells along differing developmental paths.

SUMO is a pervasive regulator of meiosis

Protein modification by SUMO helps orchestrate the elaborate events of meiosis to faithfully produce haploid gametes. To date, only a handful of meiotic SUMO targets have been identified. Here, we delineate a multidimensional SUMO-modified meiotic proteome in budding yeast, identifying 2747 conjugation sites in 775 targets, and defining their relative levels and dynamics. Modified sites cluster in disordered regions and only a minority match consensus motifs. Target identities and modification dynamics imply that SUMOylation regulates all levels of chromosome organization and each step of meiotic prophase I. Execution-point analysis confirms these inferences, revealing functions for SUMO in S-phase, the initiation of recombination, chromosome synapsis and crossing over. K15-linked SUMO chains become prominent as chromosomes synapse and recombine, consistent with roles in these processes. SUMO also modifies ubiquitin, forming hybrid oligomers with potential to modulate ubiquitin signaling. We conclude that SUMO plays diverse and unanticipated roles in regulating meiotic chromosome metabolism.

Most mammalian, yeast and other eukaryote cells have two sets of chromosomes, one from each parent, which contain all the cell’s DNA. Sex cells – like the sperm and egg – however, have half the number of chromosomes and are formed by a specialized type of cell division known as meiosis. At the start of meiosis, each cell replicates its chromosomes so that it has twice the amount of DNA. The cell then undergoes two rounds of division to form sex cells which each contain only one set of chromosomes. Before the cell divides, the two duplicated sets of chromosomes pair up and swap sections of their DNA. This exchange allows each new sex cell to have a unique combination of DNA, resulting in offspring that are genetically distinct from their parents.

This complex series of events is tightly regulated, in part, by a protein called the 'small ubiquitin-like modifier' (or SUMO for short), which attaches itself to other proteins and modifies their behavior. This process, known as SUMOylation, can affect a protein’s stability, where it is located in the cell and how it interacts with other proteins. However, despite SUMO being known as a key regulator of meiosis, only a handful of its protein targets have been identified.

To gain a better understanding of what SUMO does during meiosis, Bhagwat et al. set out to find which proteins are targeted by SUMO in budding yeast and to map the specific sites of modification. The experiments identified 2,747 different sites on 775 different proteins, suggesting that SUMO regulates all aspects of meiosis. Consistently, inactivating SUMOylation at different times revealed SUMO plays a role at every stage of meiosis, including the replication of DNA and the exchanges between chromosomes. In depth analysis of the targeted proteins also revealed that SUMOylation targets different groups of proteins at different stages of meiosis and interacts with other protein modifications, including the ubiquitin system which tags proteins for destruction.

The data gathered by Bhagwat et al. provide a starting point for future research into precisely how SUMO proteins control meiosis in yeast and other organisms. In humans, errors in meiosis are the leading cause of pregnancy loss and congenital diseases. Most of the proteins identified as SUMO targets in budding yeast are also present in humans. So, this research could provide a platform for medical advances in the future. The next step is to study mammalian models, such as mice, to confirm that the regulation of meiosis by SUMO is the same in mammals as in yeast.

Sharing Marks: H3K4 Methylation and H2B Ubiquitination as Features of Meiotic Recombination and Transcription

Meiosis is a specialized cell division that gives raise to four haploid gametes from a single diploid cell. During meiosis, homologous recombination is crucial to ensure genetic diversity and guarantee accurate chromosome segregation. Both the formation of programmed meiotic DNA double-strand breaks (DSBs) and their repair using homologous chromosomes are essential and highly regulated pathways. Similar to other processes that take place in the context of chromatin, histone posttranslational modifications (PTMs) constitute one of the major mechanisms to regulate meiotic recombination. In this review, we focus on specific PTMs occurring in histone tails as driving forces of different molecular events, including meiotic recombination and transcription. In particular, we concentrate on the influence of H3K4me3, H2BK123ub, and their corresponding molecular machineries that write, read, and erase these histone marks. The Spp1 subunit within the Complex of Proteins Associated with Set1 (COMPASS) is a critical regulator of H3K4me3-dependent meiotic DSB formation. On the other hand, the PAF1c (RNA polymerase II associated factor 1 complex) drives the ubiquitination of H2BK123 by Rad6-Bre1. We also discuss emerging evidence obtained by cryo-electron microscopy (EM) structure determination that has provided new insights into how the "cross-talk" between these two marks is accomplished.

Schizosaccharomyces pombe DNA translocases Rrp1 and Rrp2 have distinct roles at centromeres and telomeres that ensure genome stability

The regulation of telomere and centromere structure and function is essential for maintaining genome integrity. Schizosaccharomyces pombe Rrp1 and Rrp2 are orthologues of Saccharomyces cerevisiae Uls1, a SWI2/SNF2 DNA translocase and SUMO-targeted ubiquitin ligase. Here, we show that Rrp1 or Rrp2 overproduction leads to chromosome instability and growth defects, a reduction in global histone levels and mislocalisation of centromere-specific histone Cnp1. These phenotypes depend on putative DNA translocase activities of Rrp1 and Rrp2, suggesting that Rrp1 and Rrp2 may be involved in modulating nucleosome dynamics. Furthermore, we confirm that Rrp2, but not Rrp1, acts at telomeres, reflecting a previously described interaction between Rrp2 and Top2. In conclusion, we identify roles for Rrp1 and Rrp2 in maintaining centromere function by modulating histone dynamics, contributing to the preservation of genome stability during vegetative cell growth.

Diverse DNA Sequence Motifs Activate Meiotic Recombination Hotspots Through a Common Chromatin Remodeling Pathway

In meiosis, multiple different DNA sequence motifs help to position homologous recombination at hotspots in the genome. How do the seemingly disparate cis-acting regulatory modules each promote locally the activity of the basal recombination machinery? We defined molecular mechanisms of action for five different hotspot-activating DNA motifs (M26, CCAAT, Oligo-C, 4095, 4156) located independently at the same site within the ade6 locus of the fission yeast Schizosaccharomyces pombe Each motif promoted meiotic recombination (i.e., is active) within this context, and this activity required the respective binding proteins (transcription factors Atf1, Pcr1, Php2, Php3, Php5, Rst2). High-resolution analyses of chromatin structure by nucleosome scanning assays revealed that each motif triggers the displacement of nucleosomes surrounding the hotspot motif in meiosis. This chromatin remodeling required the respective sequence-specific binding proteins, was constitutive for two motifs, and was enhanced meiotically for three others. Hotspot activity of each motif strongly required the ATP-dependent chromatin remodeling enzyme Snf22 (Snf2/Swi2), with lesser dependence on Gcn5, Mst2, and Hrp3. These findings support a model in which most meiotic recombination hotspots are positioned by the binding of transcription factors to their respective DNA sites. The functional redundancy of multiple, sequence-specific protein-DNA complexes converges upon shared chromatin remodeling pathways that help provide the basal recombination machinery (Spo11/Rec12 complex) access to its DNA substrates within chromatin.

Chromatin remodeler Fft3 plays a dual role at blocked DNA replication forks

Here, we investigate the function of fission yeast Fun30/Smarcad1 family of SNF2 ATPase-dependent chromatin remodeling enzymes in DNA damage repair. There are three Fun30 homologues in fission yeast, Fft1, Fft2, and Fft3. We find that only Fft3 has a function in DNA repair and it is needed for single-strand annealing of an induced double-strand break. Furthermore, we use an inducible replication fork barrier system to show that Fft3 has two distinct roles at blocked DNA replication forks. First, Fft3 is needed for the resection of nascent strands, and second, it is required to restart the blocked forks. The latter function is independent of its ATPase activity.

Nucleosome Remodeling by Fun30SMARCAD1 in the DNA Damage Response

Many cellular pathways are dedicated to maintain the integrity of the genome. In eukaryotes, the underlying DNA transactions occur in the context of chromatin. Cells utilize chromatin and its dynamic nature to regulate those genome integrity pathways. Accordingly, chromatin becomes restructured and modified around DNA damage sites. Here, we review the current knowledge of a chromatin remodeler Fun30^SMARCAD1, which plays a key role in genome maintenance. Fun30^SMARCAD1 promotes DNA end resection and the repair of DNA double-stranded breaks (DSBs). Notably, however, Fun30^SMARCAD1 plays additional roles in maintaining heterochromatin and promoting transcription. Overall, Fun30^SMARCAD1 is involved in distinct processes and the specific roles of Fun30^SMARCAD1 at DSBs, replication forks and sites of transcription appear discordant at first view. Nonetheless, a picture emerges in which commonalities within these context-dependent roles of Fun30^SMARCAD1 exist, which may help to gain a more global understanding of chromatin alterations induced by Fun30^SMARCAD1.