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Sir2 Takes Affirmative Action to Ensure Equal Opportunity in Replication Origin Licensing COMMENTARY Armelle Lengronnea and Philippe Paseroa,1

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Sir2 Takes Affirmative Action to Ensure Equal Opportunity in Replication Origin Licensing COMMENTARY Armelle Lengronnea and Philippe Paseroa,1 COMMENTARY Sir2 takes affirmative action to ensure equal opportunity in replication origin licensing COMMENTARY Armelle Lengronnea and Philippe Paseroa,1 In all eukaryotes, chromosomal DNA replication ini- initiation factors (11–13). It is generally believed that tiates at multiple sites distributed along chromosomes these factors are recycled after initiation to activate ad- called origins of replication. The initiation of DNA syn- ditional origins, ensuring therefore a staggered activa- thesis is a two-step process involving the loading of the tion of origins throughout the S phase. More recently, minichromosome maintenance (MCM) double hexamer studies in budding yeast have added another layer of on origins during the G1 phase of the cell cycle and the complexity to this regulation by showing that repeated activation of licensed origins during the S phase (1). This elements containing replication origins, such as the ri- strict temporal separation of origin licensing from firing bosomal DNA (rDNA) array, compete with single-copy ensures that a given origin is activated once and only loci for limiting initiation factors (14). In the absence of once per cell cycle, thereby preventing overreplication Sir2, increased initiation at the 150 to 200 rDNA origins of the genome (Fig. 1A). To avoid underreplication, further increases the competition for limiting initiation cells must also ensure that initiation events are evenly factors and reduces initiation at unique loci (9, 14–16). distributed along chromosomes. This is achieved by As a consequence, sir2Δ cells reach G2/M with replica- licensing many more origins than actually needed tion gaps, leading to increased genomic instability (15). (2, 3). Consequently, only a fraction of potential initiation In PNAS, Hoggard et al. (17) propose an alternative sites are activated during the S phase, the remaining model for how the histone deacetylase Sir2 prevents onesservingasbackuporigins when fork progression is genomic instability in budding yeast, through a mech- impeded (4, 5). Origin activation follows a defined rep- anism that is independent of the derepression of lication timing program, in which open chromatin do- rDNA origins. This model builds on the fact that rep- mains replicate early in S phase and heterochromatin lication origins are located within nucleosome-free re- replicates late (1). This sequential activation of replica- gions to facilitate the loading of MCMs by the origin tion origins allows a tight coordination between DNA recognition complex (ORC), Cdc6, and Cdt1 (18, 19). synthesis and the production of histones and dNTPs. It has been reported that nucleosomes interfere with Moreover, the replication program can be interrupted MCM loading in cdc6-4 mutants, which have a re- when forks encounter DNA lesions and restarted once duced ability to assemble prereplication complexes replication stress is relieved (6). The correct execution (pre-RCs) (20). In the absence of Sir2, higher levels of the replication timing program is therefore essential of H4K16 acetylation would increase the mobility to maintain the integrity of eukaryotic genomes. of origin-proximal nucleosomes (Fig. 1A), facilitating Although the general rules governing origin firing MCM loading and cell growth (20, 21). However, are now well understood, the molecular mechanisms why does this increased licensing activity rescuing that determine the timing and efficiency of initiation at growth defects in cdc6-4 mutants lead to more geno- individual origins are still debated. In metazoan and in mic instability in a wild-type context? To answer this fission yeast, multiple MCM complexes can be loaded question, the authors measured the chromatin levels at a replication origin, increasing its probability to of Mcm2, Sir3, and H4K16ac in wild-type and sir2Δ initiate DNA synthesis (7). In contrast, only one MCM cells by chromatin immunoprecipitation and next- double hexamer is loaded at origins in budding yeast generation sequencing. They also monitored the con- and origin efficiency does not correlate with MCM lev- sequences on the kinetics of DNA replication using a els (8). However, origin activation is modulated by epi- label-free approach called SortSeq. In sir2Δ cells, they genetic mechanisms involving the histone deacetylases observed a higher level of both H4K16ac and Mcm2 at Rpd3 and Sir2 (9, 10). Origin firing also depends on the early origins relative to late origins, indicating that ability of prereplication complexes to attract limiting Sir2 dampens pre-RC assembly at early origins. They aInstitut de Gen ´ ´etiqueHumaine, CNRS and Universit ´ede Montpellier, Equipe Labelis ´ ee ´ Ligue contre le Cancer, 34396 Montpellier, France Author contributions: A.L. and P.P. wrote the paper. The authors declare no competing interest. Published under the PNAS license. See companion article, “Sir2 mitigates an intrinsic imbalance in origin licensing efficiency between early- and late-replicating euchromatin,” 10.1073/pnas.2004664117. 1To whom correspondence may be addressed. Email: [email protected]. First published June 30, 2020. www.pnas.org/cgi/doi/10.1073/pnas.2010001117 PNAS | July 21, 2020 | vol. 117 | no. 29 | 16723–16725 Downloaded by guest on September 26, 2021 Fig. 1. The histone deacetylase Sir2 dampens the activity of early origins to favor an equal distribution of initiation events throughout the budding yeast genome. (A) The activation of replication origins in budding yeast depends on the binding of the origin recognition complex (ORC) to autonomous replicating sequences (ARS) and the loading of Mcm2-7 double hexamers in a Cdc6- and Cdt1-dependent manner to form the prereplication complex (pre-RC). Although ORC is bound to origins throughout the cell cycle, pre-RCs can only be assembled in G1. Cyclin-dependent and Dbf4-dependent kinases (CDK and DDK) trigger origin firing upon entry into S phase. Since origin activation depends on a set of limiting initiation factors, initiation events occur sequentially throughout the length of the S phase. The licensing of replication origins occurs in the context of chromatin. Nucleosomes flanking early origins (but not late origins) are acetylated on H4K16, which increases their mobility and facilitates MCM loading. By deacetylating these nucleosomes, Sir2 would reduce the advantage of early origins over late origins for pre-RC loading, ensuring therefore a more even distribution of licensed origins throughout the genome (17). (B) In the absence of Sir2, suboptimal pre-RC assembly at late origins increases the risk of underreplication in late-replicating regions and the persistence of replication gaps (17). Since sir2Δ cells show an increased activation of ribosomal DNA (rDNA) origins, the reduced activation of late origins could also be due to a competition for limiting initiation factors (14, 15). Replication defects observed in sir2Δ mutants could therefore be due to a combination of origin licensing and activation defects, even though the relative contribution of both mechanisms remains to be established. Note that late origins refer here to euchromatic late origins and not to telomeric origins, which are directly repressed by Sir2. also found that late origins were less efficiently activated, which is deacetylase involved in the repression of late origins firing that acts consistent with earlier studies (14) and explains the persistence of both locally and through an rDNA-mediated mechanism (10, 14). replication gaps at the end of S phase (Fig. 1B). This reduced Another interesting question concerns the mechanism by which activation of late origins in sir2Δ mutants was not suppressed in Sir2 reduces the rate of initiation at early origins relative to late strains bearing a single-nucleotide polymorphism in rDNA origin, origins. It has been proposed that origin-proximal nucleosomes weakening its activity (22). Altogether, these data indicate that interfere with pre-RC assembly, unless if they are acetylated on Sir2 does not only act through the rDNA to regulate the activation H4K16 and can be pushed aside to accommodate the loading of the of single-copy origins but also acts directly at the level of individ- MCM double hexamer (20). Although this model is plausible, other ual origins. These results also argue for a model in which mechanisms could be envisaged. For instance, it has been recently Sir2 promotes the licensing of late origins by reducing the ability reported that Sir2 represses origin activation at the rDNA by silencing of early origins to load MCMs, contributing therefore to a contin- noncoding transcription through rDNA origins. Indeed, increased uum of origin activation throughout the S phase. transcription in sir2Δ cells displaces MCMs to a nucleosome-free re- This provocative model raises a number of questions and opens gion, increasing the probability of origin firing (16). Since pervasive directions for future research. The first question concerns the rela- transcription is a widespread process, similar mechanisms could also tive contribution of Sir2’s distinct functions in origin licensing (17) operate at single-copy early origins. However, recent evidence indi- and in origin activation through an rDNA-mediated process (14, cates that pervasive transcription has rather a negative effect on 15). To rule out a contribution of the rDNA in the replication defects single-copy origins by inactivating pre-RCs (23, 24). Transcriptionally observed in sir2Δ mutant cells, Hoggard et al. monitored the activ- inactivated origins can be repetitively
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