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Establishing the program of origin firing during S phase in fission Yeast.

ABSTRACT: Initiation of eukaryotic DNA synthesis occurs at origins of replication that are utilized with characteristic times and frequencies during S phase. We have investigated origin usage by evaluating the kinetics of replication factor binding in fission yeast and show that similar to metazoa, ORC binding is periodic during the cell cycle, increasing during mitosis and peaking at M/G1. At an origin, the timing of ORC binding in M and pre-RC assembly in G1 correlates with the timing of firing during S, and the level of pre-IC formation reflects origin efficiency. Extending mitosis allows ORC to become more equally associated with origins and leads to genome-wide changes in origin usage, while overproduction of pre-IC factors increases replication of both efficient and inefficient origins. We propose that differential recruitment of ORC to origins during mitosis followed by competition among origins for limiting replication factors establishes the timing and efficiency of origin firing.

SUBMITTER: Wu PY

PROVIDER: S-EPMC2787407 | biostudies-literature | 2009 Mar

REPOSITORIES: biostudies-literature

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Establishing the program of origin firing during S phase in fission Yeast.

Wu Pei-Yun Jenny PY Nurse Paul P

Cell 20090301 5

Initiation of eukaryotic DNA synthesis occurs at origins of replication that are utilized with characteristic times and frequencies during S phase. We have investigated origin usage by evaluating the kinetics of replication factor binding in fission yeast and show that similar to metazoa, ORC binding is periodic during the cell cycle, increasing during mitosis and peaking at M/G1. At an origin, the timing of ORC binding in M and pre-RC assembly in G1 correlates with the timing of firing during S ...[more]

PMID: 19269364

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Establishing the program of origin firing during S phase in fission yeast

Project description:Our data have revealed a correlation between the timing of ORC and MCM binding to origins and the timing of replication. We hypothesized that origins bind ORC during M with varying affinities, and that delays in ORC binding and pre-RC formation at late-firing origins result in low efficiencies due to these origins subsequently competing less effectively for limiting replication factors. We investigated if equalizing ORC binding results in changes in origin efficiencies. Because the increase in ORC binding occurs during M, we surmised that extending M might result in origins accumulating ORC more equally, leading to more equal distribution of pre-RC and pre-IC assembly among origins. As a consequence, early origins might become less efficient and late origins more efficient. To extend M in cells, we used the drug MBC (Carbendazim), which prevents microtubule polymerization and disrupts the mitotic spindle. Cells were then allowed to undergo S phase in the presence of BrdU, and labeled fragments were isolated by immunoprecipitation of genomic DNA and hybridized to Affymetrix tiling arrays. These results were compared to the replication profile of synchronized cells treated with HU but not with MBC, providing a control for normal origin efficiencies across the genome. This method allows the relative efficiency of origin usage genome-wide to be determined based on signal intensities from array hybridizations. Keywords: comparison between cells treated with a drug and normal cells

2009-03-06 | GSE14864 | GEO

The spatial and temporal organization of origin firing during the S-phase of fission yeast.

Project description:Eukaryotes duplicate their genomes using multiple replication origins, but the organization of origin firing along chromosomes and during S-phase is not well understood. Using fission yeast, we report the first genome-wide analysis of the spatial and temporal organization of replication origin firing, analyzed using single DNA molecules that can approach the full length of chromosomes. At S-phase onset, origins fire randomly and sparsely throughout the chromosomes. Later in S-phase, clusters of fired origins appear embedded in the sparser regions, which form the basis of nuclear replication foci. The formation of clusters requires proper histone methylation and acetylation, and their locations are not inherited between cell cycles. The rate of origin firing increases gradually, peaking just before mid S-phase. Toward the end of S-phase, nearly all the available origins within the unreplicated regions are fired, contributing to the timely completion of genome replication. We propose that the majority of origins do not fire as a part of a deterministic program. Instead, origin firing, both individually and as clusters, should be viewed as being mostly stochastic.

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Fission yeast Eso1p is required for establishing sister chromatid cohesion during S phase.

Project description:Sister chromatid cohesion is essential for cell viability. We have isolated a novel temperature-sensitive lethal mutant named eso1-H17 that displays spindle assembly checkpoint-dependent mitotic delay and abnormal chromosome segregation. At the permissive temperature, the eso1-H17 mutant shows mild sensitivity to UV irradiation and DNA-damaging chemicals. At the nonpermissive temperature, the mutant is arrested in M phase with a viability loss due to a failure to establish sister chromatid cohesion during S phase. The lethal M-phase arrest phenotype, however, is suppressed by inactivation of a spindle checkpoint. The eso1(+) gene is not essential for the onset and progression of DNA replication but has remarkable genetic interactions with those genes regulating the G(1)-S transition and DNA replication. The N-terminal two-thirds of Eso1p is highly homologous to DNA polymerase eta of budding yeast and humans, and the C-terminal one-third is homologous to budding yeast Eco1p (also called Ctf7p), which is required for the establishment of sister chromatid cohesion. Deletion analysis and determination of the mutation site reveal that the function of the Eco1p/Ctf7p-homologous domain is necessary and sufficient for sister chromatid cohesion. On the other hand, deletion of the DNA polymerase eta domain in Eso1p increases sensitivity to UV irradiation. These results indicate that Eso1p plays a dual role during DNA replication. The C-terminal region acts to establish sister chromatid cohesion, and the N-terminal region presumably catalyzes translesion DNA synthesis when template DNA contains lesions that block regular DNA replication.

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Rif1 is a global regulator of timing of replication origin firing in fission yeast.

Project description:One of the long-standing questions in eukaryotic DNA replication is the mechanisms that determine where and when a particular segment of the genome is replicated. Cdc7/Hsk1 is a conserved kinase required for initiation of DNA replication and may affect the site selection and timing of origin firing. We identified rif1?, a null mutant of rif1(+), a conserved telomere-binding factor, as an efficient bypass mutant of fission yeast hsk1. Extensive deregulation of dormant origins over a wide range of the chromosomes occurs in rif1? in the presence or absence of hydroxyurea (HU). At the same time, many early-firing, efficient origins are suppressed or delayed in firing timing in rif1?. Rif1 binds not only to telomeres, but also to many specific locations on the arm segments that only partially overlap with the prereplicative complex assembly sites, although Rif1 tends to bind in the vicinity of the late/dormant origins activated in rif1?. The binding to the arm segments occurs through M to G1 phase in a manner independent of Taz1 and appears to be essential for the replication timing program during the normal cell cycle. Our data demonstrate that Rif1 is a critical determinant of the origin activation program on the fission yeast chromosomes.

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Rif1 is a global regulator of timing of replication origin firing in fission yeast

Project description:One of the long-standing questions in eukaryotic DNA replication is the mechanisms that determine where and when a particular segment of the genome is replicated. Cdc7/Hsk1 is a conserved kinase required for initiation of DNA replication, and may affect the site selection and timing of origin firing. We identified rif1∆, a null mutant of rif1+, a conserved telomere binding factor, as an efficient bypass mutant of fission yeast hsk1. Extensive deregulation of dormant origins over a wide range of the chromosomes occurs in rif1∆ in the presence or absence of HU. At the same time, many early-firing, efficient origins are suppressed or delayed in firing timing in rif1∆. Rif1 binds not only to telomeres but also to many specific locations on the arm segments that only partially overlap with the pre-Replicative Complex assembly sites, although Rif1 tends to bind in the vicinity of the late/ dormant origins activated in rif1∆. The binding to the arm segments occurs through M to G1 phase in a manner independent of Taz1 and appears to be essential for the replication timing program during normal cell cycle. Our data demonstrate that Rif1 is a critical determinant of origin activation program on the fission yeast chromosomes. BrdU incorporation profiles at early S-phase in Wild vs rif1∆. Cdc45 binding profiles at early S-phase in Wild vs rif1∆. Rif1 binidng sites aroud G1/S boudary and at M-phase. Mcm4 binding sites in wild, hsk1-89 temperature mutant and rif1∆.

2011-12-13 | GSE34369 | GEO

Cyclin-dependent kinase inhibits reinitiation of a normal S-phase program during G2 in fission yeast.

Project description:To achieve faithful replication of the genome once in each cell cycle, reinitiation of S phase is prevented in G(2) and origins are restricted from refiring within S phase. We have investigated the block to rereplication during G(2) in fission yeast. The DNA synthesis that occurs when G(2)/M cyclin-dependent kinase (CDK) activity is depleted has been assumed to be repeated rounds of S phase without mitosis, but this has not been demonstrated to be the case. We show here that on G(2)/M CDK depletion in G(2), repeated S phases are induced, which are correlated with normal G(1)/S transcription and attainment of doublings in cell size. Mostly normal mitotic S-phase origins are utilized, although at different efficiencies, and replication is essentially equal across the genome. We conclude that CDK inhibits reinitiation of S phase during G(2), and if G(2)/M CDK is depleted, replication results from induction of a largely normal S-phase program with only small differences in origin usage and efficiency.

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Dephosphorylation of the pre-initiation complex during S-phase is critical for origin firing dynamics

Project description:Genome stability requires complete DNA duplication exactly once before division. In eukaryotes, cyclin-dependent kinase (CDK) plays a dual role in this regulation by inhibiting helicase loading factors and activating origin firing. CDK drives initiation by phosphorylation of two substrates, Sld2 and Sld3, forming the transient and limiting intermediate called the pre-initiation complex (pre-IC). The importance and mechanism of dissociation of the pre-IC from origins is not understood. Here we show in the budding yeast Saccharomyces cerevisiae that CDK phosphorylations of Sld3 and Sld2 are specifically and rapidly turned-over during interphase by the PP2A and PP4 phosphatases. Inhibition of dephosphorylation of Sld3/Sld2 causes dramatic defects in replication initiation genome-wide, retention of the pre-IC at origins and cell death. These studies not only provide a mechanism to ensure that Sld3/Sld2 are dephosphorylated before helicase loading factors but also establish a novel positive role for phosphatases in eukaryotic origin firing.

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Cyclin A-Cdk1 regulates the origin firing program in mammalian cells.

Project description:Somatic mammalian cells possess well-established S-phase programs with specific regions of the genome replicated at precise times. The ATR-Chk1 pathway plays a central role in these programs, but the mechanism for how Chk1 regulates origin firing remains unknown. We demonstrate here the essential role of cyclin A2-Cdk1 in the regulation of late origin firing. Activity of cyclin A2-Cdk1 was hardly detected at the onset of S phase, but it was obvious at middle to late S phase under unperturbed condition. Chk1 depletion resulted in increased expression of Cdc25A, subsequent hyperactivation of cyclin A2-Cdk1, and abnormal replication at early S phase. Hence, the ectopic expression of cyclin A2-Cdk1AF (constitutively active mutant) fusion constructs resulted in abnormal origin firing, causing the premature appearance of DNA replication at late origins at early S phase. Intriguingly, inactivation of Cdk1 in temperature-sensitive Cdk1 mutant cell lines (FT210) resulted in a prolonged S phase and inefficient activation of late origin firing even at late S phase. Our results thus suggest that cyclin A2-Cdk1 is a key regulator of S-phase programs.

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Checkpoint inhibition of origin firing prevents inappropriate replication in G2/M phase.

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