Specialization of B-type cyclins for mitosis or meiosis in S. cerevisiae

The B-type cyclin Clb4 prevents meiosis I sister centromere separation in budding yeast

In meiosis, one round of DNA replication followed by two rounds of chromosome segregation halves the ploidy of the original cell. Accurate chromosome segregation in meiosis I depends on recombination between homologous chromosomes. Sister centromeres attach to the same spindle pole in this division and only segregate in meiosis II. We used budding yeast to select for mutations that produced viable spores in the absence of recombination. The most frequent mutations inactivated CLB4, which encodes one of four B-type cyclins. In two wild yeast isolates, Y55 and SK1, but not the W303 laboratory strain, deleting CLB4 causes premature sister centromere separation and segregation in meiosis I and frequent termination of meiosis after a single division, demonstrating a novel role for Clb4 in meiotic chromosome dynamics and meiotic progression. This role depends on the genetic background since meiosis in W303 is largely independent of CLB4.

Rewiring of the phosphoproteome executes two meiotic divisions in budding yeast

The cell cycle is ordered by a controlled network of kinases and phosphatases. To generate gametes via meiosis, two distinct and sequential chromosome segregation events occur without an intervening S phase. How canonical cell cycle controls are modified for meiosis is not well understood. Here, using highly synchronous budding yeast populations, we reveal how the global proteome and phosphoproteome change during the meiotic divisions. While protein abundance changes are limited to key cell cycle regulators, dynamic phosphorylation changes are pervasive. Our data indicate that two waves of cyclin-dependent kinase (Cdc28Cdk1) and Polo (Cdc5Polo) kinase activity drive successive meiotic divisions. These two distinct phases of phosphorylation are ensured by the meiosis-specific Spo13 protein, which rewires the phosphoproteome. Spo13 binds to Cdc5Polo to promote phosphorylation in meiosis I, particularly of substrates containing a variant of the canonical Cdc5Polo motif. Overall, our findings reveal that a master regulator of meiosis directs the activity of a kinase to change the phosphorylation landscape and elicit a developmental cascade.

Meiotic Cytokinesis in Saccharomyces cerevisiae: Spores That Just Need Closure

In the budding yeast Saccharomyces cerevisiae, sporulation occurs during starvation of a diploid cell and results in the formation of four haploid spores forming within the mother cell ascus. Meiosis divides the genetic material that is encapsulated by the prospore membrane that grows to surround the haploid nuclei; this membrane will eventually become the plasma membrane of the haploid spore. Cellularization of the spores occurs when the prospore membrane closes to capture the haploid nucleus along with some cytoplasmic material from the mother cell, and thus, closure of the prospore membrane is the meiotic cytokinetic event. This cytokinetic event involves the removal of the leading-edge protein complex, a complex of proteins that localizes to the leading edge of the growing prospore membrane. The development and closure of the prospore membrane must be coordinated with other meiotic exit events such as spindle disassembly. Timing of the closure of the prospore membrane depends on the meiotic exit pathway, which utilizes Cdc15, a Hippo-like kinase, and Sps1, an STE20 family GCKIII kinase, acting in parallel to the E3 ligase Ama1-APC/C. This review describes the sporulation process and focuses on the development of the prospore membrane and the regulation of prospore membrane closure.

Meiosis in budding yeast

Meiosis is a specialized cell division program that is essential for sexual reproduction. The two meiotic divisions reduce chromosome number by half, typically generating haploid genomes that are packaged into gametes. To achieve this ploidy reduction, meiosis relies on highly unusual chromosomal processes including the pairing of homologous chromosomes, assembly of the synaptonemal complex, programmed formation of DNA breaks followed by their processing into crossovers, and the segregation of homologous chromosomes during the first meiotic division. These processes are embedded in a carefully orchestrated cell differentiation program with multiple interdependencies between DNA metabolism, chromosome morphogenesis, and waves of gene expression that together ensure the correct number of chromosomes is delivered to the next generation. Studies in the budding yeast Saccharomyces cerevisiae have established essentially all fundamental paradigms of meiosis-specific chromosome metabolism and have uncovered components and molecular mechanisms that underlie these conserved processes. Here, we provide an overview of all stages of meiosis in this key model system and highlight how basic mechanisms of genome stability, chromosome architecture, and cell cycle control have been adapted to achieve the unique outcome of meiosis.

Meiotic cells escape prolonged spindle checkpoint activity through kinetochore silencing and slippage

To prevent chromosome mis-segregation, a surveillance mechanism known as the spindle checkpoint delays the cell cycle if kinetochores are not attached to spindle microtubules, allowing the cell additional time to correct improper attachments. During spindle checkpoint activation, checkpoint proteins bind the unattached kinetochore and send a diffusible signal to inhibit the anaphase promoting complex/cyclosome (APC/C). Previous work has shown that mitotic cells with depolymerized microtubules can escape prolonged spindle checkpoint activation in a process called mitotic slippage. During slippage, spindle checkpoint proteins bind unattached kinetochores, but the cells cannot maintain the checkpoint arrest. We asked if meiotic cells had as robust of a spindle checkpoint response as mitotic cells and whether they also undergo slippage after prolonged spindle checkpoint activity. We performed a direct comparison between mitotic and meiotic budding yeast cells that signal the spindle checkpoint through two different assays. We find that the spindle checkpoint delay is shorter in meiosis I or meiosis II compared to mitosis, overcoming a checkpoint arrest approximately 150 minutes earlier in meiosis than in mitosis. In addition, cells in meiosis I escape spindle checkpoint signaling using two mechanisms, silencing the checkpoint at the kinetochore and through slippage. We propose that meiotic cells undertake developmentally-regulated mechanisms to prevent persistent spindle checkpoint activity to ensure the production of gametes.

Distinct Aurora B pools at the inner centromere and kinetochore have different contributions to meiotic and mitotic chromosome segregation

Proper chromosome segregation depends on establishment of bioriented kinetochore-microtubule attachments, which often requires multiple rounds of release and reattachment. Aurora B and C kinases phosphorylate kinetochore proteins to release tensionless attachments. Multiple pathways recruit Aurora B/C to the centromere and kinetochore. We studied how these pathways contribute to anaphase onset timing and correction of kinetochore-microtubule attachments in budding yeast meiosis and mitosis. We find that the pool localized by the Bub1/Bub3 pathway sets the normal duration of meiosis and mitosis, in differing ways. Our meiosis data suggests that disruption of this pathway leads to PP1 kinetochore localization, which dephosphorylates Cdc20 for premature anaphase onset. For error correction, the Bub1/Bub3 and COMA pathways are individually important in meiosis but compensatory in mitosis. Finally, we find that the haspin and Bub1/3 pathways function together to ensure error correction in mouse oogenesis. Our results suggest that each recruitment pathway localizes spatially distinct kinetochore-localized Aurora B/C pools that function differently between meiosis and mitosis.

Cell Cycle Progression Influences Biofilm Formation in Saccharomyces cerevisiae 1308

Biofilm-immobilized continuous fermentation is a novel fermentation strategy that has been utilized in ethanol fermentation. Continuous fermentation contributes to the self-proliferation of Saccharomyces cerevisiae biofilms. Previously, we successfully described the cell cycle differences between biofilm-immobilized fermentation and calcium alginate-immobilized fermentation. In the present study, we investigated the relationship between biofilm formation and the cell cycle. We knocked down CLN3, SIC1, and ACE2 and found that Δcln3 and Δsic1 exhibited a predominance of G₂/M phase cells, increased biofilm formation, and significantly increased extracellular polysaccharide formation and expression of genes in the FLO gene family during immobilisation fermentation. Δace2 exhibited a contrasting performance. These findings suggest that the increase in the proportion of cells in the G₂/M phase of the cell cycle facilitates biofilm formation and that the cell cycle influences biofilm formation by regulating cell adhesion and polysaccharide formation. This opens new avenues for basic research and may also help to provide new ideas for biofilm prevention and optimization. IMPORTANCE Immobilised fermentation can be achieved using biofilm resistance, resulting in improved fermentation efficiency and yield. The link between the cell cycle and biofilms deserves further study since reports are lacking in this area. This study showed that the ability of Saccharomyces cerevisiae to produce biofilm differed when cell cycle progression was altered. Further studies suggested that cell cycle regulatory genes influenced biofilm formation by regulating cell adhesion and polysaccharide formation. Findings related to cell cycle regulation of biofilm formation set the stage for biofilm in Saccharomyces cerevisiae and provide a theoretical basis for the development of a new method to improve biofilm-based industrial fermentation.

Cyclin/Forkhead-mediated coordination of cyclin waves: an autonomous oscillator rationalizing the quantitative model of Cdk control for budding yeast

Networks of interacting molecules organize topology, amount, and timing of biological functions. Systems biology concepts required to pin down 'network motifs' or 'design principles' for time-dependent processes have been developed for the cell division cycle, through integration of predictive computer modeling with quantitative experimentation. A dynamic coordination of sequential waves of cyclin-dependent kinases (cyclin/Cdk) with the transcription factors network offers insights to investigate how incompatible processes are kept separate in time during the eukaryotic cell cycle. Here this coordination is discussed for the Forkhead transcription factors in light of missing gaps in the current knowledge of cell cycle control in budding yeast. An emergent design principle is proposed where cyclin waves are synchronized by a cyclin/Cdk-mediated feed-forward regulation through the Forkhead as a transcriptional timer. This design is rationalized by the bidirectional interaction between mitotic cyclins and the Forkhead transcriptional timer, resulting in an autonomous oscillator that may be instrumental for a well-timed progression throughout the cell cycle. The regulation centered around the cyclin/Cdk-Forkhead axis can be pivotal to timely coordinate cell cycle dynamics, thereby to actuate the quantitative model of Cdk control.

Biofilm-based fermentation: a novel immobilisation strategy for Saccharomyces cerevisiae cell cycle progression during ethanol production

Biofilm-based fermentation, as a new immobilisation strategy, is beneficial for industrial fermentation due to its excellent environmental resistance, high productivity and continuous fermentation relative to calcium alginate-immobilised fermentation. These two techniques differ mainly regarding cell stages. Here, we describe the cell phenotype of Saccharomyces cerevisiae biofilm-based fermentation and compare cell cycle stages with those during immobilisation in calcium alginate. Most cells in the biofilm-based fermentation adhered to the cotton-fibre carrier of the biofilm and were in the G2/M phase whereas alginate-embedded cells were in the G1/G0 phase. Deletion of the RIM15 gene, which regulates cell cycle progression according to nutritional status, hampered the cell cycle arrest observed in alginate-embedded cells, enhanced biofilm formation and improved fermentation ability. The improved biofilm formation shown by the rim15△ strain could be attributed to an increase in the expression level of the adhesion protein FLO11 and synthesis of trehalose. These findings suggest that the extracellular environment is mainly responsible for the difference between biofilm-based fermentation and alginate-embedded fermentation, and that RIM15 plays an essential role in cell cycle progression. KEY POINTS: • In the biofilm, S. cerevisiae cell populations were mostly in the G2/M phase while alginate-embedded cells were arrested in the G1/G0 phase. • The RIM15 gene partially influenced the cell cycle progression observed during ethanol fermentation. • Biofilm-based cells were actively adsorbed on the physical carrier. • Biofilm immobilisation could maintain cell division activity explaining its fermentation efficiency.

CDK Regulation of Meiosis: Lessons from S. cerevisiae and S. pombe

Meiotic progression requires precise orchestration, such that one round of DNA replication is followed by two meiotic divisions. The order and timing of meiotic events is controlled through the modulation of the phosphorylation state of proteins. Key components of this phospho-regulatory system include cyclin-dependent kinase (CDK) and its cyclin regulatory subunits. Over the past two decades, studies in budding and fission yeast have greatly informed our understanding of the role of CDK in meiotic regulation. In this review, we provide an overview of how CDK controls meiotic events in both budding and fission yeast. We discuss mechanisms of CDK regulation through post-translational modifications and changes in the levels of cyclins. Finally, we highlight the similarities and differences in CDK regulation between the two yeast species. Since CDK and many meiotic regulators are highly conserved, the findings in budding and fission yeasts have revealed conserved mechanisms of meiotic regulation among eukaryotes.

Clb3-centered regulations are recurrent across distinct parameter regions in minimal autonomous cell cycle oscillator designs

Some biological networks exhibit oscillations in their components to convert stimuli to time-dependent responses. The eukaryotic cell cycle is such a case, being governed by waves of cyclin-dependent kinase (cyclin/Cdk) activities that rise and fall with specific timing and guarantee its timely occurrence. Disruption of cyclin/Cdk oscillations could result in dysfunction through reduced cell division. Therefore, it is of interest to capture properties of network designs that exhibit robust oscillations. Here we show that a minimal yeast cell cycle network is able to oscillate autonomously, and that cyclin/Cdk-mediated positive feedback loops (PFLs) and Clb3-centered regulations sustain cyclin/Cdk oscillations, in known and hypothetical network designs. We propose that Clb3-mediated coordination of cyclin/Cdk waves reconciles checkpoint and oscillatory cell cycle models. Considering the evolutionary conservation of the cyclin/Cdk network across eukaryotes, we hypothesize that functional ("healthy") phenotypes require the capacity to oscillate autonomously whereas dysfunctional (potentially "diseased") phenotypes may lack this capacity.

Cyclin-Specific Docking Mechanisms Reveal the Complexity of M-CDK Function in the Cell Cycle

Cyclin-dependent kinases (CDKs) coordinate hundreds of molecular events during the cell cycle. Multiple cyclins are involved, but the global role of cyclin-specific phosphorylation has remained unsolved. We uncovered a cyclin docking motif, LxF, that mediates binding of replication factor Cdc6 to mitotic cyclin. This interaction leads to phospho-adaptor Cks1-mediated inhibition of M-CDK to facilitate Cdc6 accumulation and sequestration in mitosis. The LxF motif and Cks1 also mediate the mutual inhibition between M-CDK and the tyrosine kinase Swe1. Additionally, the LxF motif is critical for targeting M-CDK to phosphorylate several mitotic regulators; for example, Spo12 is targeted via LxF to release the phosphatase Cdc14. The results complete the full set of G1, S, and M-CDK docking mechanisms and outline the unified role of cyclin specificity and CDK activity thresholds. Cooperation of cyclin and Cks1 docking creates a variety of CDK thresholds and switching orders, including combinations of last in, first out (LIFO) and first in, first out (FIFO) ordering.

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Mitotic cyclin Clb2 binds a specific linear motif, LxF, in targets or inhibitors

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LxF interaction enhances mitotic CDK substrate phosphorylation

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Phospho-adaptor Cks1 and the LxF docking mediate CDK inhibition by Cdc6 and Swe1

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Cyclin-specific targeting enables finetuning of CDK function

Multiple kinases inhibit origin licensing and helicase activation to ensure reductive cell division during meiosis

Meiotic cells undergo a single round of DNA replication followed by two rounds of chromosome segregation (the meiotic divisions) to produce haploid gametes. Both DNA replication and chromosome segregation are similarly regulated by CDK oscillations in mitotic cells. Yet how these two events are uncoupled between the meiotic divisions is unclear. Using Saccharomyces cerevisiae, we show that meiotic cells inhibit both helicase loading and helicase activation to prevent DNA replication between the meiotic divisions. CDK and the meiosis–specific kinase Ime2 cooperatively inhibit helicase loading, and their simultaneous inhibition allows inappropriate helicase reloading. Further analysis uncovered two previously unknown mechanisms by which Ime2 inhibits helicase loading. Finally, we show that CDK and the polo–like kinase Cdc5 trigger degradation of Sld2, an essential helicase–activation protein. Together, our data demonstrate that multiple kinases inhibit both helicase loading and activation between the meiotic divisions, thereby ensuring reductive cell division.

Roles of CDK and DDK in Genome Duplication and Maintenance: Meiotic Singularities

Cells reproduce using two types of divisions: mitosis, which generates two daughter cells each with the same genomic content as the mother cell, and meiosis, which reduces the number of chromosomes of the parent cell by half and gives rise to four gametes. The mechanisms that promote the proper progression of the mitotic and meiotic cycles are highly conserved and controlled. They require the activities of two types of serine-threonine kinases, the cyclin-dependent kinases (CDKs) and the Dbf4-dependent kinase (DDK). CDK and DDK are essential for genome duplication and maintenance in both mitotic and meiotic divisions. In this review, we aim to highlight how these kinases cooperate to orchestrate diverse processes during cellular reproduction, focusing on meiosis-specific adaptions of their regulation and functions in DNA metabolism.

Coordination of Double Strand Break Repair and Meiotic Progression in Yeast by a Mek1-Ndt80 Negative Feedback Loop

During meiosis, homologous chromosomes are physically connected by crossovers and sister chromatid cohesion. Interhomolog crossovers are generated by the highly regulated repair of programmed double strand breaks (DSBs). The meiosis-specific kinase Mek1 is critical for this regulation. Mek1 downregulates the mitotic recombinase Rad51, indirectly promoting interhomolog strand invasion by the meiosis-specific recombinase Dmc1. Mek1 also promotes the formation of crossovers that are distributed throughout the genome by interference and is the effector kinase for a meiosis-specific checkpoint that delays entry into Meiosis I until DSBs have been repaired. The target of this checkpoint is a meiosis-specific transcription factor, Ndt80, which is necessary to express the polo-like kinase CDC5 and the cyclin CLB1 thereby allowing completion of recombination and meiotic progression. This work shows that Mek1 and Ndt80 negatively feedback on each other such that when DSB levels are high, Ndt80 is inactive due to high levels of Mek1 activity. As DSBs are repaired, chromosomes synapse and Mek1 activity is reduced below a threshold that allows activation of Ndt80. Ndt80 transcription of CDC5 results in degradation of Red1, a meiosis-specific protein required for Mek1 activation, thereby abolishing Mek1 activity completely. Elimination of Mek1 kinase activity allows Rad51-mediated repair of any remaining DSBs. In this way, cells do not enter Meiosis I until recombination is complete and all DSBs are repaired.

The Genomes of Three Uneven Siblings: Footprints of the Lifestyles of Three Trichoderma Species

The genus Trichoderma contains fungi with high relevance for humans, with applications in enzyme production for plant cell wall degradation and use in biocontrol. Here, we provide a broad, comprehensive overview of the genomic content of these species for "hot topic" research aspects, including CAZymes, transport, transcription factors, and development, along with a detailed analysis and annotation of less-studied topics, such as signal transduction, genome integrity, chromatin, photobiology, or lipid, sulfur, and nitrogen metabolism in T. reesei, T. atroviride, and T. virens, and we open up new perspectives to those topics discussed previously. In total, we covered more than 2,000 of the predicted 9,000 to 11,000 genes of each Trichoderma species discussed, which is >20% of the respective gene content. Additionally, we considered available transcriptome data for the annotated genes. Highlights of our analyses include overall carbohydrate cleavage preferences due to the different genomic contents and regulation of the respective genes. We found light regulation of many sulfur metabolic genes. Additionally, a new Golgi 1,2-mannosidase likely involved in N-linked glycosylation was detected, as were indications for the ability of Trichoderma spp. to generate hybrid galactose-containing N-linked glycans. The genomic inventory of effector proteins revealed numerous compounds unique to Trichoderma, and these warrant further investigation. We found interesting expansions in the Trichoderma genus in several signaling pathways, such as G-protein-coupled receptors, RAS GTPases, and casein kinases. A particularly interesting feature absolutely unique to T. atroviride is the duplication of the alternative sulfur amino acid synthesis pathway.

A single cyclin-CDK complex is sufficient for both mitotic and meiotic progression in fission yeast

The dominant model for eukaryotic cell cycle control proposes that cell cycle progression is driven by a succession of CDK complexes with different substrate specificities. However, in fission yeast it has been shown that a single CDK complex generated by the fusion of the Cdc13 cyclin with the CDK protein Cdc2 can drive the mitotic cell cycle. Meiosis is a modified cell cycle programme in which a single S-phase is followed by two consecutive rounds of chromosome segregation. Here we systematically analyse the requirements of the different fission yeast cyclins for meiotic cell cycle progression. We also show that a single Cdc13-Cdc2 complex, in the absence of the other cyclins, can drive the meiotic cell cycle. We propose that qualitatively different CDK complexes are not absolutely required for cell cycle progression either during mitosis or meiosis, and that a single CDK complex can drive both cell cycle programmes.

Positive feedback of NDT80 expression ensures irreversible meiotic commitment in budding yeast

In budding yeast, meiotic commitment is the irreversible continuation of the developmental path of meiosis. After reaching meiotic commitment, cells finish meiosis and gametogenesis, even in the absence of the meiosis-inducing signal. In contrast, if the meiosis-inducing signal is removed and the mitosis-inducing signal is provided prior to reaching meiotic commitment, cells exit meiosis and return to mitosis. Previous work has shown that cells commit to meiosis after prophase I but before entering the meiotic divisions. Since the Ndt80 transcription factor induces expression of middle meiosis genes necessary for the meiotic divisions, we examined the role of the NDT80 transcriptional network in meiotic commitment. Using a microfluidic approach to analyze single cells, we found that cells commit to meiosis in prometaphase I, after the induction of the Ndt80-dependent genes. Our results showed that high-level expression of NDT80 is important for the timing and irreversibility of meiotic commitment. A modest reduction in NDT80 levels delayed meiotic commitment based on meiotic stages, although the timing of each meiotic stage was similar to that of wildtype cells. A further reduction of NDT80 resulted in the surprising finding of inappropriately uncommitted cells: withdrawal of the meiosis-inducing signal and addition of the mitosis-inducing signal to cells at stages beyond metaphase I caused return to mitosis, leading to multi-nucleate cells. Since Ndt80 enhances its own transcription through positive feedback, we tested whether positive feedback ensured the irreversibility of meiotic commitment. Ablating positive feedback in NDT80 expression resulted in a complete loss of meiotic commitment. These findings suggest that irreversibility of meiotic commitment is a consequence of the NDT80 transcriptional positive feedback loop, which provides the high-level of Ndt80 required for the developmental switch of meiotic commitment. These results also illustrate the importance of irreversible meiotic commitment for maintaining genome integrity by preventing formation of multi-nucleate cells.

Ipl1/Aurora kinase suppresses S-CDK-driven spindle formation during prophase I to ensure chromosome integrity during meiosis

Cells coordinate spindle formation with DNA repair and morphological modifications to chromosomes prior to their segregation to prevent cell division with damaged chromosomes. Here we uncover a novel and unexpected role for Aurora kinase in preventing the formation of spindles by Clb5-CDK (S-CDK) during meiotic prophase I and when the DDR is active in budding yeast. This is critical since S-CDK is essential for replication during premeiotic S-phase as well as double-strand break induction that facilitates meiotic recombination and, ultimately, chromosome segregation. Furthermore, we find that depletion of Cdc5 polo kinase activity delays spindle formation in DDR-arrested cells and that ectopic expression of Cdc5 in prophase I enhances spindle formation, when Ipl1 is depleted. Our findings establish a new paradigm for Aurora kinase function in both negative and positive regulation of spindle dynamics.

Inhibition of the Smc5/6 complex during meiosis perturbs joint molecule formation and resolution without significantly changing crossover or non-crossover levels

Meiosis is a specialized cell division used by diploid organisms to form haploid gametes for sexual reproduction. Central to this reductive division is repair of endogenous DNA double-strand breaks (DSBs) induced by the meiosis-specific enzyme Spo11. These DSBs are repaired in a process called homologous recombination using the sister chromatid or the homologous chromosome as a repair template, with the homolog being the preferred substrate during meiosis. Specific products of inter-homolog recombination, called crossovers, are essential for proper homolog segregation at the first meiotic nuclear division in budding yeast and mice. This study identifies an essential role for the conserved Structural Maintenance of Chromosomes (SMC) 5/6 protein complex during meiotic recombination in budding yeast. Meiosis-specific smc5/6 mutants experience a block in DNA segregation without hindering meiotic progression. Establishment and removal of meiotic sister chromatid cohesin are independent of functional Smc6 protein. smc6 mutants also have normal levels of DSB formation and repair. Eliminating DSBs rescues the segregation block in smc5/6 mutants, suggesting that the complex has a function during meiotic recombination. Accordingly, smc6 mutants accumulate high levels of recombination intermediates in the form of joint molecules. Many of these joint molecules are formed between sister chromatids, which is not normally observed in wild-type cells. The normal formation of crossovers in smc6 mutants supports the notion that mainly inter-sister joint molecule resolution is impaired. In addition, return-to-function studies indicate that the Smc5/6 complex performs its most important functions during joint molecule resolution without influencing crossover formation. These results suggest that the Smc5/6 complex aids primarily in the resolution of joint molecules formed outside of canonical inter-homolog pathways.

Most eukaryotic cells are diploid, which means that they contain two copies of each chromosome – one from each parent. In order to preserve the chromosome number from generation to generation, diploid organisms employ a process called meiosis to form gametes containing only one copy of each chromosome. During sexual reproduction, two gametes (sperm and eggs in mammals) fuse to form a zygote with the same chromosome number as the parents. This zygote will develop into a new organism that has genetic characteristics unique from, but still related to, both parents. The reduction of chromosome number and the reshuffling of genetic traits during meiosis depend on the repair of naturally occurring DNA breaks. Improper break repair during meiosis may block meiosis altogether or form genetically instable gametes, leading to fertility problems or defects in the offspring. The study presented here demonstrates the importance of the evolutionarily conserved Smc5/6 protein complex in upholding the integrity of meiotic repair processes. Our results show that cells deficient in components of the Smc5/6 complex lead to inviable meiotic products. Cells lacking functional Smc5/6 complex are unable to direct DNA repair to the proper template and accumulate abnormal repair intermediates, which inhibit the reductive division.

Control analysis of the eukaryotic cell cycle using gene copy-number series in yeast tetraploids

Background

In the model eukaryote, Saccharomyces cerevisiae, previous experiments have identified those genes that exert the most significant control over cell growth rate. These genes are termed HFC for high flux control. Such genes are overrepresented within pathways controlling the mitotic cell cycle.

Results

We postulated that the increase/decrease in growth rate is due to a change in the rate of progression through specific cell cycle steps. We extended and further developed an existing logical model of the yeast cell cycle in order elucidate how the HFC genes modulated progress through the cycle. This model can simulate gene dosage-variation and calculate the cycle time, determine the order and relative speed at which events occur, and predict arrests and failures to correctly execute a step. To experimentally test our model’s predictions, we constructed a tetraploid series of deletion mutants for a set of eight genes that control the G2/M transition. This system allowed us to vary gene copy number through more intermediate levels than previous studies and examine the impact of copy-number variation on growth, cell-cycle phenotype, and response to different cellular stresses.

Conclusions

For the majority of strains, the predictions agreed with experimental observations, validating our model and its use for further predictions. Where simulation and experiment diverged, we uncovered both novel tetraploid-specific phenotypes and a switch in the determinative execution point of a key cell-cycle regulator, the Cdc28 kinase, from the G1/S to the S/G2 boundaries.

Cdk1 modulation ensures the coordination of cell-cycle events during the switch from meiotic prophase to mitosis

BACKGROUND

Budding yeast cells that enter the developmental path of meiosis do not commit to finishing meiosis until after prophase I and the realization of such meiosis-specific events as pairing of homologous chromosomes and initiation of recombination. If the meiosis-inducing signal is withdrawn prior to commitment, cells exit meiosis and return to mitosis. The timing of this transition poses a singular problem for maintaining genome integrity. Cells in meiotic prophase have already replicated their DNA, but they have not undergone the morphological changes intrinsic to mitosis, including budding. Successful re-entry into mitosis requires that these cells bud but not rereplicate their DNA, reversing the normal order of mitosis. This study focuses on the cellular mechanisms that permit this dramatically altered order of cell-cycle events.

RESULTS

By developing a microfluidics assay to monitor individual cells, we show that the successful transition from meiotic prophase to mitosis requires the modulation of Cdk1 activity to coordinate cell-cycle events. The S. cerevisiae Wee1 homolog Swe1 prevents the formation of multinucleate cells by restraining M phase CDK activity to allow bud formation prior to nuclear division. The remaining S phase CDK activity promotes bud formation and prevents origin licensing so that DNA cannot rereplicate between bud formation and nuclear division. Once a bud has formed, M phase CDK drives cells through a normal mitotic division.

CONCLUSIONS

Our study uncovers the essential requirement of Swe1 to modulate CDK activity to coordinate cell-cycle events and maintain genome integrity during the transition from meiotic prophase to mitosis.

Helping chromosomes and chromatids stay on track

The prevention of premature interactions between microtubules and kinetochores is essential to ensuring that meiosis produces gametes with the correct number of chromosomes.

Sporulation in the budding yeast Saccharomyces cerevisiae

In response to nitrogen starvation in the presence of a poor carbon source, diploid cells of the yeast Saccharomyces cerevisiae undergo meiosis and package the haploid nuclei produced in meiosis into spores. The formation of spores requires an unusual cell division event in which daughter cells are formed within the cytoplasm of the mother cell. This process involves the de novo generation of two different cellular structures: novel membrane compartments within the cell cytoplasm that give rise to the spore plasma membrane and an extensive spore wall that protects the spore from environmental insults. This article summarizes what is known about the molecular mechanisms controlling spore assembly with particular attention to how constitutive cellular functions are modified to create novel behaviors during this developmental process. Key regulatory points on the sporulation pathway are also discussed as well as the possible role of sporulation in the natural ecology of S. cerevisiae.

Among B-type cyclins only CLB5 and CLB6 promote premeiotic S phase in Saccharomyces cerevisiae

The Saccharomyces cerevisiae cyclin Clb5 is required for premeiotic S phase, meiotic recombination, and successful progression through meiosis. Clb5 is not essential for mitotic proliferation because Clb1-Clb4 can support DNA replication in clb5 clb6 mutants. Clb1, Clb3, and Clb4 accumulate in clb5 clb6 cells during meiotic differentiation yet fail to promote premeiotic DNA replication. When expressed under the regulation of the CLB5 promoter, Clb1 and Clb3 accumulate and are active in the early stages of meiotic differentiation but cannot induce premeiotic DNA replication, suggesting that they do not target Cdk1 to the necessary substrates. The Clb5 hydrophobic patch (HP) residues are important for Clb5 function but this motif alone does not provide the specificity required for Clb5 to induce premeiotic S phase. Domain exchange experiments demonstrated that the amino terminus of Clb5 when fused to Clb3 confers upon Clb3 the ability to induce premeiotic S phase. Chimeric cyclins containing smaller regions of the Clb5 amino terminus displayed reduced ability to activate premeiotic DNA replication despite being more abundant and having greater associated histone H1 kinase activity than endogenous Clb5. These observations suggest that Clb5 has a unique ability to trigger premeiotic S phase and that the amino-terminal region of Clb5 contributes to its specificity and regulates the functions performed by the cyclin-Cdk complex.

Cyclin-dependent kinase promotes formation of the synaptonemal complex in yeast meiosis

Cyclin-dependent protein kinases (CDKs) are required for various cell cycle events both in mitosis and in meiosis. During the meiotic prophase of Saccharomyces cerevisiae, only one CDK, Cdc28, which forms a complex with B-type cyclins, Clb5 or Clb6, promotes not only the onset of premeiotic DNA replication but also the formation of meiotic double-strand breaks (DSBs). In this study, we showed that Cdc28 exhibits punctate staining on chromosomes during meiotic prophase I. Chromosomal localization of Cdc28, dependent on Clb5 and/or Clb6, is frequently observed in zygotene and pachytene, when formation of the synaptonemal complex (SC) occurs. Interestingly, the CDK localization is independent of DSB formation, but rather dependent on meiosis-specific chromosome components such as Red1, Hop1 and a cohesin subunit Rec8. Compromised CDK activity in meiotic prophase leads to defective SC formation without affecting DSB formation. These results suggest that CDK-dependent phosphorylation regulates meiotic chromosome morphogenesis.

The cyclin-A CYCA1;2/TAM is required for the meiosis I to meiosis II transition and cooperates with OSD1 for the prophase to first meiotic division transition

Meiosis halves the chromosome number because its two divisions follow a single round of DNA replication. This process involves two cell transitions, the transition from prophase to the first meiotic division (meiosis I) and the unique meiosis I to meiosis II transition. We show here that the A-type cyclin CYCA1;2/TAM plays a major role in both transitions in Arabidopsis. A series of tam mutants failed to enter meiosis II and thus produced diploid spores and functional diploid gametes. These diploid gametes had a recombined genotype produced through the single meiosis I division. In addition, by combining the tam-2 mutation with AtSpo11-1 and Atrec8, we obtained plants producing diploid gametes through a mitotic-like division that were genetically identical to their parents. Thus tam alleles displayed phenotypes very similar to that of the previously described osd1 mutant. Combining tam and osd1 mutations leads to a failure in the prophase to meiosis I transition during male meiosis and to the production of tetraploid spores and gametes. This suggests that TAM and OSD1 are involved in the control of both meiotic transitions.

Functional conservation of the meiotic genes SDS and RCK in male meiosis in the monocot rice

The Arabidopsis SDS (SOLO DANCERS) and RCK (ROCK-N-ROLLERS) genes are important for male meiosis, but it is still unknown whether they represent conserved functions in plants. We have performed phylogenetic analyses of SDS and RCK and their respective homologs, and identified their putative orthologs in poplar and rice. Quantitative real-time RT-PCR analysis indicated that rice SDS and RCK are expressed preferentially in young flowers, and transgenic RNAi rice lines with reduced expression of these genes exhibited normal vegetative development, but showed significantly reduced fertility with partially sterile flowers and defective pollens. SDS deficiency also caused a decrease in pollen amounts. Further cytological examination of male meiocytes revealed that the SDS deficiency led to defects in homolog interaction and bivalent formation in meiotic prophase I, and RCK deficiency resulted in defective meiotic crossover formation. These results indicate that rice SDS and RCK genes have similar functions to their Arabidopsis orthologs. Because rice and Arabidopsis, respectively, are members of monocots and eudicots, two largest groups of flowering plants, our results suggest that the functions of SDS and RCK are likely conserved in flowering plants.

Sic1-induced DNA rereplication during meiosis

Orderly progression through meiosis requires strict regulation of DNA metabolic events, so that a single round of DNA replication is systematically followed by a recombination phase and 2 rounds of chromosome segregation. We report here the disruption of this sequence of events in Saccharomyces cerevisiae through meiosis-specific induction of the cyclin-dependent kinase (CDK) inhibitor Sic1 mutated at multiple phosphorylation sites. Accumulation of this stabilized version of Sic1 led to significant DNA rereplication in the absence of normal chromosome segregation. Deletion of DMC1 abolished DNA rereplication, but additional deletion of RAD17 restored the original phenotype. Therefore, activation of the meiotic recombination checkpoint, which arrests meiotic progression at pachytene, suppressed DNA rereplication resulting from Sic1 stabilization. In contrast to deletion of DMC1, deletion of NDT80, which encodes a transcription factor required for pachytene exit, did not inhibit DNA rereplication. Our results provide strong evidence that CDK activity is required to prevent inappropriate initiation of DNA synthesis before the meiotic divisions.

The anaphase promoting complex targeting subunit Ama1 links meiotic exit to cytokinesis during sporulation in Saccharomyces cerevisiae

Ascospore formation in yeast is accomplished through a cell division in which daughter nuclei are engulfed by newly formed plasma membranes, termed prospore membranes. Closure of the prospore membrane must be coordinated with the end of meiosis II to ensure proper cell division. AMA1 encodes a meiosis-specific activator of the anaphase promoting complex (APC). The activity of APC(Ama1) is inhibited before meiosis II, but the substrates specifically targeted for degradation by Ama1 at the end of meiosis are unknown. We show here that ama1Delta mutants are defective in prospore membrane closure. Ssp1, a protein found at the leading edge of the prospore membrane, is stabilized in ama1Delta mutants. Inactivation of a conditional form of Ssp1 can partially rescue the sporulation defect of the ama1Delta mutant, indicating that an essential function of Ama1 is to lead to the removal of Ssp1. Depletion of Cdc15 causes a defect in meiotic exit. We find that prospore membrane closure is also defective in Cdc15 and that this defect can be overcome by expression of a form of Ama1 in which multiple consensus cyclin-dependent kinase phosphorylation sites have been mutated. These results demonstrate that APC(Ama1) functions to coordinate the exit from meiosis II with cytokinesis.

Cyclins in meiosis: lost in translation

In a recent issue of Cell, Carlile and Amon examine the regulation of four budding yeast B-type cyclins, crucial for regulating and distinguishing meoisis I and meoisis II divisions, and find a surprising diversity of behaviors and modes of regulation. In particular, Clb3 is regulated by a striking translational repression specific to meoisis I.

The Saccharomyces cerevisiae CLB5 promoter contains two middle sporulation elements (MSEs) that are differentially regulated during sporulation

The B-type cyclins Clb5 and Clb6 are essential activators of DNA replication during sporulation in Saccharomyces cerevisiae. The expression of CLB5 is maximally induced during the middle phase of sporulation by the transcription factor Ndt80. We have performed an analysis of the CLB5 promoter and have identified two middle sporulation elements (MSEs) that act as binding sites for Ndt80. Although both MSE sequences bind Ndt80 in vitro, they display differential effectiveness in their ability to function as cis-acting regulatory sequences in vivo. Mutation of both MSE sequences in the CLB5 promoter profoundly reduces the induction of CLB5 transcription during the middle phase of sporulation but results in no obvious defect in progression through meiosis and sporulation, implying that the Ndt80-dependent induction of CLB5 is not required for effective DNA replication or chromosome division.

Meiosis I is established through division-specific translational control of a cyclin

In budding yeast, key meiotic events such as DNA replication, recombination, and the meiotic divisions are controlled by Clb cyclin-dependent kinases (Clb-CDKs). Using a novel synchronization procedure, we have characterized the activity of these Clb-CDKs and observed a surprising diversity in their regulation during the meiotic divisions. Clb1-CDK activity is restricted to meiosis I, and Clb3-CDK activity to meiosis II, through 5'UTR-mediated translational control of its transcript. The analysis of cells inappropriately producing Clb3-CDKs during meiosis I furthermore defines Clb3 as an inhibitor of the meiosis I chromosome segregation program. Our results demonstrate an essential role for Clb-CDK regulation in establishing the meiotic chromosome segregation pattern.

Shugoshin promotes sister kinetochore biorientation in Saccharomyces cerevisiae

Chromosome segregation must be executed accurately during both mitotic and meiotic cell divisions. Sgo1 plays a key role in ensuring faithful chromosome segregation in at least two ways. During meiosis this protein regulates the removal of cohesins, the proteins that hold sister chromatids together, from chromosomes. During mitosis, Sgo1 is required for sensing the absence of tension caused by sister kinetochores not being attached to microtubules emanating from opposite poles. Here we describe a differential requirement for Sgo1 in the segregation of homologous chromosomes and sister chromatids. Sgo1 plays only a minor role in segregating homologous chromosomes at meiosis I. In contrast, Sgo1 is important to bias sister kinetochores toward biorientation. We suggest that Sgo1 acts at sister kinetochores to promote their biorientation.

Checking Your Breaks: Surveillance Mechanisms of Meiotic Recombination

Numerous DNA double-strand breaks (DSBs) are introduced into the genome in the course of meiotic recombination. This poses a significant hazard to the genomic integrity of the cell. Studies in a number of organisms have unveiled the existence of surveillance mechanisms or checkpoints that couple the formation and repair of DSBs to cell cycle progression. Through these mechanisms, aberrant meiocytes are delayed in their meiotic progression, thereby facilitating repair of meiotic DSBs, or are culled through programmed cell death, thereby protecting the germline from aneuploidies that could lead to spontaneous abortions, birth defects and cancer predisposition in the offspring. Here we summarize recent progress in our understanding of these checkpoints. This review focuses on the surveillance mechanisms of the budding yeast S. cerevisiae, where the molecular details are best understood, but will frequently compare and contrast these mechanisms with observations in other organisms.

Negative control contributes to an extensive program of meiotic splicing in fission yeast

Despite a high frequency of introns in the fission yeast Schizosaccharomyces pombe, regulated splicing is virtually unknown. We present evidence that splicing constitutes a major mechanism for controlling gene expression during meiosis, as 12 of 96 transcripts tested, which encode known components as well as previously uncharacterized ORFs, retain introns until specific times during differentiation. The meiotically spliced pre-mRNAs include two cyclins, rem1 (discovered by Ayte and Nurse) and crs1. Consistent with the use of regulated splicing to block protein production, expression of crs1 in vegetative cells is toxic. Analyses of gene chimeras indicate that splicing is prevented in mitotically growing cells via inhibition, in contrast to the positive control of meiotic splicing in budding yeast. Most strikingly, splicing of crs1 and rem1 is regulated by sequences located outside the coding regions, far from the target introns, a phenomenon previously observed only in metazoans.

Ime2p and Cdc28p: co-pilots driving meiotic development

Meiosis can be considered an elaboration of the cell division cycle in the sense that meiosis combines cell-cycle processes with programs specific to meiosis. Each phase of the cell division cycle is driven forward by cell-cycle kinases (Cdk) and coordinated with other phases of the cycle through checkpoint functions. Meiotic differentiation is also controlled by these two types of regulation; however, recent study in the budding yeast S. cerevisiae indicates that progression of meiosis is also controlled by a master regulator specific to meiosis, namely the Ime2p kinase. Below, I describe the overlapping roles of Ime2p and Cdk during meiosis in yeast and speculate on how these two kinases cooperate to drive the progression of meiosis.

Cyclin B-cdk activity stimulates meiotic rereplication in budding yeast

Haploidization of gametes during meiosis requires a single round of premeiotic DNA replication (meiS) followed by two successive nuclear divisions. This study demonstrates that ectopic activation of cyclin B/cyclin-dependent kinase in budding yeast recruits up to 30% of meiotic cells to execute one to three additional rounds of meiS. Rereplication occurs prior to the meiotic nuclear divisions, indicating that this process is different from the postmeiotic mitoses observed in other fungi. The cells with overreplicated DNA produced asci containing up to 20 spores that were viable and haploid and demonstrated Mendelian marker segregation. Genetic tests indicated that these cells executed the meiosis I reductional division and possessed a spindle checkpoint. Finally, interfering with normal synaptonemal complex formation or recombination increased the efficiency of rereplication. These studies indicate that the block to rereplication is very different in meiotic and mitotic cells and suggest a negative role for the recombination machinery in allowing rereplication. Moreover, the production of haploids, regardless of the genome content, suggests that the cell counts replication cycles, not chromosomes, in determining the number of nuclear divisions to execute.

Activity of phosphoforms and truncated versions of Ndt80, a checkpoint-regulated sporulation-specific transcription factor of Saccharomyces cerevisiae

Ndt80 contributes to the highly regulated cascade of sequential gene expression that directs spore formation in Saccharomyces cerevisiae. This DNA-binding transcriptional activator, which is responsible for the expression of a set of middle sporulation-specific genes, is a target of the meiotic recombination checkpoint. Triggering of this checkpoint prevents phosphorylation and accumulation of active Ndt80. In this study we have investigated the requirements for the activation function of Ndt80 by exploring the role of phosphorylation in the regulation of its activity and by examining the effect of C-terminal truncations. Of three phosphoforms of Ndt80 that we resolved, which we refer to as P approximately Ndt80", P approximately Ndt80', and P approximately Ndt80 in order of increasing electrophoretic mobility, the P approximately Ndt80" and P approximately Ndt80' isoforms correlated with active Ndt80. In particular, P approximately Ndt80" was present in lysates from wild-type sporulating cells and in cells that bypassed checkpoint-mediated arrest as a result of mutations in RAD17, SUM1, or SWE1, or overexpression of NDT80. P approximately Ndt80' was the slowest-migrating isoform that accumulated in Delta ime2/Delta ime2 Delta sum1/Delta sum1 cells in sporulation medium and in mitotic cells that ectopically expressed NDT80. Nonphosphorylated Ndt80 and P approximately Ndt80, which had a slightly lower mobility than nonphosphorylated Ndt80 and was the predominant phosphoform present in checkpoint-arrested cells, correlated with inactive Ndt80. These data are consistent with the notion that extensive phosphorylation, but not Ime2-dependent phosphorylation, of Ndt80 is required for its activity. Examination of the effect of increasingly extensive truncation of the C terminal region of Ndt80 revealed that some functions of Ndt80 were more sensitive to a reduction in its activity than others. In particular, we found that a truncated version of Ndt80 that lacked the last 110 residues was able to promote expression of some middle sporulation-specific genes, but could not direct spore formation. Full activity, however, could be restored to this version of Ndt80 by increasing its level of expression.

Cell-cycle-regulatory elements and the control of cell differentiation in the budding yeast

The stable differentiation of cells into other cell types typically involves dramatic reorganization of cellular structures and functions. This often includes remodeling of the cell cycle and the apparatus that controls it. Here we review our understanding of the role and regulation of cell cycle control elements during cell differentiation in the yeast, Saccharomyces cerevisiae. Although the process of differentiation may be more overtly obvious in metazoan organisms, those systems are by nature more difficult to study at a mechanistic level. We consider the relatively well-understood mechanisms by which mating-type switching and the pheromone-induced differentiation of gametes are coupled to the cell cycle as well as the more obscure mechanisms that govern the remodeling of the cell cycle during meiosis and filamentous growth. In some cases, the cell cycle is a primary stimulus for differentiation whereas, in other cases, the signals that promote differentiation alter the cell cycle. Thus, despite relative simplicity of these processes in yeast, the nature of the interplay between the cell cycle and differentiation is diverse.

Control of landmark events in meiosis by the CDK Cdc28 and the meiosis-specific kinase Ime2

Meiosis is thought to require the protein kinase Ime2 early for DNA replication and the cyclin-dependent kinase Cdc28 late for chromosome segregation. To elucidate the roles of these kinases, we inhibited their activities early and late using conditional mutants that are sensitive to chemical inhibitors. Our studies reveal that both Cdc28 and Ime2 have critical roles in meiotic S phase and M phase. Early inhibition of analog-sensitive cdc28-as1 blocked DNA replication, revealing a previously undetected role for Cdc28. Yet Cdc28 was dispensable for one of its functions in the mitotic cell cycle, degradation of Sic1. Late addition of inhibitor to ime2-as1 revealed unexpected roles of Ime2 in the initiation and execution of chromosome segregation. The requirement of Ime2 for M phase is partially explained by its stimulation of the key meiotic transcription factor Ndt80, which is needed in turn for high Cdc28 activity. In accordance with a late role for Ime2, we observed an increase in its activity during M phase that depended on Cdc28 and Ndt80. We speculate that several unique features of the meiotic cell division reflect a division of labor and regulatory coordination between Ime2 and Cdc28.

Division of the nucleolus and its release of CDC14 during anaphase of meiosis I depends on separase, SPO12, and SLK19

Disjunction of maternal and paternal centromeres during meiosis I requires crossing over between homologous chromatids, which creates chiasmata that hold homologs together. It also depends on a mechanism ensuring that maternal and paternal sister kinetochore pairs attach to oppositely oriented microtubules. Proteolytic cleavage of cohesin's Rec8 subunit by separase destroys cohesion between sister chromatid arms at anaphase I and thereby resolves chiasmata. The Spo12 and Slk19 proteins have been implicated in regulating meiosis I kinetochore orientation and/or in preventing cleavage of Rec8 at centromeres. We show here that the role of these proteins is instead to promote nucleolar segregation, including release of the Cdc14 phosphatase required for Cdk1 inactivation and disassembly of the anaphase I spindle. Separase is also required but surprisingly not its protease activity. It has two mechanistically different roles during meiosis I. Loss of the protease-independent function alone results in a second meiotic division occurring on anaphase I spindles in spo12delta and slk19delta mutants.

Phosphorylation and maximal activity of Saccharomyces cerevisiae meiosis-specific transcription factor Ndt80 is dependent on Ime2

The Saccharomyces cerevisiae meiosis-specific transcription factor Ndt80 is responsible for the induction of a class of genes referred to as middle sporulation genes. Among the members of this family are the B-type cyclins and other genes whose products are required for meiotic chromosome division and spore morphogenesis. Inactivation of NDT80 leads to a failure to induce the middle sporulation genes and a subsequent arrest in pachytene. The expression of NDT80 is itself highly regulated. The initial transcription of NDT80 is dependent upon the protein kinase Ime2; once Ndt80 protein accumulates, it activates its own promoter, thus generating an autoactivation loop. In addition to being transcriptionally regulated, Ndt80 protein is posttranslationally regulated. Phosphorylation of Ndt80 occurs coincident with its activation as a transcription factor. If expressed prematurely in meiosis, Ndt80 accumulates initially in an unmodified form that is subsequently modified by phosphorylation. In contrast, Ndt80 expressed in ime2 mutant strains does not become modified and has a reduced ability to activate transcription of its target genes. Ime2 can also phosphorylate Ndt80 in vitro, further supporting a direct role for Ime2 in the phosphorylation of Ndt80. These data indicate that Ime2 plays a novel and previously unexpected role in promoting chromosome dissemination and progress through meiotic development by activating Ndt80.

Role of Ndt80, Sum1, and Swe1 as targets of the meiotic recombination checkpoint that control exit from pachytene and spore formation in Saccharomyces cerevisiae

The meiotic recombination checkpoint, which is triggered by defects in recombination or chromosome synapsis, arrests sporulating cells of Saccharomyces cerevisiae at pachytene by preventing accumulation of active Clb-Cdc28. We compared the effects of manipulating the three known targets of the meiotic recombination checkpoint, NDT80, SWE1, and SUM1, in dmc1-arrested cells. Ndt80 is an activator of a set of middle sporulation-specific genes (MSGs), which includes CLB genes and genes involved in spore wall formation; Swe1 inhibits Clb-Cdc28 activity; and Sum1 is a repressor of NDT80 and some MSGs. Activation of the checkpoint leads to inhibition of Ndt80 activity and to stabilization of Swe1 and Sum1. Thus, dmc1-arrested cells fail to express MSGs, arrest at pachytene, and do not form spores. Our study shows that dmc1/dmc1 sum1/sum1 cells expressed MSGs prematurely and at high levels, entered the meiotic divisions efficiently, and in some cases formed asci containing mature spores. In contrast, dmc1/dmc1 swe1/swe1 cells expressed MSGs at a very low level, were inefficient and delayed in entry into the meiotic divisions, and never formed mature spores. We found that cells of dmc1/dmc1 sum1/sum1 ndt80/ndt80 and dmc1/dmc1 swe1/swe1 ndt80/ndt80 strains arrested at pachytene and that dmc1/dmc1 or dmc1/dmc1 swe1/swe1 cells overexpressing NDT80 were less efficient in bypassing checkpoint-mediated arrest than dmc1/dmc1 sum1/sum1 cells. Our results are consistent with previous suggestions that increased Clb-Cdc28 activity, caused by mutation of SWE1 or by an NDT80-dependent increase in CLB expression, allows dmc1/dmc1 cells to exit pachytene and that subsequent upregulation of Ndt80 activity by a feedback mechanism promotes entry into the meiotic divisions. Spore morphogenesis, however, requires efficient and timely activation of MSGs, which we speculate was achieved in dmc1/dmc1 sum1/sum1 cells by premature expression of NDT80.

Spo13 protects meiotic cohesin at centromeres in meiosis I

In the absence of Spo13, budding yeast cells complete a single meiotic division during which sister chromatids often separate. We investigated the function of Spo13 by following chromosomes tagged with green fluorescent protein. The occurrence of a single division in spo13Delta homozygous diploids depends on the spindle checkpoint. Eliminating the checkpoint accelerates meiosis I in spo13Delta cells and allows them to undergo two divisions in which sister chromatids often separate in meiosis I and segregate randomly in meiosis II. Overexpression of Spo13 and the meiosis-specific cohesin Rec8 in mitotic cells prevents separation of sister chromatids despite destruction of Pds1 and activation of Esp1. This phenotype depends on the combined overexpression of both proteins and mimics one aspect of meiosis I chromosome behavior. Overexpressing the mitotic cohesin, Scc1/Mcd1, does not substitute for Rec8, suggesting that the combined actions of Spo13 and Rec8 are important for preventing sister centromere separation in meiosis I.

Genes involved in the initiation of DNA replication in yeast

Replication and segregation of the information contained in genomic DNA are strictly regulated processes that eukaryotic cells alternate to divide successfully. Experimental work on yeast has suggested that this alternation is achieved through oscillations in the activity of a serine/threonine kinase complex, CDK, which ensures the timely activation of DNA synthesis. At the same time, this CDK-mediated activation sets up the basis of the mechanism that ensures ploidy maintenance in eukaryotes. DNA synthesis is initiated at discrete sites of the genome called origins of replication on which a prereplicative complex (pre-RC) of different protein subunits is formed during the G1 phase of the cell division cycle. Only after pre-RCs are formed is the genome competent to be replicated. Several lines of evidence suggest that CDK activity prevents the assembly of pre-RCs ensuring single rounds of genome replication during each cell division cycle. This review offers a descriptive discussion of the main molecular events that a unicellular eukaryote such as the budding yeast Saccharomyces cerevisiae undergoes to initiate DNA replication.

Homolog interaction during meiotic prophase I in Arabidopsis requires the SOLO DANCERS gene encoding a novel cyclin-like protein

Interactions between homologs in meiotic prophase I, such as recombination and synapsis, are critical for proper homolog segregation and involve the coordination of several parallel events. However, few regulatory genes have been identified; in particular, it is not clear what roles the proteins similar to the mitotic cell cycle regulators might play during meiotic prophase I. We describe here the isolation and characterization of a new Arabidopsis mutant called solo dancers that exhibits a severe defect in homolog synapsis, recombination and bivalent formation in meiotic prophase I, subsequently resulting in seemingly random chromosome distribution and formation of abnormal meiotic products. We further demonstrate that the mutation affects a meiosis-specific gene encoding a novel protein of 578 amino acid residues with up to 31% amino acid sequence identity to known cyclins in the C-terminal portion. These results argue strongly that homolog interactions during meiotic prophase I require a novel meiosis-specific cyclin in Arabidopsis.

Inhibition of APC-mediated proteolysis by the meiosis-specific protein kinase Ime2

Proteolysis triggered by the anaphase-promoting complex (APC) is needed for sister chromatid separation and the exit from mitosis. APC is a ubiquitin ligase whose activity is tightly controlled during the cell cycle. To identify factors involved in the regulation of APC-mediated proteolysis, a Saccharomyces cerevisiae GAL-cDNA library was screened for genes whose overexpression prevented degradation of an APC target protein, the mitotic cyclin Clb2. Genes encoding G1, S, and mitotic cyclins were identified, consistent with previous data showing that the cyclin-dependent kinase Cdk1 associated with different cyclins is a key factor for inhibiting APC(Cdh1) activity from late-G1 phase until mitosis. In addition, the meiosis-specific protein kinase Ime2 was identified as a negative regulator of APC-mediated proteolysis. Ectopic expression of IME2 in G1 arrested cells inhibited the degradation of mitotic cyclins and of other APC substrates. IME2 expression resulted in the phosphorylation of Cdh1 in G1 cells, indicating that Ime2 and Cdk1 regulate APC(Cdh1) in a similar manner. The expression of IME2 in cycling cells inhibited bud formation and caused cells to arrest in mitosis. We show further that Ime2 itself is an unstable protein whose proteolysis occurs independently of the APC and SCF (Skp1/Cdc53/F-box) ubiquitin ligases. Our findings suggest that Ime2 represents an unstable, meiosis-specific regulator of APC(Cdh1).

Testing a mathematical model of the yeast cell cycle

We derived novel, testable predictions from a mathematical model of the budding yeast cell cycle. A key qualitative prediction of bistability was confirmed in a strain simultaneously lacking cdc14 and G1 cyclins. The model correctly predicted quantitative dependence of cell size on gene dosage of the G1 cyclin CLN3, but it incorrectly predicted strong genetic interactions between G1 cyclins and the anaphase-promoting complex specificity factor Cdh1. To provide constraints on model generation, we determined accurate concentrations for the abundance of all nine cyclins as well as the inhibitor Sic1 and the catalytic subunit Cdc28. For many of these we determined abundance throughout the cell cycle by centrifugal elutriation, in the presence or absence of Cdh1. In addition, perturbations to the Clb-kinase oscillator were introduced, and the effects on cyclin and Sic1 levels were compared between model and experiment. Reasonable agreement was obtained in many of these experiments, but significant experimental discrepancies from the model predictions were also observed. Thus, the model is a strong but incomplete attempt at a realistic representation of cell cycle control. Constraints of the sort developed here will be important in development of a truly predictive model.