Honors & Awards
Career Award at the Scientific Interface, Burroughs Wellcome Fund (2008)
Postdoctoral Fellowship (F32), NIH (2006-2008)
PhD, University of Cambridge, Applied Mathematics (2004)
CASM Pt III, University of Cambridge, Applied Mathematics (2001)
BS, MIT, Mathematics (1999)
BS, MIT, Physics (1999)
Current Research and Scholarly Interests
Underlying the wonderful diversity of natural forms is the ability of an organism to grow into its appropriate shape. Regulation ensures that cells grow, divide and differentiate so that the organism and its constitutive parts are properly proportioned and of suitable size. Although the size-control mechanism active in an individual cell is of fundamental importance to this process, it is difficult to isolate and study in complex multi-cellular systems and remains poorly understood. It is therefore of interest to study size control in unicellular organisms, which are governed by simpler physiology: proliferate rapidly whenever environmental conditions permit. Therefore, the laboratory studies size control in budding yeast, a genetically tractable eukaryotic organism. Previous studies of the budding yeast cell cycle, which couples growth and division, have revealed mechanisms shared by both yeasts and humans. This leads me to believe our findings will be of general interest, particularly since mammalian size control genes are frequently mutated in cancers.
In the 1970s, Lee Hartwell and colleagues attributed size control to a point between cell birth and DNA replication. Upon passage through the size-control checkpoint, budding yeast irreversibly commit to division. This checkpoint was therefore called the Start of the cell cycle. The last 30 years have seen rapid advances in our understanding of the molecular interactions in the cell cycle. Yet, in spite of all this exquisite molecular detail fundamental questions remain unanswered: What makes this transition irreversible? Where in this sequence of molecular interactions does size control occur? How does a cell compute its own size? The laboratory aims to address such systems-level questions.
A central aim of the burgeoning field of systems biology is to understand the principles governing genetic control networks. I believe finding the principles underlying genetic circuits will occur through detailed studies and then comparisons of several natural systems. Due to its extensive development as an experimental system, the budding yeast cell cycle is poised to become central to this enterprise. A systematic understanding of biological control circuits should allow us to more readily discern the function of natural systems and aid us in engineering synthetic systems.
- Principles of Cell Cycle Control
BIO 171, BIO 271, CSB 271 (Aut)
Independent Studies (12)
- Advanced Research Laboratory in Experimental Biology
BIO 199 (Aut, Win, Spr, Sum)
- Directed Reading in Biology
BIO 198 (Aut, Win, Spr, Sum)
- Directed Reading in Biophysics
BIOPHYS 399 (Aut, Win, Spr, Sum)
- Directed Reading in Cancer Biology
CBIO 299 (Win, Spr)
- Graduate Research
BIO 300 (Aut, Win, Spr, Sum)
- Graduate Research
BIOPHYS 300 (Aut, Win, Spr, Sum)
- Graduate Research
CBIO 399 (Aut, Win, Spr, Sum)
- Out-of-Department Advanced Research Laboratory in Experimental Biology
BIO 199X (Sum)
- Out-of-Department Directed Reading
BIO 198X (Aut, Win, Spr, Sum)
- Out-of-Department Graduate Research
BIO 300X (Win, Spr, Sum)
- Teaching in Cancer Biology
CBIO 260 (Spr)
- Teaching of Biology
BIO 290 (Aut, Win, Spr)
- Advanced Research Laboratory in Experimental Biology
- Prior Year Courses
Postdoctoral Faculty Sponsor
David Jukam, Mardo Koivomaegi, Kurt Schmoller, Seyed Ali Mohammad Shariati, Matthew Swaffer, Shicong Xie, Evgeny Zatulovskiy
Devon Chandler-Brown, Jonathan Turner
Doctoral Dissertation Advisor (AC)
Daniel Berenson, Devon Chandler-Brown, Benjamin Topacio, Jonathan Turner
COLD SPRING HARBOR PERSPECTIVES IN BIOLOGY
2016; 8 (4)
Cells of a given type maintain a characteristic cell size to function efficiently in their ecological or organismal context. They achieve this through the regulation of growth rates or by actively sensing size and coupling this signal to cell division. We focus this review on potential size-sensing mechanisms, including geometric, external cue, and titration mechanisms. Mechanisms that titrate proteins against DNA are of particular interest because they are consistent with the robust correlation of DNA content and cell size. We review the literature, which suggests that titration mechanisms may underlie cell-size sensing inXenopusembryos, budding yeast, andEscherichia coli, whereas alternative mechanisms may function in fission yeast.
View details for DOI 10.1101/cshperspect.a019083
View details for Web of Science ID 000373486900002
View details for PubMedID 26254313
Mitosis is swell.
journal of cell biology
2015; 211 (4): 733-735
Cell volume and dry mass are typically correlated. However, in this issue, Zlotek-Zlotkiewicz et al. (2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201505056) and Son et al. (2015. J. Cell Biol. http://dx.doi.org/10.1083/jcb.201505058) use new live-cell techniques to show that entry to mitosis coincides with rapid cell swelling, which is reversed before division.
View details for DOI 10.1083/jcb.201511007
View details for PubMedID 26598610
Dilution of the cell cycle inhibitor Whi5 controls budding-yeast cell size.
2015; 526 (7572): 268-272
Cell size fundamentally affects all biosynthetic processes by determining the scale of organelles and influencing surface transport. Although extensive studies have identified many mutations affecting cell size, the molecular mechanisms underlying size control have remained elusive. In the budding yeast Saccharomyces cerevisiae, size control occurs in G1 phase before Start, the point of irreversible commitment to cell division. It was previously thought that activity of the G1 cyclin Cln3 increased with cell size to trigger Start by initiating the inhibition of the transcriptional inhibitor Whi5 (refs 6-8). Here we show that although Cln3 concentration does modulate the rate at which cells pass Start, its synthesis increases in proportion to cell size so that its total concentration is nearly constant during pre-Start G1. Rather than increasing Cln3 activity, we identify decreasing Whi5 activity--due to the dilution of Whi5 by cell growth--as a molecular mechanism through which cell size controls proliferation. Whi5 is synthesized in S/G2/M phases of the cell cycle in a largely size-independent manner. This results in smaller daughter cells being born with higher Whi5 concentrations that extend their pre-Start G1 phase. Thus, at its most fundamental level, size control in budding yeast results from the differential scaling of Cln3 and Whi5 synthesis rates with cell size. More generally, our work shows that differential size-dependency of protein synthesis can provide an elegant mechanism to coordinate cellular functions with growth.
View details for DOI 10.1038/nature14908
View details for PubMedID 26390151
A genetically encoded Forster resonance energy transfer sensor for monitoring in vivo trehalose-6-phosphate dynamics
2015; 474: 1-7
Trehalose-6-phosphate is a pivotal regulator of sugar metabolism, growth, and osmotic equilibrium in bacteria, yeasts, and plants. To directly visualize the intracellular levels of intracellular trehalose-6-phosphate, we developed a series of specific Förster resonance energy transfer (FRET) sensors for in vivo microscopy. We demonstrated real-time monitoring of regulation in the trehalose pathway of Escherichia coli. In Saccharomyces cerevisiae, we could show that the concentration of free trehalose-6-phosphate during growth on glucose is in a range sufficient for inhibition of hexokinase. These findings support the hypothesis of trehalose-6-phosphate as the effector of a negative feedback system, similar to the inhibition of hexokinase by glucose-6-phosphate in mammalian cells and controlling glycolytic flux.
View details for DOI 10.1016/j.ab.2014.12.019
View details for Web of Science ID 000352170800001
Compartmentalization of a Bistable Switch Enables Memory to Cross a Feedback-Driven Transition
2015; 160 (6): 1182-1195
Cells make accurate decisions in the face of molecular noise and environmental fluctuations by relying not only on present pathway activity, but also on their memory of past signaling dynamics. Once a decision is made, cellular transitions are often rapid and switch-like due to positive feedback loops in the regulatory network. While positive feedback loops are good at promoting switch-like transitions, they are not expected to retain information to inform subsequent decisions. However, this expectation is based on our current understanding of network motifs that accounts for temporal, but not spatial, dynamics. Here, we show how spatial organization of the feedback-driven yeast G1/S switch enables the transmission of memory of past pheromone exposure across this transition. We expect this to be one of many examples where the exquisite spatial organization of the eukaryotic cell enables previously well-characterized network motifs to perform new and unexpected signal processing functions.
View details for DOI 10.1016/j.cell.2015.02.032
View details for Web of Science ID 000351951800017
Histone titration against the genome sets the DNA-to-cytoplasm threshold for the Xenopus midblastula transition.
Proceedings of the National Academy of Sciences of the United States of America
2015; 112 (10): E1086-95
During early development, animal embryos depend on maternally deposited RNA until zygotic genes become transcriptionally active. Before this maternal-to-zygotic transition, many species execute rapid and synchronous cell divisions without growth phases or cell cycle checkpoints. The coordinated onset of transcription, cell cycle lengthening, and cell cycle checkpoints comprise the midblastula transition (MBT). A long-standing model in the frog, Xenopus laevis, posits that MBT timing is controlled by a maternally loaded inhibitory factor that is titrated against the exponentially increasing amount of DNA. To identify MBT regulators, we developed an assay using Xenopus egg extract that recapitulates the activation of transcription only above the DNA-to-cytoplasm ratio found in embryos at the MBT. We used this system to biochemically purify factors responsible for inhibiting transcription below the threshold DNA-to-cytoplasm ratio. This unbiased approach identified histones H3 and H4 as concentration-dependent inhibitory factors. Addition or depletion of H3/H4 from the extract quantitatively shifted the amount of DNA required for transcriptional activation in vitro. Moreover, reduction of H3 protein in embryos induced premature transcriptional activation and cell cycle lengthening, and the addition of H3/H4 shortened post-MBT cell cycles. Our observations support a model for MBT regulation by DNA-based titration and suggest that depletion of free histones regulates the MBT. More broadly, our work shows how a constant concentration DNA binding molecule can effectively measure the amount of cytoplasm per genome to coordinate division, growth, and development.
View details for DOI 10.1073/pnas.1413990112
View details for PubMedID 25713373
- Modularity and predictability in cell signaling and decision making MOLECULAR BIOLOGY OF THE CELL 2014; 25 (22): 3445-3450
Docking interactions: cell-cycle regulation and beyond.
2014; 24 (14): R647-9
In budding yeast, the mating pathway activates Far1 to inhibit G1 cyclins in complex with the cyclin-dependent kinase (Cln-Cdk). Yet, the molecular mechanism has remained largely unclear for over 20 years. A recent report helps shed light on this regulation.
View details for DOI 10.1016/j.cub.2014.05.060
View details for PubMedID 25050961
Start and the restriction point
CURRENT OPINION IN CELL BIOLOGY
2013; 25 (6): 717-723
Commitment to division requires that cells sense, interpret, and respond appropriately to multiple signals. In most eukaryotes, cells commit to division in G1 before DNA replication. Beyond a point, known as Start in yeast and the restriction point in mammals, cells will proceed through the cell cycle despite changes in upstream signals. In metazoans, misregulated G1 control can lead to developmental problems or disease, so it is important to understand how cells decipher the myriad external and internal signals that contribute to the fundamental all-or-none decision to divide. Extensive study of G1 control in the budding yeast Saccharomyces cerevisiae and mammalian culture systems has revealed highly similar networks regulating commitment. However, protein sequences of functional orthologs often indicate a total lack of conservation suggesting significant evolution of G1 control. Here, we review recent studies defining the conserved and diverged features of G1 control and highlight systems-level aspects that may be common to other biological regulatory networks.
View details for DOI 10.1016/j.ceb.2013.07.010
View details for Web of Science ID 000328796200007
View details for PubMedID 23916770
CONSTRAINTS ON THE ADULT-OFFSPRING SIZE RELATIONSHIP IN PROTISTS
2013; 67 (12): 3537-3544
The relationship between adult and offspring size is an important aspect of reproductive strategy. Although this filial relationship has been extensively examined in plants and animals, we currently lack comparable data for protists, whose strategies may differ due to the distinct ecological and physiological constraints on single-celled organisms. Here, we report measurements of adult and offspring sizes in 3888 species and subspecies of foraminifera, a class of large marine protists. Foraminifera exhibit a wide range of reproductive strategies; species of similar adult size may have offspring whose sizes vary 100-fold. Yet, a robust pattern emerges. The minimum (5th percentile), median, and maximum (95th percentile) offspring sizes exhibit a consistent pattern of increase with adult size independent of environmental change and taxonomic variation over the past 400 million years. The consistency of this pattern may arise from evolutionary optimization of the offspring size-fecundity trade-off and/or from cell-biological constraints that limit the range of reproductive strategies available to single-celled organisms. When compared with plants and animals, foraminifera extend the evidence that offspring size covaries with adult size across an additional five orders of magnitude in organism size.
View details for DOI 10.1111/evo.12210
View details for Web of Science ID 000327572400013
Nuclear Repulsion Enables Division Autonomy in a Single Cytoplasm
2013; 23 (20): 1999-2010
Current models of cell-cycle control, based on classic studies of fused cells, predict that nuclei in a shared cytoplasm respond to the same CDK activities to undergo synchronous cycling. However, synchrony is rarely observed in naturally occurring syncytia, such as the multinucleate fungus Ashbya gossypii. In this system, nuclei divide asynchronously, raising the question of how nuclear timing differences are maintained despite sharing a common milieu.We observe that neighboring nuclei are highly variable in division-cycle duration and that neighbors repel one another to space apart and demarcate their own cytoplasmic territories. The size of these territories increases as a nucleus approaches mitosis and can influence cycling rates. This nonrandom nuclear spacing is regulated by microtubules and is required for nuclear asynchrony, as nuclei that transiently come in very close proximity will partially synchronize. Sister nuclei born of the same mitosis are generally not persistent neighbors over their lifetimes yet remarkably retain similar division cycle times. This indicates that nuclei carry a memory of their birth state that influences their division timing and supports that nuclei subdivide a common cytosol into functionally distinct yet mobile compartments.These findings support that nuclei use cytoplasmic microtubules to establish "cells within cells." Individual compartments appear to push against one another to compete for cytoplasmic territory and insulate the division cycle. This provides a mechanism by which syncytial nuclei can spatially organize cell-cycle signaling and suggests size control can act in a system without physical boundaries.
View details for DOI 10.1016/j.cub.2013.07.076
View details for Web of Science ID 000326317300021
View details for PubMedID 24094857
Control of cell cycle transcription during G1 and S phases
NATURE REVIEWS MOLECULAR CELL BIOLOGY
2013; 14 (8): 518-528
The accurate transition from G1 phase of the cell cycle to S phase is crucial for the control of eukaryotic cell proliferation, and its misregulation promotes oncogenesis. During G1 phase, growth-dependent cyclin-dependent kinase (CDK) activity promotes DNA replication and initiates G1-to-S phase transition. CDK activation initiates a positive feedback loop that further increases CDK activity, and this commits the cell to division by inducing genome-wide transcriptional changes. G1-S transcripts encode proteins that regulate downstream cell cycle events. Recent work is beginning to reveal the complex molecular mechanisms that control the temporal order of transcriptional activation and inactivation, determine distinct functional subgroups of genes and link cell cycle-dependent transcription to DNA replication stress in yeast and mammals.
View details for DOI 10.1038/nrm3629
View details for Web of Science ID 000322118200013
View details for PubMedID 23877564
Feedforward Regulation Ensures Stability and Rapid Reversibility of a Cellular State
2013; 50 (6): 856-868
Cellular transitions are important for all life. Such transitions, including cell fate decisions, often employ positive feedback regulation to establish and stabilize new cellular states. However, positive feedback is unlikely to underlie stable cell-cycle arrest in yeast exposed to mating pheromone because the signaling pathway is linear, rather than bistable, over a broad range of extracellular pheromone concentration. We show that the stability of the pheromone-arrested state results from coherent feedforward regulation of the cell-cycle inhibitor Far1. This network motif is effectively isolated from the more complex regulatory network in which it is embedded. Fast regulation of Far1 by phosphorylation allows rapid cell-cycle arrest and reentry, whereas slow Far1 synthesis reinforces arrest. We expect coherent feedforward regulation to be frequently implemented at reversible cellular transitions because this network motif can achieve the ostensibly conflicting aims of arrest stability and rapid reversibility without loss of signaling information.
View details for DOI 10.1016/j.molcel.2013.04.014
View details for Web of Science ID 000321319900009
- An Algorithm to Automate Yeast Segmentation and Tracking PLOS ONE 2013; 8 (3)
- Cell growth and cell cycle control. Molecular biology of the cell 2013; 24 (6): 678-?
A SHIFT IN THE LONG-TERM MODE OF FORAMINIFERAN SIZE EVOLUTION CAUSED BY THE END-PERMIAN MASS EXTINCTION
2013; 67 (3): 816-827
Size is among the most important traits of any organism, yet the factors that control its evolution remain poorly understood. In this study, we investigate controls on the evolution of organismal size using a newly compiled database of nearly 25,000 foraminiferan species and subspecies spanning the past 400 million years. We find a transition in the pattern of foraminiferan size evolution from correlation with atmospheric pO2 during the Paleozoic (400-250 million years ago) to long-term stasis during the post-Paleozoic (250 million years ago to present). Thus, a dramatic shift in the evolutionary mode coincides with the most severe biotic catastrophe of the Phanerozoic (543 million years ago to present). Paleozoic tracking of pO2 was confined to Order Fusulinida, whereas Paleozoic lagenides, miliolids, and textulariids were best described by the stasis model. Stasis continued to best describe miliolids and textulariids during post-Paleozoic time, whereas random walk was the best supported mode for the other diverse orders. The shift in evolutionary dynamics thus appears to have resulted primarily from the selective elimination of fusulinids at the end of the Permian Period. These findings illustrate the potential for mass extinction to alter macroevolutionary dynamics for hundreds of millions of years.
View details for DOI 10.1111/j.1558-5646.2012.01807.x
View details for Web of Science ID 000315894800018
View details for PubMedID 23461330
An algorithm to automate yeast segmentation and tracking.
2013; 8 (3)
Our understanding of dynamic cellular processes has been greatly enhanced by rapid advances in quantitative fluorescence microscopy. Imaging single cells has emphasized the prevalence of phenomena that can be difficult to infer from population measurements, such as all-or-none cellular decisions, cell-to-cell variability, and oscillations. Examination of these phenomena requires segmenting and tracking individual cells over long periods of time. However, accurate segmentation and tracking of cells is difficult and is often the rate-limiting step in an experimental pipeline. Here, we present an algorithm that accomplishes fully automated segmentation and tracking of budding yeast cells within growing colonies. The algorithm incorporates prior information of yeast-specific traits, such as immobility and growth rate, to segment an image using a set of threshold values rather than one specific optimized threshold. Results from the entire set of thresholds are then used to perform a robust final segmentation.
View details for DOI 10.1371/journal.pone.0057970
View details for PubMedID 23520484
LATE PALEOZOIC FUSULINOIDEAN GIGANTISM DRIVEN BY ATMOSPHERIC HYPEROXIA
2012; 66 (9): 2929-2939
Atmospheric hyperoxia, with pO(2) in excess of 30%, has long been hypothesized to account for late Paleozoic (360-250 million years ago) gigantism in numerous higher taxa. However, this hypothesis has not been evaluated statistically because comprehensive size data have not been compiled previously at sufficient temporal resolution to permit quantitative analysis. In this study, we test the hyperoxia-gigantism hypothesis by examining the fossil record of fusulinoidean foraminifers, a dramatic example of protistan gigantism with some individuals exceeding 10 cm in length and exceeding their relatives by six orders of magnitude in biovolume. We assembled and examined comprehensive regional and global, species-level datasets containing 270 and 1823 species, respectively. A statistical model of size evolution forced by atmospheric pO(2) is conclusively favored over alternative models based on random walks or a constant tendency toward size increase. Moreover, the ratios of volume to surface area in the largest fusulinoideans are consistent in magnitude and trend with a mathematical model based on oxygen transport limitation. We further validate the hyperoxia-gigantism model through an examination of modern foraminiferal species living along a measured gradient in oxygen concentration. These findings provide the first quantitative confirmation of a direct connection between Paleozoic gigantism and atmospheric hyperoxia.
View details for DOI 10.1111/j.1558-5646.2012.01626.x
View details for Web of Science ID 000308405100020
View details for PubMedID 22946813
Cell Size Control in Yeast
2012; 22 (9): R350-R359
Cell size is an important adaptive trait that influences nearly all aspects of cellular physiology. Despite extensive characterization of the cell-cycle regulatory network, the molecular mechanisms coupling cell growth to division, and thereby controlling cell size, have remained elusive. Recent work in yeast has reinvigorated the size control field and suggested provocative mechanisms for the distinct functions of setting and sensing cell size. Further examination of size-sensing models based on spatial gradients and molecular titration, coupled with elucidation of the pathways responsible for nutrient-modulated target size, may reveal the fundamental principles of eukaryotic cell size control.
View details for DOI 10.1016/j.cub.2012.02.041
View details for Web of Science ID 000303967600019
View details for PubMedID 22575477
Evolution of networks and sequences in eukaryotic cell cycle control
PHILOSOPHICAL TRANSACTIONS OF THE ROYAL SOCIETY B-BIOLOGICAL SCIENCES
2011; 366 (1584): 3532-3544
The molecular networks regulating the G1-S transition in budding yeast and mammals are strikingly similar in network structure. However, many of the individual proteins performing similar network roles appear to have unrelated amino acid sequences, suggesting either extremely rapid sequence evolution, or true polyphyly of proteins carrying out identical network roles. A yeast/mammal comparison suggests that network topology, and its associated dynamic properties, rather than regulatory proteins themselves may be the most important elements conserved through evolution. However, recent deep phylogenetic studies show that fungal and animal lineages are relatively closely related in the opisthokont branch of eukaryotes. The presence in plants of cell cycle regulators such as Rb, E2F and cyclins A and D, that appear lost in yeast, suggests cell cycle control in the last common ancestor of the eukaryotes was implemented with this set of regulatory proteins. Forward genetics in non-opisthokonts, such as plants or their green algal relatives, will provide direct information on cell cycle control in these organisms, and may elucidate the potentially more complex cell cycle control network of the last common eukaryotic ancestor.
View details for DOI 10.1098/rstb.2011.0078
View details for Web of Science ID 000296981900006
View details for PubMedID 22084380
Commitment to a Cellular Transition Precedes Genome-wide Transcriptional Change
2011; 43 (4): 515-527
In budding yeast, commitment to cell division corresponds to activating the positive feedback loop of G1 cyclins controlled by the transcription factors SBF and MBF. This pair of transcription factors has over 200 targets, implying that cell-cycle commitment coincides with genome-wide changes in transcription. Here, we find that genes within this regulon have a well-defined distribution of transcriptional activation times. Combinatorial use of SBF and MBF results in a logical OR function for gene expression and partially explains activation timing. Activation of G1 cyclin expression precedes the activation of the bulk of the G1/S regulon, ensuring that commitment to cell division occurs before large-scale changes in transcription. Furthermore, we find similar positive feedback-first regulation in the yeasts S. bayanus and S. cerevisiae, as well as human cells. The widespread use of the feedback-first motif in eukaryotic cell-cycle control, implemented by nonorthologous proteins, suggests its frequent deployment at cellular transitions.
View details for DOI 10.1016/j.molcel.2011.06.024
View details for Web of Science ID 000294151000005
View details for PubMedID 21855792
Distinct Interactions Select and Maintain a Specific Cell Fate
2011; 43 (4): 528-539
The ability to specify and maintain discrete cell fates is essential for development. However, the dynamics underlying selection and stability of distinct cell types remain poorly understood. Here, we provide a quantitative single-cell analysis of commitment dynamics during the mating-mitosis switch in budding yeast. Commitment to division corresponds precisely to activating the G1 cyclin positive feedback loop in competition with the cyclin inhibitor Far1. Cyclin-dependent phosphorylation and inhibition of the mating pathway scaffold Ste5 are required to ensure exclusive expression of the mitotic transcriptional program after cell cycle commitment. Failure to commit exclusively results in coexpression of both cell cycle and pheromone-induced genes, and a morphologically mixed inviable cell fate. Thus, specification and maintenance of a cellular state are performed by distinct interactions, which are likely a consequence of disparate reaction rates and may be a general feature of the interlinked regulatory networks responsible for selecting cell fates.
View details for DOI 10.1016/j.molcel.2011.06.025
View details for Web of Science ID 000294151000006
View details for PubMedID 21855793
- Cell signaling. To divide or not to divide. Science 2009; 324 (5926): 476-477
Positive feedback of G1 cyclins ensures coherent cell cycle entry
2008; 454 (7202): 291-U12
In budding yeast, Saccharomyces cerevisiae, the Start checkpoint integrates multiple internal and external signals into an all-or-none decision to enter the cell cycle. Here we show that Start behaves like a switch due to systems-level feedback in the regulatory network. In contrast to current models proposing a linear cascade of Start activation, transcriptional positive feedback of the G1 cyclins Cln1 and Cln2 induces the near-simultaneous expression of the approximately 200-gene G1/S regulon. Nuclear Cln2 drives coherent regulon expression, whereas cytoplasmic Cln2 drives efficient budding. Cells with the CLN1 and CLN2 genes deleted frequently arrest as unbudded cells, incurring a large fluctuation-induced fitness penalty due to both the lack of cytoplasmic Cln2 and insufficient G1/S regulon expression. Thus, positive-feedback-amplified expression of Cln1 and Cln2 simultaneously drives robust budding and rapid, coherent regulon expression. A similar G1/S regulatory network in mammalian cells, comprised of non-orthologous genes, suggests either conservation of regulatory architecture or convergent evolution.
View details for DOI 10.1038/nature07118
View details for Web of Science ID 000257665300029
View details for PubMedID 18633409
The effects of molecular noise and size control on variability in the budding yeast cell cycle
2007; 448 (7156): 947-U12
Molecular noise in gene expression can generate substantial variability in protein concentration. However, its effect on the precision of a natural eukaryotic circuit such as the control of cell cycle remains unclear. We use single-cell imaging of fluorescently labelled budding yeast to measure times from division to budding (G1) and from budding to the next division. The variability in G1 decreases with the square root of the ploidy through a 1N/2N/4N ploidy series, consistent with simple stochastic models for molecular noise. Also, increasing the gene dosage of G1 cyclins decreases the variability in G1. A new single-cell reporter for cell protein content allows us to determine the contribution to temporal G1 variability of deterministic size control (that is, smaller cells extending G1). Cell size control contributes significantly to G1 variability in daughter cells but not in mother cells. However, even in daughters, size-independent noise is the largest quantitative contributor to G1 variability. Exit of the transcriptional repressor Whi5 from the nucleus partitions G1 into two temporally uncorrelated and functionally distinct steps. The first step, which depends on the G1 cyclin gene CLN3, corresponds to noisy size control that extends G1 in small daughters, but is of negligible duration in mothers. The second step, whose variability decreases with increasing CLN2 gene dosage, is similar in mothers and daughters. This analysis decomposes the regulatory dynamics of the Start transition into two independent modules, a size sensing module and a timing module, each of which is predominantly controlled by a different G1 cyclin.
View details for DOI 10.1038/nature06072
View details for Web of Science ID 000248912900051
View details for PubMedID 17713537