Wang S, Zickler D, Kleckner N, Zhang L. Meiotic crossover patterns: obligatory crossover, interference and homeostasis in a single process. Cell Cycle. 2015;14 (3) :305-14.Abstract
During meiosis, crossover recombination is tightly regulated. A spatial patterning phenomenon known as interference ensures that crossovers are well-spaced along the chromosomes. Additionally, every pair of homologs acquires at least one crossover. A third feature, crossover homeostasis, buffers the system such that the number of crossovers remains steady despite decreases or increases in the number of earlier recombinational interactions. Here we summarize recent work from our laboratory supporting the idea that all 3 of these aspects are intrinsic consequences of a single basic process and suggesting that the underlying logic of this process corresponds to that embodied in a particular (beam-film) model.
Zickler D, Kleckner N. Recombination, Pairing, and Synapsis of Homologs during Meiosis. Cold Spring Harb Perspect Biol. 2015.Abstract
Recombination is a prominent feature of meiosis in which it plays an important role in increasing genetic diversity during inheritance. Additionally, in most organisms, recombination also plays mechanical roles in chromosomal processes, most notably to mediate pairing of homologous chromosomes during prophase and, ultimately, to ensure regular segregation of homologous chromosomes when they separate at the first meiotic division. Recombinational interactions are also subject to important spatial patterning at both early and late stages. Recombination-mediated processes occur in physical and functional linkage with meiotic axial chromosome structure, with interplay in both directions, before, during, and after formation and dissolution of the synaptonemal complex (SC), a highly conserved meiosis-specific structure that links homolog axes along their lengths. These diverse processes also are integrated with recombination-independent interactions between homologous chromosomes, nonhomology-based chromosome couplings/clusterings, and diverse types of chromosome movement. This review provides an overview of these diverse processes and their interrelationships.
Vasnier C, De Muyt A, Zhang L, Tessé S, Kleckner NE, Zickler D, Espagne E. Absence of SUN-domain protein Slp1 blocks karyogamy and switches meiotic recombination and synapsis from homologs to sister chromatids. Proc Natl Acad Sci U S A. 2014;111 (38) :E4015-23.Abstract
Karyogamy, the process of nuclear fusion is required for two haploid gamete nuclei to form a zygote. Also, in haplobiontic organisms, karyogamy is required to produce the diploid nucleus/cell that then enters meiosis. We identify sun like protein 1 (Slp1), member of the mid-Sad1p, UNC-84-domain ubiquitous family, as essential for karyogamy in the filamentous fungus Sordaria macrospora, thus uncovering a new function for this protein family. Slp1 is required at the last step, nuclear fusion, not for earlier events including nuclear movements, recognition, and juxtaposition. Correspondingly, like other family members, Slp1 localizes to the endoplasmic reticulum and also to its extensions comprising the nuclear envelope. Remarkably, despite the absence of nuclear fusion in the slp1 null mutant, meiosis proceeds efficiently in the two haploid "twin" nuclei, by the same program and timing as in diploid nuclei with a single dramatic exception: the normal prophase program of recombination and synapsis between homologous chromosomes, including loading of recombination and synaptonemal complex proteins, occurs instead between sister chromatids. Moreover, the numbers of recombination-initiating double-strand breaks (DSBs) and ensuing recombinational interactions, including foci of the essential crossover factor Homo sapiens enhancer of invasion 10 (Hei10), occur at half the diploid level in each haploid nucleus, implying per-chromosome specification of DSB formation. Further, the distribution of Hei10 foci shows interference like in diploid meiosis. Centromere and spindle dynamics, however, still occur in the diploid mode during the two meiotic divisions. These observations imply that the prophase program senses absence of karyogamy and/or absence of a homolog partner and adjusts the interchromosomal interaction program accordingly.
Kleckner N, Fisher JK, Stouf M, White MA, Bates D, Witz G. The bacterial nucleoid: nature, dynamics and sister segregation. Curr Opin Microbiol. 2014;22 :127-37.Abstract
Recent studies reveal that the bacterial nucleoid has a defined, self-adherent shape and an underlying longitudinal organization and comprises a viscoelastic matrix. Within this shape, mobility is enhanced by ATP-dependent processes and individual loci can undergo ballistic off-equilibrium movements. In Escherichia coli, two global dynamic nucleoid behaviors emerge pointing to nucleoid-wide accumulation and relief of internal stress. Sister segregation begins with local splitting of individual loci, which is delayed at origin, terminus and specialized interstitial snap regions. Globally, as studied in several systems, segregation is a multi-step process in which internal nucleoid state plays critical roles that involve both compaction and expansion. The origin and terminus regions undergo specialized programs partially driven by complex ATP burning mechanisms such as a ParAB Brownian ratchet and a septum-associated FtsK motor. These recent findings reveal strong, direct parallels among events in different systems and between bacterial nucleoids and mammalian chromosomes with respect to physical properties, internal organization and dynamic behaviors.
Gladyshev E, Kleckner N. Direct recognition of homology between double helices of DNA in Neurospora crassa. Nat Commun. 2014;5 :3509.Abstract
Chromosomal regions of identical or nearly identical DNA sequence can preferentially associate with one another in the apparent absence of DNA breakage. Molecular mechanism(s) underlying such homology-dependent pairing phenomena remain(s) unknown. Using Neurospora crassa repeat-induced point mutation (RIP) as a model system, we show that a pair of DNA segments can be recognized as homologous, if they share triplets of base pairs arrayed with the matching periodicity of 11 or 12 base pairs. This pattern suggests direct interactions between slightly underwound co-aligned DNA duplexes engaging once per turn and over many consecutive turns. The process occurs in the absence of MEI3, the only RAD51/DMC1 protein in N. crassa, demonstrating independence from the canonical homology recognition pathway. A new perspective is thus provided for further analysis of the breakage-independent recognition of homology that underlies RIP and, potentially, other processes where sequence-specific pairing of intact chromosomes is involved.
De Muyt A, Zhang L, Piolot T, Kleckner N, Espagne E, Zickler D. E3 ligase Hei10: a multifaceted structure-based signaling molecule with roles within and beyond meiosis. Genes Dev. 2014;28 (10) :1111-23.Abstract
Human enhancer of invasion-10 (Hei10) mediates meiotic recombination and also plays roles in cell proliferation. Here we explore Hei10's roles throughout the sexual cycle of the fungus Sordaria with respect to localization and effects of null, RING-binding, and putative cyclin-binding (RXL) domain mutations. Hei10 makes three successive types of foci. Early foci form along synaptonemal complex (SC) central regions. At some of these positions, depending on its RING and RXL domains, Hei10 mediates development and turnover of two sequential types of recombination complexes, each demarked by characteristic amplified Hei10 foci. Integration with ultrastructural data for recombination nodules further reveals that recombination complexes differentiate into three types, one of which corresponds to crossover recombination events during or prior to SC formation. Finally, Hei10 positively and negatively modulates SUMO localization along SCs by its RING and RXL domains, respectively. The presented findings suggest that Hei10 integrates signals from the SC, associated recombination complexes, and the cell cycle to mediate both the development and programmed turnover/evolution of recombination complexes via SUMOylation/ubiquitination. Analogous cell cycle-linked assembly/disassembly switching could underlie localization and roles for Hei10 in centrosome/spindle pole body dynamics and associated nuclear trafficking. We suggest that Hei10 is a unique type of structure-based signal transduction protein.
Zhang L, Espagne E, De Muyt A, Zickler D, Kleckner NE. Interference-mediated synaptonemal complex formation with embedded crossover designation. Proc Natl Acad Sci U S A. 2014;111 (47) :E5059-68.Abstract
Biological systems exhibit complex patterns at length scales ranging from the molecular to the organismic. Along chromosomes, events often occur stochastically at different positions in different nuclei but nonetheless tend to be relatively evenly spaced. Examples include replication origin firings, formation of chromatin loops along chromosome axes and, during meiosis, localization of crossover recombination sites ("crossover interference"). We present evidence in the fungus Sordaria macrospora that crossover interference is part of a broader pattern that includes synaptonemal complex (SC) nucleation. This pattern comprises relatively evenly spaced SC nucleation sites, among which a subset are crossover sites that show a classical interference distribution. This pattern ensures that SC forms regularly along the entire length of the chromosome as required for the maintenance of homolog pairing while concomitantly having crossover interactions locally embedded within the SC structure as required for both DNA recombination and structural events of chiasma formation. This pattern can be explained by a threshold-based designation and spreading interference process. This model can be generalized to give diverse types of related and/or partially overlapping patterns, in two or more dimensions, for any type of object.
Zhang L, Wang S, Yin S, Hong S, Kim KP, Kleckner N. Topoisomerase II mediates meiotic crossover interference. Nature. 2014;511 (7511) :551-6.Abstract
Spatial patterning is a ubiquitous feature of biological systems. Meiotic crossovers provide an interesting example, defined by the classic phenomenon of crossover interference. Here we identify a molecular pathway for interference by analysing crossover patterns in budding yeast. Topoisomerase II plays a central role, thus identifying a new function for this critical molecule. SUMOylation (of topoisomerase II and axis component Red1) and ubiquitin-mediated removal of SUMOylated proteins are also required. The findings support the hypothesis that crossover interference involves accumulation, relief and redistribution of mechanical stress along the protein/DNA meshwork of meiotic chromosome axes, with topoisomerase II required to adjust spatial relationships among DNA segments.
Zhang L, Liang Z, Hutchinson J, Kleckner N. Crossover patterning by the beam-film model: analysis and implications. PLoS Genet. 2014;10 (1) :e1004042.Abstract
Crossing-over is a central feature of meiosis. Meiotic crossover (CO) sites are spatially patterned along chromosomes. CO-designation at one position disfavors subsequent CO-designation(s) nearby, as described by the classical phenomenon of CO interference. If multiple designations occur, COs tend to be evenly spaced. We have previously proposed a mechanical model by which CO patterning could occur. The central feature of a mechanical mechanism is that communication along the chromosomes, as required for CO interference, can occur by redistribution of mechanical stress. Here we further explore the nature of the beam-film model, its ability to quantitatively explain CO patterns in detail in several organisms, and its implications for three important patterning-related phenomena: CO homeostasis, the fact that the level of zero-CO bivalents can be low (the "obligatory CO"), and the occurrence of non-interfering COs. Relationships to other models are discussed.
Fisher JK, Kleckner N. Magnetic force micropiston: An integrated force/microfluidic device for the application of compressive forces in a confined environment. Rev Sci Instrum. 2014;85 (2) :023704.Abstract
Cellular biology takes place inside confining spaces. For example, bacteria grow in crevices, red blood cells squeeze through capillaries, and chromosomes replicate inside the nucleus. Frequently, the extent of this confinement varies. Bacteria grow longer and divide, red blood cells move through smaller and smaller passages as they travel to capillary beds, and replication doubles the amount of DNA inside the nucleus. This increase in confinement, either due to a decrease in the available space or an increase in the amount of material contained in a constant volume, has the potential to squeeze and stress objects in ways that may lead to changes in morphology, dynamics, and ultimately biological function. Here, we describe a device developed to probe the interplay between confinement and the mechanical properties of cells and cellular structures, and forces that arise due to changes in a structure's state. In this system, the manipulation of a magnetic bead exerts a compressive force upon a target contained in the confining space of a microfluidic channel. This magnetic force microfluidic piston is constructed in such a way that we can measure (a) target compliance and changes in compliance as induced by changes in buffer, extract, or biochemical composition, (b) target expansion force generated by changes in the same parameters, and (c) the effects of compression stress on a target's structure and function. Beyond these issues, our system has general applicability to a variety of questions requiring the combination of mechanical forces, confinement, and optical imaging.
Mirkin EV, Chang FS, Kleckner N. Protein-mediated chromosome pairing of repetitive arrays. J Mol Biol. 2014;426 (3) :550-7.Abstract
Chromosomally integrated arrays of lacO and tetO operator sites visualized by LacI and TetR repressor proteins fused with GFP (green fluorescent protein) (or other fluorescent proteins) are widely used to monitor the behavior of chromosomal loci in various systems. However, insertion of such arrays and expression of the corresponding proteins is known to perturb genomic architecture. In several cases, juxtaposition of such arrays located on different chromosomes has been inferred to reflect pairing of the corresponding loci. Here, we report that a version of TetR-GFP mutated to disrupt GFP dimerization (TetR-A206KGFP or "TetR-kGFP") abolishes pairing of tetO arrays in vivo and brings spatial proximity of chromosomal loci marked with those arrays back to the wild-type level. These data argue that pairing of arrays is caused by GFP dimerization and thus presents an example of protein-assisted interaction in chromosomes. Arrays marked with another protein, TetR-tdTomato, which has a propensity to form intramolecular dimers instead of intermolecular dimers, also display reduced level of pairing, supporting this idea. TetR-kGFP provides an improved system for studying chromosomal loci with a low pairing background.
Kleckner N, Zickler D, Witz G. Molecular biology. Chromosome capture brings it all together. Science. 2013;342 (6161) :940-1.
Danilowicz C, Peacock-Villada A, Vlassakis J, Facon A, Feinstein E, Kleckner N, Prentiss M. The differential extension in dsDNA bound to Rad51 filaments may play important roles in homology recognition and strand exchange. Nucleic Acids Res. 2013.Abstract
RecA and Rad51 proteins play an important role in DNA repair and homologous recombination. For RecA, X-ray structure information and single molecule force experiments have indicated that the differential extension between the complementary strand and its Watson-Crick pairing partners promotes the rapid unbinding of non-homologous dsDNA and drives strand exchange forward for homologous dsDNA. In this work we find that both effects are also present in Rad51 protein. In particular, pulling on the opposite termini (3' and 5') of one of the two DNA strands in a dsDNA molecule allows dsDNA to extend along non-homologous Rad51-ssDNA filaments and remain stably bound in the extended state, but pulling on the 3'5' ends of the complementary strand reduces the strand-exchange rate for homologous filaments. Thus, the results suggest that differential extension is also present in dsDNA bound to Rad51. The differential extension promotes rapid recognition by driving the swift unbinding of dsDNA from non-homologous Rad51-ssDNA filaments, while at the same time, reducing base pair tension due to the transfer of the Watson-Crick pairing of the complementary strand bases from the highly extended outgoing strand to the slightly less extended incoming strand, which drives strand exchange forward.
Mirkin EV, Chang FS, Kleckner N. Dynamic trans interactions in yeast chromosomes. PLoS One. 2013;8 (9) :e75895.Abstract
Three-dimensional organization of the genome is important for regulation of gene expression and maintenance of genomic stability. It also defines, and is defined by, contacts between different chromosomal loci. Interactions between loci positioned on different chromosomes, i.e. "trans" interactions are one type of such contacts. Here, we describe a case of inducible trans interaction in chromosomes of the budding yeast S. cerevisiae. Special DNA sequences, inserted in two ectopic chromosomal loci positioned in trans, pair with one another in an inducible manner. The spatial proximity diagnostic of pairing is observable by both chromosome capture analysis (3C) and epifluorescence microscopy in whole cells. Protein synthesis de novo appears to be required for this process. The three-dimensional organization of the yeast nucleus imposes a constraint on such pairing, presumably by dictating the probability with which the two sequences collide with one another.
Fisher JK, Bourniquel A, Witz G, Weiner B, Prentiss M, Kleckner N. Four-dimensional imaging of E. coli nucleoid organization and dynamics in living cells. Cell. 2013;153 (4) :882-95.Abstract
Visualization of living E. coli nucleoids, defined by HupA-mCherry, reveals a discrete, dynamic helical ellipsoid. Three basic features emerge. (1) Nucleoid density coalesces into longitudinal bundles, giving a stiff, low-DNA-density ellipsoid. (2) This ellipsoid is radially confined within the cell cylinder. Radial confinement gives helical shape and directs global nucleoid dynamics, including sister segregation. (3) Longitudinal density waves flux back and forth along the nucleoid, with 5%-10% of density shifting within 5 s, enhancing internal nucleoid mobility. Furthermore, sisters separate end-to-end in sequential discontinuous pulses, each elongating the nucleoid by 5%-15%. Pulses occur at 20 min intervals, at defined cell-cycle times. This progression includes sequential installation and release of programmed tethers, implying cyclic accumulation and relief of intranucleoid mechanical stress. These effects could comprise a chromosome-based cell-cycle engine. Overall, the presented results suggest a general conceptual framework for bacterial nucleoid morphogenesis and dynamics.
Hong S, Sung Y, Yu M, Lee M, Kleckner N, Kim KP. The logic and mechanism of homologous recombination partner choice. Mol Cell. 2013;51 (4) :440-53.Abstract
Recombinational repair of spontaneous double-strand breaks (DSBs) exhibits sister bias. DSB-initiated meiotic recombination exhibits homolog bias. Physical analysis in yeast reveals that, in both cases, the recombination reaction intrinsically gives homolog bias. From this baseline default, cohesin intervenes to confer sister bias, likely independent of cohesion. In meiosis, cohesin's sister-biasing effect is counteracted by RecA homolog Rad51 and its mediators, plus meiotic RecA homolog Dmc1, which thereby restore intrinsic homolog bias. Meiotic axis complex Red1/Mek1/Hop1 participates by cleanly switching recombination from mitotic to meiotic mode, concomitantly activating Dmc1. We propose that a Rad51/DNA filament at one DSB end captures the intact sister, creating an anchor pad. This filament extends across the DSB site on the intact partner, precluding intersister strand exchange, thus forcing use of the homolog. Cohesin and Dmc1 interactively modulate this extension, with program-appropriate effects. In accord with this model, Rad51-mediated recombination in vivo requires the presence of a sister.
Conover AJ, Danilowicz C, Gunaratne R, Coljee VW, Kleckner N, Prentiss M. Changes in the tension in dsDNA alter the conformation of RecA bound to dsDNA-RecA filaments. Nucleic Acids Res. 2011;39 (20) :8833-43.Abstract
The RecA protein is an ATPase that mediates recombination via strand exchange. In strand exchange a single-stranded DNA (ssDNA) bound to RecA binding site I in a RecA/ssDNA filament pairs with one strand of a double-stranded DNA (dsDNA) and forms heteroduplex dsDNA in site I if homology is encountered. Long sequences are exchanged in a dynamic process in which initially unbound dsDNA binds to the leading end of a RecA/ssDNA filament, while heteroduplex dsDNA unbinds from the lagging end via ATP hydrolysis. ATP hydrolysis is required to convert the active RecA conformation, which cannot unbind, to the inactive conformation, which can unbind. If dsDNA extension due to RecA binding increases the dsDNA tension, then RecA unbinding must decrease tension. We show that in the presence of ATP hydrolysis decreases in tension induce decreases in length whereas in the absence of hydrolysis, changes in tension have no systematic effect. These results suggest that decreases in force enhance dissociation by promoting transitions from the active to the inactive RecA conformation. In contrast, increases in tension reduce dissociation. Thus, the changes in tension inherent to strand exchange may couple with ATP hydrolysis to increase the directionality and stringency of strand exchange.
Joshi MC, Bourniquel A, Fisher J, Ho BT, Magnan D, Kleckner N, Bates D. Escherichia coli sister chromosome separation includes an abrupt global transition with concomitant release of late-splitting intersister snaps. Proc Natl Acad Sci U S A. 2011;108 (7) :2765-70.Abstract
The basis for segregation of sister chromosomes in bacteria is not established. We show here that two discrete ~150-kb regions, both located early in the right replichore, exhibit prolonged juxtaposition of sister loci, for 20 and 30 min, respectively, after replication. Flanking regions, meanwhile, separate. Thus, the two identified regions comprise specialized late-splitting intersister connections or snaps. Sister snap loci separate simultaneously in both snap regions, concomitant with a major global nucleoid reorganization that results in emergence of a bilobed nucleoid morphology. Split snap loci move rapidly apart to a separation distance comparable with one-half the length of the nucleoid. Concomitantly, at already split positions, sister loci undergo further separation to a comparable distance. The overall consequence of these and other effects is that thus far replicated sister chromosomes become spatially separated (individualized) into the two nucleoid lobes, while the terminus region (and likely, all unreplicated portions of the chromosome) moves to midcell. These and other findings imply that segregation of Escherichia coli sister chromosomes is not a smooth continuous process but involves at least one and likely, two major global transition(s). The presented patterns further suggest that accumulation of internal intranucleoid forces and constraining of these forces by snaps play central roles in global chromosome dynamics. They are consistent with and supportive of our previous proposals that individualization of sisters in E. coli is driven primarily by internally generated pushing forces and is directly analogous to sister individualization at the prophase to prometaphase transition of the eukaryotic cell cycle.
Zhang L, Kim KP, Kleckner NE, Storlazzi A. Meiotic double-strand breaks occur once per pair of (sister) chromatids and, via Mec1/ATR and Tel1/ATM, once per quartet of chromatids. Proc Natl Acad Sci U S A. 2011;108 (50) :20036-41.Abstract
Meiotic recombination initiates via programmed double-strand breaks (DSBs). We investigate whether, at a given initiation site, DSBs occur independently among the four available chromatids. For a single DSB "hot spot", the proportions of nuclei exhibiting zero, one, or two (or more) observable events were defined by tetrad analysis and compared with those predicted by different DSB distribution scenarios. Wild-type patterns are incompatible with independent distribution of DSBs among the four chromatids. In most or all nuclei, DSBs occur one-per-pair of chromatids, presumptively sisters. In many nuclei, only one DSB occurs per four chromatids, confirming the existence of trans inhibition where a DSB on one chromosome interactively inhibits DSB formation on the partner chromosome. Several mutants exhibit only a one-per-pair constraint, a phenotype we propose to imply loss of trans inhibition. Signal transduction kinases Mec1 (ATR) and Tel1 (ATM) exhibit this phenotype and thus could be mediators of this effect. Spreading trans inhibition can explain even spacing of total recombinational interactions and implies that establishment of interhomolog interactions and DSB formation are homeostatic processes. The two types of constraints on DSB formation provide two different safeguards against recombination failure during meiosis.
Feinstein E, Danilowicz C, Conover A, Gunaratne R, Kleckner N, Prentiss M. Single-molecule studies of the stringency factors and rates governing the polymerization of RecA on double-stranded DNA. Nucleic Acids Res. 2011;39 (9) :3781-91.Abstract
RecA is a key protein in homologous recombination. During recombination, one single-stranded DNA (ssDNA) bound to site I in RecA exchanges Watson-Crick pairing with a sequence-matched ssDNA that was part of a double-stranded DNA molecule (dsDNA) bound to site II in RecA. After strand exchange, heteroduplex dsDNA is bound to site I. In vivo, direct polymerization of RecA on dsDNA through site I does not occur, though it does in vitro. The mechanisms underlying the difference have been unclear. We use single-molecule experiments to decouple the two steps involved in polymerization: nucleation and elongation. We find that elongation is governed by a fundamental clock that is insensitive to force and RecA concentration from 0.2 and 6 µM, though rates depend on ionic conditions. Thus, we can probe nucleation site stability by creating nucleation sites at high force and then measuring elongation as a function of applied force. We find that in the presence of ATP hydrolysis a minimum force is required for polymerization. The minimum force decreases with increasing RecA or ATP concentrations. We propose that force reduces the off-rate for nucleation site binding and that nucleation site stability is the stringency factor that prevents in vivo polymerization.