Da-Qiao Ding

Dynamics of Homologous Chromosome Pairing during Meiotic Prophase in Fission Yeast

Pairing of homologous chromosomes is important for homologous recombination and correct chromosome segregation during meiosis. It has been proposed that telomere clustering, nuclear oscillation, and recombination during meiotic prophase facilitate homologous chromosome pairing in fission yeast. Here we examined the contributions of these chromosomal events to homologous chromosome pairing, by directly observing the dynamics of chromosomal loci in living cells of fission yeast. Homologous loci exhibited a dynamic process of association and dissociation during the time course of meiotic prophase. Lack of nuclear oscillation reduced association frequency for both centromeric and arm regions of the chromosome. Lack of telomere clustering or recombination reduced association frequency at arm regions, but not significantly at centromeric regions. Our results indicate that homologous chromosomes are spatially aligned by oscillation of telomere-bundled chromosomes and physically linked by recombination at chromosome arm regions; this recombination is not required for association of homologous centromeres.

Meiotic nuclear reorganization: switching the position of centromeres and telomeres in the fission yeast Schizosaccharomyces pombe

In fission yeast meiotic prophase, telomeres are clustered near the spindle pole body (SPB; a centrosome‐equivalent structure in fungi) and take the leading position in chromosome movement, while centromeres are separated from the SPB. This telomere position contrasts with mitotic nuclear organization, in which centromeres remain clustered near the SPB and lead chromosome movement. Thus, nuclear reorganization switching the position of centromeres and telomeres must take place upon entering meiosis. In this report, we analyze the nuclear location of centromeres and telomeres in genetically well‐characterized meiotic mutant strains. An intermediate structure for telomere‐centromere switching was observed in haploid cells induced to undergo meiosis by synthetic mating pheromone; fluorescence in situ hybridization revealed that in these cells, both telomeres and centromeres were clustered near the SPB. Further analyses in a series of mutants showed that telomere‐centromere switching takes place in two steps; first, association of telomeres with the SPB and, second, dissociation of centromeres from the SPB. The first step can take place in the haploid state in response to mating pheromone, but the second step does not take place in haploid cells and probably depends on conjugation‐related events. In addition, a linear minichromosome was also co‐localized with authentic telomeres instead of centromeres, suggesting that telomere clustering plays a role in organizing chromosomes within a meiotic prophase nucleus.

Large-scale screening of intracellular protein localization in living fission yeast cells by the use of a GFP-fusion genomic DNA library

Intracellular localization is an important part of the characterization of a gene product. In an attempt to search for genes based on the intracellular localization of their products, we constructed a green fluorescent protein (GFP)‐fusion genomic DNA library of S. pombe.

A NOVEL FISSION YEAST GENE, KMS1+, IS REQUIRED FOR THE FORMATION OF MEIOTIC PROPHASE-SPECIFIC NUCLEAR ARCHITECTURE

Abstract In the meiotic prophase nucleus of the fission yeast Schizosaccharomyces pombe, chromosomes are arranged in an oriented manner: telomeres cluster in close proximity to the spindle pole body (SPB), while centromeres form another cluster at some distance from the SPB. We have isolated a mutant, kms1, in which the structure of the meiotic prophase nucleus appears to be distorted. Using specific probes to localize the SPB and telomeres, multiple signals were observed in the mutant nuclei, in contrast to the case in wild-type. Genetic analysis showed that in the mutant, meiotic recombination frequency was reduced to about one-quarter of the wild-type level and meiotic segregation was impaired. This phenotype strongly suggests that the telomere-led rearrangement of chromosomal distribution that normally occurs in the fission yeast meiotic nucleus is an important prerequisite for the efficient pairing of homologous chromosomes. The kms1 mutant was also impaired in karyogamy, suggesting that the kms1+ gene is involved in SPB function. However, the kms1+ gene is dispensable for mitotic growth. The predicted amino acid sequence of the gene product shows no significant similarity to known proteins.

Meiosis-specific noncoding RNA mediates robust pairing of homologous chromosomes in meiosis.

Find Your Partner Gametes generally have haploid genomes, so that when they fuse during fertilization, they reconstitute a diploid organism. Meiosis is required to make haploid gametes, which involves a reduction division where two homologous chromosomes first pair and are then segregated away from each other. Working in the fission yeast Schizosaccharomyces pombe, Ding et al. (p. 732; see the Perspective by Dernburg) explored how homologous chromosomes recognized each other and found that a non-coding RNA locus, sme2, helps to drive the pairing of homologous chromosomes. An RNA transcript helps to bring together homologous chromosomes during cell division. Pairing and recombination of homologous chromosomes are essential for ensuring reductional segregation in meiosis. However, the mechanisms by which chromosomes recognize their homologous partners are poorly understood. Here, we report that the sme2 gene encodes a meiosis-specific noncoding RNA that mediates homologous recognition in the fission yeast Schizosaccharomyces pombe. The sme2 locus shows robust pairing from early in meiotic prophase. The sme2 RNA transcripts accumulate at their respective gene loci and greatly enhance pairing of homologous loci: Deletion of the sme2 sequence eliminates this robust pairing, whereas transposition to other chromosomal sites confers robust pairing at those ectopic sites. Thus, we propose that RNA transcripts retained on the chromosome play an active role in recognition of homologous chromosomes for pairing.

Meiotic cohesins modulate chromosome compaction during meiotic prophase in fission yeast

The meiotic cohesin Rec8 is required for the stepwise segregation of chromosomes during the two rounds of meiotic division. By directly measuring chromosome compaction in living cells of the fission yeast Schizosaccharomyces pombe, we found an additional role for the meiotic cohesin in the compaction of chromosomes during meiotic prophase. In the absence of Rec8, chromosomes were decompacted relative to those of wild-type cells. Conversely, loss of the cohesin-associated protein Pds5 resulted in hypercompaction. Although this hypercompaction requires Rec8, binding of Rec8 to chromatin was reduced in the absence of Pds5, indicating that Pds5 promotes chromosome association of Rec8. To explain these observations, we propose that meiotic prophase chromosomes are organized as chromatin loops emanating from a Rec8-containing axis: the absence of Rec8 disrupts the axis, resulting in disorganized chromosomes, whereas reduced Rec8 loading results in a longitudinally compacted axis with fewer attachment points and longer chromatin loops.

Hexanucleotide motifs mediate recruitment of the RNA elimination machinery to silent meiotic genes

The selective elimination system blocks the accumulation of meiosis-specific mRNAs during the mitotic cell cycle in fission yeast. These mRNAs harbour a region, the determinant of selective removal (DSR), which is recognized by a YTH-family RNA-binding protein, Mmi1. Mmi1 directs target transcripts to destruction in association with nuclear exosomes. Hence, the interaction between DSR and Mmi1 is crucial to discriminate mitosis from meiosis. Here, we show that Mmi1 interacts with repeats of the hexanucleotide U(U/C)AAAC that are enriched in the DSR. Disruption of this ‘DSR core motif’ in a target mRNA inhibits its elimination. Tandem repeats of the motif can function as an artificial DSR. Mmi1 binds to it in vitro. Thus, a core motif cluster is responsible for the DSR activity. Furthermore, certain variant hexanucleotide motifs can augment the function of the DSR core motif. Notably, meiRNA, which composes the nuclear Mei2 dot required to suppress Mmi1 activity during meiosis, carries numerous copies of the core/augmenting motifs on its tail and is indeed degraded by the Mmi1/exosome system, indicating its likely role as decoy bait for Mmi1.

Characterization of fission yeast meiotic mutants based on live observation of meiotic prophase nuclear movement

Abstract.We characterized four meiotic mutants of the fission yeast Schizosaccharomyces pombe by live observation of nuclear movement. Nuclei were stained with either the DNA-specific fluorescent dye Hoechst 33342 or jellyfish green fluorescent protein (GFP) fused with the N-terminal portion of DNA polymerase α. We first followed nuclear dynamics in wild-type cells to determine the temporal sequence of meiotic events: nuclear fusion in the conjugated zygote is immediately followed by oscillatory nuclear movements that continue for 146 min; then, after coming to rest, the nucleus remains in the center of the cell for 26 min before the first meiotic division. Next we examined nuclear dynamics in four meiotic mutants: mei1 (also called mat2), mei4, dhc1, and taz1. Mei1 and mei4 both arrest during meiotic prophase; our observations, however, show that the timing of mei1 arrest is quite different from that of mei4: the mei1 mutant arrests after nuclear fusion but before starting the oscillatory nuclear movements, while the mei4 mutant arrests after the nucleus has completed the oscillatory movements but before the first meiotic division. We also show examples of the dynamic phenotypes of dhc1 and taz1, both of which complete meiosis but exhibit impaired nuclear movement and reduced frequencies of homologous recombination: the dhc1 mutant exhibits no nuclear movement after nuclear fusion, while the taz1 mutant exhibits severely impaired nuclear movement after nuclear fusion.

From meiosis to postmeiotic events: Alignment and recognition of homologous chromosomes in meiosis

Recombination of homologous chromosomes is essential for correct reductional segregation of homologous chromosomes, which characterizes meiosis. To accomplish homologous recombination, chromosomes must find their homologous partners and pair with them within the spatial constraints of the nucleus. Although various mechanisms have developed in different organisms, two major steps are involved in the process of pairing: first, alignment of homologous chromosomes to bring them close to each other for recognition; and second, recognition of the homologous partner of each chromosome so that they can form an intimate pair. Here, we discuss the various mechanisms used for alignment and recognition of homologous chromosomes in meiosis.

A cohesin-based structural platform supporting homologous chromosome pairing in meiosis

The pairing and recombination of homologous chromosomes during the meiotic prophase is necessary for the accurate segregation of chromosomes in meiosis. However, the mechanism by which homologous chromosomes achieve this pairing has remained an open question. Meiotic cohesins have been shown to affect chromatin compaction; however, the impact of meiotic cohesins on homologous pairing and the fine structures of cohesion-based chromatin remain to be determined. A recent report using live-cell imaging and super-resolution microscopy demonstrated that the lack of meiotic cohesins alters the chromosome axis structures and impairs the pairing of homologous chromosomes. These results suggest that meiotic cohesin-based chromosome axis structures are crucial for the pairing of homologous chromosomes.