Keita Aoki

Comparative Functional Genomics of the Fission Yeasts

A combined analysis of genome sequence, structure, and expression gives insights into fission yeast biology. The fission yeast clade—comprising Schizosaccharomyces pombe, S. octosporus, S. cryophilus, and S. japonicus—occupies the basal branch of Ascomycete fungi and is an important model of eukaryote biology. A comparative annotation of these genomes identified a near extinction of transposons and the associated innovation of transposon-free centromeres. Expression analysis established that meiotic genes are subject to antisense transcription during vegetative growth, which suggests a mechanism for their tight regulation. In addition, trans-acting regulators control new genes within the context of expanded functional modules for meiosis and stress response. Differences in gene content and regulation also explain why, unlike the budding yeast of Saccharomycotina, fission yeasts cannot use ethanol as a primary carbon source. These analyses elucidate the genome structure and gene regulation of fission yeast and provide tools for investigation across the Schizosaccharomyces clade.

Genome-wide identification and characterization of replication origins by deep sequencing

BackgroundDNA replication initiates at distinct origins in eukaryotic genomes, but the genomic features that define these sites are not well understood.ResultsWe have taken a combined experimental and bioinformatic approach to identify and characterize origins of replication in three distantly related fission yeasts: Schizosaccharomyces pombe, Schizosaccharomyces octosporus and Schizosaccharomyces japonicus. Using single-molecule deep sequencing to construct amplification-free high-resolution replication profiles, we located origins and identified sequence motifs that predict origin function. We then mapped nucleosome occupancy by deep sequencing of mononucleosomal DNA from the corresponding species, finding that origins tend to occupy nucleosome-depleted regions.ConclusionsThe sequences that specify origins are evolutionarily plastic, with low complexity nucleosome-excluding sequences functioning in S. pombe and S. octosporus, and binding sites for trans-acting nucleosome-excluding proteins functioning in S. japonicus. Furthermore, chromosome-scale variation in replication timing is conserved independently of origin location and via a mechanism distinct from known heterochromatic effects on origin function. These results are consistent with a model in which origins are simply the nucleosome-depleted regions of the genome with the highest affinity for the origin recognition complex. This approach provides a general strategy for understanding the mechanisms that define DNA replication origins in eukaryotes.

Breakage of the nuclear envelope by an extending mitotic nucleus occurs during anaphase in Schizosaccharomyces japonicus.

During open mitosis in higher eukaryotic cells, the nuclear envelope completely breaks down and then mitotic chromosomes are exposed in the cytoplasm. By contrast, mitosis in lower eukaryotes, including fungi, proceeds with the nucleus enclosed in an intact nuclear envelope. The mechanism of mitosis has been studied extensively in yeast, a closed mitosis organism. Here, we describe a form of mitosis in which the nuclear envelope is torn by elongation of the nucleus in the fission yeast Schizosaccharomyces japonicus. The mitotic nucleus of Sz. japonicus adopted a fusiform shape in anaphase, and its following extension caused separation. Finally, a tear in the nuclear envelope occurred in late anaphase. At the same time, a polarized‐biased localization of nuclear pores was seen in the fusiform‐shaped nuclear envelope, suggesting a compromise in the mechanical integrity of the lipid membrane. It has been known that nuclear membrane remains intact in some metazoan mitosis. We found that a similar tear of the nuclear envelope was also observed in late mitosis of the Caenorhabditis elegans embryo. These findings provide insight into the diversity of mitosis and the biological significance of breakdown of the nuclear envelope.

Novel episomal vectors and a highly efficient transformation procedure for the fission yeast Schizosaccharomyces japonicus

Schizosaccharomyces japonicus is a fission yeast for which new genetic tools have recently been developed. Here, we report novel plasmid vectors with high transformation efficiency and an electroporation method for Sz. japonicus. We isolated 44 replicating segments from 12 166 transformants of Sz. japonicus genomic fragments and found a chromosomal fragment, RS1, as a new replicating sequence that conferred high transformation activity to Sz. japonicus cells. This sequence was cloned into a pUC19 vector with ura4+ of Sz. pombe (pSJU11) or the kan gene on the kanMX6 module (pSJK11) as selection markers. These plasmids transformed Sz. japonicus cells in the early‐log phase by electroporation at a frequency of 123 cfu/µg for pSJK11 and 301 cfu/µg for pSJU11, which were higher than previously reported autonomously replicating sequences. Although a portion of plasmids remained in host cells by integration into the chromosome via RS1 segment, the plasmids could be recovered from transformants. The plasmid copy number was estimated to be 1.88 copies per cell by Southern blot analysis using a Sz. pombe ura4+ probe. The plasmid containing ade6+ suppressed the auxotrophic growth of the ade6‐domE mutant, indicating that the plasmid would be useful for suppressor screening and complementation assays in Sz. japonicus. Furthermore, pSJU11 transformed Sz. pombe cells with the same frequency as the pREP2 plasmid. This study is a report to demonstrate practical use of episomal plasmid vectors for genetic research in Sz. japonicus. RS1 has been submitted to the DDBJ/EMBL/GenBank database (Accession No. AB547343). Copyright

Synchronous Activation of Cell Division by Light or Temperature Stimuli in the Dimorphic Yeast Schizosaccharomyces japonicus

ABSTRACT Many fungi respond to light and regulate fungal development and behavior. A blue light-activated complex has been identified in Neurospora crassa as the product of the wc-1 and wc-2 genes. Orthologs of WC-1 and WC-2 have hitherto been found only in filamentous fungi and not in yeast, with the exception of the basidiomycete pathogenic yeast Cryptococcus. Here, we report that the fission yeast Schizosaccharomyces japonicus responds to blue light depending on Wcs1 and Wcs2, orthologs of components of the WC complex. Surprisingly, those of ascomycete S. japonicus are more closely related to those of the basidiomycete. S. japonicus reversibly changes from yeast to hyphae in response to environmental stresses. After incubation at 30°C, a colony of yeast was formed, and then hyphal cells extended from the periphery of the colony. When light cycles were applied, distinct dark- and bright-colored hyphal cell stripes were formed because the growing hyphal cells had synchronously activated cytokinesis. In addition, temperature cycles of 30°C for 12 h and 35°C for 12 h or of 25°C for 12 h and 30°C for 12 h during incubation in the dark induced a response in the hyphal cells similar to that of light. The stripe formation of the temperature cycles was independent of the wcs genes. Both light and temperature, which are daily external cues, have the same effect on growing hyphal cells. A dual sensing mechanism of external cues allows organisms to adapt to daily changes of environmental alteration.

Regulation of nuclear envelope dynamics via APC/C is necessary for the progression of semi‐open mitosis in Schizosaccharomyces japonicus

Three types of mitosis, which are open, closed or semi‐open mitosis, function in eukaryotic cells, respectively. The open mitosis involves breakage of the nuclear envelope before nuclear division, whereas the closed mitosis proceeds with an intact nuclear envelope. To understand the mechanism and significance of three types of mitotic division in eukaryotes, we investigated the process of semi‐open mitosis, in which the nuclear envelope is only partially broken, in the fission yeast Schizosaccharomyces japonicus. In anaphase‐promoting complex/cyclosome (APC/C) mutants of Sz. japonicus, the nuclear envelope remained relatively intact during anaphase, resulting in impaired semi‐open mitosis. As a suppressor of apc2 mutant, a mutation of Oar2, which was a 3‐oxoacyl‐[acyl carrier protein] reductase, was obtained. The level of the Oar2, which had two destruction‐box motifs recognized by APC/C, was increased in APC/C mutants. Furthermore, the defective semi‐open mitosis observed in an apc2 mutant was restored by mutated oar2+. Based on these findings, we propose that APC/C regulates the dynamics of the nuclear envelope through degradation of Oar2 dependent on APC/C during the metaphase‐to‐anaphase transition of semi‐open mitosis in Sz. japonicus.

Construction of an insertion marker collection of Sz. japonicus (IMACS) for genetic mapping and a fosmid library covering its genome

Measuring relative genetic distances is one of the best ways to locate genetic loci. Here we report the construction of a strains set for genetic mapping in Schizosaccharomyces japonicus, which belongs to the genus Schizosaccharomyces together with the well‐studied fission yeast Sz. pombe. We constructed 29 strains that bear a positive‐negative selection marker at different loci. The marker was inserted every 500 kb in the genome of Sz. japonicus. Each marker thus becomes a ‘scale mark’ of a chromosome that behaves like a yardstick. By determining the genetic distances from the inserted markers, the relative location of a genomic mutation can be determined. We also constructed a fosmid library that covers an entire genome of Sz. japonicus. These tools together would facilitate identification and cloning of the gene. Copyright

Schizosaccharomyces japonicus: A Distinct Dimorphic Yeast Among the Fission Yeasts

Genomic sequencing data and morphological properties demonstrate evolutionary relationships among groups of the fission yeast, Schizosaccharomyces Phylogenetically, S. japonicus is the furthest removed from other species of fission yeast. The basic characteristics of cell proliferation are shared among all fission yeast, including the process of binary fission during vegetative growth, conjugation and karyogamy with horsetail movement, mating-type switching, and sporulation. However, S. japonicus also exhibits characteristics that are unique to filamentous fungi. S. japonicus is a nonpathogenic yeast that exhibits dimorphism. Depending on the environmental conditions, S. japonicus transforms from yeast cells into filamentous cells (hyphae), and blue light triggers synchronous septation of hyphal cells. A rough version of the whole-genome sequence is now available, facilitating genetic manipulation of S. japonicus. Furthermore, the extensive genetic knowledge available for S. pombe is aiding the development of genetic tools for analyzing S. japonicus. S. japonicus will help shed light on the evolutionary relationships among the fission yeast.

Transformation of Schizosaccharomyces japonicus

This protocol describes the use of electroporation to transform Schizosaccharomyces japonicus with plasmids or linear DNA. Plasmids are helpful for the complementation testing of mutations and for the expression of specific genes. Linear DNA fragments integrated into chromosomal DNA by homologous recombination are useful for gene deletion or to fuse a gene with a tag sequence (e.g., encoding a fluorescent protein). To introduce DNA into S. japonicus, electroporation methods are recommended because S. japonicus is sensitive to lithium acetate (LiOAc).

Release of condensin from mitotic chromosomes requires the Ran-GTP gradient in the reorganized nucleus

ABSTRACT After mitosis, nuclear reorganization occurs together with decondensation of mitotic chromosomes and reformation of the nuclear envelope, thereby restoring the Ran-GTP gradient between the nucleus and cytoplasm. The Ran-GTP gradient is dependent on Pim1/RCC1. Interestingly, a defect in Pim1/RCC1 in Schizosaccharomyces pombe causes postmitotic condensation of chromatin, namely hypercondensation, suggesting a relationship between the Ran-GTP gradient and chromosome decondensation. However, how Ran-GTP interacts with chromosome decondensation is unresolved. To examine this interaction, we used Schizosaccharomyces japonicus, which is known to undergo partial breakdown of the nuclear membrane during mitosis. We found that Pim1/RCC1 was localized on nuclear pores, but this localization failed in a temperature-sensitive mutant of Pim1/RCC1. The mutant cells exhibited hypercondensed chromatin after mitosis due to prolonged association of condensin on the chromosomes. Conceivably, a condensin-dephosphorylation defect might cause hypercondensed chromatin, since chromosomal localization of condensin is dependent on phosphorylation by cyclin-dependent kinase (CDK). Indeed, CDK-phospho-mimic mutation of condensin alone caused untimely condensin localization, resulting in hypercondensed chromatin. Together, these results suggest that dephosphorylation of CDK sites of condensin might require the Ran-GTP gradient produced by nuclear pore-localized Pim1/RCC1. Summary: A mutant of Pim1/RCC1 caused hypercondensed chromatin after mitosis due to prolonged association of condensin on chromosomes, suggesting that dephosphorylation of CDK sites of condensin might require Ran-GTP after mitosis.