Schizosaccharomyces japonicus: A Distinct Dimorphic Yeast among the Fission Yeasts

Divergence of cytokinesis and dimorphism control by myosin II regulatory light chain in fission yeasts

Non-muscle myosin II activation by regulatory light chain (Rlc1Sp) phosphorylation at Ser35 is crucial for cytokinesis during respiration in the fission yeast Schizosaccharomyces pombe. We show that in the early divergent and dimorphic fission yeast S. japonicus non-phosphorylated Rlc1Sj regulates the activity of Myo2Sj and Myp2Sj heavy chains during cytokinesis. Intriguingly, Rlc1Sj-Myo2Sj nodes delay yeast to hyphae onset but are essential for mycelial development. Structure-function analysis revealed that phosphorylation-induced folding of Rlc1Sp α1 helix into an open conformation allows precise regulation of Myo2Sp during cytokinesis. Consistently, inclusion of bulky tryptophan residues in the adjacent α5 helix triggered Rlc1Sp shift and supported cytokinesis in absence of Ser35 phosphorylation. Remarkably, unphosphorylated Rlc1Sj lacking the α1 helix was competent to regulate S. pombe cytokinesis during respiration. Hence, early diversification resulted in two efficient phosphorylation-independent and -dependent modes of Rlc1 regulation of myosin II activity in fission yeasts, the latter being conserved through evolution.

A high-quality reference genome for the fission yeast Schizosaccharomyces osmophilus

Fission yeasts are an ancient group of fungal species that diverged from each other from tens to hundreds of million years ago. Among them is the preeminent model organism Schizosaccharomyces pombe, which has significantly contributed to our understandings of molecular mechanisms underlying fundamental cellular processes. The availability of the genomes of S. pombe and 3 other fission yeast species S. japonicus, S. octosporus, and S. cryophilus has enabled cross-species comparisons that provide insights into the evolution of genes, pathways, and genomes. Here, we performed genome sequencing on the type strain of the recently identified fission yeast species S. osmophilus and obtained a complete mitochondrial genome and a nuclear genome assembly with gaps only at rRNA gene arrays. A total of 5,098 protein-coding nuclear genes were annotated and orthologs for more than 95% of them were identified. Genome-based phylogenetic analysis showed that S. osmophilus is most closely related to S. octosporus and these 2 species diverged around 16 million years ago. To demonstrate the utility of this S. osmophilus reference genome, we conducted cross-species comparative analyses of centromeres, telomeres, transposons, the mating-type region, Cbp1 family proteins, and mitochondrial genomes. These analyses revealed conservation of repeat arrangements and sequence motifs in centromere cores, identified telomeric sequences composed of 2 types of repeats, delineated relationships among Tf1/sushi group retrotransposons, characterized the evolutionary origins and trajectories of Cbp1 family domesticated transposases, and discovered signs of interspecific transfer of 2 types of mitochondrial selfish elements.

Specific Functional Features of the Cell Integrity MAP Kinase Pathway in the Dimorphic Fission Yeast Schizosaccharomyces japonicus

Mitogen activated protein kinase (MAPK) signaling pathways execute essential functions in eukaryotic organisms by transducing extracellular stimuli into adaptive cellular responses. In the fission yeast model Schizosaccharomyces pombe the cell integrity pathway (CIP) and its core effector, MAPK Pmk1, play a key role during regulation of cell integrity, cytokinesis, and ionic homeostasis. Schizosaccharomyces japonicus, another fission yeast species, shows remarkable differences with respect to S. pombe, including a robust yeast to hyphae dimorphism in response to environmental changes. We show that the CIP MAPK module architecture and its upstream regulators, PKC orthologs Pck1 and Pck2, are conserved in both fission yeast species. However, some of S. pombe's CIP-related functions, such as cytokinetic control and response to glucose availability, are regulated differently in S. japonicus. Moreover, Pck1 and Pck2 antagonistically regulate S. japonicus hyphal differentiation through fine-tuning of Pmk1 activity. Chimeric MAPK-swapping experiments revealed that S. japonicus Pmk1 is fully functional in S. pombe, whereas S. pombe Pmk1 shows a limited ability to execute CIP functions and promote S. japonicus mycelial development. Our findings also suggest that a modified N-lobe domain secondary structure within S. japonicus Pmk1 has a major influence on the CIP signaling features of this evolutionarily diverged fission yeast.

Quorum sensing and stress-activated MAPK signaling repress yeast to hypha transition in the fission yeast Schizosaccharomyces japonicus

Quorum sensing (QS), a mechanism of microbial communication dependent on cell density, governs developmental decisions in many bacteria and in some pathogenic and non-pathogenic fungi including yeasts. In these simple eukaryotes this response is mediated by the release into the growth medium of quorum-sensing molecules (QSMs) whose concentration increases proportionally to the population density. To date the occurrence of QS is restricted to a few yeast species. We show that a QS mediated by the aromatic alcohols phenylethanol and tryptophol represses the dimorphic yeast to hypha differentiation in the fission yeast S. japonicus in response to an increased population density. In addition, the stress activated MAPK pathway (SAPK), which controls cell cycle progression and adaptation to environmental changes in this organism, constitutively represses yeast to hypha differentiation both at transcriptional and post-translational levels. Moreover, deletion of its main effectors Sty1 MAPK and Atf1 transcription factor partially suppressed the QS-dependent block of hyphal development under inducing conditions. RNAseq analysis showed that the expression of nrg1⁺, which encodes a putative ortholog of the transcription factor Nrg1 that represses yeast to hypha dimorphism in C. albicans, is downregulated both by QS and the SAPK pathway. Remarkably, Nrg1 may act in S. japonicus as an activator of hyphal differentiation instead of being a repressor. S. japonicus emerges as an attractive and amenable model organism to explore the QS mechanisms that regulate cellular differentiation in fungi.

Quorum sensing is a relevant mechanism of communication dependent on population density that controls cell development and pathogenesis in microorganisms including fungi. We describe a quorum sensing mediated by the release of aromatic alcohols in the growth medium that blocks hyphal development in the fission yeast Schizosaccharomyces japonicus. This is the first description of such a mechanism in the fission yeast lineage, and confirms its expansion along Ascomycota fungi. The stress-responsive pathway (SAPK), which regulates fungal growth and differentiation, limits hyphal growth in S. japonicus in a constitutive fashion, and nonfunctional SAPK mutants are partially insensitive to quorum sensing and able to form hyphae in high cell density cultures. Nrg1, an important factor that blocks hyphal development in the pathogen Candida albicans, activates hyphal growth in S. japonicus, and its expression is counteracted by both quorum sensing and the SAPK pathway. Nrg1 function may thus have diverged evolutionary in this organism from being a repressor to an activator of hyphal development. S. japonicus emerges as a suitable model organism to explore the intricate mechanisms regulating fungal differentiation.

Yeast-to-hypha transition of Schizosaccharomyces japonicus in response to environmental stimuli

Many fungal species are dimorphic, exhibiting both unicellular yeast-like and filamentous forms. Schizosaccharomyces japonicus, a member of the fission yeast clade, is one such dimorphic fungus. Here, we first identify fruit extracts as natural, stress-free, starvation-independent inducers of filamentation, which we use to describe the properties of the dimorphic switch. During the yeast-to-hypha transition, the cell evolves from a bipolar to a unipolar system with 10-fold accelerated polarized growth but constant width, vacuoles segregated to the nongrowing half of the cell, and hyper-lengthening of the cell. We demonstrate unusual features of S. japonicus hyphae: these cells lack a Spitzenkörper, a vesicle distribution center at the hyphal tip, but display more rapid cytoskeleton-based transport than the yeast form, with actin cables being essential for the transition. S. japonicus hyphae also remain mononuclear and undergo complete cell divisions, which are highly asymmetric: one daughter cell inherits the vacuole, the other the growing tip. We show that these elongated cells scale their nuclear size, spindle length, and elongation rates, but display altered division size controls. This establishes S. japonicus as a unique system that switches between symmetric and asymmetric modes of growth and division.

The Ras1-Cdc42 pathway is involved in hyphal development of Schizosaccharomyces japonicus

Dimorphic yeasts transform into filamentous cells or hyphae in response to environmental cues. The mechanisms for the hyphal transition of dimorphic yeasts have mainly been studied in Candida albicans, an opportunistic human fungal pathogen. The Ras1-MAPK pathway is a major signal transduction pathway for hyphal transition in C. albicans. Recently, the non-pathogenic dimorphic yeast Schizosaccharomyces japonicus has also been used for genetic analyses of hyphal induction. We confirmed that Ras1-MAPK and other MAPK pathways exist in Sz. japonicus. To examine how hyphal transition is induced by environmental stress-triggered signal transduction, we studied the hyphal transition of deletion mutants of MAPK pathways in Sz. japonicus. We found that the MAPK pathways are not involved in hyphal induction, although the mating response is dependent on these pathways. However, only Ras1 deletion caused a severe defect in hyphal development via both DNA damage and environmental stressors. In fact, genes on the Cdc42 branch of the Ras1 (Ras1-Cdc42) pathway, efc25Sj, scd1Sj and scd2Sj, are required for hyphal development. Cell morphology analysis indicated that the apical growth of hyphal cells was inhibited in Ras1-Cdc42-pathway deletion mutants. Thus, the control of cell polarity by the Ras1-Cdc42 pathway is crucial for hyphal development.

Introduction to Fission Yeast as a Model System

Here, we briefly outline the history of fission yeast, its life cycle, and aspects of its biology that make it a useful model organism for studying problems of eukaryotic molecular and cell biology.