Budding in Yeast

5 Key excerpts on "Budding in Yeast"

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  eBook - ePub

  Yeast Cell Envelopes Biochemistry Biophysics and Ultrastructure

  Volume II

  • Leo H Arnold(Author)
  • 2018(Publication Date)
  • CRC Press

    (Publisher)

  Chapter 3 MOLECULAR BIOLOGY OF BUDDING M. L. Slater TABLE OF CONTENTS

  I.	Introduction
  II.	Bud Emergence
  A.  Genetic Control and Evolution
  B.  Ultrastructure
  C.  Nutrition
  III.	Bud Growth
  IV.	Septum Formation
  V.	Conclusions
  References

  I. INTRODUCTION

  The great majority of yeast species undergo vegetative reproduction by budding. In this chapter Saccharomyces cerevisiae is taken as the model because of the preponderance of data on this species. A perusal of the micrographs of Volume I, Chapter 2 , will give some idea of the number of untouched morphological problems associated with more exotic architecture.

  The molecular biology of budding is inherently interesting in terms of the underlying mechanisms which attend three dimensional changes in the cell envelope. Moreover, the yeast cell provides some unique features of eukaryotic cell division. For example, the great dissimilarity in mass between mother and separated daughter cell (under modest growth rates) has no parallel in organisms that divide by binary fission.

  1 ,2

  Elucidation of budding mechanisms can be approached with a variety of techniques which are primarily biochemical and genetic in scope. However, one should emphasize at the outset that the bud is identifiable by light microscopy and no other procedure is required in this regard. The bud is recognized by its shape rather than its composition, and that shape is intimately associated with the structure of the cell wall.3 The composition of the wall can explain its ability to maintain cell-shape, and consequently the distinction between the bud and the rest of the cell. But how the shape is determined is not so easily explained!

  The events of budding as observed in the light microscope are summarized diagrammatically in Figure 1 . Consider an unbudded cell (1A) in a growth-supporting medium. This cell first enlarges as an ellipsoid (1B) and then a bud emerges (1C). The bud continues to increase and finally a septum appears (1D). Separation follows and yields mother (IE) and daughter (IF) cells. The large mother cell can undergo another round via the shorter path indicated in Figure 1 , (i.e., E → C → D) whereas the daughter cell must first enlarge before a bud appears (i.e., it engages in the longer path (F → A → B → C → D). We can mention here, and develop later, that the unequal distribution of mass between mother and daughter at separation is dependent on growth rate. The difference is much less pronounced under maximal rates of growth.

  1 ,2

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  eBook - ePub

  Molecular Biology of Fungal Development

  • Heinz D. Osiewacz(Author)
  • 2002(Publication Date)
  • CRC Press

    (Publisher)

  In the yeast form (YF), cells divide by budding, followed by complete separation of the mature bud from the mother cell. Pseudohyphae (PH) originate from the budding of elongated cells that remain attached to each other, resulting in the formation of pseudohyphal filaments or pseudomycelium. Hyphae (H) and hyphal mycelium develop by continuous tip growth of hyphal cells, followed by fission of cells through the formation of septa. The molecular mechanisms underlying dimorphism are far from being understood in detail, despite the fact that a wealth of information on the cellular and molecular biology of model yeasts like S. cerevisiae has become available [ 5 ]. A profound understanding of yeast-mycelial dimorphism requires the molecular analysis of all growth forms of yeasts and of the signals and regulatory networks that control interconversion among them. In a first step, easily tractable model organisms serve as initial source of molecular information. At a later stage, however, molecular studies must include analysis of dimorphic yeast species from a broad range of taxa. 2 GROWTH FORMS OF YEAST 2.1 The Yeast Form By definition, yeasts prefer unicellular growth as their favorite mode of vegetative reproduction. The typical yeast cell reproduces by either budding, exemplified by S. cerevisiae, or fission, as in S. pombe [ 5 ]. Budding and fission both confer a unique relationship between cell growth and the sequence of events that constitute the cell division cycle. Budding is initiated by the emergence of a bud as a small protuberance from the cell surface (Fig. 1). Until cytokinesis, further growth is restricted to the bud. The bud lengthens in parallel to DNA synthesis (during S phase) and then swells to achieve its characteristic form during mitosis (M phase). Once the bud receives a daughter nucleus, it separates from the mother cell via septation and enters G1 as an independent daughter cell

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  eBook - ePub

  Yeast

  Molecular and Cell Biology

  • Horst Feldmann(Author)
  • 2012(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  7 Yeast Growth and the Yeast Cell Cycle

  7.1 Modes of Propagation

  As already briefly indicated, yeast can follow two modes of reproduction: (i) asexual budding , the most common mode of vegetative reproduction in yeasts, or (ii) mating of haploid cells of opposite mating-type that can propagate vegetatively or – under starving conditions – be induced to sporulate. In budding cells, the chromosomes are duplicated in a mitotic cycle, and distributed between mothers and daughters followed by cell separation, while sporulation involves meiosis to generate four (haploid) ascospores. Various unique aspects of these lifestyles of yeast, including budding, cell polarity, spindle formation, cytokinesis, cell division, and sporulation, have been intensively studied at the cellular and molecular levels.

  7.1.1 Vegetative Reproduction

  Budding is the most common mode of vegetative growth in yeasts and multilateral budding is a typical reproductive characteristic of ascomycetous yeasts, including Saccharomyces cerevisiae . The eukaryotic cell cycle involves both continuous events (cell growth) and periodic events (DNA synthesis and mitosis). Commencement and progression of these events in yeast can formally be distinguished into pathways for DNA synthesis and nuclear division, spindle formation, bud emergence and nuclear migration, and cytokinesis. However, from a molecular viewpoint these processes are intimately coupled.

  The cell cycle can be defined as the period between division of a mother cell and subsequent division of its daughter progeny. The regulatory mechanisms that order and coordinate the progress of the cell cycle have been intensely studied (overviews: Mata and Nurse, 1998; Futcher, 2000; Lauren et al. , 2001). The cell cycle (Figure 7.1 ) consists of two separable phases – interphase and mitosis. While in interphase three sections (G1 , S, and G2

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  eBook - ePub

  Fungal Biology

  • J. W. Deacon(Author)
  • 2013(Publication Date)
  • Wiley-Blackwell

    (Publisher)

  It occurs during G1 in S. cerevisiae and it is the stage where the cell integrates all the information from intracellular and environmental signals to determine whether the cell cycle will continue, or enter stationary phase, or the cell will undergo sexual reproduction (Chapter 5). Many cell division cycle (CDC) genes have been identified in the budding yeast S. cerevisiae, and homologous genes that regulate development have also been found in the distantly related fission yeast, Schizosaccharomyces pombe. Analysis of the genes and gene products of these two organisms has helped us to understand how cells establish their polarity of growth (Fig. 4.12). In budding yeast (sequence shown in Fig. 4.12a) the first bud develops at one of the poles of the cell. When this bud has developed and separated from the mother cell, the next bud arises at a point adjacent to the bud scar. It grows initially by polar growth (stage 4) but then by wall growth over most of the bud surface (stage 5). At the time of bud emergence, a “tag” or “landmark” is laid down at the site where a new bud will form, and a ring of septin proteins is deposited at this point. At a later stage of development (stage 6) the cellular machinery, including actin microfilaments, will direct vesicles and wall precursors to this site, resulting in the localized development of a septum to separate the daughter cell. The fission yeast, Schizosaccharomyces pombe (Fig. 4.12b), grows in a different way from Saccharomyces, because it produces cylindrical cells that extend at both ends, with tags at the poles of the cell. When the cell has reached a critical volume the nucleus divides, and tags then direct the production of a septum at the site where the cell will divide (stages 5–7). Both Saccharomyces and Schizosaccharomyces can undergo mating under the influence of pheromones produced by cells of opposite mating types, so this represents yet another developmental stage involving the establishment of polar sites (Fig

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  eBook - ePub

  The Chemistry and Biology of Winemaking

  • Ian S Hornsey(Author)
  • 2015(Publication Date)
  • Royal Society of Chemistry

    (Publisher)

  25 After the initiation of START, environmental factors, such as external stress and poor nutrient levels, are no longer able to prevent cell division. Completion of START requires that a cell grows beyond a minimum size, and commits that cell to a round of mitotic division, as opposed to alternative developmental fates, such as conjugation, or, for diploid cells, meiosis. Commitment to mitotic division sets in train the pathways required for the initiation of DNA synthesis and the transition to S-phase. If nutrients are in sufficient supply (they are rarely a limiting factor in most stages of a wine fermentation), the S-phase is entered, and, in addition to embarking upon DNA synthesis, an embryo bud is initiated. After the completion of DNA synthesis, and entry into phase G2, extensive bud growth ensues, and nuclear events culminate in mitosis (M-phase). The final steps of the cell cycle are septation, which separates the cytoplasm of mother and daughter cells (cytokinesis), and total separation of the two cells, although complete separation is not a prerequisite for continued reproduction. Ineffective mother–daughter cell separation can result in chains of cells being formed, which is a characteristic phenotype of some individual strains. Total separation of the bud leaves a permanent mark on the mother cell, the bud scar and a temporary birth mark on the daughter cell. Some workers recognise a fifth phase to the cell cycle, G0, which represents cells in a state of ‘suspended animation’ (see below).

  Figure 2 The cell cycle of Saccharomyces cerevisiae (

  after Harold24

  )(Reproduced by kind permission of the Society for General Microbiology)

  The genesis of a bud begins with the selection of a locus on the surface of the mother cell, where outgrowth will later take place. Placement of the bud is governed by a set of rules, which can be predicted from the genotype; for example: haploid ‘α’ and ‘a’ bud axially, as do homozygous diploids, but heterozygous diploids (‘αa’) bud in a bipolar manner. When the markers are activated, they induce a localised assembly of an annular structure that marks the new bud initial; this includes a ring of chitin, another of 10 nm neck filaments and several additional proteins. The daughter bud normally emerges within this ring. A newborn yeast cell grows and deposits cell wall polysaccharides uniformly all over its surface until it attains a critical size, whence it initiates a bud; thereafter, deposition of new wall material is confined to the growing bud. Expansion of the bud wall is patterned in space and in time: to begin with, growth takes place chiefly at the tip of the forming bud, then it becomes uniformly distributed and eventually expansion halts. Expansion of the wall is a secretory process, membrane-bound vesicles, which seem to originate in the Golgi apparatus, playing a prominent role. It is generally understood that these vesicles carry precursors and enzymes, such as mannosylated proteins, chitin synthase and β-glucan synthase, for wall biosynthesis. When the vesicles reach the plasmalemma, in the growth area, they fuse and undergo exocytosis. It appears that the prominent cables of actin microfilaments, that course through the mother cell cytoplasm and reach the bud, serve as ‘tracks’ upon which wall precursor vesicles move towards the site of exocytosis. The cables seem to steer the vesicles to specific fusion sites, which appear as prominent plaques on the plasmalemma that contain actin and actin-binding proteins. Figure 3

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