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Chapter 9 : Establishment of Cell Identity in Pathogenic Fungi

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Abstract:

This chapter examines how cell identity influences mating-type determination, particularly in fungal pathogens. It explores cases where cell identity plays roles outside of mating type, affects cell morphology, and influences pathogenesis. The chapter begins with a description of cell type determination in the budding yeast and uses this as a platform for exploring mechanisms in the human fungal pathogens and as well as the plant fungal pathogen . It concludes with a short description of the influence of cell identity on the behaviors of several other plant and human fungal pathogens and how cell identity in fungi is evolving to encompass a more diverse array of fungal behaviors. A fungal pathogen in which cell identity determination has come to the fore is in the basidiomycete fungus . There are two obvious possibilities for specifying haploid cell identity: either the pheromone and pheromone receptor alleles in each mating type have diverged from one another sufficiently to confer pheromone-receptor binding specificity, or there are other factors or mechanisms at play in determining haploid cell types. The authors will continue to discover new strategies undertaken by the pathogenic fungi to develop and integrate multiple cell identities in the effort to survive.

Citation: Hull C. 2006. Establishment of Cell Identity in Pathogenic Fungi, p 133-141. In Heitman J, Filler S, Edwards, Jr. J, Mitchell A (ed), Molecular Principles of Fungal Pathogenesis. ASM Press, Washington, DC. doi: 10.1128/9781555815776.ch9

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Restriction Fragment Length Polymorphism
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Figures

Image of Figure 1.
Figure 1.

Cell type determination in (A) The mating-type () locus of contains two alleles (<1 kb). from cells encodes the homeodomain regulator 1. α from α cells encodes the α-domain protein α1 and the homeodomain protein α2. In diploid cells both and α are present. (B) The transcriptional regulatory circuit of In cells the protein 1 is produced but has no known effect, -specific genes (sg) are constitutively active, and the α-specific genes (αsg) are not expressed. In α cells, α2 represses -specific genes and α1 activates α-specific genes, thus establishing the α cell type. In /α diploid cells, α2 continues to repress -specific genes, but it also interacts with 1 to form a transcriptional regulatory complex that represses haploid-specific genes (hsg), including α1. This repression leads to the specification of the diploid cell type, making further sexual development (i.e., meiosis and sporulation) possible.

Citation: Hull C. 2006. Establishment of Cell Identity in Pathogenic Fungi, p 133-141. In Heitman J, Filler S, Edwards, Jr. J, Mitchell A (ed), Molecular Principles of Fungal Pathogenesis. ASM Press, Washington, DC. doi: 10.1128/9781555815776.ch9
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Image of Figure 2.
Figure 2.

Cell type determination in (A) The mating-type-like ) locus of (~9 kb) contains transcriptional regulators and three other gene types. encodes the homeodomain protein 1 and the HMG box protein 2. encodes the α-domain protein α1 and the homeodomain protein α2. Both and α contain diverged alleles of a poly(A) polymerase (), an oxysterol binding protein (), and a phosphotidylinositol kinase (). The roles of , , and in cell type determination are not known. (B) The transcriptional circuit of Cells containing only information (/ or /α at the locus) mate as cells because the predicted transcription factor 2 activates genes required for -type mating. Cells containing only α information (α/α or Δ/α at the locus) mate as α cells because the predicted transcription factor α1 activates genes required for α-type mating. Cells containing both and α information (/α at the locus) do not mate with other cells because 1 and α2 work in concert to repress genes required for mating and white-opaque switching. Cells that cannot switch from white to opaque cannot mate, and this switch is repressed by 1-α2.

Citation: Hull C. 2006. Establishment of Cell Identity in Pathogenic Fungi, p 133-141. In Heitman J, Filler S, Edwards, Jr. J, Mitchell A (ed), Molecular Principles of Fungal Pathogenesis. ASM Press, Washington, DC. doi: 10.1128/9781555815776.ch9
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Image of Figure 3.
Figure 3.

Cell type determination in (A) There are two independent, unlinked mating-type loci in One locus () contains the cell-type-specific pheromone gene and pheromone receptor gene in one allele (, ~4.6 kb), and the other allele (, ~8.5 kb) contains the pheromone gene and pheromone receptor gene also contains and , whose products show no similarity to other proteins and play no apparent role in mating but do seem to affect pathogenesis by controlling mitochondrial function ( ). The second locus (, ~3.6 kb) encodes the homeodomain proteins bE and bW, of which there are 25 different forms, resulting in many different mating types in a tetrapolar mating-type system. (B) Regulation of cell identity in Half circles represent haploid cell surfaces. Haploid cell types are specified by the expression of (●) and (forked receptor on cell surface) or and (round receptor on cell surface). Soluble pheromones are sensed by surface receptors, activating cell fusion when two cells of the opposite mating type encounter one another. After cell fusion, the components of the locus (bE and bW) interact with one another in specific combinations (e.g., bW1-bE2) to indicate the dikaryotic state and make the cells competent for further sexual development in corn plants.

Citation: Hull C. 2006. Establishment of Cell Identity in Pathogenic Fungi, p 133-141. In Heitman J, Filler S, Edwards, Jr. J, Mitchell A (ed), Molecular Principles of Fungal Pathogenesis. ASM Press, Washington, DC. doi: 10.1128/9781555815776.ch9
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Image of Figure 4.
Figure 4.

Cell type determination in (A) The mating-type () locus of is unusually large (>100 kb) and contains many genes whose functions in cell type determination are unknown. A schematic of the locus for var. (serotype D) is shown. encodes three copies of the MF pheromone, the predicted pheromone receptor Ste3, and the homeodomain protein Sxi2. encodes three copies of the MFα pheromone, the predicted pheromone receptor Ste3α, and the homeodomain protein Sxi1α. Fifteen additional genes have related but diverged alleles in both and α. (B) Regulation of cell identity in The determinants of haploid cell identity are unknown; however, a likely mechanism is the expression of cell-type-specific pheromones and receptors. The expression of and the pheromone receptor could specify the mating type, and the expression of α and the pheromone receptor could specify the α mating type. Soluble pheromones would be sensed by surface receptors, activating cell fusion when two cells of the opposite mating type encounter one another. After cell fusion, the homeodomain proteins Sxi1α and Sxi2 are predicted to interact with one another and regulate transcription through an unknown mechanism to establish the dikaryotic state. The newly designated dikaryon is then competent to undergo extended filamentous growth, meiosis, and sporulation.

Citation: Hull C. 2006. Establishment of Cell Identity in Pathogenic Fungi, p 133-141. In Heitman J, Filler S, Edwards, Jr. J, Mitchell A (ed), Molecular Principles of Fungal Pathogenesis. ASM Press, Washington, DC. doi: 10.1128/9781555815776.ch9
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