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Category: Fungi and Fungal Pathogenesis
The mat Genes of Schizosaccharomyces pombe: Expression, Homothallic Switch, and Silencing, Page 1 of 2
< Previous page | Next page > /docserver/preview/fulltext/10.1128/9781555815837/9781555814212_Chap08-1.gif /docserver/preview/fulltext/10.1128/9781555815837/9781555814212_Chap08-2.gifAbstract:
Schizosaccharomyces pombe (fission yeast) is essentially a haploid organism. The cylindrical cells grow at the poles and divide symmetrically at a central septum. This chapter reviews the current understanding of the fission yeast mating-type system. Facilitated by the repetitive arrangement of homology boxes at three closely spaced subloci, the mat region of S. pombe is prone to genetic rearrangements, which can yield various types of heterothallic derivatives. The activation of mating-type genes requires profound changes in the transcriptional program in prestationary cells. P-factor is a 23-amino-acid unmodified peptide, made by proteolytic cleavage while a larger precursor protein is transported along the conventional secretory pathway. Pheromone signaling induces transcription of a large number of genes required for mating and meiosis. The nutritional state is being monitored by additional 7TM-protein-coupled receptor components. More recently, the biased guidance of strand invasion has been attributed to a mat-specific protein complex, Swi5-Swi2, assumed to be equivalent to the Swi5-Sfr1 complex required for the homology search in general recombination. The donor cassettes are embedded in a 20-kb stretch of heterochromatin, which is devoid of coding genes other than the internal parts of mat2-M and mat3-P themselves. The critical inducer of meiosis in S. pombe is the RNA-binding Mei2 protein, but the mechanism of Mei2 action is still unclear. The tight coupling to ascospore formation puts meiosis in a critical position as to long-term survival in a dormant state.
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The life cycle of S. pombe. Vegetative cells are predominantly haploid (1n). Rarely occurring diploid growth of zygotes (2n) can be selected for in the laboratory. Homothallic switch of mating type (P ↔ M) occurs during vegetative growth.
Cassette organization in the mat region of S. pombe. (A) Overall arrangement on the right arm of chromosome 2 (not to scale); (B) open reading frames at mat1-P and mat1-M; (C) cis-acting elements at mat1-M (!, site of imprint); (D) overall organization of the silent mat2/3 domain; (E) local organization of mat2-P and mat3-M.
Heterothallic strains by rearrangement (spacer regions not to scale). The wild-type configuration of the homothallic h 90 strain (middle lane) can lead to h -S by deletion (Del), or h +N by insertion (In), between homology boxes of different mat cassettes. The h +N insertion results from aberrant switching events (see Fig. 8.5 C). Silent cassettes are shaded; imprintable sites to initiate mating-type switches are indicated (!).
The cycle of homothallic mating-type switching in S. pombe, passing four stages of asymmetric cell division (Pu → Pu + Ps; Ps → Ps + Mu; Mu → Mu + Ms; Ms → Ms + Pu). When switchable cells divide (Ps or Ms), one of their respective daughters will have switched mating type. On depleted medium, this allows conjugation and ascospore formation with the unswitched sister cell. If unswitchable cells divide (Pu or Mu), both daughter cells retain the same mating type and are not mutually inhibited by mating pheromones, even though one of these becomes imprinted. This leads to a single switch in the subsequent quadruplet of cousin cells.
Mating-type switching at the imprinted mat1-P cassette. (A) A replication fork approaching from the left is halted outside mat1 at RTS1. The leading strand approaching from the right is halted at the imprint (!). (B) Facilitated by various recombinational repair factors, the leading 3′ end is liberated from H1 at mat1, swapping templates with H1 at mat3-M (i). Repair synthesis pursues throughout mat3-M, to be terminated at the H2/H3 boundary (ii). The processed 3′ end reenters mat1 at H2 (iii), to be joined with the arrested lagging strand at RTS1. This strand is duplicated immediately thereafter (not shown), to complete the newly formed mat1-M cassette. The intact strand of the resident mat1-P cassette is duplicated as well and imprinted anew (not shown), whereas the previously imprinted strand is degraded. (C) If switching of mat1-P to M fails to be terminated at the H2/H3 boundary of mat3-M, repair synthesis continues throughout the entire K region, only to be resolved at the H2/H3 boundary of mat2-P. This aberrant switching event results in the heterothallic h +N configuration.
Cell-type-specific genes, controlled by the mat gene products