- First isolated from an East African beer variety (Swahili: pombe, beer)
- Genome: 12.6Mbp, 5400 genes located on 3 chromosomes
- Nitrogen starvation is required for mating and meiosis in Schizosaccharomyces pombe.
- Yeast, Volume 14, Issue 16 , Pages 1529 - 1566
Similarity to yeast
- Undergoes differentiation during which opposite mating types communicate by pheromones.
- h+ and h- complementary mating types which produce P and M factors.
- S. pombe is a haploid organism. Only starvation can induce mating and meiosis of diploid cells.
Consequence of coregualtion of mating and meiosis is that both require a pheromone signal. Mat1-Pm gene (required for entry into meiosis) is transcribed in reponse to pheromone signal. The expression state of the first class of genes is determined by the cell type, that is whether the mat1-Pc or the mat1-Mc gene product is present in the cell. The first type of differentiation generates two different cell types (or mating types), called P and M, respectively. A population of S. pombe cells is a mixture of these two types, because the cells frequently switch from P to M and from M to P when dividing. The second differentiation process is activated when the cells become starved for nutrients. This induces mating between a P and an M cell to generate a zygote, which subsequently enters meiosis and sporulates.
Certain components of fission yeast pheromone signalling system resemble the ones found in budding yeast:
- M factor has similar chemical structure to alpha factor
- P is unknown?
Summary of sexual differentiation:
- As long as nutrients are sufficient, the cells grow by haploid mitosis.
- Under conditions of nutritional starvation, mating is induced between the two haploid cell types, P and M, resulting in the generation of a diploid zygote.
- The zygote subsequently undergoes meiosis and sporulation, giving rise to an ascus with four haploid spores.
Mating pathway components
Rgs1 (Regulator of G-protein Signalling (RGS) family) is a Ste11 target gene. Rgs1 expression requires both an Ste11-mediated nitrogen starvation signal and the pheromone-induced activation of the Byr2/Byr1/Spk1 MAPK pathway.
Endogenous Rgs1 tagged with GFP localizes to the nucleus and cytoplasm, and this localization pattern is not altered during pheromone treatment. Importantly, Rgs1 function requires its C-terminal RGS domain, as well as a central DEP domain and a novel homology domain present in its N-terminal region (Fungal-DR domain).
Rgs1 negatively regulates pheromone signalling during mating, acting in a negative feedback loop that is essential for the mating process.
The ste8+ gene encodes a 659 amino acid putative protein kinase, which is identical to the previously identified byr2 suppressor of the ras1 defect ("Bypass of Yeast Ras").
In fission yeast, the mat1-Pm gene, which is required for entry into meiosis, is expressed in response to a pheromone signal. Ste8 is required for mat1-Pm transcription.
Ste8 shares homology with several Ser/Thr kinases. Ste8 has 32% sequence identity to Ste11. Sequence similarity is much more extensive in C-terminal protein kinase domain. A large region of 33 AA between protein kinase domains II and III seems to be specific to Ste11 (absent from Ste8 and other kinases). High degree of sequence similarity between protein kinase domains implies functional homology.
Expression of the S. cerevisiae STE11 gene in S. pombe ste8 mutants restores the ability to transcribe mat1-Pm in response to pheromone. Complementation is complete: both conjugation and sporulation obeserved. Ste11 causes strong induction of mat1-Pm transcription in response to N-starvation (even stronger than in wild type cells).
Ste11 has been proposed to be a positive tanscriptional regulator of genes induced in N starvation. It positively regulates transcription of several genes in sexual pathway and mei2 gene. 10bp motif (TR box) upstream of these genes binds Ste11. Ste8 in Ste11- is not induced in reponse to N starvation.
MAP kinase kinase kinase acting upstream of the MAPKK Byr1p and the MAP kinase Spk1p in the pheromone signaling pathway.
- aka Ste1
- a MAPKK
- paralog of Skh1 MEK kinase from mkh signalling pathway (Mkh1-Spm1 MAP kinase cascade)? 
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