18 REARRANGEMENT OF DNA (Full Edition)
2 The mating pathway is triggered by pheromone-receptor interactions
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- The mating type is a property of haploid yeast that makes it able to fuse to form a diploid only with a cell of the opposite mating type.
- A pheromone is a small molecule secreted by one mating type of an organism in order to interact with a member of the opposite mating type.
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Yeast of a given mating type secrete a small polypeptide that binds to a receptor on cells of the opposite mating type.
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The yeast S. cerevisiae can
propagate in either the haploid or diploid condition. Conversion
between these states takes place by mating (fusion of haploid spores to
give a diploid) and by sporulation (meiosis of diploids to give haploid
spores). The ability to engage in these activities is determined by the
mating type of the strain.
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Figure 18.1
Mating type controls several activities.
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The properties of the two mating types are summarized in Figure 18.1.
We may view them as resting on the rationale that there is no point in
mating unless the haploids are of different genetic types; and
sporulation is productive only when the diploid is heterozygous and
therefore able to generate recombinants.
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The mating type of a (haploid) cell is determined by the genetic information present at the MAT locus.
Cells that carry the MATa allele at this locus are type a;
likewise, cells that carry the MATα
allele are type α.
Cells of opposite types can mate; cells of the same type cannot.
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Recognition of cells of opposite mating type is
accomplished by the secretion of pheromones.
α cells secrete the small
polypeptide α-factor;
a cells secrete a-factor.
The α-factor is a
peptide of 13 amino acids; the a-factor
is a peptide of 12 amino acids that is modified by addition of a
farnesyl (lipid-like) group and carboxymethylation. Each of these
peptides is synthesized in the form of a precursor polypeptide that is
cleaved to release the mature peptide sequence.
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A cell of one mating type carries a surface receptor for
the pheromone of the opposite type. When an a cell and an
α
cell encounter one another, the pheromones bind to their receptors to
arrest the cells in the G1 phase of the cell cycle, and various
morphological changes occur. In a successful mating, the cell cycle
arrest is followed by cell and nuclear fusion to produce an a/α
diploid cell.
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Figure 18.2
The yeast life cycle proceeds through mating of MATa and MATα haploids to give heterozygous diploids that sporulate to generate haploid spores.
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The a/α
cell carries both the MATa and MATα
alleles and has the ability to sporulate. Figure 18.2
demonstrates how this design maintains the normal haploid/diploid life
cycle. Note that only heterozygous diploids can sporulate; homozygous
diploids (either a/a or α/α)
cannot sporulate.
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Much of the information about the yeast
mating type pathway was deduced from the properties of mutations that
eliminate the ability of a and/or α cells to mate.
The genes identified by such mutations are called STE (for sterile).
Mutations in the genes STE2 and STE3 are specific for individual mating types;
but mutations in the other STE genes eliminate mating in both a
and α
cells. This situation is explained by the fact that the events that
follow the interaction of factor with receptor are identical for both
types (for review see 172).
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Mating is a symmetrical process that is
initiated by the interaction of pheromone secreted by one cell type
with the receptor carried by the other cell type. The only
genes that are uniquely required for the response pathway in a
particular mating type are those coding for the receptors.
Either the a factor-receptor or the α
factor-receptor interaction switches on the same response pathway.
Mutations that eliminate steps in the common pathway have the same
effects in both cell types (584).
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172 Nasmyth, K.
(1982).
Molecular genetics of yeast mating type.
Annu. Rev. Genet. 16, 439-500.
PubMed Journal
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584 Bender, A. and Sprague, G. F., Jr.
(1986).
Yeast peptide pheromones, a-factor and alpha-factor, activate a common response mechanism in their target cells.
Cell 47, 929-937.
PubMed Journal
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©Jones and Bartlett Publishers (2007)
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