Download MAT

• Lecture 2: Read about the yeast MAT locus in
Molecular Biology of the Gene. Watson et al. Chapter
10. Plus section on yeast as a model system
• Read chapter 22 and chapter 10 [section on MATing
type gene expression] in Molecular Cell Biology by
Lodish et al [http://www.ncbi.nlm.nih.gov/books/bv.fcgi?rid=mcb]
here it is section 14.1]
Yeasts of opposite mating types
can MATE
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Yeast cells can proliferate both as
haploids (1n) and as diploids (2n); 2n
cells are 1.2-fold bigger
Haploid cells have one of two mating
types:
a or alpha
Two haploid cells can mate to form a
zygote; since yeast cannot move, cells
must grow towards each other (shmoos)
The diploid zygote starts dividing from the
junction
Under nitrogen starvation diploid cells
undergo meiosis and sporulation to form
an ascus with four haploid spores
Thus, although yeast is unicellular, we
can distinguish different cell types with
different genetic programmes:
– Haploid MATa versus MATalpha
– Haploid versus Diploid
(MATa/alpha)
– Spores
– Mothers and daughters
Mating
• Haploid cells
produce specific
mating factors, which
bind to specific cellsurface receptors
• Changes gene
expression
• Induces cell fusion,
which produces a
diploid cell
Yeast sexual cycle
• Caught in the act: cell attachment,
cell fusion and nuclear fusion in an
electron micrograph
• Haploid cells produce mating
peptide pheromones, i.e. a-factor
and alpha-factor, to which the
mating partner responds to prepare
for mating
Budding and Schmooed cells
(A) Cells of Saccharomyces cerevisiae are usually spherical.
(B) They become polarized when treated with mating factor
from cells of the opposite mating type. The polarized cells
are called “shmoos.” (C) Al Capp's famous cartoon character,
the original Shmoo. (A and B, courtesy of Michael Snyder; C,
© 1948 Capp Enterprises, Inc., all rights reserved.)
Yeast sexual cycle
• Central to sexual communication is
the pheromone response pathway
• This pathway is a complex system
that controls the response of yeast
cells to a- or alpha-factor
• All blocks of that pathway consist
of components conserved from
yeast to human
• The pathway consists of a specific
pheromone receptor, that binds aor alpha-factor
• Binding stimulates a signalling
pathway that causes cell cycle
arrest and coordinates expression
of genes important for mating
• The pathway is also needed to
orient the cell towards its mating
Pheromone-induced signalling
Cytoskeleton changes to all cell fusion
Transcriptional changes to
promote mating
Mating-type (MAT) locus
A major genetic determinant of mating is
the MAT LOCUS situated on
chromosome III
Two alleles that determine mating type
• MAT
• MATa
How do these alleles determine mating
type?
Mating-type allele switching
• Mating type is determined by which
allele (a or ) is in the MAT locus
• Mating-type specific allele (a or ) is
introduced into MAT by recombination
• Silent (not transcribed) copies of each
allele are “stored” at HML and HMRa
Genetic determination of yeast
cell type
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The mating type locus encodes regulatory proteins, i.e. transcription
factors
The MATa locus encodes the a1 transcriptional activator (a2 has no
known function) The a2 null also shows no growth, mating or sporulation
defect
The MATalpha locus encodes the alpha1 activator and the alpha2
repressor
The mating type locus functions as a master regulator locus: it controls
expression of many genes
Transcription factors
• MAT locus alleles encodes cell-specific
transcription factors
– MAT encodes Mat 1 and Mat 2
– MATa encodes Mata1
• MCM1 encodes a general transcription
factor found in haploid and diploid cells.
Regulation of cell-specific
genes
• Mat 1 (and Mcm1) activate -specific
genes
• Mat 2 (and Mcm1) repress a-specific
genes
• Mata1 and Mat 2 repress haploidspecific genes (no role for Mata1in
haploid)
cell
Mat 1 and Mat 2 produced
• Mat 1 and Mcm1 activate -specific
genes
• Mat 2 and Mcm1 repress a-specific
genes
a cell
Mata1 produced
• Mcm1 activates a-specific and haploidspecific genes
a/ cell (diploid)
• Mat 2 and Mcm1 repress a-specific
genes
• Mata1 and Mat 2 repress haploidspecific genes and repress MAT 1
Cell-specific genes
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-specific genes are those needed to
produce alpha-factor and the gene for the afactor receptor
• a-specific genes are those needed for afactor production and the gene for the alphafactor receptor.
• Haploid-specific genes include the RME gene
encoding the meiosis repressor and the HO
endonuclease (which catalyzes switching)
• and those that encode proteins involved in
the response to pheromone
Some Transcription factors
can act as activators or
repressors
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MCM1 is a yeast transcriptional
activator with a DNA-binding
domain shared by other
members of the MADS-box
family, including SRF.
MCM1 binds cooperatively with
the MATalpha1 protein to
activate transcription of the
alpha-specific genes,
or with the MATalpha2 protein
to repress transcription of the aspecific genes.
Haploids can switch mating
type!
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Haploid cells can switch their mating type, i.e. from a to alpha or from
alpha to a
This is due to two silent mating type loci on the same chromosome,
which become activated when translocated to the MAT locus
The translocation is a gene conversion event initiated by the HO
nuclease
The switch ensures that in Nature yeast cells return quickly to the
diploid, more stable stage
Laboratory yeast strains lack the HO nuclease and hence have stable
haploid phases
Mating Type Switching in
Yeast
HO endonuclease
• Mating-type switching involves
recombination of a transcriptionally
silent allele into a transcriptionally active
locus
• Recombination requires double-strand
break at MAT catalyzed by HO
endonuclease in a process of gene
conversion.
Mating-type switching
• Cells switch
mating type
(almost)
every
generation
• Only mother
cells (not
buds) switch
Ash1p is restricted to
daughter cells due to
mRNA localisation
Myosin fibrils locate Ash mRNA to the bud
Transcriptional silencing
How are the HML and HMRa loci kept
transcriptionally silent?
• Chromatin structure of HML and
HMRa and the surrounding region is
condensed (heterochromatin)
• Histones are deacetylated
• Blocks transcriptional machinery access