GENETICS 301
LECTURE 4
Yeast Genetics
-in the wild, budding yeast exist as diploids
-when starved of certain nutrients, the diploids undergo meiosis
and sporulate producing haploid spores of two mating types MATa
and MATα
−MATa cells secrete a-pheromone and MATα cells secrete alpha
pheromone to induce mating reaction in each other
Components of the signalling pathway
One receptor for each factor
*
STE2- alpha-factor receptor
*
STE3- a-factor receptor
*
receptors (and factors) are not homologous to each other
*
(virtually) all other STE genes effect both a and alpha cell
types (exceptions: processing enzymes involved in modifying
pheromones)
•
receptor binding in each cell type feeds into the same
pathway
-Signalling in mating reaction causes cells to undergo polarized
morphogenesis called mating projections or “shmoos” that help the
cells to fuse
-Mating signals also arrest cells in G1 before DNA is replicated
-two visual methods to determine mating type of cells
-halo assay using patch of cells you want to test for mating
type and a lawn of cells that are sensitized to either a-pheromone
or alpha-pheromone
-mating test using strain of known mating type and at least
one auxotrophic marker (eg. trp1) the strain to be tested doesn’t
carry, the strain to be tested should also have another auxotrophic
marker (eg. ura3)
-replica plate a patch of the strain to be tested onto a lawn of
the tester strain (known MATa or MATα) and allow to mate
-select for diploids by replica plating onto synthetic media
plates lacking both nutrients required for each auxotrophy (eg.
lacking uracil and tryptophan)
Mating type switching
-wild-type haploids switch mating types every generation
(homothallic both mating types from one cell)
-since opposite mating type cells can mate to form diploids under
most conditions, homothallic strains only briefly exist as haploids
-budding yeast lab strains remain as haploids unless deliberately
mated because they are unable to switch mating types
(heterothallic, must mix two strains to mate) due to a mutation in a
gene called HO
Cassette Model for Mating Type Switching
-There are silent versions of the MATa genes and MATalpha genes
at the two ends of the chromosome the MAT locus is on
-these silent versions replace the MAT locus and are expressed
only at the MAT locus