Introduction to Fungi, Third Edition

268 HEMIASCOMYCETES
Fig10.5 The structure of the mating type idiomorphs a (top) and a (bottom) of Saccharomyces cerevisiae.The two alleles differ only
in their central (Y) regions which contain parts of two genes a1anda2ora1anda2.The entire lengths of these genes and their
directions of transcription are indicated by arrows.The function of a2 is unknown. In diploid cells, the lack of expression of a1andthe
formation of a dimeric a2/a1protein suppresses the expression of mating type-specific proteins including hormones and their
receptors. nt ¼ nucleotides.Redrawn from Haber (1998) Annual Reviews of Genetics 32,withpermission.ß1998 Annual Reviews,
www.annualreviews.org.
the plasma membrane receptor Ste2p, which
can bind a-pheromone from the environment.
In diploid cells, expression of a1 and thus of
a-specific proteins is repressed. The a2 gene
encodes a repressor protein which interacts
with several other regulatory proteins,
including Mcm1p, to repress the expression of
a-specific genes, including those encoding the
a-pheromone and the a-factor receptor Ste3p. In
the absence of the a1 and a2 gene products,
haploid cells have an a-phenotype with respect to
mating behaviour because a-genes are constitutively
expressed. The function of a2 is unknown,
and the a1 gene product is active only in diploid
cells, combining with the a2 protein to repress
haploid-specific genes including those encoding
the two pheromones and their receptors.
Another gene repressed by the a2/a1 dimer
is RME1, which encodes a repressor of meiosis.
Meiosis can therefore only take place if a diploid
a/a nucleus exists in which Rme1p is repressed
by the a2/a1 dimer, and if nutrient conditions
are limiting. The signal for nutrient limitation
is sensed and transmitted by a cyclic AMPdependent
signalling chain (Klein et al., 1994).
Therefore, the most important difference
between a/a diploids and homozygous diploids
or haploids is that only the a/a diploids can initiate
meiosis under nutrient-limiting conditions,
leading to the production of ascospores which
are more resistant to adverse conditions than
vegetative cells. It is likely that this enhanced
survival of ascospores is the reason why haploid
or homozygous populations of S. cerevisiae and
some other ascomycetes (e.g. Schizosaccharomyces
pombe) possess the intriguing ability to switch
their mating type, thereby acquiring the ability
to undergo meiosis. An excellent account of the
experiments and ideas leading to the unravelling
of the mating factor switch in S. cerevisiae has
been given by Haber (1998), and we borrow
heavily from it in the following summary.
Strathern and Herskowitz (1979) observed
that the ability to switch their mating type is
acquired only by cells which have previously
divided at least once. The pattern established by
a single germinating a-type haploid ascospore is
shown in Fig. 10.6: the first daughter cell has the
mating type a, but then the mother cell switches
its mating type prior to its second division,
so that two a-cells result. Meanwhile, the first
daughter (a-type) cell undergoes its first division
so that a cluster of two a- and two a-type cells
is produced. Conjugation can occur, and the
two zygotes can carry on dividing as diploid
yeast cells with the additional option to
undergo meiosis and produce asci if required.
The mating type switch is brought about by the