Growth, Differentiation and Sexuality

III. Downstream Regulation
of Development
A. Pheromone Signalling
The yeast S. cerevisiae provides us with the most
completely understood model of how pheromone
binding activates an intracellular phosphorylation
cascade and leads to induction of genes required
for mating (see recent review by Lengeler et al.
2000). The corresponding signalling pathway in basidiomycetes
is dealt with in Chap. 19 of this volume,
and only a brief mention is made here with
the object of seeing, with examples from U. maydis
and C. cinereus, how the transcription factors
targeted by pheromone signalling can regulate different
subsets of genes to bring about the different
developmental responses that we see during sexual
development.
In S. cerevisiae, receptor binding by a pheromone
activates a heterotrimeric G protein coupled
to its internal surface and initiates a phosphorelay
through a MAPK cascade to activate the target
transcription factor Ste12p, a member of the
homeodomain family of transcription factors.
Ste12p binds pheromone response elements
(PREs) in the promoters of genes required for
mating. The first kinase in the pathway is Ste20p,
and the MAPK module is composed of the three
kinases Ste11p (MAPKKK), Ste7p (MAPKK) and
Fus3p (MAPK). Several components of a similar U.
maydis MAPK pathway have been identified (see
Lengeler et al. 2000), in particular homologues of
the MAPK module and a target transcription factor
Prf1 (pheromone response factor, Hartmann et al.
1996). Pheromone stimulation leads to induction
of both the pheromone and receptor genes, and
within the promoters of these genes are elements
(PREs) that have been shown to bind Prf1. Prf1
is also essential for induction of the b genes,
and PREs can be found in the DNA sequences
that contain these genes (Urban et al. 1996).
Inactivation of components of the MAPK module
leads to inability to make conjugation tubes; cell
fusion can still occur but cells cannot induce the b
genes necessary for filamentous growth and, even
if these are driven from a constitutive promoter,
the hyphae are non-pathogenic (Kaffarnik et al.
2003; Müller et al. 2003).
Another highly conserved signalling pathway
implicated in mating in U. maydis is the cAMP
pathway, with key elements a Gα protein that initiates
downstream activation of adenylate cyclase
Mating Type Genes in Basidiomycetes 369
which, together with cAMP, phosphorylates cAMPdependent
protein kinase A (PKA; Gold et al. 1994,
1997; Regenfelder et al. 1997; Dürrenberger et al.
1998; see Chap. 15, this volume). Prf1 integrates
signals from these two pathways to coordinate the
activities of the a and b genes (Hartmann et al.
1996, 1999). It has been shown recently that both
PKA and the MAPK phosphorylate Prf1, and that
this permits promoter discrimination whilst still
using the same PRE DNA-binding sites. Depending
on its phosphorylation status, Prf1 is able to
activate either a or b genes; induction of a genes
requires PKA sites to be phosphorylated but not the
MAPK sites, whereas induction of b genes requires
the integrity of both PKA and MAPK sites (Kaffarnik
et al. 2003). Evidence points to pheromone
signal activating both pathways.
Prf1, unlike S. cerevisiae Ste12p, is a member
of a transcription factor family that has an HMG
DNA-binding domain (Hartmann et al. 1996).
Two transcription factors have been identified
in C. cinereus that like Prf1 of U. maydis, have
an HMG DNA-binding domain, Hmg1 (Milner,
Aime and Casselton, unpublished data) and Pcc1
(Murata et al. 1998), and are both required for
pheromone-induced mating functions. Hmg1 is
necessary for nuclear migration in mating but not
for clamp cell fusion. Deletion of the hmg1 gene in
one mate means that it can no longer receive nuclei
but it can still donate nuclei into its non-mutant
partner that has a functional transcription factor.
When both mates lack a functional copy of hmg1,
nuclear migration is totally blocked, but hyphal
fusion leads to formation of a dikaryon with
perfectly fused clamp cells (Milner, Aime and
Casselton, unpublished data). Pcc1 is required for
clamp cell fusion but not nuclear migration (own
unpublished data). There are thus two different
transcription factors for different outputs of the
pheromone signal – Hmg1 for nuclear migration
and Pcc1 for clamp cell fusion; discrimination
between them depends on whether or not the A
(clamp cell) pathway has been activated.
B. Heterodimer Targets
Many different morphological and physiological
changes may accompany the switch from an
asexual to a dikaryotic phase of the life cycle,
other than fruiting, and overall regulation of
some of these can be attributed to one or other
set of mating type genes (Brachmann et al. 2001;