Growth, Differentiation and Sexuality

400 H.A.B. Wösten and J.G.H. Wessels
homeodomain complexes of MATa1 and MATα2of
S. cerevisae (Goutte and Johnson 1988) and the bE
and bW heterodimers of U. maydis (Romeis et al.
2000).
Another gene which seems to be part of the
A-regulated pathway is pcc1 (Murata et al. 1998).
A homokaryotic strain with a mutated copy of this
gene formed pseudoclamps. Moreover, after prolonged
time, it formed fully differentiated fruiting
bodies. From this and the fact that pcc1 is expressed
in a wild-type homokaryon, it was concluded that
thisgenemaybearepressorofthefruitingpathway
in the absence of a functional A complex. The pcc1
gene encodes a protein with a HMG box motif and
a nuclear localization signal, indicating that it is
a transcription factor. The HMG box is also present
in ste11 of Schizosaccharomyces pombe (Sugimoto
et al. 1991) and prf1 of U. maydis (Hartmann et al.
1996). These genes encode pheromone response
factors which bind to a pheromone-responsive element
in the promoter of target genes. Among these
genes are prf1 and ste11 themselves (Sugimoto et al.
1991; Aono et al. 1994; Hartmann et al. 1996; Urban
et al. 1996). A pheromone-responsive element similar
to those of U. maydis and S. pombe was found in
thepromoterofthepcc1 gene. This, and the fact that
pcc1 is up-regulated by a compatible B mating interaction
suggest that pcc1 is a possible pheromone
response factor of C. cinereus (Murata et al. 1998;
see Chap. 17, this volume). Since the gene is also
up-regulated by an activated A gene, Murata et al.
(1998) suggested that pcc1 plays an important role
in coordinating the activities of the A and B genes.
This interpretation is strengthened by the fact that
the pcc1 mutant homokaryon formed fully differentiated
fruiting bodies, albeit after a prolonged
time.Itwassuggestedthattherepressoractivityof
pcc1 is released by a compatible A gene interaction
via clp1 (Kamada 2002).
Like pcc-1 and possibly clp1, theFBF gene
of S. commune is involved in clamp formation
and in fruiting (Springer and Wessels 1989).
A recessive mutation of FBF (fbf ) completely
blocks dikaryotic fruiting. The mutation occurs
spontaneously at high frequency, causing sterility
in about 10% of mycelia regenerated from protoplasts
of a MATA con MATB con homokaryon. The
fbf mutation is also responsible for the frequent
occurrence of sterile sectors in fruiting colonies of
this homokaryon. The phenotype of the mutation
is particularly clear when mycelia are grown as
a lawn from mycelial fragments. Under these
conditions, the homokaryotic MATA con MATB con
forms numerous fruiting bodies and few aerial
hyphae (Fig. 19.3E), its phenotype being similar
to that of the normal heterokaryotic dikaryon
(Fig. 19.3C). The MATA con MATB con fbf mycelium
forms no fruiting bodies but instead produces
copious aerial mycelium (Fig. 19.3F). The fbf mutation,
which has no phenotype in monokaryons,
completely suppresses fruiting when homozygous
in a MATA-on MATB-on heterokaryon. Notably, it
also affects the formation of clamp connections;
the clamps do not fuse. fbf may be related to
a spontaneously occurring recessive mutation,
called coh1 (Perkins and Raper 1970), because
no complementation occurred in an fbf ×coh1
cross (Springer and Wessels 1989). Wessels (in The
Mycota, Vol. I, 1st edn., Chap. 21) suggested that
fbf couldalsoberelatedtoFRT1 of S. commune
(Horton and Raper 1991). However, this seems not
to be the case (Horton et al. 1999).
FRT1 was initially identified as a gene which
induced fruiting when transformed into certain
homokaryons of S. commune (Horton and Raper
1991). Experimental evidence indicated that the
strains which fruited upon introduction of FRT1
contained an endogenous FRT1 allele of a different
kind (designated FRT1-2 as opposed to FRT1-1;
Horton et al. 1999). By contrast, strains possessing
a similar allele did not fruit when transformed
with FRT1-1 (Horton and Raper 1991). Fruiting
in heterokaryotic dikaryons derived from these
fruiting homokaryons was also accelerated. FRT1-1
encodes a putative nucleotide-binding protein of
192 amino acids with a P-loop motif (Horton and
Raper 1995). This P-loop motif was demonstrated
to be essential for the fruiting-inducing activity
of the protein. Surprisingly, homokaryotic strains
in which the FRT1 gene was disrupted were more
fluffy, compared to wild-type strains. The aerial
hyphae of the disruptant strains were aggregated
(Horton et al. 1999), resembling the first stages
of fruiting-body development (van der Valk and
Marchant 1978; Raudaskoski and Vauras 1982).
Indeed, not only were levels of the monokaryonspecific
SC3 mRNA increased in the haploid fruiter
but also those of the dikaryon-specific mRNAs of
SC1, SC4 and SC7 (see below). From these results,
it was hypothesized that FRT1 is part of a signal
transduction pathway which represses expression
of dikaryon-specific genes in the monokaryon
(Horton and Raper 1995). The expression of
the dikaryon-specific genes in a homokaryon in
which the FRT1 gene was deleted is apparently
not sufficient to initiate fruiting-body formation.