Growth, Differentiation and Sexuality

tions at the end of the pathways. The isolation of
more components of these same cascades and their
detailed study will probably largely reproduce what
is already known from other organisms. Therefore,
it will be important to identify modifications and
requirements specific for A. nidulans sporulation.
f) Transcription Factors and Other Regulators
Two of the most prominent transcription factors
involved in asexual sporulation in A. nidulans are
the Cys2-His2 Zn(II) finger transcription factor
BrlA, and the ATTS/TEA DNA-binding domain
transcriptional regulator AbaA. They were both
identified in genetic screenings almost 40 years ago
(Clutterbuck 1969; Fig. 14.3). Both corresponding
mutant strains initiate the developmental
programme of the wild type but fail to produce
mature conidiospores. brlA mutant strains fail
to proceed from stalk to vesicle formation, and
thus the aberrant conidiophore resembles a bristle
(Mirabito et al. 1989; Prade and Timberlake 1993).
brlA represents one of the few genes which are
necessary and sufficient for conidiation (Adams
et al. 1988). Deletion of the gene prevents conidiaton
from taking place, and overexpression of
brlA in liquid culture leads to the induction of
developmental genes, a block of vegetative growth,
and the production of phialide-like structures
and some conidia (Mirabito et al. 1989; Sewall
Fungal Asexual Sporulation 273
1994). Complex conidiophores, however, are not
produced. The brlA genelocusisquitecomplex,
because it encodes two overlapping transcripts and
the derived proteins, BrlAα and BrlAβ,differin23
amino acids at the N terminus (cf. BrlAβ has the
additional amino acids; Fig. 14.5). One indication
for the complex transcriptional regulation may
betherelativelylargepromoterregionofabout
2 kb (Han and Adams 2001). The expression of
BrlAβ is not transcriptionally regulated during
development, but depends on a micro-open
reading frame in the upstream non-translated
region (Han et al. 1993; Timberlake 1993). Factors
required for the release of the translational block
are not known yet. In a heterologous yeast system,
Chang and Timberlake (1992) were able to define
a Brl-response element (BRE) in the promoter of
the rodA gene. It is not clear yet whether BrlAα
and BrlAβ regulate different target genes.
In abaA mutants, development proceeds to
the phialide stage (Mirabito et al. 1989; Andrianopoulos
and Timberlake 1994; Fig. 14.3). These
spore-producing cells fail to do so, and rather
produce hypha-like structures with swellings along
the hypha. These structures resemble a mechanical
calculator, an abacus. After initial cloning and
functional characterization of the brlA and abaA
genes, the AbaA protein was successfully expressed
in Escherichia coli, and promoter-binding
studies were carried out (Andrianopoulos and
Fig. 14.5. A–D The brlA gene. A In
brlA deletion mutants, only elongated
stalks are formed. B In brlAβ mutants,
secondary conidiophores may arise
from the vesicles of aberrant primary
conidiophores. C In brlAα mutants,
development proceeds further but
conidia are not produced. D Diagram
of the brlA locus. The transcripts are
indicated with an arrow and the open
reading frames are shown by shaded
boxes. The N-terminal extension of the
BrlAβ protein, disrupted by the intron,
is indicated by a closed box. Modified
after Prade and Timberlake (1993)