Growth, Differentiation and Sexuality

III. Transcriptional Cascades
for Pathogenic Development
The multiallelic b locus encodes a pair of homeodomain
transcription factors (bE and bW) which
areactiveonlyasheterodimerswithmonomers
encoded by different alleles (e.g. bE1/bW2; Kronstad
and Leong 1990; Schulz et al. 1990; Gillissen
et al. 1992; Kämper et al. 1995). The active bE/bW
heterodimer is formed after two haploid cells have
fused, and triggers a transcriptional cascade resulting
in the formation of infectious dikaryotic
hyphae. Since structural aspects of the b locus and
Fig. 18.5. Function of the a and b mating type loci during
mating. The a1 and a2 alleles encode pheromones and receptor
molecules which regulate directed growth and fusion
of sporidia. After cell fusion, the bE and bW homeodomain
proteins encoded by the multiallelic b locus form a heterodimeric
complex, but only when derived from different
alleles. The bE/bW heterodimer binds to a conserved sequence
motif (bbs) in the promoter regions of class 1 genes.
Class 2 genes are indirectly regulated by b via regulatory
proteins which are encoded by class 1 genes. b-induced
and b-repressed genes are indicated by arrowheads and
bars respectively. An asterisks indicates that deletion mutants
have been generated. With the exception of kpp6,none
of these genes are required for pathogenicity. The following
functions can be assigned to proteins encoded by b-
Regulatory and Structural Networks in Ustilago maydis 381
the encoded proteins are covered in the article by
Casselton and Challen (Chap. 17, this volume), we
will restrict ourselves to the regulatory cascade triggered
by the bE/bW complex (Fig. 18.5).
The central role of the bE/bW heterodimer
for pathogenic development was demonstrated
by the construction of haploid strains with
ahybridb locus expressing different alleles of
bE and bW (Bölker et al. 1995). Such strains are
solopathogenic, i.e. they are able to infect the host
plant without a mating partner. Solopathogenic
haploid strains have become an invaluable resource
for studying gene functions independently of fusion
events. At the same time, such strains serve
dependent genes: polX (frb 52), hypothetical DNA polymerase
X; dik6, no similarities; lga2, interferes with mitochondrial
fusion; egl1, endoglucanase; dik1, no similarities;
rep1, repellent; hum1, hydrophobin; pdi1 (frb23), hypothetical
protein disulfide isomerase; kpp6, MAPK; exc1 (frb133),
hypothetical exochitinase; frb63, unknown;ant1 (frb172),
hypothetical K + /H + antiporter; pma1 (frb323), hypothetical
plasma membrane ATPase; atr1 (frb34), hypothetical
acyl transferase; frb110, unknown (homology to potential
polypeptide from Neurospora crassa); cap1 (frb136), unknown
homology to capsule-associated protein from Cryptococcus
neoformans, frb124,unknown;mfa1/2, pheromone
precursor; pra1/2, pheromone receptor (Bohlmann et al.
1994; Schauwecker et al. 1995; Urban et al. 1996a,b; Wösten
et al. 1996; Brachmann et al. 2001; Bortfeld et al. 2004)