Growth, Differentiation and Sexuality

and a multi-drug resistance protein respectively
(Aichinger et al. 2003). The mig1 and mig2
(maize-induced) gene clusters comprise two
and five highly homologous co-regulated genes
respectively, which are highly expressed during the
biotrophic stage and encode small, cysteine-rich
secreted proteins sharing no homologies to known
proteins (Basse et al. 2002b). udet1 was also
identified as being up-regulated during fungal
growth in planta, and was subsequently shown to
encode a steroid 5α reductase by complementing
a det2 mutant of Arabidopsis thaliana. Inplants,
these enzymes are needed for the synthesis of
brassinosteroids, hormones which control plant
development. However, udet1 mutants were unaffected
in pathogenic development (Basse et al.
2002a), which rules out that Udet1 is involved
in synthesis of the trigger for tumour induction.
Deletion of either the entire pig or the entire mig2
cluster was also without effect on pathogenicity
(Basse et al. 2002b; Aichinger et al. 2003).
The mig genes as well as ssp1, a gene encoding
a potential dioxygenase predominantly expressed
in teliospores (Huber et al. 2002), are negatively
regulated by the histone deacetylase Hda1 (Huber
et al. 2002; Torreblanca et al. 2003). Hda1 is
thought to form a complex with a previously identified
regulatory protein, Rum1 (Quadbeck-Seeger
et al. 2000), and was shown to regulate a distinct
set of genes presumably by modulation of their
chromatin structure (Reichmann et al. 2002). The
deletion of either rum1 or hda1 leads to a defined
developmental block prior to the formation
of teliospores (Quadbeck-Seeger et al. 2000; Reichmann
et al. 2002). It is likely that modulation of
chromatin structure represents an additional level
of global gene regulation which is used for coregulating
specific sets of genes.
Preliminary data from array analyses indicate
that a set of more than 500 genes is plant-regulated.
This set includes various genes which encode transporters
for carbon compounds and amino acids,
possibly reflecting adaptation of U. maydis to
conditions encountered in the plant environment
(M. Vranes and J. Kämper, unpublished data). Research
on those genes whose expression is confined
to the biotrophic phase is currently focused on their
regulators (Farfsing et al. 2005), as it is anticipated
that deletions in the corresponding regulatory
genes will affect pathogenic development. We
expect that the systematic analysis of these genes
will provide important insights into the process of
fungal adaptation during biotrophic growth.
Regulatory and Structural Networks in Ustilago maydis 383
IV. Small GTPase Networks
for Cytokinesis and Dimorphism
GTP-binding proteins of the Ras superfamily act
as molecular switches and are involved in diverse
biological processes (Bourne et al. 1990). They exist
in two different states, the active GTP-bound
conformation and the inactive GDP-bound form.
Switching between active and inactive forms is regulated
by interaction with specific accessory proteins.
Guanine nucleotide exchange factors (GEFs)
activate Ras-like GTPases by catalysing the release
of GDP, which is then immediately replaced with
GTP (Cherfils and Chardin 1999). Inactivation of
these molecular switches requires interaction with
GTPase activating proteins (GAPs) which stimulate
the low intrinsic GTPase activity resulting in
the hydrolysis of GTP to GDP (Bernards and Settleman
2004). Most members of the family of Ras-like
proteins are associated with cellular membranes
(Fig. 18.6). Membrane binding is mediated by hydrophobic
membrane anchors consisting of a Cterminal
farnesylation, as in the case of Ras, or
geranylgeranylation, as in the case of Rho/Rac proteins
(Zhang and Casey 1996). An additional level of
regulation occurs by Rho/Rac-specific guanine nucleotide
dissociation inhibitors (Rho-GDIs) which
can bind to the prenylated C terminus. Binding
of Rho-GDI results in the relocalization of GTPases
from the membrane to the cytoplasm, by sequestering
the hydrophobic membrane anchor (Olofsson
1999).
Fig. 18.6. Signalling network involving small GTPases of
the Rho/Rac family in U. maydis. The highly related GT-
Pases Rac1 and Cdc42 trigger both specific and common
pathways. Potential GEFs (Don1 and Cdc24) and target kinases
(Don3 and Cla4) are indicated by ovals and hexagons
respectively. The distinct phenotypes of Cdc42 and Rac1
deletion mutants stress their involvement in specific as well
as overlapping signalling events