Wednesday March 31
Parallel session 4: Fungal Physiology and Biochemistry
PS4.1
Gerhard Braus
Georg-August-Universität Göttingen
gbraus@gwdg.de
The homothallic filamentous ascomycete
A. nidulans is able to form fruitbodies (cleistothecia) either by
mating of two strains or by selfing in the absence of a partner. The
three-dimensional
A. nidulans cleistothecium is the most complicated structure this
fungus is able to form. Differentiation and secondary metabolism are correlated
processes in fungi that respond to various parameters including light,
nutrients, aeration or pheromones. Our work on several proteins will be
described, which are involved in the crosstalk between developmental regulation
and secondary metabolism control in
Aspergillus nidulans. They include the heterotrimeric
velvet complex VelB/VeA/LaeA, where VeA bridges VelB to the nuclear
master regulator of secondary metabolism LaeA, the eight subunit COP9
signalosome complex controlling protein turnover, and the MAP kinase-related
protein kinase ImeB.
PS4.2
Valeria Mapelli[2]
Emese Kápolna[1] Peter René Hillestrřm[1] Erik Huusfeldt
Larsen[1] Lisbeth Olsson[2]
1National
Food Institute -
valeria.mapelli@chalmers.se
Selenium (Se) is an essential element for many organisms as it is present under
the form of Se-cysteine in Se-proteins. 25 Se-proteins are known in humans and
are all involved in protection of cells from oxidative stress. The main
sources of Se for animals are edible plants able to accumulate Se from the soil
in inorganic and organic forms. Some of the Se organic forms bioavailable for
animals have been proven to have cancer-preventing effects if regularly
introduced into the diet. Since Se content in plants is highly susceptible to
environmental factors, the intake of Se is often insufficient to result in
beneficial effects. Therefore, the use of Se-enriched yeast as food supplement
is made available to avoid Se shortage. The yeast
Saccharomyces cerevisiae does not require Se as essential element,
but is able to metabolise and accumulate Se. Due to the very similar properties
of Se and sulphur (S), S- and Se-compounds share the same assimilation and
metabolic routes, but the competition is in favour of S-species, as the high
reactivity of Se leads to the formation of toxic compounds. Due to the delicate
balance between beneficial and toxic effects of Se, the study of Se metabolism
in yeast is a crucial point towards the establishment of a yeast cell factory
for the production of bioactive Se-compounds. The present study shows how the
presence of Se influences cell physiology and metabolism. On this basis, we show
how the coupling of metabolic engineering and bioprocess optimization represents
a successful strategy towards the production of organic Se-molecule with high
anti-cancer potential. The Se-metabolome has been carefully mapped.
PS4.3
Arabinan and L-arabinose metabolism in
Trichoderma reesei
Benjamin Metz, Eda Akel, Christian P. Kubicek and Bernhard Seiboth
Research Area Gene Technology and Applied Biochemistry,
bseiboth@mail.tuwien.ac.at
The efficient use of complex plant material as carbon source for the production
of different bio-based products requires an improved transformation of the
different plant cell wall constituents. The saprotrophic fungus
T. reesei (Hypocrea
jecorina) has been well established for the biotechnological production of
cellulases and xylanases and for the degradation of the respective polymers.
However, the enzymes and their regulation involved in the degradation of other
plant carbohydrate polymers including the L-arabinose polymer arabinan are less
well understood.
In the genome sequence of H. jecorina
four genes including three α-L-arabinofuranosidase genes (afb1,
afb2,
afb3) and a β-xylosidase with a
separate α-l-arabinofuranosidase activity (bxl1)
are found but no endoarabinanase. The resulting degradation product L-arabinose
is taken-up and further degraded by a fungal specific degradation pathway which
is interconnected with the D-xylose pathway. The following sequence of enzymes
was established starting with an L-arabinose reductase, followed by an L-arabinitol
dehydrogenase LAD1, an L-xylulose reductase LXR1, a xylitol dehydrogenase XDH1
and a xylulokinase XKI1. The L-arabinose reductase step in
T. reesei is catalyzed by the aldose
reductase XYL1 which is also involved in the degradation of D-xylose. Cloning of
a fungal LXR1 enzyme responsible for NADPH dependant reduction of L-xylulose to
xylitol was previously reported but our analysis revealed that LXR1 is not
involved in L-arabinose catabolism. We have therefore tested different other LXR
candidates and have identified one LXR whose deletion reduces the growth on L-arabinose
and L-arabitol.
Growth on arabinan, and its monomer L-arabinose requires the operation of the
general cellulase and xylanase regulator XYR1. This impairment of growth in the
xyr1 deleted strain can be overcome
by constitutive expression of the aldose reductase XYL1. Transcriptional
analysis reveals that abf1-3 and
bxl1 are induced by L-arabinose and
L-arabinitol. Transcription of
abf2
and bxl1 is dependent on XYR1 and
cannot be compensated for by constitutive expression of XYL1. Induction of all
four arabinofuranosidases is strongly enhanced in a
lad1 deleted strain and severely
impaired in the xyl1 deleted strain.
We conclude that the transcription of the arabinofuranosidase genes requires an
early pathway intermediate (L-arabinitol or L-arabinose), the first enzyme of
the pathway XYL1, and in the case of abf2
and bxl1 also the function of the
cellulase regulator XYR1.