Wednesday March 31


Parallel session 4: Fungal Physiology and Biochemistry



Coordination of fungal development and secondary metabolism in Aspergilus nidulans

Gerhard Braus

Georg-August-Universität Göttingen


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.





Metabolism of selenium in Saccharomyces cerevisiae and improved biosynthesis of bioactive organic Se-compounds

Valeria Mapelli[2] Emese Kápolna[1] Peter René Hillestrřm[1] Erik Huusfeldt Larsen[1] Lisbeth Olsson[2]

1National Food Institute - Danish Technical University,2Chalmers University of Technology


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.


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, Institute of Chemical Engineering, TU Vienna, Getreidemarkt 9, A-1060 Wien, Austria


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.




return to table of contents