Fungal L-xylulose reductases

A large number of filamentous fungi grow on dead or decaying plant material and contribute thus to the rapid turn- over of lignin and diverse polysaccharides including cellulose, hemicellulose and pectin. Hemicelluloses and pectins are heteropolysaccharides composed mainly of pentoses, hexoses, or sugar acids. Their primary structure depends on the source and can vary even between different tissues of a single plant. The pentose L-arabinose is one of the main components of both hemicelluloses and pectins. An L-arabinose catabolic pathway is therefore of importance for many microorganisms and has also an impact on microbial fermentations in which cheap plant biomass is converted into industrially useful products such as fine chemicals or enzymes. Fungi have developed a specific pathway for the catabolism of pentoses such as L-arabinose or D-xylose. Most of the genes involved in the L-arabinose pathway of filamentous fungi have recently been cloned and functionally characterized. L-xylulose reductase catalyzes the third step in this five step pathway by converting L-xylulose to xylitol. An L-xylulose reductase gene (lxr1) from the filamentous fungus Hypocrea jecorina (anamorph: Trichoderma reesei) was cloned previously but the role of this gene in the catabolism of L-arabinose has now been questioned by a number of recent findings. Based on our data this gene encodes rather for a D-mannitol 2-dehydrogenase which is linked to developmental processes such as sporulation and condiospore germination. Enzymes with L-xylulose reductase are in addition involved in a number of other processes including catabolism of D-galactose and cellulase induction. The present proposal therefore aims at the identification and characterization of these enzymes with L-xylulose reductase activity in H. jecorina and a characterization of their role in the different metabolic pathways including L-arabinose, D-galactose and D-mannitol metabolism. Our organism of choice for this study is the filamentous fungus H. jecorina, a saprophyte with a broad biotechnological application due to its excellent secretion capacity for different extracellular enzymes, some produced industrially on a technical scale for textile or food industry. Sequencing of the H. jecorina genome was recently completed and provides therefore an excellent basis for future research.

Fungi play a vital role as saprotrophs in our ecosystems due to their ability to decompose dead or decaying organisms. Numerous fungi are especially adapted to degrade polymeric plant cell walls by a highly specific inventory of different extracellular enzymes to their sugar monomers and are able to convert these break-down products by a diversity of intracellular metabolic pathways to energy and carbon for fungal growth and development. Plant biomass in the form of plant cell walls is the major renewable biomass on earth and today different strategies are developed to produce biofuels (bioethanol) and other bio-based chemicals from different plant feedstocks such as trees and grasses. The composition of plant cell wall of trees and grasses is complex and consists of different polysaccharides mainly cellulose, different hemicelluloses and pectins and the polymer lignin. The need to fully convert these plant cell walls has accelerated research to investigate the degradation of the individual components of the different polysaccharides. The pentose L-arabinose occurs in both hemicellulose and pectin and fungi produce a number of extracellular arabinanases to specifically release L-arabinose from these plant polymers. Most of the components of the L-arabinose pathway have been characterized in fungi such as Trichoderma reesei. L-xylulose reductase (LXR) catalyzes the third step in this five step pathway by converting L- xylulose to xylitol. Our research showed that a previously described enzyme is not involved in this process but is involved in other metabolic pathways related to sporulation and germination. By using genome wide transcript analysis we could show that Trichoderma reesei is especially adapted to its environment by producing different enzymes including cellulases and hemicellulases on its spore surface which allow the fungus to initiate germination in its ecological niche. We have also identified new candidates for an L-xylulose reductase which are involved in L-arabinose degradation. These genes can be introduced into the yeast Saccharomyces cerevisiae strains to improve conversion of these pentoses to ethanol.