Abstract
BackgroundEconomical production of fuels and chemicals from plant biomass requires the efficient use of sugars derived from the plant cell wall. Neurospora crassa, a model lignocellulosic degrading fungus, is capable of breaking down the complex structure of the plant cell wall. In addition to cellulases and hemicellulases, N. crassa secretes lytic polysaccharide monooxygenases (LPMOs), which cleave cellulose by generating oxidized sugars—particularly aldonic acids. However, the strategies N. crassa employs to utilize these sugars are unknown.ResultsWe identified an aldonic acid utilization pathway in N. crassa, comprised of an extracellular hydrolase (NCU08755), cellobionic acid transporter (CBT-1, NCU05853) and cellobionic acid phosphorylase (CAP, NCU09425). Extracellular cellobionic acid could be imported directly by CBT-1 or cleaved to gluconic acid and glucose by a β-glucosidase (NCU08755) outside the cells. Intracellular cellobionic acid was further cleaved to glucose 1-phosphate and gluconic acid by CAP. However, it remains unclear how N. crassa utilizes extracellular gluconic acid. The aldonic acid pathway was successfully implemented in Saccharomyces cerevisiae when N. crassa gluconokinase was co-expressed, resulting in cellobionic acid consumption in both aerobic and anaerobic conditions.ConclusionsWe successfully identified a branched aldonic acid utilization pathway in N. crassa and transferred its essential components into S. cerevisiae, a robust industrial microorganism.Electronic supplementary materialThe online version of this article (doi:10.1186/s13068-015-0303-2) contains supplementary material, which is available to authorized users.
Highlights
Economical production of fuels and chemicals from plant biomass requires the efficient use of sugars derived from the plant cell wall
Neurospora crassa consumption of aldonic acids As a cellulose degrading fungus, N. crassa is capable of utilizing Avicel, a microcrystalline cellulose
Intermediate products of Avicel utilization include cellodextrins, aldonic acids and glucose—none of which accumulated in the supernatant of N. crassa grown in Avicel
Summary
Economical production of fuels and chemicals from plant biomass requires the efficient use of sugars derived from the plant cell wall. Neurospora crassa, a model lignocellulosic degrading fungus, is capable of breaking down the complex structure of the plant cell wall. The recalcitrant nature of the plant cell wall is challenging for the enzymatic and chemical hydrolysis strategies required to release renewable sugars from plant biomass [1]. Cellodextrin and xylodextrin utilization pathways were previously identified as major strategies used by N. crassa and other fungi to utilize complex biomass [2, 3] In both cases, secreted enzymes first break down the cellulose and hemicellulose to soluble cellodextrins and xylodextrins, respectively. These are transported into the cells by cellodextrin and xylodextrin transporters and—in the case of xylodextrins—reduced before they are further processed to monomeric sugars by intracellular hydrolases
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