Insights from the Fungus Fusarium oxysporum Point to High Affinity Glucose Transporters as Targets for Enhancing Ethanol Production from Lignocellulose

Abstract

Ethanol is the most-widely used biofuel in the world today. Lignocellulosic plant biomass derived from agricultural residue can be converted to ethanol via microbial bioprocessing. Fungi such as Fusarium oxysporum can simultaneously saccharify straw to sugars and ferment sugars to ethanol. But there are many bottlenecks that need to be overcome to increase the efficacy of microbial production of ethanol from straw, not least enhancement of the rate of fermentation of both hexose and pentose sugars. This research tested the hypothesis that the rate of sugar uptake by F. oxysporum would enhance the ethanol yields from lignocellulosic straw and that high affinity glucose transporters can enhance ethanol yields from this substrate. We characterized a novel hexose transporter (Hxt) from this fungus. The F. oxysporum Hxt represents a novel transporter with homology to yeast glucose signaling/transporter proteins Rgt2 and Snf3, but it lacks their C-terminal domain which is necessary for glucose signalling. Its expression level decreased with increasing glucose concentration in the medium and in a glucose uptake study the Km(glucose) was 0.9 mM, which indicated that the protein is a high affinity glucose transporter. Post-translational gene silencing or over expression of the Hxt in F. oxysporum directly affected the glucose and xylose transport capacity and ethanol yielded by F. oxysporum from straw, glucose and xylose. Thus we conclude that this Hxt has the capacity to transport both C5 and C6 sugars and to enhance ethanol yields from lignocellulosic material. This study has confirmed that high affinity glucose transporters are ideal candidates for improving ethanol yields from lignocellulose because their activity and level of expression is high in low glucose concentrations, which is very common during the process of consolidated processing.

Rapid and Effective Oxidative Pretreatment of Woody Biomass at Mild Reaction Conditions and Low Oxidant Loadings

“…A better understanding of the molecular basis of alcohol stress and tolerance in such fungi is important if enhanced tolerance and production is to be achieved. Previous work [54] coupled with this study point towards sugar transporters as key targets for improving fungalenabled CBP.…”

“…Interestingly, FOXG_09625 showed 29% protein homology to the SSH -identified hexose transporter and no homology to the high affinity glucose transporter indicating potentially different roles for all three genes in either tolerance or production. Yet, a recent study [54] has indicated FOXG_09625 is associated (directly/indirectly) with ethanol production since mutants overexpressing a hexose transporter resulted in increased ethanol yield coupled with compensatory changes in the expression of other transporters notably the up-regulation of FOXG_09625. Investigations, including overexpression/knockout studies of FOXG_09625, while not possible due to a limitation in Figure 4.…”

Advances in consolidated bioprocessing systems for bioethanol and butanol production from biomass: a comprehensive review

“…However, in order to become an economically suitable CBP organism, a better understanding of the hydrolysing and ethanol production pathways of this fungus as well as the mechanisms involved and bottlenecks faced is required. Fortunately, recently some studies have been devoted to understanding details of ethanol production in F. oxysporum which will be useful for development of CBP ethanol production by this fungus (Ali et al, 2012(Ali et al, , 2013(Ali et al, , 2014Hennesy et al, 2013;Anasontzis et al, 2014).…”

“…Molecular analysis confirmed that a coding region homologous to a putative sugar transporter (FOXG_09625) was disrupted. Ali et al (2013) identified a novel hexose transporter (Hxt) in F. oxysporum which could positively affect sugar uptake by F. oxysporum and would enhance the ethanol yields from lignocellulosic biomass. Overexpression of the sugar transporter (Hxt) in F. oxysporum significantly enhanced the glucose and xylose transport capacity and ethanol yield (39% increase), when straw, glucose and xylose were used as carbon source.…”

Use of cultivars of low cost, agroindustrial and urban waste in the production of cellulosic ethanol in Brazil: A proposal to utilization of microdistillery

“…As an example, modification of the hexose transporter (Hxt) of F. oxysporum has allowed high affinity of the glucose transport and an increasing of 33% in the ethanol production [224].…”