Genetic improvement of native xylose-fermenting yeasts for ethanol production

Abstract

Lignocellulosic substrates are the largest source of fermentable sugars for bioconversion to fuel ethanol and other valuable compounds. To improve the economics of biomass conversion, it is essential that all sugars in potential hydrolysates be converted efficiently into the desired product(s). While hexoses are fermented into ethanol and some high-value chemicals, the bioconversion of pentoses in hydrolysates remains inefficient. This remains one of the key challenges in lignocellulosic biomass conversion. Native pentose-fermenting yeasts can ferment both glucose and xylose in lignocellulosic biomass to ethanol. However, they perform poorly in the presence of hydrolysate inhibitors, exhibit low ethanol tolerance and glucose repression, and ferment pentoses less efficiently than the main hexoses glucose and mannose. This paper reviews classical and molecular strain improvement strategies applied to native pentose-fermenting yeasts for improved ethanol production from xylose and lignocellulosic substrates. We focus on Pachysolen tannophilus, Scheffersomyces (Candida) shehatae, Scheffersomyces (Pichia) stipitis, and Spathaspora passalidarum which are good ethanol producers among the native xylose-fermenting yeasts. Strains obtained thus far are not robust enough for efficient ethanol production from lignocellulosic hydrolysates and can benefit from further improvements.

Characterization of an Anthracene Intermediate in Dynemicin Biosynthesis

“…[2–4] Considerable effort in the past 15 years has been devoted to defining the function of the highly reducing PKS that anchors all enediyne BGCs. [5, 6] It is now generally accepted that several, if not all, of these multidomain iterative enzymes initiate their biosynthetic pathways by assembling β-hydroxyhexaene octaketide 1 (Figure 1) bound to their acyl-carrier protein domain. [7,8] This labile intermediate can be cleaved by a separately encoded thioesterase (TE), accompanied by decarboxylation and dehydration to afford heptaene 2 (Figure 1), a universally observed enediyne shunt product.…”

Characterization of a Cellulose and Xylose Utilization Bacillus cereus Yc-A1

“…CMC solid medium contain 2% agar in CMC-Na liquid medium. Filter paper hydrolyzing medium contain 1×6 cm filter paper strips, 0.02% CaCL 2 .2H 2 O, 0.2% (NH 4 ) 2 SO 4 , 0.1% KH 2 PO 4 , 0.05% MgSO 4 .7H 2 O, 0.15%Na 2 SO 4 , 0.05% peptone, 0.05% yeast extract [11]. YPD medium contain 2% glucose, 2% peptone, 1% yeast extract, 0.25% agar.…”

“…YPD medium contain 2% glucose, 2% peptone, 1% yeast extract, 0.25% agar. Xylose liquid medium contain 6% Xylose, 0.02% (NH 4 ) 2 SO 4 , 0.5% KH 2 PO 4 , 0.04% MgSO 4 .7H 2 O, pH5.0. Glucose liquid medium contain 6% glucose, 0.02% (NH 4 ) 2 SO 4 , 0.5% KH 2 PO 4 , 0.04% MgSO 4 .7H 2 O, pH5.0.…”

“…Xylose liquid medium contain 6% Xylose, 0.02% (NH 4 ) 2 SO 4 , 0.5% KH 2 PO 4 , 0.04% MgSO 4 .7H 2 O, pH5.0. Glucose liquid medium contain 6% glucose, 0.02% (NH 4 ) 2 SO 4 , 0.5% KH 2 PO 4 , 0.04% MgSO 4 .7H 2 O, pH5.0. Mixed sugar medium contain 1.5% glucose, 4.5% xylose, 0.5% peptone, 0.5% yeast extract, 0.5% KH 2 PO 4 , 0.02% (NH 4 ) 2 SO 4, 0.04% MgSO 4 .7H 2 O, pH5.0.…”

“…Glucose liquid medium contain 6% glucose, 0.02% (NH 4 ) 2 SO 4 , 0.5% KH 2 PO 4 , 0.04% MgSO 4 .7H 2 O, pH5.0. Mixed sugar medium contain 1.5% glucose, 4.5% xylose, 0.5% peptone, 0.5% yeast extract, 0.5% KH 2 PO 4 , 0.02% (NH 4 ) 2 SO 4, 0.04% MgSO 4 .7H 2 O, pH5.0. mineral salt was sterilized by filtration, other medium was sterilized at 121°C for 20min.…”