Comparison of Spathaspora passalidarum and recombinant Saccharomyces cerevisiae for integration of first- and second-generation ethanol production

Expression of a periplasmic β-glucosidase from Yarrowia lipolytica allows efficient cellobiose-xylose co-fermentation by industrial xylose-fermenting Saccharomyces cerevisiae strains

This study aimed to compare the effects of cellobiose hydrolysis, whether occurring inside or outside the cell, on the ability of Saccharomyces cerevisiae strains to ferment this sugar and then apply the most effective strategy to industrial S. cerevisiae strains. Firstly, two recombinant laboratory S. cerevisiae strains were engineered: CEN.PK-X-Bgl1YL, expressing the periplasmic β-glucosidase BGL1 from Yarrowia lipolytica; and CEN.PK-X-B7-T2, co-expressing the intracellular β-glucosidase SpBGL7 from Spathaspora passalidarum and the cellobiose transporter MgCBT2 from Meyerozyma guilliermondii. Both engineered strains were able to grown in media with cellobiose and to ferment this disaccharide. However, CEN.PK-X-Bgl1YL, which hydrolyzes cellobiose extracellularly, exhibited faster growth and superior batch fermentation performance. Furthermore, enzymatic and transport activities revealed that sugar uptake was possibly the limiting factor in cellobiose fermentation by CEN.PK-X-B7-T2. Since extracellular hydrolysis with the periplasmic β-glucosidase was more efficient for cellobiose fermentation, we integrated the BGL1 gene into two industrial xylose-fermenting S. cerevisiae strains. The resulting strains (MP-C5H1-Bgl1YL and MP-P5-Bgl1YL) efficiently co-consumed ∼ 22 g L- 1 of cellobiose and ∼ 22 g L- 1 of xylose in 24 h, achieving high ethanol production levels (∼ 17 g L- 1 titer, ∼ 0.50 g L- 1 h- 1 volumetric productivity, and 0.40 g g- 1 ethanol yield). Our findings suggest that the expression of periplasmic β-glucosidases in S. cerevisiae could be an effective strategy to overcome the disaccharide transport problem, thus enabling efficient cellobiose fermentation or even cellobiose-xylose co-fermentation.

First- and second-generation integrated process for bioethanol production: Fermentation of molasses diluted with hemicellulose hydrolysate by recombinant Saccharomyces cerevisiae

The integration of first- (1G) and second-generation (2G) ethanol production by adding sugarcane juice or molasses to lignocellulosic hydrolysates offers the possibility to overcome the problem of inhibitors (acetic acid, furfural, hydroxymethylfurfural and phenolic compounds), and add nutrients (such as salts, sugars and nitrogen sources) to the fermentation medium, allowing the production of higher ethanol titers. In this work, an 1G2G production process was developed with hemicellulosic hydrolysate (HH) from a diluted sulfuric acid pretreatment of sugarcane bagasse and sugarcane molasses. The industrial Saccharomyces cerevisiae CAT-1 was genetically modified for xylose consumption and used for co-fermentation of sucrose, fructose, glucose, and xylose. The fed-batch fermentation with high cell density that mimics an industrial fermentation was performed at bench scale fermenter, achieved high volumetric ethanol productivity of 1.59 g L-1 h-1, 0.39 g g-1 of ethanol yield, and 44.5 g L-1 ethanol titer, and shown that the yeast was able to consume all the sugars present in must simultaneously. With the results, it was possible to establish a mass balance for the global process: from pretreatment to the co-fermentation of molasses and HH, and it was possible to establish an effective integrated process (1G2G) with sugarcane molasses and HH co-fermentation employing a recombinant yeast.

Strategies for Efficient Expression of Heterologous Monosaccharide Transporters in Saccharomyces cerevisiae

In previous work, we developed a Saccharomyces cerevisiae strain (DLG-K1) lacking the main monosaccharide transporters (hxt-null) and displaying high xylose reductase, xylitol dehydrogenase and xylulokinase activities. This strain proved to be a useful chassis strain to study new glucose/xylose transporters, as SsXUT1 from Scheffersomyces stipitis. Proteins with high amino acid sequence similarity (78–80%) to SsXUT1 were identified from Spathaspora passalidarum and Spathaspora arborariae genomes. The characterization of these putative transporter genes (SpXUT1 and SaXUT1, respectively) was performed in the same chassis strain. Surprisingly, the cloned genes could not restore the ability to grow in several monosaccharides tested (including glucose and xylose), but after being grown in maltose, the uptake of ¹⁴C-glucose and ¹⁴C-xylose was detected. While SsXUT1 lacks lysine residues with high ubiquitinylation potential in its N-terminal domain and displays only one in its C-terminal domain, both SpXUT1 and SaXUT1 transporters have several such residues in their C-terminal domains. A truncated version of SpXUT1 gene, deprived of the respective 3′-end, was cloned in DLG-K1 and allowed growth and fermentation in glucose or xylose. In another approach, two arrestins known to be involved in the ubiquitinylation and endocytosis of sugar transporters (ROD1 and ROG3) were knocked out, but only the rog3 mutant allowed a significant improvement of growth and fermentation in glucose when either of the XUT permeases were expressed. Therefore, for the efficient heterologous expression of monosaccharide (e.g., glucose/xylose) transporters in S. cerevisiae, we propose either the removal of lysines involved in ubiquitinylation and endocytosis or the use of chassis strains hampered in the specific mechanism of membrane protein turnover.