Ethanol production using a newly isolated Saccharomyces cerevisiae strain directly assimilating intact inulin with a high degree of polymerization

Glucose Starvation Stimulates the Promoting Strength of a Novel Evolved Suc2 Promoter

Promoters are essential for designing Saccharomyces cerevisiae cell factories. Identifying novel promoters tuned to produce specific metabolites under increasingly diverse industrial stresses is required to improve the economic feasibility of whole fermentation processes. In this study, a positively evolved Suc2 promoter (SUC 2p) with promoter activity stronger than that of the wild-type Suc2 promoter (SUC 2wtp) was obtained. Quantitative real-time PCR, fluorescence analysis, Western blotting, and a β-galactosidase activity assay revealed that SUC 2p is a medium-strength promoter compared with eight reported promoters at a medium glucose concentration (2% (w/v)). Different glucose concentrations modulated the strength of SUC 2p. Low glucose concentrations (0.05 and 0.5% (w/v)) enhanced the promoter strength of SUC 2p dramatically, with promoter activity higher than that of reported strong promoters. Glucose starvation resulted in the formation of a new Msn2/4 binding site on SUC 2p. Our work should facilitate the development of promoters with novel fine-tuning properties and the construction of S. cerevisiae cell factories suitable for the industrial production of essential chemicals under glucose-deprived conditions.

Microbial production of mannitol by Lactobacillus brevis 3-A5 from concentrated extract of Jerusalem artichoke tubers

In the present study, the conversion of the extract of Jerusalem artichoke tubers for mannitol production by Lactobacillus brevis 3-A5 was investigated. When the bacterium utilized enzymatic hydrolysates of Jerusalem artichoke extract as the main substrates in batch fermentation, the significant decrease in mannitol productivity was observed when the initial concentration of reducing sugar increased. Then, a strategy of continuous fed-batch fermentation was adopted for improving mannitol production with enzymatic hydrolysates of Jerusalem artichoke extract as main substrates. Although the concentration of mannitol could reach 199.86 g/L at the end of the fermentation, the productivity for the overall process of the fermentation was only 1.67 g/L/H. To improve the mannitol productivity with both higher yield and concentration, the simultaneous enzymatic saccharification and fermentation (SSF) was studied. In SSF, the mannitol production reached 176.50 g/L in 28 H with a productivity of 6.30 g/L/H and a yield of 0.68 g/g total sugar. Our study provides a cost-effective and eco-friendly method for mannitol production from a cheap biomass.

Utilization of inulin-containing waste in industrial fermentations to produce biofuels and bio-based chemicals

Inulins are polysaccharides that belong to an important class of carbohydrates known as fructans and are used by many plants as a means of storing energy. Inulins contain 20 to several thousand fructose units joined by β-2,1 glycosidic bonds, typically with a terminal glucose unit. Plants with high concentrations of inulin include: agave, asparagus, coffee, chicory, dahlia, dandelion, garlic, globe artichoke, Jerusalem artichoke, jicama, onion, wild yam, and yacón. To utilize inulin as its carbon and energy source directly, a microorganism requires an extracellular inulinase to hydrolyze the glycosidic bonds to release fermentable monosaccharides. Inulinase is produced by many microorganisms, including species of Aspergillus, Kluyveromyces, Penicillium, and Pseudomonas. We review various inulinase-producing microorganisms and inulin feedstocks with potential for industrial application as well as biotechnological efforts underway to develop sustainable practices for the disposal of residues from processing inulin-containing crops. A multi-stage biorefinery concept is proposed to convert cellulosic and inulin-containing waste produced at crop processing operations to valuable biofuels and bioproducts using Kluyveromyces marxianus, Yarrowia lipolytica, Rhodotorula glutinis, and Saccharomyces cerevisiae as well as thermochemical treatments.

Engineering a natural Saccharomyces cerevisiae strain for ethanol production from inulin by consolidated bioprocessing

BACKGROUND

The yeast Saccharomyces cerevisiae is an important eukaryotic workhorse in traditional and modern biotechnology. At present, only a few S. cerevisiae strains have been extensively used as engineering hosts. Recently, an astonishing genotypic and phenotypic diversity of S. cerevisiae was disclosed in natural populations. We suppose that some natural strains can be recruited as superior host candidates in bioengineering. This study engineered a natural S. cerevisiae strain with advantages in inulin utilization to produce ethanol from inulin resources by consolidated bioprocess. Rational engineering strategies were employed, including secretive co-expression of heterologous exo- and endo-inulinases, repression of a protease, and switch between haploid and diploid strains.

RESULTS

Results from co-expressing endo- and exo-inulinase genes showed that the extracellular inulinase activity increased 20 to 30-fold in engineered S. cerevisiae strains. Repression of the protease PEP4 influenced cell physiology in late stationary phase. Comparison between haploid and diploid engineered strains indicated that diploid strains were superior to haploid strains in ethanol production albeit not in production and secretion of inulinases. Ethanol fermentation from both inulin and Jerusalem artichoke tuber powder was dramatically improved in most engineered strains. Ethanol yield achieved in the ultimate diploid strain JZD-InuMKCP was close to the theoretical maximum. Productivity achieved in the strain JZD-InuMKCP reached to 2.44 and 3.13 g/L/h in fermentation from 200 g/L inulin and 250 g/L raw Jerusalem artichoke tuber powder, respectively. To our knowledge, these are the highest productivities reported up to now in ethanol fermentation from inulin resources.

CONCLUSIONS

Although model S. cerevisiae strains are preferentially used as hosts in bioengineering, some natural strains do have specific excellent properties. This study successfully engineered a natural S. cerevisiae strain for efficient ethanol production from inulin resources by consolidated bioprocess, which indicated the feasibility of natural strains used as bioengineering hosts. This study also presented different properties in enzyme secretion and ethanol fermentation between haploid and diploid engineering strains. These findings provided guidelines for host selection in bioengineering.

Invertase Suc2-mediated inulin catabolism is regulated at the transcript level in Saccharomyces cerevisiae

Background

Invertase Suc2 was recently identified as a key hydrolase for inulin catabolism in Saccharomyces cerevisiae, whereas the Suc2 activity degrading inulin varies greatly in different S. cerevisiae strains. The molecular mechanism causing such variation remained obscure. The aim of this study is to investigate how Suc2 activity is regulated in S. cerevisiae.

Results

The effect of SUC2 expression level on inulin hydrolysis was investigated by introducing different SUC2 genes or their corresponding promoters in S. cerevisiae strain BY4741 that can only weakly catabolize inulin. Both inulinase and invertase activities were increased with the rising SUC2 expression level. Variation in the promoter sequence has an obvious effect on the transcript level of the SUC2 gene. It was also found that the high expression level of SUC2 was beneficial to inulin degradation and ethanol yield.

Conclusions

Suc2-mediated inulin catabolism is regulated at transcript level in S. cerevisiae. Our work should be valuable for engineering advanced yeast strains in application of inulin for ethanol production.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-015-0243-3) contains supplementary material, which is available to authorized users.

Efficient simultaneous saccharification and fermentation of inulin to 2,3-butanediol by thermophilic Bacillus licheniformis ATCC 14580

2,3-Butanediol (2,3-BD) is an important starting material for the manufacture of bulk chemicals. For efficient and large-scale production of 2,3-BD through fermentation, low-cost substrates are required. One such substrate, inulin, is a polydisperse fructan found in a wide variety of plants. In this study, a levanase with high inulinase activity and high pH and temperature stability was identified in Bacillus licheniformis strain ATCC 14580. B. licheniformis strain ATCC 14580 was found to efficiently produce 2,3-BD from fructose at 50°C. Then, the levanase was used for simultaneous saccharification and fermentation (SSF) of inulin to 2,3-BD. A fed-batch SSF yielded 103.0 g/liter 2,3-BD in 30 h, with a high productivity of 3.4 g/liter · h. The results suggest that the SSF process developed with the thermophilic B. licheniformis strain used might be a promising alternative for efficient 2,3-BD production from the favorable substrate inulin.