Abstract
BackgroundNumerous studies have examined the direct fermentation of cellulosic materials by cellulase-expressing yeast; however, ethanol productivity in these systems has not yet reached an industrial level. Certain microorganisms, such as the cellulolytic fungus Trichoderma reesei, produce expansin-like proteins, which have a cellulose-loosening effect that may increase the breakdown of cellulose. Here, to improve the direct conversion of cellulose to ethanol, yeast Saccharomyces cerevisiae co-displaying cellulase and expansin-like protein on the cell surface were constructed and examined for direct ethanol fermentation performance.ResultsThe cellulase and expansin-like protein co-expressing strain showed 246 mU/g-wet cell of phosphoric acid swollen cellulose (PASC) degradation activity, which corresponded to 2.9-fold higher activity than that of a cellulase-expressing strain. This result clearly demonstrated that yeast cell-surface expressed cellulase and expansin-like protein act synergistically to breakdown cellulose. In fermentation experiments examining direct ethanol production from PASC, the cellulase and expansin-like protein co-expressing strain produced 3.4 g/L ethanol after 96 h of fermentation, a concentration that was 1.4-fold higher than that achieved by the cellulase-expressing strain (2.5 g/L).ConclusionsThe PASC degradation and fermentation ability of an engineered yeast strain was markedly improved by co-expressing cellulase and expansin-like protein on the cell surface. To our knowledge, this is the first report to demonstrate the synergetic effect of co-expressing cellulase and expansin-like protein on a yeast cell surface, which may be a promising strategy for constructing direct ethanol fermenting yeast from cellulose.
Highlights
Numerous studies have examined the direct fermentation of cellulosic materials by cellulase-expressing yeast; ethanol productivity in these systems has not yet reached an industrial level
Synergism between cellulase-expressing and expansin-like protein-expressing yeast strains To confirm the synergetic effect of yeast cell surfacedisplayed expansin-like proteins (SWOI and AoelpI) and cellulases (EGII, CBHII, and BGL) on cellulose degradation, the phosphoric acid swollen cellulose (PASC) degradation ability of a mix of the cellulase-expressing strain MT8-1/cocδBEC1 and a yeast strain MT8-1/δSWO or MT8-1/δPAO displaying the expansin-like protein SWOI or Aoep1 was evaluated (Figure 1)
These results suggested that yeast cell-surface expressed expansin-like protein and cellulase acted synergistically in the degradation of cellulose
Summary
Numerous studies have examined the direct fermentation of cellulosic materials by cellulase-expressing yeast; ethanol productivity in these systems has not yet reached an industrial level. Certain microorganisms, such as the cellulolytic fungus Trichoderma reesei, produce expansin-like proteins, which have a cellulose-loosening effect that may increase the breakdown of cellulose. To improve the direct conversion of cellulose to ethanol, yeast Saccharomyces cerevisiae co-displaying cellulase and expansin-like protein on the cell surface were constructed and examined for direct ethanol fermentation performance. Yamada et al [11] reported direct ethanol production from phosphoric acid swollen cellulose (PASC) and pretreated rice straw by a yeast strain constructed using a novel expression optimization method to co-display Trichoderma reesei EGII and CBHII, and Aspergillus aculeatus BGL1 on the cell surface. The efficiency of ethanol production from cellulose remains too low for industrial lignocellulosic ethanol production processes
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