Abstract
BackgroundThe recalcitrant nature of cellulosic materials and the high cost of enzymes required for efficient hydrolysis are the major impeding steps to their practical usage for ethanol production. Ideally, a recombinant microorganism, possessing the capability to utilize cellulose for simultaneous growth and ethanol production, is of great interest. We have reported recently the use of a yeast consortium for the functional presentation of a mini-cellulosome structure onto the yeast surface by exploiting the specific interaction of different cohesin-dockerin pairs. In this study, we engineered a yeast consortium capable of displaying a functional mini-cellulosome for the simultaneous growth and ethanol production on phosphoric acid swollen cellulose (PASC).ResultsA yeast consortium composed of four different populations was engineered to display a functional mini-cellulosome containing an endoglucanase, an exoglucanase and a β-glucosidase. The resulting consortium was demonstrated to utilize PASC for growth and ethanol production. The final ethanol production of 1.25 g/L corresponded to 87% of the theoretical value and was 3-fold higher than a similar yeast consortium secreting only the three cellulases. Quantitative PCR was used to enumerate the dynamics of each individual yeast population for the two consortia. Results indicated that the slight difference in cell growth cannot explain the 3-fold increase in PASC hydrolysis and ethanol production. Instead, the substantial increase in ethanol production is consistent with the reported synergistic effect on cellulose hydrolysis using the displayed mini-cellulosome.ConclusionsThis report represents a significant step towards the goal of cellulosic ethanol production. This engineered yeast consortium displaying a functional mini-cellulosome demonstrated not only the ability to grow on the released sugars from PASC but also a 3-fold higher ethanol production than a similar yeast consortium secreting only the three cellulases. The use of more complex cellulosomal structures may further improve the overall efficiency for ethanol production.
Highlights
The recalcitrant nature of cellulosic materials and the high cost of enzymes required for efficient hydrolysis are the major impeding steps to their practical usage for ethanol production
We have recently reported the use of a yeast consortium for the functional presentation of a mini-cellulosome structure onto the yeast surface by exploiting the specific interaction of the different cohesin-dockerin pairs employed [10]
Surface display of the mini-scaffoldin Scaf-ctf using the constitutive Aga1 anchor system To enable the direct growth and ethanol production on phosphoric acid swollen cellulose (PASC) by the synthetic yeast consortium, the Aga1-Aga2 anchor system used in the previous study [10] which required galactose for induced expression was replaced by a constitutively expressed Aga1 anchor system using a strong PGK promoter (Figure 1A)
Summary
The recalcitrant nature of cellulosic materials and the high cost of enzymes required for efficient hydrolysis are the major impeding steps to their practical usage for ethanol production. A recombinant microorganism, possessing the capability to utilize cellulose for simultaneous growth and ethanol production, is of great interest. Ethanol, which is generally expected to be the first major commercial product of this emerging cellulosic biofuel technology, the recalcitrant nature of cellulosic materials and the high cost of enzymes required for efficient hydrolysis are the major limiting steps to the more widespread exploitation of this natural resource [3]. An ideal microorganism for CBP should possess the capability of efficient enzyme production and simultaneous cellulose saccharification and ethanol fermentation. Attempts have been made to engineer S. cerevisiae for cellulose hydrolysis under anaerobic conditions with only varying degrees of success [6-8]
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