Abstract
Background: Zymomonas mobilis is an efficient ethanol fermentation strain, but its narrow substrate range limits its fermentation in lignocellulose hydrolysate. As a potential consolidated bioprocessing (CBP) stain for bioethanol production, the ability of cellulose utilization was necessary. In this study, extracellular expression of β-glucosidase on Z. mobilis was studied as the first step for construction of a practical CBP strain to reduce the use of β-glucosidase in the cellulase components. Results: The heterologous β-glucosidase from Bacillus polymyxa was expressed in the ethanologenic strain Z. mobilis (ZM4) and secreted extracellularly by an endogenous signal peptide and a fusion protein. The signal peptide SP1086 of the endoglucanase gene ZMO1086 from Z. mobilis was identified and facilitated 12 % of the endoglucanase encoded by ZMO1086 from Z. mobilis ZM4 and 16 % of the β-glucosidase encoded by bglB gene secreted out of the membrane of Z. mobilis ZM4. Another method for enhancement of the β-glucosidase secretion is to fuse the β-glucosidase encoded by bglB with the levansucrase encoded by sacB from Z. mobilis ZM4 to achieve the secretive expression. Its expression level was enhanced two times but only showed a 2 % secretion ratio in this situation. Conclusions: The SP1086 signal peptide showed an obviously secreting capacity of the β-glucosidase protein. The fusion protein with SacB also showed the secretion effect, but it was less efficient.
Highlights
Zymomonas mobilis is an efficient ethanol fermentation strain, but its narrow substrate range limits its fermentation in lignocellulose hydrolysate
The ZMO1086 gene fragment was located at the downstream of the promoter Peno, and the signal peptide SP1086 was included in the CDS of ZMO1086 at the 5′ end
The results indicated that the secretive expression of ZMO1086 using the signal peptide in Z. mobilis ZM4 was successful the secretion ratio was not satisfactory
Summary
Zymomonas mobilis is an efficient ethanol fermentation strain, but its narrow substrate range limits its fermentation in lignocellulose hydrolysate. Extracellular expression of β-glucosidase on Z. mobilis was studied as the first step for construction of a practical CBP strain to reduce the use of β-glucosidase in the cellulase components. Consolidated bioprocessing (CBP) integrates cellulase production, cellulose hydrolysis, and ethanol fermentation into one single unit; the expensive cellulase enzyme production step is removed or the usage is reduced [1]. When cellulase enzyme is produced by CBP cells, the cellulase should be in effective contact with a solid cellulosic substrate for the hydrolysis to occur. To achieve this goal, the cellulase enzymes are sufficiently expressed and the enzymes should be secreted extracellularly onto the periplasm space of the cell or directly to the extracellular medium.
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