Secretive expression of heterologous β-glucosidase in Zymomonas mobilis

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.