Coordinately Express Hemicellulolytic Enzymes in Kluyveromyces Marxianus to Improve the Saccharification and Ethanol Production from Corncobs

Abstract

BackgroundHemicelluloses act as one factor contributing to the recalcitrance of lignocelluloses that prevent cellulases to degrade the cellulose efficiently even in low quantities. Supplement of hemicellulases can enhance performance of commercial cellulases in the enzymatic hydrolyses of lignocellulose. Kluyveromyce marxianu is an attractive yeast for cellulosic ethanol fermentation, as well as a promising host for heterologous protein production, since it has remarkable thermotolerance, high growth rate, and broad substrate spectrum etc. In this study, we attempted to coordinately express multiple hemicellulases in K. marxianus through a 2A-mediated ribosomes skipping to self-cleave polyproteins, and investigated their capabilities for saccharification and ethanol production from corncobs.ResultsTwo polycistronic genes IMPX and IMPαX were constructed to test the self-cleavage of P2A sequence from Foot and Mouth Disease virus (FMDV) in K. marxianus. The IMPX gene consisted of a β-mannanase gene M330 (without the stop codon), a P2A sequence and a β-xylanase gene Xyn-CDBFV in turn, while in the IMPαX gene there was an additional α-factor signal sequence in frame with the N-terminus of Xyn-CDBFV. The extracellular β-mannanase activities of IMPX and IMPαX strains were 21.34 and 15.50 U/mL repectively. By contrast, the IMPαX strain secreted 136.17 U/mLof the β-xylanase, which was much higher than that of IMPX strain 42.07 U/mL. Based on these, two recombinant strains, the IXαR and IMPαXPαR, were constructed to coordinately and secretorily express two xylantic enzymes a β-D-xylosidase RuXyn1 and Xyn-CDBFV, or three hemicellulolytic enzymes including M330, Xyn-CDBFV and RuXyn1. In fed-batch fermentations, extracellular activities of β-xylanase and β-xylosidase in the IMPαX strain were 1664.2 and 0.90 U/mL, while productions of secretory β-mannanase, β-xylanase, and β-xylosidase in the IMPαXPαR strain were 159.8, 2210.5, and 1.25 U/ml of respectively. Hemicellulolytic enzymes of these two strains enhanced the yields of both glucose and xylose from diluted acid pretreated (DAP) corncobs when acted synergistically with commercial cellulases. In hybrid saccharification and fermentation (HSF) of DAP corncobs, hemicellulases of the IMPαXPαR strain increased the ethanol yields by 8.7% at 144 h. When using aqueous ammonia pretreated (AAP) corncobs as HSF feedstocks, the IMPαXPαR strain increased both ethanol and xylose yields, which were about 12.7% and 18.2% more than that of the control at 120 h. Our results indicated that coordinately expression of hemicellulolytic enzymes in K. marxianus could promote the saccharification and ethanol production from corncobs.ConclusionsThe FMDV P2A sequence showed high efficiency in self-cleavage of polyproteins in K. marxianus, and could be used for secretory expression of multiple enzymes in present of their own signal sequences. The IMPαXPαR strain that coexpressed three hemicellulolytic enzymes improved the saccharification and ethanol production from corncobs, and could be used as a promising strain for ethanol production from lignocelluloses.