Metabolic engineering of Clostridium tyrobutyricum for n-butanol production through co-utilization of glucose and xylose

The glucose-mediated carbon catabolite repression (CCR) in Clostridium tyrobutyricum impedes efficient utilization of xylose present in lignocellulosic biomass hydrolysates. In order to relieve the CCR and enhance xylose utilization, three genes (xylT, xylA, and xylB) encoding a xylose proton-symporter, a xylose isomerase and a xylulokinase, respectively, from Clostridium acetobutylicum ATCC 824 were co-overexpressed with aldehyde/alcohol dehydrogenase (adhE2) in C. tyrobutyricum (Δack). Compared to the strain Ct(Δack)-pM2 expressing only adhE2, the mutant Ct(Δack)-pTBA had a higher xylose uptake rate and was able to simultaneously consume glucose and xylose at comparable rates for butanol production. Ct(Δack)-pTBA produced more butanol (12.0 vs. 3.2 g/L) with a higher butanol yield (0.12 vs. 0.07 g/g) and productivity (0.17 vs. 0.07 g/L · h) from both glucose and xylose, while Ct(Δack)-pM2 consumed little xylose in the fermentation. The results confirmed that the CCR in C. tyrobutyricum could be overcome through overexpressing xylT, xylA, and xylB. The mutant was also able to co-utilize glucose and xylose present in soybean hull hydrolysate (SHH) for butanol production, achieving a high butanol titer of 15.7 g/L, butanol yield of 0.24 g/g, and productivity of 0.29 g/L · h. This study demonstrated the potential application of Ct(Δack)-pTBA for industrial biobutanol production from lignocellulosic biomass.

Adaptive evolution for fast growth on glucose and the effects on the regulation of glucose transport system in Clostridium tyrobutyricum

Laboratory adaptive evolution of microorganisms offers the possibility of relating acquired mutations to increased fitness of the organism under the conditions used. By combining a fibrous-bed bioreactor, we successfully developed a simple and valuable adaptive evolution strategy in repeated-batch fermentation mode with high initial substrate concentration and evolved Clostridium tyrobutyricum mutant with significantly improved butyric acid volumetric productivity up to 2.25 g/(L h), which is the highest value in batch fermentation reported so far. Further experiments were conducted to pay attention to glucose transport system in consideration of the high glucose consumption rate resulted from evolution. Complete characterization and comparison of the glucose phosphoenolpyruvate (PEP)-dependent phosphotransferase system (PTS) were carried out in the form of toluene-treated cells and cell-free extracts derived from both C. tyrobutyricum wide-type and mutant, while an alternative glucose transport route that requires glucokinase was confirmed by the phenomena of resistance to the glucose analogue 2-deoxyglucose and ATP-dependent glucose phosphorylation. Our results suggest that C. tyrobutyricum mutant is defective in PTS activity and compensates for this defect with enhanced glucokinase activity, resulting in the efficient uptake and consumption of glucose during the whole metabolism.

Construction and characterization of ack deleted mutant of Clostridium tyrobutyricum for enhanced butyric acid and hydrogen production

Clostridium tyrobutyricum produces butyrate, acetate, H(2), and CO(2) as its main fermentation products from glucose and xylose. To improve butyric acid and hydrogen production, integrational mutagenesis was used to create a metabolically engineered mutant with inactivated ack gene, encoding acetate kinase (AK) associated with the acetate formation pathway. A non-replicative plasmid containing the acetate kinase gene (ack) fragment was constructed and introduced into C. tyrobutyricum by electroporation. Integration of the plasmid into the homologous region on the chromosome should inactivate the target ack gene and produce ack-deleted mutant, PAK-Em. Enzyme activity assays showed that the AK activity in PAK-Em decreased by approximately 50%; meanwhile, phosphotransacetylase (PTA) and hydrogenase activities each increased by approximately 40%. The sodium dodecyl sulfate-polyacrylamide gel electrophoresis (SDS-PAGE) results showed that the expression of protein with approximately 32 kDa molecular mass was reduced significantly in the mutant. Compared to the wild type, the mutant grew more slowly at pH 6.0 and 37 degrees C, with a lower specific growth rate of 0.14 h(-1) (vs 0.21 h(-1) for the wild type), likely due to the partially impaired PTA-AK pathway. However, the mutant produced 23.5% more butyrate (0.42 vs 0.34 g/g glucose) at a higher final concentration of 41.7 g/L (vs 19.98 g/L) as a result of its higher butyrate tolerance as indicated in the growth kinetics study using various intial concentrations of butyrate in the media. The mutant also produced 50% more hydrogen (0.024 g/g) from glucose than the wild type. Immobilized-cell fermentation of PAK-Em in a fibrous-bed bioreactor (FBB) further increased the final butyric acid concentration (50.1 g/L) and the butyrate yield (0.45 g/g glucose). Furthermore, in the FBB fermentation at pH 5.0 with xylose as the substrate, only butyric acid was produced by the mutant, whereas the wild type produced large amounts of acetate (0.43 g/g xylose) and lactate (0.61 g/g xylose) and little butyrate (0.05 g/g xylose), indicating a dramatic metabolic pathway shift caused by the ack deletion in the mutant.

Construction and characterization of pta gene-deleted mutant of Clostridium tyrobutyricum for enhanced butyric acid fermentation

Clostridium tyrobutyricum ATCC 25755 is an acidogenic bacterium, producing butyrate and acetate as its main fermentation products. In order to decrease acetate and increase butyrate production, integrational mutagenesis was used to disrupt the gene associated with the acetate formation pathway in C. tyrobutyricum. A nonreplicative integrational plasmid containing the phosphotransacetylase gene (pta) fragment cloned from C. tyrobutyricum by using degenerate primers and an erythromycin resistance cassette were constructed and introduced into C. tyrobutyricum by electroporation. Integration of the plasmid into the homologous region on the chromosome inactivated the target pta gene and produced the pta-deleted mutant (PTA-Em), which was confirmed by Southern hybridization. SDS-PAGE and two-dimensional protein electrophoresis results indicated that protein expression was changed in the mutant. Enzyme activity assays using the cell lysate showed that the activities of PTA and acetate kinase (AK) in the mutant were reduced by more than 60% for PTA and 80% for AK. The mutant grew more slowly in batch fermentation with glucose as the substrate but produced 15% more butyrate and 14% less acetate as compared to the wild-type strain. Its butyrate productivity was approximately 2-fold higher than the wild-type strain. Moreover, the mutant showed much higher tolerance to butyrate inhibition, and the final butyrate concentration was improved by 68%. However, inactivation of pta gene did not completely eliminate acetate production in the fermentation, suggesting the existence of other enzymes (or pathways) also leading to acetate formation. This is the first-reported genetic engineering study demonstrating the feasibility of using a gene-inactivation technique to manipulate the acetic acid formation pathway in C. tyrobutyricum in order to improve butyric acid production from glucose.