Installing xylose assimilation and cellodextrin phosphorolysis pathways in obese Yarrowia lipolytica facilitates cost-effective lipid production from lignocellulosic hydrolysates

BACKGROUND

Yarrowia lipolytica, one of the most charming chassis cells in synthetic biology, is unable to use xylose and cellodextrins.

RESULTS

Herein, we present work to tackle for the first time the engineering of Y. lipolytica to produce lipids from cellodextrins and xylose by employing rational and combinatorial strategies. This includes constructing a cellodextrin-phosphorolytic Y. lipolytica by overexpressing Neurospora crassa cellodextrin transporter, Clostridium thermocellum cellobiose/cellodextrin phosphorylase and Saccharomyces cerevisiae phosphoglucomutase. The effect of glucose repression on xylose consumption was relieved by installing a xylose uptake facilitator combined with enhanced PPP pathway and increased cytoplasmic NADPH supply. Further enhancing lipid production and interrupting its consumption conferred the obese phenotype to the engineered yeast. The strain is able to co-ferment glucose, xylose and cellodextrins efficiently, achieving a similar μmax of 0.19 h-1, a qs of 0.34 g-s/g-DCW/h and a YX/S of 0.54 DCW-g/g-s on these substrates, and an accumulation of up to 40% of lipids on the sugar mixture and on wheat straw hydrolysate.

CONCLUSIONS

Therefore, engineering Y. lipolytica capable of assimilating xylose and cellodextrins is a vital step towards a simultaneous saccharification and fermentation (SSF) process of LC biomass, allowing improved substrate conversion rate and reduced production cost due to low demand of external glucosidase.

In vitro and in vivo exploration of the cellobiose and cellodextrin phosphorylases panel in Ruminiclostridium cellulolyticum: implication for cellulose catabolism

BACKGROUND

In anaerobic cellulolytic micro-organisms, cellulolysis results in the action of several cellulases gathered in extracellular multi-enzyme complexes called cellulosomes. Their action releases cellobiose and longer cellodextrins which are imported and further degraded in the cytosol to fuel the cells. In Ruminiclostridium cellulolyticum, an anaerobic and cellulolytic mesophilic bacteria, three cellodextrin phosphorylases named CdpA, CdpB, and CdpC, were identified in addition to the cellobiose phosphorylase (CbpA) previously characterized. The present study aimed at characterizing them, exploring their implication during growth on cellulose to better understand the life-style of cellulolytic bacteria on such substrate.

RESULTS

The three cellodextrin phosphorylases from R. cellulolyticum displayed marked different enzymatic characteristics. They are specific for cellodextrins of different lengths and present different k _cat values. CdpC is the most active enzyme before CdpA, and CdpB is weakly active. Modeling studies revealed that a mutation of a conserved histidine residue in the phosphate ion-binding pocket in CdpB and CdpC might explain their activity-level differences. The genes encoding these enzymes are scattered over the chromosome of R. cellulolyticum and only the expression of the gene encoding the cellobiose phosphorylase and the gene cdpA is induced during cellulose growth. Characterization of four independent mutants constructed in R. cellulolyticum for each of the cellobiose and cellodextrin phosphorylases encoding genes indicated that only the cellobiose phosphorylase is essential for growth on cellulose.

CONCLUSIONS

Unexpectedly, the cellobiose phosphorylase but not the cellodextrin phosphorylases is essential for the growth of the model bacterium on cellulose. This suggests that the bacterium adopts a "short" dextrin strategy to grow on cellulose, even though the use of long cellodextrins might be more energy-saving. Our results suggest marked differences in the cellulose catabolism developed among cellulolytic bacteria, which is a result that might impact the design of future engineered strains for biomass-to-biofuel conversion.

A seven-gene cluster in Ruminiclostridium cellulolyticum is essential for signalization, uptake and catabolism of the degradation products of cellulose hydrolysis

BACKGROUND

Like a number of anaerobic and cellulolytic Gram-positive bacteria, the model microorganism Ruminiclostridium cellulolyticum produces extracellular multi-enzymatic complexes called cellulosomes, which efficiently degrade the crystalline cellulose. Action of the complexes on cellulose releases cellobiose and longer cellodextrins but to date, little is known about the transport and utilization of the produced cellodextrins in the bacterium. A better understanding of the uptake systems and fermentation of sugars derived from cellulose could have a major impact in the field of biofuels production.

RESULTS

We characterized a putative ABC transporter devoted to cellodextrins uptake, and a cellobiose phosphorylase (CbpA) in R. cellulolyticum. The genes encoding the components of the ABC transporter (a binding protein CuaA and two integral membrane proteins) and CbpA are expressed as a polycistronic transcriptional unit induced in the presence of cellobiose. Upstream, another polycistronic transcriptional unit encodes a two-component system (sensor and regulator), and a second binding protein CuaD, and is constitutively expressed. The products might form a three-component system inducing the expression of cuaABC and cbpA since we showed that CuaR is able to recognize the region upstream of cuaA. Biochemical analysis showed that CbpA is a strict cellobiose phosphorylase inactive on longer cellodextrins; CuaA binds to all cellodextrins (G2-G5) tested, whereas CuaD is specific to cellobiose and presents a higher affinity to this sugar. This results are in agreement with their function in transport and signalization, respectively. Characterization of a cuaD mutant, and its derivatives, indicated that the ABC transporter and CbpA are essential for growth on cellobiose and cellulose.

CONCLUSIONS

For the first time in a Gram-positive strain, we identified a three-component system and a conjugated ABC transporter/cellobiose phosphorylase system which was shown to be essential for the growth of the model cellulolytic bacterium R. cellulolyticum on cellobiose and cellulose. This efficient and energy-saving system of transport and phosphorolysis appears to be the major cellobiose utilization pathway in R. cellulolyticum, and seems well adapted to cellulolytic life-style strain. It represents a new way to enable engineered strains to utilize cellodextrins for the production of biofuels or chemicals of interest from cellulose.