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Arabinose Induced Catabolite Repression as Mechanism for Pentose Hierarchy Control in Clostridium acetobutylicum ATCC 824

ABSTRACT: Background: Bacterial fermentation of carbohydrates from sustainable lignocellulosic biomass into biofuels by the anaerobic bacterium Clostridium acetobutylicum is a promising alternative energy source to fossil fuels. Understanding the complex metabolic pathways it employs and how they are regulated will contribute to improved biofuel production. Recently, it has been demonstrated that xylose is not appreciably fermented in the presence of arabinose, suggesting a hierarchy of pentose utilization in this organism. Our goal is to uncover if transcriptional regulation contributes to this hierarchy. Results: Growth and sugar consumption rates showed that arabinose, like glucose, actively represses xylose utilization in cultures fermenting xylose. RNA-Seq revealed there was a large overlap in differentially regulated genes after addition of arabinose or glucose, suggesting a common mechanism of regulation. A putative ORF was identified that may be important for transcriptional regulation in response to the nutritional state of the cells. Conclusions: Decreased xylose consumption, increased acetate production, and transcription of the phosphoketolase gene (CA_C1343) revealed a transition of pentose catabolism from the pentose phosphate pathway to the phosphoketolase pathway after addition of arabinose. Together, these results substantiate the claim that arabinose is utilized preferentially over xylose in C. acetobutylicum. Furthermore, they demonstrate that this phenomenon is modulated in part at the transcriptional level, and they provide valuable insight into potential mechanisms for altering pentose utilization to modulate fermentation products for biofuel production.

ORGANISM(S): Clostridium acetobutylicum ATCC 824

PROVIDER: GSE107804 | GEO | 2017/12/20

SECONDARY ACCESSION(S): PRJNA421436

REPOSITORIES: GEO

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Project description:Background Lignocellulosic biomass is a promising renewable feedstock for biofuel production. Acetate is one of the major inhibitors liberated from hemicelluloses during hydrolysis. An understanding of the toxic effects of acetate on the fermentation microorganism and the efficient utilization of mixed sugars of glucose and xylose in the presence of hydrolysate inhibitors is crucial for economic biofuel production. Results A new microarray was designed including both coding sequences and intergenic regions to investigate the acetate stress responses of Zymomonas mobilis 8b when using single carbon sources of glucose or xylose, or mixed sugars of both glucose and xylose. With the supplementation of exogenous acetate, 8b can utilize all the glucose with a similar ethanol yield, although the growth, final biomass, and ethanol production rate were reduced. However, xylose utilization was inhibited in both media containing xylose or a mixed sugar of glucose and xylose, although the performance of 8b was better in mixed sugar than xylose-only media. The presence of acetate caused genes related to biosynthesis, the flagellar system, and glycolysis to be downregulated, and genes related to stress responses and energy metabolism to be upregulated. Unexpectedly, xylose seems to pose more stress on 8b, recruiting more genes for xylose utilization, than does acetate. Several gene candidates based on transcriptome results were selected for genetic manipulation, and a TonB-dependent receptor knockout mutant was confirmed to have a slight advantage regarding acetate tolerance. Conclusions Our results indicate Z. mobilis utilized a different mechanism for xylose utilization, with an even more severe impact on Z. mobilis than that caused by acetate treatment. Our study also suggests redox imbalance caused by stressful conditions may trigger a metabolic reaction leading to the accumulation of toxic intermediates such as xylitol, but Z. mobilis manages its carbon and energy metabolism through the control of individual reactions to mitigate the stressful conditions. We have thus provided extensive transcriptomic datasets and gained insights into the molecular responses of Z. mobilis to the inhibitor acetate when grown in different sugar sources, which will facilitate future metabolic modeling studies and strain improvement efforts for better xylose utilization and acetate tolerance.

Project description:Pseudomonas alloputida KT2440 (previously misclassified as P. putida KT2440 based on 16S rRNA gene homology) has emerged as an ideal host strain for plan t biomass valorization. However, P. alloputida KT2440 is unable to natively utilize abundant pentose sugars (e.g., xylose and arabinose) in hydrolysate streams, which may account for up to 25% of lignocellulosic biomass. In the last decades, microbes have been engineered to utilize the pentose sugars. However, most of the engineered strains were either slow-growing or displayed phenotypes that could not be replicated. In this work, we successfully isolated five Pseudomonas species with the native capability to utilize glucose, xylose and p-coumarate as a sole carbon source. These isolates were in two clusters; one set of isolates (M2 and M5) and the second set of isolates (BP6 and BP7) showed 85.6% and 96.2% ANI, respectively, to P. alloputida KT24440. BP8 showed 84.6% ANI to P. putida KT2440 and does not belong to any neighboring type strains indicating a new species. Notably, the isolates showed robust growth solely on xylose and higher growth rates (m, 0.36-0.49 h-1) when compared to only known xylose-utilizing Pseudomonas taiwanenesis VLB120 (m, 0.28 h-1) as a control. Unexpectedly, among five isolates, M2 and M5 grew solely on arabinose as well. Comprehensive analysis of genomics, transcriptomics and proteomics revealed the isolates utilize xylose and arabinose via Weimberg pathway (xylD-xylX-xylA) and oxidative pathway (araD-araX-araA), respectively. Furthermore, a preliminary result demonstrated the production of flaviolin solely on xylose and arabinose in the isolate, showing noteworthy potential to be an alternative host for lignocellulosic feedstocks into valuable products. This is the first report on isolating Pseudomonas strains natively capable of utilizing all of the major carbon sources in lignocellulosic biomass, and leading to higher consumption of available substrates and therefore maximizing the product yield.

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Arabinose Induced Catabolite Repression as Mechanism for Pentose Hierarchy Control in Clostridium acetobutylicum ATCC 824

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