Production of d-xylonic acid using a non-recombinant Corynebacterium glutamicum strain.

It was found that Corynebacterium glutamicum ΔiolR devoid of the transcriptional regulator IolR accumulates high amounts of d-xylonate when cultivated in the presence of d-xylose. Detailed analyses of constructed deletion mutants revealed that the putative myo-inositol 2-dehydrogenase IolG also acts as d-xylose dehydrogenase and is mainly responsible for d-xylonate oxidation in this organism. Process development for d-xylonate production was initiated by cultivating C. glutamicum ΔiolR on defined d-xylose/d-glucose mixtures under batch and fed-batch conditions. The resulting yield matched the theoretical maximum of 1 mol mol-1 and high volumetric productivities of up to 4 g L-1 h-1 could be achieved. Subsequently, a novel one-pot sequential hydrolysis and fermentation process based on optimized medium containing hydrolyzed sugarcane bagasse was developed. Cost-efficiency and abundance of second-generation substrates, good performance indicators, and enhanced market access using a non-recombinant strain open the perspective for a commercially viable bioprocess for d-xylonate production in the near future.

Corynebacterium glutamicum Chassis C1*: Building and Testing a Novel Platform Host for Synthetic Biology and Industrial Biotechnology.

Targeted top-down strategies for genome reduction are considered to have a high potential for providing robust basic strains for synthetic biology and industrial biotechnology. Recently, we created a library of 26 genome-reduced strains of Corynebacterium glutamicum carrying broad deletions in single gene clusters and showing wild-type-like biological fitness. Here, we proceeded with combinatorial deletions of these irrelevant gene clusters in two parallel orders, and the resulting library of 28 strains was characterized under various environmental conditions. The final chassis strain C1* carries a genome reduction of 13.4% (412 deleted genes) and shows wild-type-like growth behavior in defined medium with d-glucose as carbon and energy source. Moreover, C1* proves to be robust against several stresses (including oxygen limitation) and shows long-term growth stability under defined and complex medium conditions. In addition to providing a novel prokaryotic chassis strain, our results comprise a large strain library and a revised genome annotation list, which will be valuable sources for future systemic studies of C. glutamicum.

The myo-inositol/proton symporter IolT1 contributes to d-xylose uptake in Corynebacterium glutamicum.

Corynebacterium glutamicum has been engineered to utilize d-xylose as sole carbon and energy source. Recently, a C. glutamicum strain has been optimized for growth on defined medium containing d-xylose by laboratory evolution, but the mutation(s) attributing to the improved-growth phenotype could not be reliably identified. This study shows that loss of the transcriptional repressor IolR is responsible for the increased growth performance on defined d-xylose medium in one of the isolated mutants. Underlying reason is derepression of the gene for the glucose/myo-inositol permease IolT1 in the absence of IolR, which could be shown to also contribute to d-xylose uptake in C. glutamicum. IolR-regulation of iolT1 could be successfully repealed by rational engineering of an IolR-binding site in the iolT1-promoter. This minimally engineered C. glutamicum strain bearing only two nucleotide substitutions mimics the IolR loss-of-function phenotype and allows for a high growth rate on d-xylose-containing media (µmax = 0.24 ±â€¯0.01 h-1).

Miniaturized and automated adaptive laboratory evolution: Evolving Corynebacterium glutamicum towards an improved d-xylose utilization.

Adaptive Laboratory Evolution (ALE) is increasingly being used as a technique for untargeted strain optimization. This work aimed at developing an automated and miniaturized ALE approach based on repetitive batch cultivations in microtiter plates. The new method is applied to the recently published strain Corynebacterium glutamicum pEKEx3-xylXABCDCc, which is capable of utilizing d-xylose via the Weimberg (WMB) pathway. As a result, the significantly improved strain WMB2evo was obtained, showing a specific growth rate of 0.26h-1 on d-xylose as sole carbon and energy source. WMB2evo grows stable during lab-scale bioreactor operation, demonstrating the high potential of this strain for future biorefinery applications. Genome sequencing of cell samples from two different ALE processes revealed potential key mutations, e.g. in the gene cg0196 (encoding for the transcriptional regulator IolR of the myo-inositol metabolism). These findings open up new perspectives for the rational engineering of C. glutamicum towards improved d-xylose utilization.

A Toolbox of Genetically Encoded FRET-Based Biosensors for Rapid l-Lysine Analysis.

Background: The fast development of microbial production strains for basic and fine chemicals is increasingly carried out in small scale cultivation systems to allow for higher throughput. Such parallelized systems create a need for new rapid online detection systems to quantify the respective target compound. In this regard, biosensors, especially genetically encoded Förster resonance energy transfer (FRET)-based biosensors, offer tremendous opportunities. As a proof-of-concept, we have created a toolbox of FRET-based biosensors for the ratiometric determination of l-lysine in fermentation broth. Methods: The sensor toolbox was constructed based on a sensor that consists of an optimized central lysine-/arginine-/ornithine-binding protein (LAO-BP) flanked by two fluorescent proteins (enhanced cyan fluorescent protein (ECFP), Citrine). Further sensor variants with altered affinity and sensitivity were obtained by circular permutation of the binding protein as well as the introduction of flexible and rigid linkers between the fluorescent proteins and the LAO-BP, respectively. Results: The sensor prototype was applied to monitor the extracellular l-lysine concentration of the l-lysine producing Corynebacterium glutamicum (C. glutamicum) strain DM1933 in a BioLector® microscale cultivation device. The results matched well with data obtained by HPLC analysis and the Ninhydrin assay, demonstrating the high potential of FRET-based biosensors for high-throughput microbial bioprocess optimization.

Formation of xylitol and xylitol-5-phosphate and its impact on growth of d-xylose-utilizing Corynebacterium glutamicum strains.

Wild-type Corynebacterium glutamicum has no endogenous metabolic activity for utilizing the lignocellulosic pentose d-xylose for cell growth. Therefore, two different engineering approaches have been pursued resulting in platform strains harbouring a functional version of either the Isomerase (ISO) or the Weimberg (WMB) pathway for d-xylose assimilation. In a previous study we found for C. glutamicum WMB by-product formation of xylitol during growth on d-xylose and speculated that the observed lower growth rates are due to the growth inhibiting effect of this compound. Based on a detailed phenotyping of the ISO, WMB and the wild-type strain of C. glutamicum, we here show that this organism has a natural capability to synthesize xylitol from d-xylose under aerobic cultivation conditions. We furthermore observed the intracellular accumulation of xylitol-5-phosphate as a result of the intracellular phosphorylation of xylitol, which was particularly pronounced in the C. glutamicum ISO strain. Interestingly, low amounts of supplemented xylitol strongly inhibit growth of this strain on d-xylose, d-glucose and d-arabitol. These findings demonstrate that xylitol is a suitable substrate of the endogenous xylulokinase (XK, encoded by xylB) and its overexpression in the ISO strain leads to a significant phosphorylation of xylitol in C. glutamicum. Therefore, in order to circumvent cytotoxicity by xylitol-5-phosphate, the WMB pathway represents an interesting alternative route for engineering C. glutamicum towards efficient d-xylose utilization.

Bioprocess automation on a Mini Pilot Plant enables fast quantitative microbial phenotyping.

BACKGROUND: The throughput of cultivation experiments in bioprocess development has drastically increased in recent years due to the availability of sophisticated microliter scale cultivation devices. However, as these devices still require time-consuming manual work, the bottleneck was merely shifted to media preparation, inoculation and finally the analyses of cultivation samples. A first step towards solving these issues was undertaken in our former study by embedding a BioLector in a robotic workstation. This workstation already allowed for the optimization of heterologous protein production processes, but remained limited when aiming for the characterization of small molecule producer strains. In this work, we extended our workstation to a versatile Mini Pilot Plant (MPP) by integrating further robotic workflows and microtiter plate assays that now enable a fast and accurate phenotyping of a broad range of microbial production hosts. RESULTS: A fully automated harvest procedure was established, which repeatedly samples up to 48 wells from BioLector cultivations in response to individually defined trigger conditions. The samples are automatically clarified by centrifugation and finally frozen for subsequent analyses. Sensitive metabolite assays in 384-well plate scale were integrated on the MPP for the direct determination of substrate uptake (specifically D-glucose and D-xylose) and product formation (specifically amino acids). In a first application, we characterized a set of Corynebacterium glutamicum L-lysine producer strains and could rapidly identify a unique strain showing increased L-lysine titers, which was subsequently confirmed in lab-scale bioreactor experiments. In a second study, we analyzed the substrate uptake kinetics of a previously constructed D-xylose-converting C. glutamicum strain during cultivation on mixed carbon sources in a fully automated experiment. CONCLUSIONS: The presented MPP is designed to face the challenges typically encountered during early-stage bioprocess development. Especially the bottleneck of sample analyses from fast and parallelized microtiter plate cultivations can be solved using cutting-edge robotic automation. As robotic workstations become increasingly attractive for biotechnological research, we expect our setup to become a template for future bioprocess development.

Improved glycerol utilization by a triacylglycerol-producing Rhodococcus opacus strain for renewable fuels.

BACKGROUND: Glycerol generated during renewable fuel production processes is potentially an attractive substrate for the production of value-added materials by fermentation. An engineered strain MITXM-61 of the oleaginous bacterium Rhodococcus opacus produces large amounts of intracellular triacylglycerols (TAGs) for lipid-based biofuels on high concentrations of glucose and xylose. However, on glycerol medium, MITXM-61 does not produce TAGs and grows poorly. The aim of the present work was to construct a TAG-producing R. opacus strain capable of high-cell-density cultivation at high glycerol concentrations. RESULTS: An adaptive evolution strategy was applied to improve the conversion of glycerol to TAGs in R. opacus MITXM-61. An evolved strain, MITGM-173, grown on a defined medium with 16 g L(-1) glycerol, produced 2.3 g L(-1) of TAGs, corresponding to 40.4% of the cell dry weight (CDW) and 0.144 g g(-1) of TAG yield per glycerol consumed. MITGM-173 was able to grow on high concentrations (greater than 150 g L(-1)) of glycerol. Cultivated in a medium containing an initial concentration of 20 g L(-1) glycerol, 40 g L(-1) glucose, and 40 g L(-1) xylose, MITGM-173 was capable of simultaneously consuming the mixed substrates and yielding 13.6 g L(-1) of TAGs, representing 51.2% of the CDM. In addition, when 20 g L(-1) glycerol was pulse-loaded into the culture with 40 g L(-1) glucose and 40 g L(-1) xylose at the stationary growth phase, MITGM-173 produced 14.3 g L(-1) of TAGs corresponding to 51.1% of the CDW although residual glycerol in the culture was observed. The addition of 20 g L(-1) glycerol in the glucose/xylose mix resulted in a TAG yield per glycerol consumed of 0.170 g g(-1) on the initial addition and 0.279 g g(-1) on the pulse addition of glycerol. CONCLUSION: We have generated a TAG-producing R. opacus MITGM-173 strain that shows significantly improved glycerol utilization in comparison to the parental strain. The present study demonstrates that the evolved R. opacus strain shows significant promise for developing a cost-effective bioprocess to generate advanced renewable fuels from mixed sugar feedstocks supplemented with glycerol.

Chassis organism from Corynebacterium glutamicum--a top-down approach to identify and delete irrelevant gene clusters.

For synthetic biology applications, a robust structural basis is required, which can be constructed either from scratch or in a top-down approach starting from any existing organism. In this study, we initiated the top-down construction of a chassis organism from Corynebacterium glutamicum ATCC 13032, aiming for the relevant gene set to maintain its fast growth on defined medium. We evaluated each native gene for its essentiality considering expression levels, phylogenetic conservation, and knockout data. Based on this classification, we determined 41 gene clusters ranging from 3.7 to 49.7 kbp as target sites for deletion. 36 deletions were successful and 10 genome-reduced strains showed impaired growth rates, indicating that genes were hit, which are relevant to maintain biological fitness at wild-type level. In contrast, 26 deleted clusters were found to include exclusively irrelevant genes for growth on defined medium. A combinatory deletion of all irrelevant gene clusters would, in a prophage-free strain, decrease the size of the native genome by about 722 kbp (22%) to 2561 kbp. Finally, five combinatory deletions of irrelevant gene clusters were investigated. The study introduces the novel concept of relevant genes and demonstrates general strategies to construct a chassis suitable for biotechnological application.

Engineering of Corynebacterium glutamicum for minimized carbon loss during utilization of D-xylose containing substrates.

Biomass-derived d-xylose represents an economically interesting substrate for the sustainable microbial production of value-added compounds. The industrially important platform organism Corynebacterium glutamicum has already been engineered to grow on this pentose as sole carbon and energy source. However, all currently described C. glutamicum strains utilize d-xylose via the commonly known isomerase pathway that leads to a significant carbon loss in the form of CO2, in particular, when aiming for the synthesis of α-ketoglutarate and its derivatives (e.g. l-glutamate). Driven by the motivation to engineer a more carbon-efficient C. glutamicum strain, we functionally integrated the Weimberg pathway from Caulobacter crescentus in C. glutamicum. This five-step pathway, encoded by the xylXABCD-operon, enabled a recombinant C. glutamicum strain to utilize d-xylose in d-xylose/d-glucose mixtures. Interestingly, this strain exhibited a tri-phasic growth behavior and transiently accumulated d-xylonate during d-xylose utilization in the second growth phase. However, this intermediate of the implemented oxidative pathway was re-consumed in the third growth phase leading to more biomass formation. Furthermore, C. glutamicum pEKEx3-xylXABCDCc was also able to grow on d-xylose as sole carbon and energy source with a maximum growth rate of µmax=0.07±0.01h(-1). These results render C. glutamicum pEKEx3-xylXABCDCc a promising starting point for the engineering of efficient production strains, exhibiting only minimal carbon loss on d-xylose containing substrates.