PCR-Based Seamless Genome Editing with High Efficiency and Fidelity in Escherichia coli

A novel C-terminal degron identified in bacterial aldehyde decarbonylases using directed evolution

BACKGROUND

Aldehyde decarbonylases (ADs), which convert acyl aldehydes into alkanes, supply promising solution for producing alkanes from renewable feedstock. However the instability of ADs impedes their further application. Therefore, the current study aimed to investigate the degradation mechanism of ADs and engineer it towards high stability.

RESULTS

Here, we describe the discovery of a degradation tag (degron) in the AD from marine cyanobacterium Prochlorococcus marinus using error-prone PCR-based directed evolution system. Bioinformatic analysis revealed that this C-terminal degron is common in bacterial ADs and identified a conserved C-terminal motif, RMSAYGLAAA, representing the AD degron (ADcon). Furthermore, we demonstrated that the ATP-dependent proteases ClpAP and Lon are involved in the degradation of AD-tagged proteins in E. coli, thereby limiting alkane production. Deletion or modification of the degron motif increased alkane production in vivo.

CONCLUSION

This work revealed the presence of a novel degron in bacterial ADs responsible for its instability. The in vivo experiments proved eliminating or modifying the degron could stabilize AD, thereby producing higher titers of alkanes.

Effective production of succinic acid from coconut water (Cocos nucifera) by metabolically engineered Escherichia coli with overexpression of Bacillus subtilis pyruvate carboxylase

Succinic acid is an important acid which is used in medicine and pharmaceutical companies. Metabolically engineered Escherichia coli strain was used for the effective production of succinic acid using Cocos nucifera water, which contained 5.00 ± 0.02 g/L glucose, 6.10 ± 0.01 g /L fructose and 6.70 ± 0.02 g /L sucrose. Fermentation of C. nucifera water with E. coli M6PM produced a final concentration of 11.78 ± 0.02 g/L succinic acid and yield of 1.23 ± 0.01 mol/mol, 0.66 ± 0.01 g/g total sugars after 72 h dual-phase fermentation in M9 medium while modeled sugar was 0.38 ± 0.02 mol/mol total sugars. It resulted in 72% of the maximum theoretical yield of succinic acid. Here we show that novel substrate of C. nucifera water resulted in effective production of succinic acid. These investigations unveil the importance of C. nucifera water as a substrate for the production of biochemicals.

Efficient One-Step Fusion PCR Based on Dual-Asymmetric Primers and Two-Step Annealing

Gene splicing by fusion PCR is a versatile and widely used methodology, especially in synthetic biology. We here describe a rapid method for splicing two fragments by one-round fusion PCR with a dual-asymmetric primers and two-step annealing (ODT) method. During the process, the asymmetric intermediate fragments were generated in the early stage. Thereafter, they were hybridized in the subsequent cycles to serve as template for the target full-length product. The process parameters such as primer ratio, elongation temperature and cycle numbers were optimized. In addition, the fusion products produced with this method were successfully applied in seamless genome editing. The fusion of two fragments by this method takes less than 0.5 day. The method is expected to facilitate various kinds of complex genetic engineering projects with enhanced efficiency.

Enhanced production of succinic acid from methanol-organosolv pretreated Strophanthus preussii by recombinant Escherichia coli

A biorefinery process for high yield production of succinic acid from biomass sugars was investigated using recombinant Escherichia coli. The major problem been addressed is utilization of waste biomass for the production of succinic acid using metabolic engineering strategy. Here, methanol extract of Strophanthus preussii was used for fermentation. The process parameters were optimized. Glucose (9 g/L), galactose (4 g/L), xylose (6 g/L) and arabinose (0.5 g/L) were the major sugars present in the methanol extract of S. preussii. E. coli K3OS with overexpression of soluble nucleotide pyridine transhydrogenase sthA and mutation of lactate dehydrogenase A (ldhA), phosphotransacetylase acetate kinase A (pta-ackA), pyruvate formate lyase B (pflB), pyruvate oxidase B (poxB), produced a final succinic acid concentration of 14.40 g/L and yield of 1.10 mol/mol total sugars after 72 h dual-phase fermentation in M9 medium. Here, we show that the maximum theoretical yield using methanol extracts of S. preussii was 64%. Hence, methanol extract of S. preussii could be used for the production of biochemicals such as succinate, malate and pyruvate.

Construction of pyruvate producing strain with intact pyruvate dehydrogenase and genome-wide transcription analysis

To obtain strain YP211 with a high tendency for accumulating pyruvate, central metabolic pathways were modified in Escherichia coli MG1655. Specifically, seven genes (ldhA, pflB, pta-ackA, poxB, ppc, frdBC) were knocked out sequentially and full pyruvate dehydrogenase was retained. In batch fermentation with M9 medium, pyruvate yield and production rate reached 0.63 g/g glucose and 1.89 g/(1 h), respectively. Meanwhile, the production of acetate, succinate, and other carboxylates was effectively controlled. To understand the physiological observations, we further completed genome-wide transcription analysis of wild-type and YP211. As the acetic acid pathways were blocked, the pathways of convertion of pyruvate to phosphoenol pyruvate and acetyl CoA were enhanced. The transcription of pck, as an alternative gene for ppc, was increased by 2.6 times. So even if gene ppc was inactivated, the tricarboxylic acid pathway was still enhanced in YP211. In order to balance intracellular NADH/NAD⁺, oxidative phosphorylation and flagellar assembly system were also up-regulated significantly. Biochemical pathways involved in pyruvate accumulation in YP211 (a). Transcriptional differences of genes related to pyruvate metabolism between strain YP211 and E. coli wild-type (b).

High production of fatty alcohols in Escherichia coli with fatty acid starvation

Background

Microbial biofuel synthesis attracting increasing attention. Great advances have been made in producing fatty alcohols from fatty acyl-CoAs and fatty acids in Escherichia coli. However, the low titers and limited knowledge regarding the basic characteristics of fatty alcohols, such as location and toxicity, have hampered large-scale industrialization. Further research is still needed.

Results

In this study, we designed a novel and efficient strategy to enhance fatty alcohol production by inducing fatty acid starvation. We report the first use of deletions of acyl-ACP thioesterases to enhance fatty alcohol production. Transcriptional analysis was conducted to investigate the mechanism of the designed strategy. Then, fatty alcohol production was further enhanced by deletion of genes from competing pathways. Fatty alcohols were shown to be extracellular products with low toxicity. The final strain, E. coli MGL2, produced fatty alcohols at the remarkable level of 6.33 g/L under fed-batch fermentation, representing the highest reported titer of fatty alcohols produced by microorganisms.

Conclusions

Deletions of genes responsible for synthesis of fatty acids and competing products are promising strategies for fatty alcohol production. Our investigation of the location and toxicity of fatty alcohols suggest bright future for fatty alcohol production in E. coli.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0524-5) contains supplementary material, which is available to authorized users.