Base editor-mediated large-scale screening of functional mutations in bacteria for industrial phenotypes.

Base editing, the targeted introduction of point mutations into cellular DNA, holds promise for improving genome-scale functional genome screening to single-nucleotide resolution. Current efforts in prokaryotes, however, remain confined to loss-of-function screens using the premature stop codons-mediated gene inactivation library, which falls far short of fully releasing the potential of base editors. Here, we developed a base editor-mediated functional single nucleotide variant screening pipeline in Escherichia coli. We constructed a library with 31,123 sgRNAs targeting 462 stress response-related genes in E. coli, and screened for adaptive mutations under isobutanol and furfural selective conditions. Guided by the screening results, we successfully identified several known and novel functional mutations. Our pipeline might be expanded to the optimization of other phenotypes or the strain engineering in other microorganisms.

CRISPRi-microfluidics screening enables genome-scale target identification for high-titer protein production and secretion.

Genome-scale target identification promises to guide microbial cell factory engineering for higher-titer production of biomolecules such as recombinant proteins (r-protein), but challenges remain due to the need not only for comprehensive genotypic perturbation but also in conjunction with high-throughput phenotypic screening strategies. Here, we developed a CRISPRi-microfluidics screening platform to systematically identify crucial gene targets that can be engineered to enhance r-protein secretion in Corynebacterium glutamicum. We created a CRISPR interference (CRISPRi) library containing 46,549 single-guide RNAs, where we aimed to unbiasedly target all genes for repression. Meanwhile, we developed a highly efficient droplet-based microfluidics system integrating the FlAsH-tetracysteine assay that enables screening of millions of strains to identify potential knockdowns conducive to nanobody VHH secretion. Among our highest-ranking candidates are a slew of previously unknown targets involved in transmembrane transport, amino-acid metabolism and redox regulation. Guided by these findings, we eventually constructed a hyperproducer for multiple proteins via combinatorial engineering of redox-response transcription factors. As the near-universal applicability of CRISPRi technology and the FlAsH-based screening platform, this procedure might be expanded to include a varied variety of microbial species and recombinant proteins.

New Method for Genome-Scale Functional Genomic Study in Bacteria with Superior Performance: CRISPR Interference Screen.

High-throughput genetic screens based on CRISPR/Cas9 technology are powerful tools to genome-wide identify gene function and genotype-phenotype association. Here, we describe a detailed protocol for conducting and evaluating pooled CRISPR screens interfering with gene expression in Escherichia coli. We provide step-by-step instructions for guide RNA library design and construction, genome-scale screening and next-generation sequencing data processing. This tool outperforms transposon sequencing (Tn-seq) with similar library sizes and short gene length. The workflow can be used in follow-up studies implemented in other bacteria systems.

A versatile toolbox for CRISPR-based genome engineering in Pichia pastoris.

Pichia pastoris has gained much attention as a popular microbial cell factory for the production of recombinant proteins and high-value chemicals from laboratory to industrial scale. However, the lack of convenient and efficient genome engineering tools has impeded further applications of Pichia pastoris towards metabolic engineering and synthetic biology. Here, we report a CRISPR-based toolbox for gene editing and transcriptional regulation in P. pastoris. Based on the previous attempts in P. pastoris, we constructed a CRISPR/Cas9 system for gene editing using the RNA Pol-III-driven expression of sgRNA. The system was used to rapidly recycle the selectable marker with an eliminable episomal plasmid and achieved up to 100% knockout efficiency. Via dCas9 fused with transcriptional repressor (Mix1/RD1152) or activator (VPR), a flexible toolbox for regulation of gene expression was developed. The reporter gene eGFP driven by yeast pGAP or pCYC1 promoter showed strong inhibition (above 70%) and up to ~ 3.5-fold activation. To implement the combinatorial genetic engineering strategy, the CRISPR system contained a single Cas9-VPR protein, and engineered gRNA was introduced in P. pastoris for simultaneous gene activation, repression, and editing (CRISPR-ARE). We demonstrated that CRISPR-ARE was highly efficient for eGFP activation, mCherry repression, and ADE2 disruption, individually or in a combinatorial manner with a stable expression of multiplex sgRNAs. The simple and multifunctional toolkit demonstrated in this study will accelerate the application of P. pastoris in metabolic engineering and synthetic biology. KEY POINTS: • An eliminable CRISPR/Cas9 system yielded a highly efficient knockout of genes. • Simplified CRISPR/dCas9-based tools enabled transcriptional regulation of targeted genes. • CRISPR-ARE system achieved simultaneous gene activation, repression, and editing in P. pastoris.

[Functional discovery and production technology for natural bioactive peptides].

Bioactive peptides play important roles in promoting human health, such as lowering blood pressure, blood sugar and blood lipid, anti-obesity, and anti-cancer. Thus, exploring functional bioactive peptides and developing efficient production technologies are of crucial importance. Herein, we review the development of function discovery and production technology for natural bioactive peptides. Presently, the top-down and bottom-up approaches are mainly used for the function discovery and production of natural active peptides. The top-down approach includes the direct extraction and identification for functional discovery, and the direct extraction, enzymatic hydrolysis and microbial fermentation for production. The bottom-up approach includes the polypeptide modification and database mining for functional discovery, and the chemical synthesis, enzyme synthesis, recombinant expression and cell-free synthesis for production. The top-down approach is usually associated with complicated process, lower efficiency, higher cost, harder quality control, and uncertain functionality, while the bottom-up approach is more suitable for the development of peptide drugs but difficult to be used for functional foods. With the technology development of sequencing and mass spectrometry, it is easier to obtain the proteomic information of various organisms at the molecular level. Based on the proteomic information, the top-down and bottom-up approaches can be combined to overcome the disadvantages of using these two approaches alone, thus providing a new strategy for the rapid development and production of natural active peptides.

Overexpression of the regulatory subunit of protein kinase A increases heterologous protein expression in Pichia pastoris.

OBJECTIVE: Translational regulation plays an important role in protein synthesis. Our goal was to screen translation-related factors to improve heterologous protein expression in Pichia pastoris. RESULTS: Twenty-eight translation-related factors were overexpressed in P. pastoris GS115 expressing enhanced green fluorescent protein (eGFP). The results showed that overexpression of Bcy1, the regulatory subunit of protein kinase A (PKA), significantly increased both eGFP expression and cell biomass by 20% under methanol induction for 120 h. Additionally, overexpression of Bcy1 elevated the growth rate by 18% and increased production of the industrial enzyme Phytase (Phy) by 26%. Transcriptome analysis indicated that the overall expression of ribosomal protein genes was significantly downregulated and that postdiauxic shift genes and stress response element genes were upregulated. CONCLUSIONS: Bcy1 regulates ribosome protein genes, postdiauxic shift genes and stress response element genes, leading to improved cell growth and heterologous protein expression. This study provides a convenient and universal factor for heterologous protein production.

Enhancing the substrate tolerance of DszC by a combination of alanine scanning and site-directed saturation mutagenesis.

The biodesulfurization 4S pathway can specifically desulfurize an aromatic S heterocyclic compound (which is difficult to desulfurize by hydrodesulfurization) and maintain the integrity of its combustion value. The four Dsz enzymes in the pathway convert the model compound dibenzothiophene (DBT) into the sulfur-free compound 2-hydroxybiphenyl (HBP). DszC is the first enzyme in the 4S pathway and is subject to feedback inhibition and substrate inhibition. This study is the first attempt to further modify the DszC mutant AKWC to improve its tolerance to DBT. Alanine scanning was performed on the dimeric surface of the DszC mutant AKWC, and the HBP yield of the BAD (AKWCP413A) strain was increased compared to the BAD (AKWC) strain. Site-directed saturation mutagenesis was performed on the 413th amino acid of AKWC, and the substrate inhibition parameter KI value of the mutant AKWCPI was 5.6 times higher than that of AKWC. When the DBT concentration was 0.25 mM, the HBP production of the recombinant strain overexpressing AKWCPI was increased by approximately 1.4-fold compared to the BL21(DE3)/BADC*+C* strain. The protein engineering of DszC further improved the substrate tolerance after overcoming the feedback inhibition, which provided a reference for the analysis of the inhibition mechanism of DszC substrate. Overexpression of DszC-beneficial mutants also greatly improved the efficiency of desulfurization.

A kinetic model to optimize and direct the dose ratio of Dsz enzymes in the 4S desulfurization pathway in vitro and in vivo.

OBJECTIVE: To enhance the biodesulfurization rate using a kinetic model that directs the ratio of Dsz enzymes. RESULTS: This study established a kinetic model that predicted the optimal ratio of Dsz enzymes in the 4S biodesulfurization system to be A:B:C = 1:2:4 and 1:4:2. When BCAD+1A+4B+2C, the conversion rate of dibenzothiophene (DBT) to 2-hydroxybiphenyl (HBP) was close to 100% in vitro. When the gene dose of dszC was increased, the HBP yield of the recombinant strain BL21(DE3)/BCAD + C reached approximately 0.012 mM in vivo, which was approximately 6-fold higher than that of the BCAD strain. CONCLUSIONS: According to the results predicted by the enzyme kinetic model, maintaining higher concentrations of DszC and DszB in the desulfurization system can effectively improve the desulfurization efficiency.

Improved Efficiency of the Desulfurization of Oil Sulfur Compounds in Escherichia coli Using a Combination of Desensitization Engineering and DszC Overexpression.

The 4S pathway of biodesulfurization, which can specifically desulfurize aromatic S-heterocyclic compounds without destroying their combustion value, is a low-cost and environmentally friendly technology that is complementary to hydrodesulfurization. The four Dsz enzymes convert the model compound dibenzothiophene (DBT) into the sulfur-free compound 2-hydroxybiphenyl (HBP). Of these four enzymes, DszC, the first enzyme in the 4S pathway, is the most severely affected by the feedback inhibition caused by HBP. This study is the first attempt to directly modify DszC to decrease its inhibition by HBP, with the results showing that the modified protein is insensitive to HBP. On the basis of the principle that the final HBP product could show a blue color with Gibbs reagent, a high-throughput screening method for its rapid detection was established. The screening method and the combinatorial mutagenesis generated the mutant AKWC (A101K/W327C) of DszC. After the IC50 was calculated, the feedback inhibition of the AKWC mutant was observed to have been substantially reduced. Interestingly, the substrate inhibition of DszC had also been reduced as a result of directed evolution. Finally, the recombinant BL21(DE3)/BADC*+C* (C* represents AKWC) strain exhibited a specific conversion rate of 214.84 µmolHBP/gDCW/h, which was 13.8-fold greater than that of the wild-type strain. Desensitization engineering and the overexpression of the desensitized DszC protein resulted in the elimination of the feedback inhibition bottleneck in the 4S pathway, which is practical and effective progress toward the production of sulfur-free fuel oil. The results of this study demonstrate the utility of desensitization of feedback inhibition regulation in metabolic pathways by protein engineering.

Enhancing co-translational folding of heterologous protein by deleting non-essential ribosomal proteins in Pichia pastoris.

BACKGROUND: Translational regulation played an important role in the correct folding of heterologous proteins to form bioactive conformations during biogenesis. Translational pausing coordinates protein translation and co-translational folding. Decelerating translation elongation speed has been shown to improve the soluble protein yield when expressing heterologous proteins in industrial expression hosts. However, rational redesign of translational pausing via synonymous mutations may not be feasible in many cases. Our goal was to develop a general and convenient strategy to improve heterologous protein synthesis in Pichia pastoris without mutating the expressed genes. RESULTS: Here, a large-scale deletion library of ribosomal protein (RP) genes was constructed for heterologous protein expression in Pichia pastoris, and 59% (16/27) RP deletants have significantly increased heterologous protein yield. This is due to the delay of 60S subunit assembly by deleting non-essential ribosomal protein genes or 60S subunit processing factors, thus globally decreased the translation elongation speed and improved the co-translational folding, without perturbing the relative transcription level and translation initiation. CONCLUSION: Global decrease in the translation elongation speed by RP deletion enhanced co-translational folding efficiency of nascent chains and decreased protein aggregates to improve heterologous protein yield. A potential expression platform for efficient pharmaceutical proteins and industrial enzymes production was provided without synonymous mutation.

Recycling of a selectable marker with a self-excisable plasmid in Pichia pastoris.

Pichia pastoris is a widely used heterologous protein production workhorse. However, with its multiple genetic modifications to solve bottlenecks for heterologous protein productivity, P. pastoris lacks selectable markers. Existing selectable marker recycling plasmids have drawbacks (e.g., slow growth and conditional lethality). Here, zeocin-resistance marker recycling vectors were constructed using the Cre/loxP recombination system. The vectors were used to (i) knock in heterologous phytase, xylanase and lipase expression cassettes, (ii) increase the phytase, xylanase and lipase gene copy number to 13, 5, and 5, respectively, with vector introduction and (iii) engineer the secretion pathway by co-overexpressing secretion helper factors (Sly1p and Sec1p) without introducing selectable markers, giving a phytase field of 0.833 g/L. The vectors allow selectable marker recycling and would be a useful tool to engineer P. pastoris for high heterologous protein productivity.

Combined strategies for improving expression of Citrobacter amalonaticus phytase in Pichia pastoris.

BACKGROUND: Phytase is used as an animal feed additive that degrades phytic acid and reduces feeding costs and pollution caused by fecal excretion of phosphorus. Some phytases have been expressed in Pichia pastoris, among which the phytase from Citrobacter amalonaticus CGMCC 1696 had high specific activity (3548 U/mg). Improvement of the phytase expression level will contribute to facilitate its industrial applications. METHODS: To improve the phytase expression, we use modification of P AOX1 and the α-factor signal peptide, increasing the gene copy number, and overexpressing HAC1 (i) to enhance folding and secretion of the protein in the endoplasmic reticulum. The genetic stability and fermentation in 10-L scaled-up fed-batch fermenter was performed to prepare for the industrial production. RESULTS: The phytase gene from C. amalonaticus CGMCC 1696 was cloned under the control of the AOX1 promoter (P AOX1 ) and expressed in P. pastoris. The phytase activity achieved was 414 U/mL. Modifications of P AOX1 and the α-factor signal peptide increased the phytase yield by 35 and 12%, respectively. Next, on increasing the copy number of the Phy gene to six, the phytase yield was 141% higher than in the strain containing only a single gene copy. Furthermore, on overexpression of HAC1 (i) (i indicating induced), a gene encoding Hac1p that regulates the unfolded protein response, the phytase yield achieved was 0.75 g/L with an activity of 2119 U/mL, 412% higher than for the original strain. The plasmids in this high-phytase expression strain were stable during incubation at 30 °C in Yeast Extract Peptone Dextrose (YPD) Medium. In a 10-L scaled-up fed-batch fermenter, the phytase yield achieved was 9.58 g/L with an activity of 35,032 U/mL. DISCUSSION: The production of a secreted protein will reach its limit at a specific gene copy number where further increases in transcription and translation due to the higher abundance of gene copies will not enhance the secretion process any further. Enhancement of protein folding in the ER can alleviate bottlenecks in the folding and secretion pathways during the overexpression of heterologous proteins in P. pastoris. CONCLUSIONS: Using modification of P AOX1 and the α-factor signal peptide, increasing the gene copy number, and overexpressing HAC1 (i) to enhance folding and secretion of the protein in the endoplasmic reticulum, we have successfully increased the phytase yield 412% relative to the original strain. In a 10-L fed-batch fermenter, the phytase yield achieved was 9.58 g/L with an activity of 35,032 U/mL. Large-scale production of phytase can be applied towards different biocatalytic and feed additive applications.