Effect of Plasmid Design and Type of Integration Event on Recombinant Protein Expression in Pichia pastoris

Scalable protein production by Komagataella phaffii enabled by ARS plasmids and carbon source-based selection

BACKGROUND

Most recombinant Komagataella phaffii (Pichia pastoris) strains for protein production are generated by genomic integration of expression cassettes. The clonal variability in gene copy numbers, integration loci and consequently product titers limit the aptitude for high throughput applications in drug discovery, enzyme engineering or most comparative analyses of genetic elements such as promoters or secretion signals. Circular episomal plasmids with an autonomously replicating sequence (ARS), an alternative which would alleviate some of these limitations, are inherently unstable in K. phaffii. Permanent selection pressure, mostly enabled by antibiotic resistance or auxotrophy markers, is crucial for plasmid maintenance and hardly scalable for production. The establishment and use of extrachromosomal ARS plasmids with key genes of the glycerol metabolism (glycerol kinase 1, GUT1, and triosephosphate isomerase 1, TPI1) as selection markers was investigated to obtain a system with high transformation rates that can be directly used for scalable production processes in lab scale bioreactors.

RESULTS

In micro-scale deep-well plate experiments, ARS plasmids employing the Ashbya gossypii TEF1 (transcription elongation factor 1) promoter to regulate transcription of the marker gene were found to deliver high transformation efficiencies and the best performances with the reporter protein (CalB, lipase B of Candida antarctica) for both, the GUT1- and TPI1-based, marker systems. The GUT1 marker-bearing strain surpassed the reference strain with integrated expression cassette by 46% upon re-evaluation in shake flask cultures regarding CalB production, while the TPI1 system was slightly less productive compared to the control. In 5 L bioreactor methanol-free fed-batch cultivations, the episomal production system employing the GUT1 marker led to 100% increased CalB activity in the culture supernatant compared to integration construct.

CONCLUSIONS

For the first time, a scalable and methanol-independent expression system for recombinant protein production for K. phaffii using episomal expression vectors was demonstrated. Expression of the GUT1 selection marker gene of the new ARS plasmids was refined by employing the TEF1 promoter of A. gossypii. Additionally, the antibiotic-free marker toolbox for K. phaffii was expanded by the TPI1 marker system, which proved to be similarly suited for the use in episomal plasmids as well as integrative expression constructs for the purpose of recombinant protein production.

OPENPichia: licence-free Komagataella phaffii chassis strains and toolkit for protein expression

The industrial yeast Komagataella phaffii (formerly named Pichia pastoris) is commonly used to synthesize recombinant proteins, many of which are used as human therapeutics or in food. However, the basic strain, named NRRL Y-11430, from which all commercial hosts are derived, is not available without restrictions on its use. Comparative genome sequencing leaves little doubt that NRRL Y-11430 is derived from a K. phaffii type strain deposited in the UC Davis Phaff Yeast Strain Collection in 1954. We analysed four equivalent type strains in several culture collections and identified the NCYC 2543 strain, from which we started to develop an open-access Pichia chassis strain that anyone can use to produce recombinant proteins to industry standards. NRRL Y-11430 is readily transformable, which we found to be due to a HOC1 open-reading-frame truncation that alters cell-wall mannan. We introduced the HOC1 open-reading-frame truncation into NCYC 2543, which increased the transformability and improved secretion of some but not all of our tested proteins. We provide our genome-sequenced type strain, the hoc1tr derivative that we named OPENPichia as well as a synthetic, modular expression vector toolkit under liberal end-user distribution licences as an unencumbered OPENPichia resource for the microbial biotechnology community.

Integration of Yeast Episomal/Integrative Plasmid Causes Genotypic and Phenotypic Diversity and Improved Sesquiterpene Production in Metabolically Engineered Saccharomyces cerevisiae

The variability in phenotypic outcomes among biological replicates in engineered microbial factories presents a captivating mystery. Establishing the association between phenotypic variability and genetic drivers is important to solve this intricate puzzle. We applied a previously developed auxin-inducible depletion of hexokinase 2 as a metabolic engineering strategy for improved nerolidol production in Saccharomyces cerevisiae, and biological replicates exhibit a dichotomy in nerolidol production of either 3.5 or 2.5 g L-1 nerolidol. Harnessing Oxford Nanopore's long-read genomic sequencing, we reveal a potential genetic cause─the chromosome integration of a 2μ sequence-based yeast episomal plasmid, encoding the expression cassettes for nerolidol synthetic enzymes. This finding was reinforced through chromosome integration revalidation, engineering nerolidol and valencene production strains, and generating a diverse pool of yeast clones, each uniquely fingerprinted by gene copy numbers, plasmid integrations, other genomic rearrangements, protein expression levels, growth rate, and target product productivities. Τhe best clone in two strains produced 3.5 g L-1 nerolidol and ∼0.96 g L-1 valencene. Comparable genotypic and phenotypic variations were also generated through the integration of a yeast integrative plasmid lacking 2μ sequences. Our work shows that multiple factors, including plasmid integration status, subchromosomal location, gene copy number, sesquiterpene synthase expression level, and genome rearrangement, together play a complicated determinant role on the productivities of sesquiterpene product. Integration of yeast episomal/integrative plasmids may be used as a versatile method for increasing the diversity and optimizing the efficiency of yeast cell factories, thereby uncovering metabolic control mechanisms.

Agar plate-based activity assay for easy and fast screening of recombinant Pichia pastoris expressing unspecific peroxygenases

Unspecific peroxygenases (UPOs) are promising biocatalysts that catalyze oxyfunctionalization reactions without the need for costly cofactors. Pichia pastoris (reclassified as Komagataella phaffii) is considered an attractive host for heterologous expression of UPOs. However, integration of UPO-expression cassettes into the genome via a single cross-over yields recombinant Pichia transformants with different UPO gene copy numbers resulting in different expression levels. Selection of the most productive Pichia transformants by a commonly used screening in liquid medium in 96-well plates is laborious and lasts up to 5 days. In this work, we developed a simple two-step agar plate-based assay to screen P. pastoris transformants for UPO activity with less effort, within shorter time, and without automated screening devices. After cell growth and protein expression on agar plates supplemented with methanol and 2,2'-azino-bis(3-ethylbenzothiazoline-6-sulfonic acid) (ABTS), an additional top agar layer supplemented with ABTS and peroxide is added. UPO activity is visualized within 15 min by formation of green zones around UPO-secreting P. pastoris transformants. The assay was validated with two UPOs, AbrUPO from Aspergillus brasiliensis and evolved PaDa-I from Agrocybe aegerita. The assay results were confirmed in a quantitative 96-deep well plate screening in liquid medium.

Heterologous Single-Strand DNA-Annealing and Binding Protein Enhance CRISPR-Based Genome Editing Efficiency in Komagataella phaffii

The industrial yeast Komagataella phaffii is a highly effective platform for heterologous protein production, owing to its high protein expression and secretion capacity. Heterologous genes and proteins are involved in multiple processes, including transcription, translation, protein folding, modification, transportation, and degradation; however, engineering these proteins and genes is challenging due to inefficient genome editing techniques. We employed Pseudomonas aeruginosa phage single-stranded DNA-annealing protein (SSAP) PapRecT and P. aeruginosa single-stranded DNA-binding protein (SSB) PaSSB to introduce SSAP-SSB-based homology recombination, which facilitated K. phaffii CRISPR-based genome engineering. Specifically, a host-independent method was developed by expressing sgRNA with PapRecT-PaSSB in a single plasmid, with which only a 50 bp short homologous arm (HA) reached a 100% positive rate for CRISPR-based gene insertion, reaching 18 colony-forming units (CFU) per μg of donor DNA. Single deletion using 1000 bp HA attained 100%, reaching 68 CFUs per μg of donor DNA. Using this efficient CRISPR-based genome editing tool, we integrated three genes (INO4, GAL4-like, and PAB1) at three different loci for overexpression to realize the collaborative regulation of human-lactalbumin (α-LA) production. Specifically, we strengthened phospholipid biosynthesis to facilitate endoplasmic reticulum membrane formation and enhanced recombinant protein transcription and translation by overexpressing transcription and translation factors. The final production of α-LA in the 3 L fermentation reached 113.4 mg L-1, two times higher than that of the strain without multiple site gene editing, which is the highest reported titer in K. phaffii. The CRISPR-based genome editing method developed in this study is suitable for the synergistic multiple-site engineering of protein and biochemical biosynthesis pathways to improve the biomanufacturing efficiency.

Optimization Workflow of Fumonisin Esterase Production for Biocatalytic Degradation of Fumonisin B1

Industrial enzyme production with the Pichia pastoris expression system requires a well-characterized production strain and a competitively priced fermentation medium to meet the expectations of the industry. The present work shows a workflow that allows the rapid and reliable screening of transformants of single copy insertion of the target production cassette. A constitutive expression system with the glyceraldehyde-3-phosphate dehydrogenase promoter (pGAP) with homology arms for the glycerol kinase 1 (GUT1) was constructed for the targeted integration of the expression plasmid in a KU70 deficient Pichia pastoris and the production of a bacterial fumonisin esterase enzyme (CFE). A robust colony qPCR method was developed for the copy number estimation of the expression cassette. Optimization of the protein production medium and the scale-up ability was aided by design of experiments (DOE) approach resulting in optimized production conditions at a semi-industrial scale. A novel fermentation medium containing 3% inactivated yeast and 2% dextrose in an ammonium-citrate buffer (IYD) was shown to be a promising alternative to YPD media (containing yeast extract, peptone, and dextrose), as similar protein titers could be obtained, while the cost of the medium was reduced 20-fold. In a demonstration-scale 48 h long fed-batch fermentation, the IYD media outperformed the small-scale YPD cultivation by 471.5 ± 22.6%.

Biological Function of Antimicrobial Peptides on Suppressing Pathogens and Improving Host Immunity

The emergence of drug-resistant genes and concerns about food safety caused by the overuse of antibiotics are becoming increasingly prominent. There is an urgent need for effective alternatives to antibiotics in the fields of livestock production and human medicine. Antimicrobial peptides can effectively replace antibiotics to kill pathogens and enhance the immune functions of the host, and pathogens cannot easily produce genes that are resistant to them. The ability of antimicrobial peptides (AMPs) to kill pathogens is associated with their structure and physicochemical properties, such as their conformation, electrical charges, hydrophilicity, and hydrophobicity. AMPs regulate the activity of immunological cells and stimulate the secretion of inflammatory cytokines via the activation of the NF-κB and MAPK signaling pathways. However, there are still some limitations to the application of AMPs in the fields of livestock production and human medicine, including a restricted source base, high costs of purification and expression, and the instability of the intestines of animals and humans. This review summarizes the information on AMPs as effective antibiotic substitutes to improve the immunological functions of the host through suppressing pathogens and regulating inflammatory responses. Potential challenges for the commercial application of AMPs in animal husbandry and human medicine are discussed.

Biosensor-guided rapid screening for improved recombinant protein secretion in Pichia pastoris

Pichia pastoris (Komagataella phaffii) is widely used for industrial production of heterologous proteins due to high secretory capabilities but selection of highly productive engineered strains remains a limiting step. Despite availability of a comprehensive molecular toolbox for construct design and gene integration, there is high clonal variability among transformants due to frequent multi-copy and off-target random integration. Therefore, functional screening of several hundreds of transformant clones is essential to identify the best protein production strains. Screening methods are commonly based on deep-well plate cultures with analysis by immunoblotting or enzyme activity assays of post-induction samples, and each heterologous protein produced may require development of bespoke assays with multiple sample processing steps. In this work, we developed a generic system based on a P. pastoris strain that uses a protein-based biosensor to identify highly productive protein secretion clones from a heterogeneous set of transformants. The biosensor uses a split green fluorescent protein where the large GFP fragment (GFP1-10) is fused to a sequence-specific protease from Tobacco Etch Virus (TEV) and is targeted to the endoplasmic reticulum. Recombinant proteins targeted for secretion are tagged with the small fragment of the split GFP (GFP11). Recombinant protein production can be measured by monitoring GFP fluorescence, which is dependent on interaction between the large and small GFP fragments. The reconstituted GFP is cleaved from the target protein by TEV protease, allowing for secretion of the untagged protein of interest and intracellular retention of the mature GFP. We demonstrate this technology with four recombinant proteins (phytase, laccase, β-casein and β-lactoglobulin) and show that the biosensor directly reports protein production levels that correlate with traditional assays. Our results confirm that the split GFP biosensor can be used for facile, generic, and rapid screening of P. pastoris clones to identify those with the highest production levels.

Affinity-based and in a single step purification of recombinant horseradish peroxidase A2A isoenzyme produced by Pichia pastoris

Horseradish peroxidase (HRP) is an oxidoreductase enzyme and oxidizes various inorganic and organic compounds. It has wide application areas such as immunological tests, probe-based test techniques, removal of phenolic pollutants from wastewater and organic synthesis. HRP is found in the root of the horseradish plant as a mixture of different isoenzymes, and it is very difficult to separate these enzymes from each other. In this regard, recombinant production is a very advantageous method in terms of producing the desired isoenzyme. This study was performed to produce HRP A2A isoenzyme extracellularly in Pichia pastoris and to purify this enzyme in a single step using a 3-amino-4-chloro benzohydrazide affinity column. First, codon-optimized HRP A2A gene was amplified and inserted into pPICZαC. So, obtained pPICZαC-HRPA2A was cloned in E. coli cells. Then, P. pastoris X-33 cells were transformed with linearized recombinant DNA and a yeast clone was cultivated for extracellular recombinant HRP A2A (rHRP A2A) enzyme production. Then, the purification of this enzyme was performed in a single step by affinity chromatography. The molecular mass of purified rHRP A2A enzyme was found to be about 40 kDa. According to characterization studies of the purified enzyme, the optimum pH and ionic strength for the rHRP A2A isoenzyme were determined to be 6.0 and 0.04 M, respectively, and o-dianisidine had the highest specificity with the lowest Km and Vmax values. Thus, this is an economical procedure to purify HRP A2A isoenzyme without time-consuming and laborious isolation from an isoenzyme mixture.

A switchable secrete-and-capture system enables efficient selection of Pichia pastoris clones producing high yields of Fab fragments

Pichia pastoris (syn. Komagataella phaffii) represents a commonly used expression system in the biotech industry. High clonal variation of transformants, however, typically results in a broad range of specific productivities for secreted proteins. To isolate rare clones with exceedingly high product titers, an extensive number of clones need to be screened. In contrast to high-throughput screenings of P. pastoris clones in microtiter plates, secrete-and-capture methodologies have the potential to efficiently isolate high-producer clones among millions of cells through fluorescence-activated cell sorting (FACS). Here, we describe a novel approach for the non-covalent binding of fragment antigen-binding (Fab) proteins to the cell surface for the isolation of high-producing clones. Eight different single-chain variable fragment (scFv)-based capture matrices specific for the constant part of the Fabs were fused to the Saccharomyces cerevisiae alpha-agglutinin (SAG1) anchor protein for surface display in P. pastoris. By encoding the capture matrix on an episomal plasmid harboring inherently unstable autonomously replicating sequences (ARS), this secrete-and-capture system offers a switchable scFv display. Efficient plasmid clearance upon removal of selective pressure enabled the direct use of isolated clones for subsequent Fab production. Flow-sorted clones (n = 276) displaying high amounts of Fabs showed a significant increase in median Fab titers detected in the cell-free supernatant (CFS) compared to unsorted clones (n = 276) when cells were cultivated in microtiter plates (factor in the range of ∼21-49). Fab titers of clones exhibiting the highest product titer observed for each of the two approaches were increased by up to 8-fold for the sorted clone. Improved Fab yields of sorted cells vs. unsorted cells were confirmed in an upscaled shake flask cultivation of selected candidates (factor in the range of ∼2-3). Hence, the developed display-based selection method proved to be a valuable tool for efficient clone screening in the early stages of our bioprocess development.

Current advances of Pichia pastoris as cell factories for production of recombinant proteins

Pichia pastoris (syn. Komagataella spp.) has attracted extensive attention as an efficient platform for recombinant protein (RP) production. For obtaining a higher protein titer, many researchers have put lots of effort into different areas and made some progress. Here, we summarized the most recent advances of the last 5 years to get a better understanding of its future direction of development. The appearance of innovative genetic tools and methodologies like the CRISPR/Cas9 gene-editing system eases the manipulation of gene expression systems and greatly improves the efficiency of exploring gene functions. The integration of novel pathways in microorganisms has raised more ideas of metabolic engineering for enhancing RP production. In addition, some new opportunities for the manufacture of proteins have been created by the application of novel mathematical models coupled with high-throughput screening to have a better overview of bottlenecks in the biosynthetic process.

Easy efficient HDR-based targeted knock-in in Saccharomyces cerevisiae genome using CRISPR-Cas9 system

During the last two decades, yeast has been used as a biological tool to produce various small molecules, biofuels, etc., using an inexpensive bioprocess. The application of Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-CRISPR-associated protein (Cas) techniques in yeast genetic and metabolic engineering has made a paradigm shift, particularly with a significant improvement in targeted chromosomal integration using synthetic donor constructs, which was previously a challenge. This study reports the CRISPR-Cas9-based highly efficient strategy for targeted chromosomal integration and in-frame expression of a foreign gene in the genome of Saccharomyces cerevisiae (S. cerevisiae) by homology-dependent recombination (HDR); our optimized methods show that CRISPR-Cas9-based chromosomal targeted integration of small constructs at multiple target sites of the yeast genome can be achieved with an efficiency of 74%. Our study also suggests that 15 bp microhomology flanked arms are sufficient for 50% targeted knock-in at minimal knock-in construct concentration. Whole-genome sequencing confirmed that there is no off-target effect. This study provides a comprehensive and streamlined protocol that will support the targeted integration of essential genes into the yeast genome for synthetic biology and other industrial purposes.Highlights• CRISPR-Cas9 based in-frame expression of foreign protein in Saccharomyces cerevisiae using Homology arm without a promoter.• As low as 15 base pairs of microhomology (HDR) are sufficient for targeted integration in Saccharomyces cerevisiae.• The methodology is highly efficient and very specific as no off-targeted effects were shown by the whole-genome sequence.

The fluorescent protein iLOV as a reporter for screening of high‐yield production of antimicrobial peptides in Pichia pastoris

The methylotrophic yeast Pichia pastoris is commonly used for the production of recombinant proteins at scale. The identification of an optimally overexpressing strain following transformation can be time and reagent consuming. Fluorescent reporters like GFP have been used to assist identification of superior producers, but their relatively big size, maturation requirements and narrow temperature range restrict their applications. Here, we introduce the use of iLOV, a flavin‐based fluorescent protein, as a fluorescent marker to identify P. pastoris high‐yielding strains easily and rapidly. The use of this fluorescent protein as a fusion partner is exemplified by the production of the antimicrobial peptide NI01, a difficult target to overexpress in its native form. iLOV fluorescence correlated well with protein expression level and copy number of the chromosomally integrated gene. An easy and simple medium‐throughput plate‐based screen directly following transformation is demonstrated for low complexity screening, while a high‐throughput method using fluorescence‐activated cell sorting (FACS) allowed for comprehensive library screening. Both codon optimization of the iLOV_NI01 fusion cassettes and different integration strategies into the P. pastoris genome were tested to produce and isolate a high‐yielding strain. Checking the genetic stability, process reproducibility and following the purification of the active native peptide are eased by visualization of and efficient cleavage from the iLOV reporter. We show that this system can be used for expression and screening of several different antimicrobial peptides recombinantly produced in P. pastoris.

One-Step In Vivo Assembly of Multiple DNA Fragments and Genomic Integration in Komagataella phaffii

The methylotrophic yeast species Komagataella phaffii (synonym: Pichia pastoris) is widely used as a host for recombinant protein production. Although several genetic engineering techniques are being employed on K. phaffii, advanced methods such as in vivo DNA assembly in this yeast species are required for synthetic biology applications. In this study, we established a technique for accomplishing one-step in vivo assembly of multiple DNA fragments and genomic integration in K. phaffii. To concurrently achieve an accurate multiple DNA assembly and a high-efficient integration into the target genomic locus in vivo, a K. phaffii strain, lacking a non-homologous end joining-related protein, DNA ligase IV (Dnl4p), that has been reported to improve gene targeting efficiency by homologous recombination, was used. Using green fluorescent protein along with the lycopene biosynthesis, we showed that our method that included a Dnl4p-defective strain permits direct and easy engineering of K. phaffii strains.

High level production of stable human serum albumin in Pichia pastoris and characterization of the recombinant product

Human serum albumin (HSA) is an important therapeutic used in clinical settings for restoration of blood volume and treatment of chemotherapy induced neutropenia. Currently sourced from human serum, it carries the risk of contamination with viruses. The production of stable extracellular recombinant (r)HSA was achieved at nearly 1 g/L at shake-flask level in Pichia pastoris (syn. Komagataella phaffii) containing a three-copy containing HSA expression cassette, prepared in vitro. The HSA specific transcripts were increased by 1.82- to 2.46-fold in the three-copy containing clones indicating increased transcript levels to result in enhanced production of extracellular rHSA. The purified rHSA displayed secondary structure, zeta potential, size distribution and biological efficacy that matched with that of the commercial HSA. Cultivation strategy was developed at bioreactor level for the single HSA expression cassette containing recombinant which led to productivity of 300 mg/L/d of rHSA with minimum proteolytic cleavage.

Strains and Molecular Tools for Recombinant Protein Production in Pichia pastoris

Within the last two decades, the methylotrophic yeast Pichia pastoris (Komagataella phaffii) has become an important alternative to E. coli or mammalian cell lines for the production of recombinant proteins. Easy handling, strong promoters, and high cell density cultivations as well as the capability of posttranslational modifications are some of the major benefits of this yeast. The high secretion capacity and low level of endogenously secreted proteins further promoted the rapid development of a versatile Pichia pastoris toolbox. This chapter reviews common and new "Pichia tools" and their specific features. Special focus is given to expression strains, such as different methanol utilization, protease-deficient or glycoengineered strains, combined with application highlights. Different promoters and signal sequences are also discussed.

Engineering of Promoters for Gene Expression in Pichia pastoris

The availability of exceptionally strong and tightly regulated promoters is a key feature of Komagataella phaffii (syn. Pichia pastoris), a widely applied yeast expression system for heterologous protein production. Most commonly, the methanol-inducible promoter of the alcohol oxidase 1 gene (P_AOX1) and the constitutive promoter of the glyceraldehyde 3 phosphate dehydrogenase gene (P_GAP) have been used. Recently, also promising novel constitutive (P_GCW14), regulated (P_GTH1, P_CAT1), and bidirectional promoters (histone promoters and synthetic hybrid variants) have been reported.As natural promoters showed so far limited tunability of expression levels and regulatory profiles, various promoter engineering efforts have been undertaken for P. pastoris . P_AOX1, P_DAS2, P_GAP, and P_GCW14 have been engineered by systematic deletion studies or random mutagenesis of upstream regulatory sequences. New engineering strategies have focused on P_AOX1 core promoter modifications by random or rational approaches and transcriptional regulatory circuits to render P_AOX1 independent of methanol induction. These promoter engineering efforts in P. pastoris have resulted in improved, sequence-diversified synthetic promoter variants allowing coordinated fine-tuning of gene expression for a multitude of biotechnological applications.

Improved Production of Streptomyces sp. FA1 Xylanase in a Dual-Plasmid Pichia pastoris System

Methanol is considered as a potential hazard in the methanol-induced yeast expression of food-related enzymes. To increase the production efficiency of recombinant proteins in Pichia pastoris without methanol induction, a novel dual-plasmid system was constructed, for the first time, by a combining the strategies of genomic integration and episomal expression. To obtain a high copy number of the target gene, the autonomously replicating sequence derived from Kluyveromyces lactis (PARS) was used to construct episomal vectors carrying the constitutive promoters P_GAP and P_GCW14. In addition, an integrative vector carrying the P_GCW14 promoter was constructed by replacing the P_GAP promoter sequence with a partial P_GCW14 promoter. Next, using xylanase XynA from Streptomyces sp. FA1 as the model enzyme, recombination strains were transformed with different combinations of integrating and episomal vectors that were constructed to investigate the changes in the protein yield. Results in shake flasks indicated that the highest enzyme yield was achieved when integrated P_GAP and episomal P_GCW14 were simultaneously transformed into the host strain. Meanwhile, the copy number of xynA increased from 1.14 ± 0.46 to 3.06 ± 0.35. The yield of XynA was successfully increased to 3925 U·mL^-1 after 102 h of fermentation in a 3.6 L fermenter, which was 16.7-fold and 2.86-fold of the yields that were previously reported for the constitutive expression and methanol-induced expression of the identical protein, respectively. Furthermore, the high-cell-density fermentation period was shortened from 132 h to 102 h compared to that of methanol-induced system. Since the risk of methanol toxicity is removed, this novel expression system would be suitable for the production of proteins related to the food and pharmaceutical industries.

The mechanism of N-acetyl-l-cysteine in improving the secretion of porcine follicle-stimulating hormone in Pichia pastoris

Our previous study revealed that N-acetyl-l-cysteine (NAC) could enhance the secretion of recombinant proteins by Pichia pastoris, but the corresponding molecular mechanisms are still unclear. In the present study, we explored whether other thiols have a similar action on the secretion of recombinant human serum albumin and porcine follicle-stimulating hormone fusion protein (HSA-pFSHβ), to reveal the mechanism of NAC on HSA-pFSHβ secretion. Transcriptome analysis showed that genes involved in oxidoreductase activity and oxidation-reduction process were upregulated in cells supplemented with NAC. The other three thiol-reducing regents including dimercaptopropanol (DT), thioglycolic acid, and mercaptolactic acid could improve HSA-pFSHβ production in the culture supernatant. Among them, only DT had similar effect as NAC on HSA-pFSHβ secretion and the increase of GSH content. Moreover, 1-20 mM GSH, 1-10 mM cysteine, or 1-20 mM N-acetyl-d-cysteine supplementation could improve the secretion of HSA-pFSHβ. Furthermore, 0.4-3.2 mM ethacrynic acid, rather than 1-16 mM BSO could inhibit the effect of NAC on the production of HSA-pFSHβ. These results indicated that NAC improved the secretion of HSA-pFSHβ by increasing the intracellular GSH content through its thiol activity rather than as a precursor for GSH synthesis. In conclusion, our results demonstrate, for the first time, that the secretion of recombinant HSA-pFSHβ in Pichia pastoris could be improved through thiol-reducing agent supplementation, and the mechanism of the effect NAC has on HSA-pFSHβ secretion is associated with improving the intracellular GSH content.

Artificial Transcription Factors for Tuneable Gene Expression in Pichia pastoris

The non-conventional yeast Pichia pastoris (syn. Komagataella phaffii) has become a powerful eukaryotic expression platform for biopharmaceutical and biotechnological applications on both laboratory and industrial scales. Despite the fundamental role that artificial transcription factors (ATFs) play in the orthogonal control of gene expression in synthetic biology, a limited number of ATFs are available for P. pastoris. To establish orthogonal regulators for use in P. pastoris, we characterized ATFs derived from Arabidopsis TFs. The plant-derived ATFs contain the binding domain of TFs from the plant Arabidopsis thaliana, in combination with the activation domains of yeast GAL4 and plant EDLL and a synthetic promoter harboring the cognate cis-regulatory motifs. Chromosomally integrated ATFs and their binding sites (ATF/BSs) resulted in a wide spectrum of inducible transcriptional outputs in P. pastoris, ranging from as low as 1- to as high as ∼63-fold induction with only small growth defects. We demonstrated the application of ATF/BSs by generating P. pastoris cells that produce β-carotene. Notably, the productivity of β-carotene in P. pastoris was ∼4.8-fold higher than that in S. cerevisiae, reaching ∼59% of the β-carotene productivity obtained in a S. cerevisiae strain optimized for the production of the β-carotene precursor, farnesyl diphosphate, by rewiring the endogenous metabolic pathways using plant-derived ATF/BSs. Our data suggest that plant-derived regulators have a high degree of transferability from S. cerevisiae to P. pastoris. The plant-derived ATFs, together with their cognate binding sites, powerfully increase the repertoire of transcriptional regulatory modules for the tuning of protein expression levels required in metabolic engineering or synthetic biology in P. pastoris.

Recombination machinery engineering facilitates metabolic engineering of the industrial yeast Pichia pastoris

The industrial yeast Pichia pastoris has been harnessed extensively for production of proteins, and it is attracting attention as a chassis cell factory for production of chemicals. However, the lack of synthetic biology tools makes it challenging in rewiring P. pastoris metabolism. We here extensively engineered the recombination machinery by establishing a CRISPR-Cas9 based genome editing platform, which improved the homologous recombination (HR) efficiency by more than 54 times, in particular, enhanced the simultaneously assembly of multiple fragments by 13.5 times. We also found that the key HR-relating gene RAD52 of P. pastoris was largely repressed in compared to that of Saccharomyces cerevisiae. This gene editing system enabled efficient seamless gene disruption, genome integration and multiple gene assembly with positive rates of 68–90%. With this efficient genome editing platform, we characterized 46 potential genome integration sites and 18 promoters at different growth conditions. This library of neutral sites and promoters enabled two-factorial regulation of gene expression and metabolic pathways and resulted in a 30-fold range of fatty alcohol production (12.6–380 mg/l). The expanding genetic toolbox will facilitate extensive rewiring of P. pastoris for chemical production, and also shed light on engineering of other non-conventional yeasts.

A multi-functional genetic manipulation system and its use in high-level expression of a β-mannanase mutant with high specific activity in Pichia pastoris

To further extend the practical application of a thermostable and acidic resistance β-mannanase (ManAK) in animal feed additives, an effective strategy that combined directed evolution and metabolic engineering was developed. Four positive mutants (P191M, P194E, S199G and S268Q) with enhanced specific activity (25.5%-60.9%) were obtained. The S199G mutant exhibited 56.7% enhancement of specific activity at 37°C and good thermostability, and this was selected for high-level expression in P. pastoris X33. A multi-functional and scarless genetic manipulation system was proposed and functionally verified (gene deletion, substitution/insertion and point mutation). This was then subjected to Rox1p (an oxygen related transcription regulator) deletion and Vitreoscilla haemoglobin (VHb) co-expression for high enzyme productivity in P. pastoris X33VIIManAKS199G . An excellent strain, named X33VIIManAKS199G ∆rox1::VHb, was achieved by combining these two factors, and then the maximum enzymatic activity was further increased to 3753 U ml-1 , which was nearly twice as much as the maximum production of ManAK in P. pastoris. This work provides a systematic and effective method to improve the enzymatic yield of β-mannanase, promotes the application of ManAK in feed additives, and also demonstrated that a scarless genetic manipulation tool is useful in P. pastoris.

Novel molecular biological tools for the efficient expression of fungal lytic polysaccharide monooxygenases in Pichia pastoris

BACKGROUND

Lytic polysaccharide monooxygenases (LPMOs) are attracting large attention due their ability to degrade recalcitrant polysaccharides in biomass conversion and to perform powerful redox chemistry.

RESULTS

We have established a universal Pichia pastoris platform for the expression of fungal LPMOs using state-of-the-art recombination cloning and modern molecular biological tools to achieve high yields from shake-flask cultivation and simple tag-less single-step purification. Yields are very favorable with up to 42 mg per liter medium for four different LPMOs spanning three different families. Moreover, we report for the first time of a yeast-originating signal peptide from the dolichyl-diphosphooligosaccharide-protein glycosyltransferase subunit 1 (OST1) form S. cerevisiae efficiently secreting and successfully processes the N-terminus of LPMOs yielding in fully functional enzymes.

CONCLUSION

The work demonstrates that the industrially most relevant expression host P. pastoris can be used to express fungal LPMOs from different families in high yields and inherent purity. The presented protocols are standardized and require little equipment with an additional advantage with short cultivation periods.

Buried Kex2 Sites in Glargine Precursor Aggregates Prevent Its Intracellular Processing in Pichia pastoris Muts Strains and the Effect of Methanol-Feeding Strategy and Induction Temperature on Glargine Precursor Production Parameters

Glargine is a long-acting insulin analog with less hypoglycemia risk. Like human insulin, glargine is a globular protein composed of two polypeptide chains linked by two disulfide bonds. Pichia pastoris KM71 Mut^s strains were engineered to produce and secrete insulin glargine through the cleavage of two Kex2 sites. Nevertheless, the recombinant product was the single-chain insulin glargine (glargine precursor) instead of the expected double-chain glargine. Molecular model analysis of the dimeric and hexameric forms of the single-chain glargine showed buried Kex2 sites that prevent intracellular glargine precursor processing. The effect of the methanol-feeding strategy (methanol limited fed-batch vs. methanol non-limited fed-batch) and the induction temperature (28 °C vs. 24 °C) on the cell growth and production parameters in bioreactor cultures was also evaluated. Exponential growth at a constant specific growth rate was observed in all the cultures. The volumetric productivities and specific substrate consumption rates were directly proportional to the specific growth rate. The lower temperature led to increased metabolic activity of the yeast cells, which increased the specific growth rate. The methanol non-limited fed-batch culture at 24 °C showed the highest values for the process parameters. After 75 h of induction, 0.122 g/L of glargine precursor was obtained from the culture medium.

Lyophilized yeast powder for adjuvant free thermostable vaccine delivery

Thermolabile nature of commercially available vaccines necessitates their storage, transportation, and dissemination under refrigerated condition. Maintenance of continuous cold chain at every step increases the final cost of vaccines. Any breach in the cold chain even for a short duration results in the need to discard the vaccines. As a result, there is a pressing need for the development of thermostable vaccines. In this proof-of-concept study, we showed that E. coli curli-green fluorescent fusion protein remains stable in freeze-dried yeast powder for more than 18 and 12 months when stored at 30 °C and 37 °C respectively. Stability of the heterologous protein remains unaffected during the process of heat-inactivation and lyophilization. The mass of lyophilized yeast powder remains almost unchanged during the entire period of storage and expressed protein remains intact even after two cycles of freeze and thaws. The protease-deficient strain appears ideal for the development of whole recombinant yeast-based vaccines. The cellular abundance of expressed antigen in dry powder after a year was comparable to freshly lyophilized cells. Scanning electron microscopy showed the intact nature of cells in powdered form even after a year of storage at 30 °C. Observation made in this study showed that freeze-dry yeast powder can play a vital role in the development of thermostable vaccines.

Key Points

• Yeast-based vaccines can overcome problem of cold chain associated with conventional vaccines

• Lyophilized yeast powder can be a simple way for long-term storage of immunogen(s)

• Protease deficient strain is important for whole recombinant yeast-based vaccines

Bioprocess performance analysis of novel methanol-independent promoters for recombinant protein production with Pichia pastoris

Background

Pichia pastoris is a powerful and broadly used host for recombinant protein production (RPP), where past bioprocess performance has often been directed with the methanol regulated AOX1 promoter (P_AOX1), and the constitutive GAP promoter (P_GAP). Since promoters play a crucial role in an expression system and the bioprocess efficiency, innovative alternatives are constantly developed and implemented. Here, a thorough comparative kinetic characterization of two expression systems based on the commercial PDF and UPP promoters (P_PDF, P_UPP) was first conducted in chemostat cultures. Most promising conditions were subsequently tested in fed-batch cultivations. These new alternatives were compared with the classical strong promoter P_GAP, using the Candida antarctica lipase B (CalB) as model protein for expression system performance.

Results

Both the P_PDF and P_UPP-based expression systems outperformed similar P_GAP-based expression in chemostat cultivations, reaching ninefold higher specific production rates (q_p). CALB transcription levels were drastically higher when employing the novel expression systems. This higher expression was also correlated with a marked upregulation of unfolded protein response (UPR) related genes, likely from an increased protein burden in the endoplasmic reticulum (ER). Based on the chemostat results obtained, best culture strategies for both P_PDF and P_UPP expression systems were also successfully implemented in 15 L fed-batch cultivations where q_p and product to biomass yield (Y_P/X*) values were similar than those obtained in chemostat cultivations.

Conclusions

As an outcome of the macrokinetic characterization presented, the novel P_PDF and P_UPP were observed to offer much higher efficiency for CalB production than the widely used P_GAP-based methanol-free alternative. Thus, both systems arise as highly productive alternatives for P. pastoris-based RPP bioprocesses. Furthermore, the different expression regulation patterns observed indicate the level of gene expression can be adjusted, or tuned, which is interesting when using Pichia pastoris as a cell factory for different products of interest.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12934-021-01564-9.

Synergistic optimisation of expression, folding, and secretion improves E. coli AppA phytase production in Pichia pastoris

Background

Pichia pastoris (Komagataella phaffii) is an important platform for heterologous protein production due to its growth to high cell density and outstanding secretory capabilities. Recent developments in synthetic biology have extended the toolbox for genetic engineering of P. pastoris to improve production strains. Yet, overloading the folding and secretion capacity of the cell by over-expression of recombinant proteins is still an issue and rational design of strains is critical to achieve cost-effective industrial manufacture. Several enzymes are commercially produced in P. pastoris, with phytases being one of the biggest on the global market. Phytases are ubiquitously used as a dietary supplement for swine and poultry to increase digestibility of phytic acid, the main form of phosphorous storage in grains.

Results

Potential bottlenecks for expression of E. coli AppA phytase in P. pastoris were explored by applying bidirectional promoters (BDPs) to express AppA together with folding chaperones, disulfide bond isomerases, trafficking proteins and a cytosolic redox metabolism protein. Additionally, transcriptional studies were used to provide insights into the expression profile of BDPs. A flavoprotein encoded by ERV2 that has not been characterised in P. pastoris was used to improve the expression of the phytase, indicating its role as an alternative pathway to ERO1. Subsequent AppA production increased by 2.90-fold compared to the expression from the state of the AOX1 promoter.

Discussion

The microbial production of important industrial enzymes in recombinant systems can be improved by applying newly available molecular tools. Overall, the work presented here on the optimisation of phytase production in P. pastoris contributes to the improved understanding of recombinant protein folding and secretion in this important yeast microbial production host.

Knockout of RSN1, TVP18 or CSC1-2 causes perturbation of Golgi cisternae in Pichia pastoris

The structural organization of the Golgi stacks in mammalian cells is intrinsically linked to function, including glycosylation, but the role of morphology is less clear in lower eukaryotes. Here we investigated the link between the structural organization of the Golgi and secretory pathway function using Pichia pastoris as a model system. To unstack the Golgi cisternae, we disrupted 18 genes encoding proteins in the secretory pathway without loss of viability. Using biosensors, confocal microscopy and transmission electron microscopy we identified three strains with irreversible perturbations in the stacking of the Golgi cisternae, all of which had disruption in genes that encode proteins with annotated function as or homology to calcium/calcium permeable ion channels. Despite this, no variation in the secretory pathway for ER size, whole cell glycomics or recombinant protein glycans was observed. Our investigations showed the robust nature of the secretory pathway in P. pastoris and suggest that Ca²⁺ concentration, homeostasis or signalling may play a significant role for Golgi stacking in this organism and should be investigated in other organisms.

Impact of overproduced heterologous protein characteristics on physiological response in Yarrowia lipolytica steady-state-maintained continuous cultures

Overproduction of recombinant secretory proteins triggers numerous physiological perturbations. Depending on a given heterologous protein characteristics, the producer cell is faced with different challenges which lead to varying responses in terms of its physiology and the target protein production rate. In the present study, we used steady-state-maintained Yarrowia lipolytica cells to investigate the impact of different heterologous proteins on the physiological behavior of the host cells. Such an approach allowed to uncouple the impact of the overproduction of a particular protein from the phenomena that result from growth phase or are caused by the heterogeneity of the analyzed populations. Altogether, eight variants of recombinant strains, individually overproducing heterologous proteins of varying molecular weight (27-65 kDa) and reporting activity (enzymatic and fluorescent) were subjected to chemostat cultivations. The steady-state-maintained cells were analyzed in terms of the substrate utilization, biomass and metabolites production, as well as the reporter protein synthesis. Simplified distribution of carbon and nitrogen between the respective products, as well as expression analysis of the heterologous genes were conducted. The here-obtained data suggest that using a more transcriptionally active promoter results in channeling more C flux towards the target protein, giving significantly higher specific amounts and production rates of the target polypeptide, at the cost of biomass accumulation, and with no significant impact on the polyols production. The extent of the reporter protein's post-translational modifications, i.e., the number of disulfide bonds and glycosylation pattern, strongly impacts the synthesis process. Specific responses in terms of the protein formation kinetics, the gene expression levels, and transcript-to-protein linearity were observed.Key Points• Eight expression systems, producing different reporter proteins were analyzed.• The cells were maintained in steady-state by continuous chemostat culturing.• Protein- and promoter-specific effects were observed.

Continuous Cultivation as a Tool Toward the Rational Bioprocess Development With Pichia Pastoris Cell Factory

The methylotrophic yeast Pichia pastoris (Komagataella phaffii) is currently considered one of the most promising hosts for recombinant protein production (RPP) and metabolites due to the availability of several tools to efficiently regulate the recombinant expression, its ability to perform eukaryotic post-translational modifications and to secrete the product in the extracellular media. The challenge of improving the bioprocess efficiency can be faced from two main approaches: the strain engineering, which includes enhancements in the recombinant expression regulation as well as overcoming potential cell capacity bottlenecks; and the bioprocess engineering, focused on the development of rational-based efficient operational strategies. Understanding the effect of strain and operational improvements in bioprocess efficiency requires to attain a robust knowledge about the metabolic and physiological changes triggered into the cells. For this purpose, a number of studies have revealed chemostat cultures to provide a robust tool for accurate, reliable, and reproducible bioprocess characterization. It should involve the determination of key specific rates, productivities, and yields for different C and N sources, as well as optimizing media formulation and operating conditions. Furthermore, studies along the different levels of systems biology are usually performed also in chemostat cultures. Transcriptomic, proteomic and metabolic flux analysis, using different techniques like differential target gene expression, protein description and ¹³C-based metabolic flux analysis, are widely described as valued examples in the literature. In this scenario, the main advantage of a continuous operation relies on the quality of the homogeneous samples obtained under steady-state conditions, where both the metabolic and physiological status of the cells remain unaltered in an all-encompassing picture of the cell environment. This contribution aims to provide the state of the art of the different approaches that allow the design of rational strain and bioprocess engineering improvements in Pichia pastoris toward optimizing bioprocesses based on the results obtained in chemostat cultures. Interestingly, continuous cultivation is also currently emerging as an alternative operational mode in industrial biotechnology for implementing continuous process operations.

Optimization of Yarrowia lipolytica-based consolidated biocatalyst through synthetic biology approach: transcription units and signal peptides shuffling

Abstract

Nowadays considerable effort is being pursued towards development of consolidated microbial biocatalysts that will be able to utilize complex, non-pretreated substrates and produce valuable compounds. In such engineered microbes, synthesis of extracellular hydrolases may be fine-tuned by different approaches, like strength of promoter, type of secretory tag, and gene copy number. In this study, we investigated if organization of a multi-element expression cassette impacts the resultant Yarrowia lipolytica transformants’ phenotype, presuming that different variants of the cassette are composed of the same regulatory elements and encode the same mature proteins. To this end, Y. lipolytica cells were transformed with expression cassettes bearing a pair of genes encoding exactly the same mature amylases, but fused to four different signal peptides (SP), and located interchangeably in either first or second position of a synthetic DNA construction. The resultant strains were tested for growth on raw and pretreated complex substrates of different plant origin for comprehensive examination of the strains’ acquired characteristics. Optimized strain was tested in batch bioreactor cultivations for growth and lipids accumulation. Based on the conducted research, we concluded that the positional order of transcription units (TU) and the type of exploited SP affect final characteristics of the resultant consolidated biocatalyst strains, and thus could be considered as additional factors to be evaluated upon consolidated biocatalysts optimization.

Key Points

• Y. lipolytica growing on raw starch was constructed and tested on different substrates.

• Impact of expression cassette design and SP on biocatalysts’ phenotype was evidenced.

• Consolidated biocatalyst process for lipids production from starch was conducted.

Electronic supplementary material

The online version of this article (10.1007/s00253-020-10644-6) contains supplementary material, which is available to authorized users.

Orthologous promoters from related methylotrophic yeasts surpass expression of endogenous promoters of Pichia pastoris

Methylotrophic yeasts such as Komagataella phaffii (syn. Pichia pastoris, Pp), Hansenula polymorpha (Hp), Candida boidinii (Cb) and Pichia methanolica (Pm) are widely used protein production platforms. Typically, strong, tightly regulated promoters of genes coding for their methanol utilization (MUT) pathways are used to drive heterologous gene expression. Despite highly similar open reading frames in the MUT pathways of the four yeasts, the regulation of the respective promoters varies strongly between species. While most endogenous Pp MUT promoters remain tightly repressed after depletion of a repressing carbon, Hp, Cb and Pm MUT promoters are derepressed to up to 70% of methanol induced levels, enabling methanol free production processes in their respective host background. Here, we have tested a series of orthologous promoters from Hp, Cb and Pm in Pp. Unexpectedly, when induced with methanol, the promoter of the HpMOX gene reached very similar expression levels as the strong methanol, inducible, and most frequently used promoter of the Pp alcohol oxidase 1 gene (P_PpAOX1). The HpFMD promoter even surpassed P_PpAOX1 up to three-fold, when induced with methanol, and reached under methanol-free/derepressed conditions similar expression as the methanol induced P_PpAOX1. These results demonstrate that orthologous promoters from related yeast species can give access to otherwise unobtainable regulatory profiles and may even considerably surpass endogenous promoters in P. pastoris.

Comprehensive comparison of Yarrowia lipolytica and Pichia pastoris for production of Candida antarctica lipase B

The large-scale production of recombinant proteins (rProt) is becoming increasingly economically important. Among the different hosts used for rProt production, yeasts are gaining popularity. The so-called non-conventional yeasts, such as the methylotrophic Pichia pastoris and the dimorphic Yarrowia lipolytica, are popular choices due to their favorable characteristics and well-established expression systems. Nevertheless, a direct comparison of the two systems for rProt production and secretion was lacking. This study therefore aimed to directly compare Y. lipolytica and P. pastoris for the production and secretion of lipase CalB in bioreactor. Y. lipolytica produced more than double the biomass and more than 5-fold higher extracellular lipase than P. pastoris. Furthermore, maximal CalB production levels were reached by Y. lipolytica in half the cultivation time required for maximal production by P. pastoris. Conversely, P. pastoris was found to express 7-fold higher levels of CalB mRNA. Secreted enhanced green fluorescent protein -in isolation and fused to CalB- and protease inhibitor MG-132 were used in P. pastoris to further investigate the reasons behind such discrepancy. The most likely explanation was ultimately found to be protein degradation by endoplasmic reticulum-associated protein degradation preceding successful secretion. This study highlighted the multifaceted nature of rProt production, prompting a global outlook for selection of rProt production systems.

Different construction strategies affected on the physiology of Pichia pastoris strains highly expressed lipase by transcriptional analysis of key genes

We demonstrated previously that expression of Rhizomucor miehei lipase (RML) in Pichia pastoris could be significantly increased by addition of gene propeptide, optimized signal peptide codons and manipulation of gene dosage. In this study, effects of various strategies on the protein synthesis and secretion pathways were analyzed. Using nine strains previously constructed, we evaluated cell culture properties, enzymatic activities, and analyzed transcriptional levels of nine genes involved in protein synthesis and secretion pathways by qPCR. We observed that (i) Addition of propeptide decreased lipase folding stress by down-regulated four UPR-related genes. (ii) Signal peptide codons optimization had no effect on host with no change in the nine detected genes. (iii) Folding stress and limited transport capacity produced when rml gene dosage exceed 2. Different limiting factors on lipase expression in strains with different construction strategies were identified. This study provides a theoretical basis for further improving RML by transforming host.

Rationale-based selection of optimal operating strategies and gene dosage impact on recombinant protein production in Komagataella phaffii (Pichia pastoris)

Its features as a microbial and eukaryotic organism have turned Komagataella phaffii (Pichia pastoris) into an emerging cell factory for recombinant protein production (RPP). As a key step of the bioprocess development, this work aimed to demonstrate the importance of tailor designing the cultivation strategy according to the production kinetics of the cell factory. For this purpose, K. phaffii clones constitutively expressing (P_GAP) Candida rugosa lipase 1 (Crl1) with different gene dosage were used as models in continuous and fed‐batch cultures. Production parameters were much greater with a multicopy clone (MCC) than with the single‐copy clone (SCC). Regarding production kinetics, the specific product generation rate (q_P) increased linearly with increasing specific growth rate (µ) in SCC; by contrast, q_P exhibited saturation in MCC. A transcriptional analysis in chemostat cultures suggested the presence of eventual post‐transcriptional bottlenecks in MCC. After the strain characterization, in order to fulfil overall development of the bioprocess, the performance of both clones was also evaluated in fed‐batch mode. Strikingly, different optimal strategies were determined for both models due to the different production kinetic patterns observed as a trade‐off for product titre, yields and productivity. The combined effect of gene dosage and adequate µ enables rational process development with a view to optimize K. phaffii RPP bioprocesses.

CRISPR-Cas9-mediated genomic multiloci integration in Pichia pastoris

BACKGROUND

Pichia pastoris (syn. Komagataella phaffii) is a widely used generally recognized as safe host for heterologous expression of proteins in both industry and academia. Recently, it has been shown to be a potentially good chassis host for the production of high-value pharmaceuticals and chemicals. Nevertheless, limited availability of selective markers and low efficiency of homologous recombination make this process difficult and time-consuming, particularly in the case of multistep biosynthetic pathways. Therefore, it is crucial to develop an efficient and marker-free multiloci gene knock-in method in P. pastoris.

RESULTS

A non-homologous-end-joining defective strain (Δku70) was first constructed using the CRISPR-Cas9 based gene deficiency approach. It was then used as a parent strain for multiloci gene integration. Ten guide RNA (gRNA) targets were designed within 100 bp upstream of the promoters or downstream of terminator, and then tested using an eGFP reporter and confirmed as suitable single-locus integration sites. Three high-efficiency gRNA targets (PAOX1UP-g2, PTEF1UP-g1, and PFLD1UP-g1) were selected for double- and triple-locus co-integration. The integration efficiency ranged from 57.7 to 70% and 12.5 to 32.1% for double-locus and triple-locus integration, respectively. In addition, biosynthetic pathways of 6-methylsalicylic acid and 3-methylcatechol were successfully assembled using the developed method by one-step integration of functional genes. The desired products were obtained, which further established the effectiveness and applicability of the developed CRISPR-Cas9-mediated gene co-integration method in P. pastoris.

CONCLUSIONS

A CRISPR-Cas9-mediated multiloci gene integration method was developed with efficient gRNA targets in P. pastoris. Using this method, multiple gene cassettes can be simultaneously integrated into the genome without employing selective markers. The multiloci integration strategy is beneficial for pathway assembly of complicated pharmaceuticals and chemicals expressed in P. pastoris.

Recent advances in plasmid-based tools for establishing novel microbial chassis

A key challenge for domesticating alternative cultivable microorganisms with biotechnological potential lies in the development of innovative technologies. Within this framework, a myriad of genetic tools has flourished, allowing the design and manipulation of complex synthetic circuits and genomes to become the general rule in many laboratories rather than the exception. More recently, with the development of novel technologies such as DNA automated synthesis/sequencing and powerful computational tools, molecular biology has entered the synthetic biology era. In the beginning, most of these technologies were established in traditional microbial models (known as chassis in the synthetic biology framework) such as Escherichia coli and Saccharomyces cerevisiae, enabling fast advances in the field and the validation of fundamental proofs of concept. However, it soon became clear that these organisms, although extremely useful for prototyping many genetic tools, were not ideal for a wide range of biotechnological tasks due to intrinsic limitations in their molecular/physiological properties. Over the last decade, researchers have been facing the great challenge of shifting from these model systems to non-conventional chassis with endogenous capacities for dealing with specific tasks. The key to address these issues includes the generation of narrow and broad host plasmid-based molecular tools and the development of novel methods for engineering genomes through homologous recombination systems, CRISPR/Cas9 and other alternative methods. Here, we address the most recent advances in plasmid-based tools for the construction of novel cell factories, including a guide for helping with "build-your-own" microbial host.

Heterologous expression of Thrombocidin-1 in Pichia pastoris: Evaluation of its antibacterial and antioxidant activity

The antimicrobial peptide Thrombocidin-1 (TC-1) isolated from human blood that derived from NAP-2 by deleting of two amino acids from C-terminal region. In this study, a C-terminal 6 _ His tagged recombinant TC-1 was expressed as a secreted peptide in Pichia pastoris, for the first time. The recombinant P. pastoris was inoculated in to BMMY culture medium, incubation with 5 μl/ml absolute methanol for 72 h at 30 °C. The TC-1 peptide was concentrated with nickel affinity chromatography and electrophoresis on 16% acrylamide gels. The molecular weight of recombinant TC-1 is approximately 8 kDa and under these conditions, the concentration of TC-1 considered 190 μg/ml that determined by the Bradford method. The antimicrobial activity test (Minimum Inhibitory Concentration and Minimum Bactericidal Concentration) was done against: Listeria monocytogenes, Escherichia coli, Klebsiella pneumonia, Staphylococcus aureus, Enterococcus faecalis and Pseudomonas aeruginosa. The growth of these pathogenic bacteria was limited when we used peptide at a concentration of as low as 19.56 μg/ml. Based on DPPH radical scavenging (DPPH-RS) activity and reducing power assays, this peptide showed relatively good antioxidant potential in comparison with standard antioxidant used in this study (BHT). Due to the existence of TC-1 in blood, which makes it safe for human consumption, and the good results of its antimicrobial and antioxidant activity, it can be introduced as a good alternative and a novel effective peptide to food industry for bio-preservation.

Engineered bidirectional promoters enable rapid multi-gene co-expression optimization

Numerous synthetic biology endeavors require well-tuned co-expression of functional components for success. Classically, monodirectional promoters (MDPs) have been used for such applications, but MDPs are limited in terms of multi-gene co-expression capabilities. Consequently, there is a pressing need for new tools with improved flexibility in terms of genetic circuit design, metabolic pathway assembly, and optimization. Here, motivated by nature's use of bidirectional promoters (BDPs) as a solution for efficient gene co-expression, we generate a library of 168 synthetic BDPs in the yeast Komagataella phaffii (syn. Pichia pastoris), leveraging naturally occurring BDPs as a parts repository. This library of synthetic BDPs allows for rapid screening of diverse expression profiles and ratios to optimize gene co-expression, including for metabolic pathways (taxadiene, β-carotene). The modular design strategies applied for creating the BDP library could be relevant in other eukaryotic hosts, enabling a myriad of metabolic engineering and synthetic biology applications.

Engineered bidirectional promoters enable rapid multi-gene co-expression optimization

Numerous synthetic biology endeavors require well-tuned co-expression of functional components for success. Classically, monodirectional promoters (MDPs) have been used for such applications, but MDPs are limited in terms of multi-gene co-expression capabilities. Consequently, there is a pressing need for new tools with improved flexibility in terms of genetic circuit design, metabolic pathway assembly, and optimization. Here, motivated by nature’s use of bidirectional promoters (BDPs) as a solution for efficient gene co-expression, we generate a library of 168 synthetic BDPs in the yeast Komagataella phaffii (syn. Pichia pastoris), leveraging naturally occurring BDPs as a parts repository. This library of synthetic BDPs allows for rapid screening of diverse expression profiles and ratios to optimize gene co-expression, including for metabolic pathways (taxadiene, β-carotene). The modular design strategies applied for creating the BDP library could be relevant in other eukaryotic hosts, enabling a myriad of metabolic engineering and synthetic biology applications.

Classic monodirectional promoters are of limited use for multiple gene co-expression. Here the authors generate a library of 168 bidirectional promoters for the yeast K. phaffii (syn. P. pastoris) with diverse expression profiles to optimize metabolic pathway design.

Comparison of Yeasts as Hosts for Recombinant Protein Production

Recombinant protein production emerged in the early 1980s with the development of genetic engineering tools, which represented a compelling alternative to protein extraction from natural sources. Over the years, a high level of heterologous protein was made possible in a variety of hosts ranging from the bacteria Escherichia coli to mammalian cells. Recombinant protein importance is represented by its market size, which reached $1654 million in 2016 and is expected to reach $2850.5 million by 2022. Among the available hosts, yeasts have been used for producing a great variety of proteins applied to chemicals, fuels, food, and pharmaceuticals, being one of the most used hosts for recombinant production nowadays. Historically, Saccharomyces cerevisiae was the dominant yeast host for heterologous protein production. Lately, other yeasts such as Komagataella sp., Kluyveromyces lactis, and Yarrowia lipolytica have emerged as advantageous hosts. In this review, a comparative analysis is done listing the advantages and disadvantages of using each host regarding the availability of genetic tools, strategies for cultivation in bioreactors, and the main techniques utilized for protein purification. Finally, examples of each host will be discussed regarding the total amount of protein recovered and its bioactivity due to correct folding and glycosylation patterns.

Metabolic engineering of Pichia pastoris

Besides its use for efficient production of recombinant proteins the methylotrophic yeast Pichia pastoris (syn. Komagataella spp.) has been increasingly employed as a platform to produce metabolites of varying origin. We summarize here the impressive methodological developments of the last years to model and analyze the metabolism of P. pastoris, and to engineer its genome and metabolic pathways. Efficient methods to insert, modify or delete genes via homologous recombination and CRISPR/Cas9, supported by modular cloning techniques, have been reported. An outstanding early example of metabolic engineering in P. pastoris was the humanization of protein glycosylation. More recently the cell metabolism was engineered also to enhance the productivity of heterologous proteins. The last few years have seen an increased number of metabolic pathway design and engineering in P. pastoris, mainly towards the production of complex (secondary) metabolites. In this review, we discuss the potential role of P. pastoris as a platform for metabolic engineering, its strengths, and major requirements for future developments of chassis strains based on synthetic biology principles.

A systematic analysis of the expression of the anti-HIV VRC01 antibody in Pichia pastoris through signal peptide optimization

Pichia pastoris (Komagataella phaffi) has been used for recombinant protein production for over 30 years with over 5000 proteins reported to date. However, yields of antibody are generally low. We have evaluated the effect of secretion signal peptides on the production of a broadly neutralizing antibody (VRC01) to increase yield. Eleven different signal peptides, including the murine IgG1 signal peptide, were combinatorially evaluated for their effect on antibody titer. Strains using different combinations of signal peptides were identified that secreted approximately 2–7 fold higher levels of VRC01 than the previous best secretor, with the highest yield of 6.50 mg L⁻¹ in shake flask expression. Interestingly it was determined that the highest yields were achieved when the murine IgG1 signal peptide was fused to the light chain, with several different signal peptides leading to high yield when fused to the heavy chain. Finally, we have evaluated the effect of using a 2A signal peptide to create a bicistronic vector in the attempt to reduce burden and increase transformation efficiency, but found it to give reduced yields compared to using two independent vectors.

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Production of VRC01 antibody in Pichia pastoris was increased 2–7 fold.

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Using the murine IgG1 signal peptide on the light chain results in higher yields.

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The use of bicistronic vectors did not increase the yield of VRC01.