Host-Informed Expression of CRISPR Guide RNA for Genomic Engineering in Komagataella phaffii

CRISPR-Cas9-based one-step multiplexed genome editing through optimizing guide RNA processing strategies in Pichia pastoris

The important methylotrophic yeast Pichia pastoris has been utilized for the production of a variety of heterologous recombinant proteins and has great potential for use in the production of value-added compounds using methanol as a substrate. However, the lack of convenient and efficient genome engineering tools has hindered further applications of P. pastoris, especially in complex and multistep metabolic engineering scenarios. Hence, we developed a rapid and convenient multi-gene editing system based on CRISPR/Cas9 by optimizing the guide RNA processing strategy, which can achieve dual-gene knockout or multi-gene integration in single step. Firstly, we found that the HgH (HH-sgRNA-HDV) structure achieved the highest single-gene knockout efficiency (95.8 %) among the three sgRNA processing cassettes, including a tRNA-sgRNA-tRNA (tgt) array, HgH structure and tRNA-sgRNA-HDV (tgH) structure. Furthermore, the dHgH structure (double HgH) enabled one-step dual-gene disruption and multi-gene integration. The efficiency of dual-site knockout ranged from 60 % to 100 %, with functional genes knockout achieving approximately 60 % (Δaox1Δgut1), while dual neutral sites knockout reached 100 %. Finally, we applied the system for one-step production of fatty acids and 5-hydroxytryptophan. The yield of FFAs reached 23 mg/L/μg protein/OD, while the yield of 5-hydroxytryptophan was 13.3 mg/L. The system will contribute to the application of P. pastoris as an attractive cell factory for multiplexed compound biosynthesis and will serve as a valuable tool for enhancing one-carbon (C1) bio-utilization.

Technological advancement spurs Komagataella phaffii as a next-generation platform for sustainable biomanufacturing

Biomanufacturing stands as a cornerstone of sustainable industrial development, necessitating a shift toward non-food carbon feedstocks to alleviate agricultural resource competition and advance a circular bioeconomy. Methanol, a renewable one‑carbon substrate, has emerged as a pivotal candidate due to its abundance, cost-effectiveness, and high reduction potential, further bolstered by breakthroughs in CO₂ hydrogenation-based synthesis. Capitalizing on this momentum, the methylotrophic yeast Komagataella phaffii has undergone transformative technological upgrades, evolving from a conventional protein expression workhorse into an intelligent bioproduction chassis. This paradigm shift is fundamentally driven by converging innovations across CRISPR-empowered advancement in genome editing and AI-powered metabolic pathway design in K. phaffii. The integration of CRISPR systems with droplet microfluidics high-throughput screening has redefined strain engineering efficiency, achieving much higher editing precision than traditional homologous recombination while compressing the "design-build-test-learn" cycle. Concurrently, machine learning-enhanced genome-scale metabolic models facilitate dynamic flux balancing, enabling simultaneous improvements in product titers, carbon yields, and volumetric productivity. Finally, technological advancement promotes the application of K. phaffii, including directing more efficiently metabolic flux toward nutrient products, and strengthening efficient synthesis of excreted proteins. As DNA synthesis automation and robotic experimentation platforms mature, next-generation breakthroughs in genome modification, cofactor engineering, and AI-guided autonomous evolution will further cement K. phaffii as a next-generation platform for decarbonizing global manufacturing paradigms. This technological trajectory positions methanol-based biomanufacturing as a cornerstone of the low-carbon circular economy.

A toolkit for facilitating markerless integration of expression cassettes in Komagataella phaffii via CRISPR/Cas9

BACKGROUND

The yeast Komagataella phaffii (formerly known as Pichia pastoris) has been widely used for functional expression of recombinant proteins, including plant and animal food proteins. CRISPR/Cas9 genome editing systems can be used for insertion of heterologous genes without the use of selection markers. The study aimed to create a convenient markerless knock-in method for integrating expression cassettes into the chromosome of K. phaffii using CRISPR/Cas9 technology. The approach was based on the hierarchical, modular, Golden Gate assembly employing the GoldenPiCS toolkit. Furthermore, the aim was to evaluate the system's efficiency and suitability for producing secreted recombinant food proteins.

RESULTS

Three Cas9/sgRNA plasmids were constructed, along with corresponding donor helper plasmids containing homology regions for chromosomal integration via homology-directed repair. The integration efficiency of an enhanced green fluorescent protein (eGFP) expression cassette was assessed at three genomic loci (04576, PFK1, and ROX1). The 04576 locus showed the highest integration efficiency, while ROX1 had the highest transformation efficiency. Whole genome sequencing revealed variable copy numbers of eGFP expression cassettes among clones, corresponding with increasing levels of fluorescence. Furthermore, the system's applicability for producing recombinant food proteins was validated by successfully expressing and secreting chicken ovalbumin. This constitutes the first report of CRISPR/Cas9 applied to produce recombinant chicken ovalbumin.

CONCLUSIONS

The adapted GoldenPiCS toolkit combined with CRISPR/Cas9 technology enabled efficient and precise genome integration in K. phaffii. This approach holds promise for expanding the production of high-value recombinant proteins. Future research should focus on optimizing integration sites and improving cloning procedures to enhance the system's efficiency and versatility.

Ribo-seq guided design of enhanced protein secretion in Komagataellaphaffii

The production of recombinant proteins requires the precise coordination of various biological processes, including protein synthesis, folding, trafficking, and secretion. The overproduction of a heterologous protein can impose various bottlenecks on these networks. Identifying and alleviating these bottlenecks can guide strain engineering efforts to enhance protein production. The methylotrophic yeast Komagataella phaffii is used for its high capacity to produce recombinant proteins. Here, we use ribosome profiling to identify bottlenecks in protein secretion during heterologous expression of human serum albumin (HSA). Validation of this analysis showed that the knockout of non-essential genes whose gene products target the ER, through co- and post-translational mechanisms, and have high ribosome utilization can increase production of a heterologous protein, HSA. A triple knockout in co-translationally translocated carbohydrate and acetate transporter Gal2p, cell wall maintenance protein Ydr134cp, and the post-translationally translocated cell wall protein Aoa65896.1 increased HSA production by 35 %. This data-driven strain engineering approach uses cell-level information to identify gene targets for phenotype improvement. This specific case identifies hits and creates strains with improved HSA production, with Ribo-seq and bioinformatic analysis to identify non-essential ER targeted proteins that are high ribosome utilizers.

Adaptation of Aglycosylated Monoclonal Antibodies for Improved Production in Komagataella phaffii

Monoclonal antibodies (mAbs) are a major class of biopharmaceuticals manufactured by well-established processes using Chinese Hamster Ovary (CHO) cells. Next-generation biomanufacturing using alternative hosts like Komagataella phaffii could improve the accessibility of these medicines, address broad societal goals for sustainability, and offer financial advantages for accelerated development of new products. Antibodies produced by K. phaffii, however, may manifest unique molecular quality attributes, like host-dependent, product-related variants, that could raise potential concerns for clinical use. We demonstrate here conservative modifications to the amino acid sequence of aglycosylated antibodies based on the human IgG1 isotype that minimize product-related variations when secreted by K. phaffii. A combination of 2-3 changes of amino acids reduced variations across six different aglycosylated versions of commercial mAbs. Expression of a modified sequence of NIST mAb in both K. phaffii and CHO cells showed comparable biophysical properties and molecular variations. These results suggest a path toward the production of high-quality mAbs that could be expressed interchangeably by either yeast or mammalian cells. Improving molecular designs of proteins to enable a range of manufacturing strategies for well-characterized biopharmaceuticals could accelerate global accessibility and innovations.

Optimized genome-wide CRISPR screening enables rapid engineering of growth-based phenotypes in Yarrowia lipolytica

CRISPR-Cas9 functional genomic screens uncover gene targets linked to various phenotypes for metabolic engineering with remarkable efficiency. However, these genome-wide screens face a number of design challenges, including variable guide RNA activity, ensuring sufficient genome coverage, and maintaining high transformation efficiencies to ensure full library representation. These challenges are prevalent in non-conventional yeast, many of which exhibit traits that are well suited to metabolic engineering and bioprocessing. To address these hurdles in the oleaginous yeast Yarrowia lipolytica, we designed a compact, high-activity genome-wide sgRNA library. The library was designed using DeepGuide, an sgRNA activity prediction algorithm and a large dataset of ∼50,000 sgRNAs with known activity. Three guides per gene enables redundant targeting of 98.8% of genes in the genome in a library of 23,900 sgRNAs. We deployed the optimized library to uncover genes essential to the tolerance of acetate, a promising alternative carbon source, and various hydrocarbons present in many waste streams. Our screens yielded several gene knockouts that improve acetate tolerance on their own and as double knockouts in media containing acetate as the sole carbon source. Analysis of the hydrocarbon screens revealed genes related to fatty acid and alkane metabolism in Y. lipolytica. The optimized CRISPR gRNA library and its successful use in Y. lipolytica led to the discovery of alternative carbon source-related genes and provides a workflow for creating high-activity, compact genome-wide libraries for strain engineering.

Functional genomic screening in Komagataella phaffii enabled by high-activity CRISPR-Cas9 library

CRISPR-based high-throughput genome-wide loss-of-function screens are a valuable approach to functional genetics and strain engineering. The yeast Komagataella phaffii is a host of particular interest in the biopharmaceutical industry and as a metabolic engineering host for proteins and metabolites. Here, we design and validate a highly active 6-fold coverage genome-wide sgRNA library for this biotechnologically important yeast containing 30,848 active sgRNAs targeting over 99% of its coding sequences. Conducting fitness screens in the absence of functional non-homologous end joining (NHEJ), the dominant DNA repair mechanism in K. phaffii, provides a quantitative means to assess the activity of each sgRNA in the library. This approach allows for the experimental validation of each guide's targeting activity, leading to more precise screening outcomes. We used this approach to conduct growth screens with glucose as the sole carbon source and identify essential genes. Comparative analysis of the called gene sets identified a core set of K. phaffii essential genes, many of which relate to metabolic engineering targets, including protein production, secretion, and glycosylation. The high activity, genome-wide CRISPR library developed here enables functional genomic screening in K. phaffii, applied here to gene essentiality classification, and promises to enable other genetic screens.

CRISPR-Cas9 knockout screen informs efficient reduction of the Komagataella phaffii secretome

BACKGROUND

The yeast Komagataella phaffii is widely used for manufacturing recombinant proteins, but secreted titers of recombinant proteins could be improved by genetic engineering. In this study, we hypothesized that cellular resources could be redirected from production of endogenous proteins to production of recombinant proteins by deleting unneeded endogenous proteins. In non-model microorganisms such as K. phaffii, however, genetic engineering is limited by lack gene annotation and knowledge of gene essentiality.

RESULTS

We identified a set of endogenous secreted proteins in K. phaffii by mass spectrometry and signal peptide prediction. Our efforts to disrupt these genes were hindered by limited annotation of essential genes. To predict essential genes, therefore, we designed, transformed, and sequenced a pooled library of guide RNAs for CRISPR-Cas9-mediated knockout of all endogenous secreted proteins. We then used predicted gene essentiality to guide iterative disruptions of up to 11 non-essential genes. Engineered strains exhibited a ~20× increase in the production of human serum albumin and a twofold increase in the production of a monoclonal antibody.

CONCLUSIONS

We demonstrated that disruption of as few as six genes can increase production of recombinant proteins. Further reduction of the endogenous proteome of K. phaffii may further improve strain performance. The pooled library of secretome-targeted guides for CRISPR-Cas9 and knowledge of gene essentiality reported here will facilitate future efforts to engineer K. phaffii for production of other recombinant proteins and enzymes.

Applications of the Methylotrophic Yeast Komagataella phaffii in the Context of Modern Biotechnology

Komagataella phaffii (formerly Pichia pastoris) is a methylotrophic yeast widely used in laboratories around the world to produce recombinant proteins. Given its advantageous features, it has also gained much interest in the context of modern biotechnology. In this review, we present the utilization of K. phaffii as a platform to produce several products of economic interest such as biopharmaceuticals, renewable chemicals, fuels, biomaterials, and food/feed products. Finally, we present synthetic biology approaches currently used for strain engineering, aiming at the production of new bioproducts.

CRISPR/Cas9-based toolkit for rapid marker recycling and combinatorial libraries in Komagataella phaffii

Komagataella phaffii, a nonconventional yeast, is increasingly attractive to researchers owing to its posttranslational modification ability, strict methanol regulatory mechanism, and lack of Crabtree effect. Although CRISPR-based gene editing systems have been established in K. phaffii, there are still some inadequacies compared to the model organism Saccharomyces cerevisiae. In this study, a redesigned gRNA plasmid carrying red and green fluorescent proteins facilitated plasmid construction and marker recycling, respectively, making marker recycling more convenient and reliable. Subsequently, based on the knockdown of Ku70 and DNA ligase IV, we experimented with integrating multiple DNA fragments at a single locus. A 26.5-kb-long DNA fragment divided into 11 expression cassettes for lycopene synthesis could be successfully integrated into a single locus at one time with a success rate of 57%. A 27-kb-long DNA fragment could also be precisely knocked out with a 50% positive rate in K. phaffii by introducing two DSBs simultaneously. Finally, to explore the feasibility of rapidly balancing the expression intensity of multiple genes in a metabolic pathway, a yeast combinatorial library was successfully constructed in K. phaffii using lycopene as an indicator, and an optimal combination of the metabolic pathway was identified by screening, with a yield titer of up to 182.73 mg/L in shake flask fermentation. KEY POINTS: • Rapid marker recycling based on the visualization of a green fluorescent protein • One-step multifragment integration and large fragment knockout in the genome • A random assembly of multiple DNA elements to create yeast libraries in K. phaffii.

Oligonucleotide-based CRISPR-Cas9 toolbox for efficient engineering of Komagataella phaffii

Komagataella phaffii (Pichia pastoris) is a methylotrophic yeast that is favored by industry and academia mainly for expression of heterologous proteins. However, its full potential as a host for bioproduction of valuable compounds cannot be fully exploited as genetic tools are lagging behind those that are available for baker's yeast. The emergence of CRISPR-Cas9 technology has significantly improved the efficiency of gene manipulations of K. phaffii, but improvements in gene-editing methods are desirable to further accelerate engineering of this yeast. In this study, we have developed a versatile vector-based CRISPR-Cas9 method and showed that it works efficiently at different genetic loci using linear DNA fragments with very short targeting sequences including single-stranded oligonucleotides. Notably, we performed site-specific point mutations and full gene deletions using short (90 nt) single-stranded oligonucleotides at very high efficiencies. Lastly, we present a strategy for transient inactivation of nonhomologous end-joining (NHEJ) pathway, where KU70 gene is disrupted by a visual marker (uidA gene). This system enables precise CRISPR-Cas9-based editing (including multiplexing) and facilitates simple reversion to NHEJ-proficient genotype. In conclusion, the tools presented in this study can be applied for easy and efficient engineering of K. phaffii strains and are compatible with high-throughput automated workflows.

Multiplex Marker-Less Genome Integration in Pichia pastoris Using CRISPR/Cas9

Pichia pastoris is known for its excellent protein expression ability. As an industrial methyl nutritional yeast, it can effectively utilize methanol as the sole carbon source, serving as a potential platform for C1 biotransformation. Unfortunately, the lack of synthetic biology tools in P. pastoris limits its broad applications, particularly when multigene pathways should be manipulated. Here, the CRISPR/Cas9 system is established to efficiently integrate multiple heterologous genes to construct P. pastoris cell factories. In this protocol, with the 2,3-butanediol (BDO) biosynthetic pathway as a representative example, the procedures to construct P. pastoris cell factories are detailed using the established CRISPR-based multiplex genome integration toolkit, including donor plasmid construction, competent cell preparation and transformation, and transformant verification. The application of the CRISPR toolkit is demonstrated by the construction of engineered P. pastoris for converting methanol to BDO. This lays the foundation for the construction of P. pastoris cell factories harboring multi-gene biosynthetic pathways for the production of high-value compounds.

A supernumerary synthetic chromosome in Komagataella phaffii as a repository for extraneous genetic material

BACKGROUND

Komagataella phaffii (Pichia pastoris) is a methylotrophic commercially important non-conventional species of yeast that grows in a fermentor to exceptionally high densities on simple media and secretes recombinant proteins efficiently. Genetic engineering strategies are being explored in this organism to facilitate cost-effective biomanufacturing. Small, stable artificial chromosomes in K. phaffii could offer unique advantages by accommodating multiple integrations of extraneous genes and their promoters without accumulating perturbations of native chromosomes or exhausting the availability of selection markers.

RESULTS

Here, we describe a linear "nano"chromosome (of 15-25 kb) that, according to whole-genome sequencing, persists in K. phaffii over many generations with a copy number per cell of one, provided non-homologous end joining is compromised (by KU70-knockout). The nanochromosome includes a copy of the centromere from K. phaffii chromosome 3, a K. phaffii-derived autonomously replicating sequence on either side of the centromere, and a pair of K. phaffii-like telomeres. It contains, within its q arm, a landing zone in which genes of interest alternate with long (approx. 1-kb) non-coding DNA chosen to facilitate homologous recombination and serve as spacers. The landing zone can be extended along the nanochromosome, in an inch-worming mode of sequential gene integrations, accompanied by recycling of just two antibiotic-resistance markers. The nanochromosome was used to express PDI, a gene encoding protein disulfide isomerase. Co-expression with PDI allowed the production, from a genomically integrated gene, of secreted murine complement factor H, a plasma protein containing 40 disulfide bonds. As further proof-of-principle, we co-expressed, from a nanochromosome, both PDI and a gene for GFP-tagged human complement factor H under the control of PAOX1 and demonstrated that the secreted protein was active as a regulator of the complement system.

CONCLUSIONS

We have added K. phaffii to the list of organisms that can produce human proteins from genes carried on a stable, linear, artificial chromosome. We envisage using nanochromosomes as repositories for numerous extraneous genes, allowing intensive engineering of K. phaffii without compromising its genome or weakening the resulting strain.

Genetic tools for metabolic engineering of Pichia pastoris

The methylotrophic yeast Pichia pastoris (also known as Komagataella phaffii) is widely used as a yeast cell factory for producing heterologous proteins. Recently, it has gained attention for its potential in producing chemicals from inexpensive feedstocks, which requires efficient genetic engineering platforms. This review provides an overview of the current advances in developing genetic tools for metabolic engineering of P. pastoris. The topics cover promoters, terminators, plasmids, genome integration sites, and genetic editing systems, with a special focus on the development of CRISPR/Cas systems and their comparison to other genome editing tools. Additionally, this review highlights the prospects of multiplex genome integration, fine-tuning gene expression, and single-base editing systems. Overall, the aim of this review is to provide valuable insights into current genetic engineering and discuss potential directions for future efforts in developing efficient genetic tools in P. pastoris.

CRISPR-Cas Technology for Bioengineering Conventional and Non-Conventional Yeasts: Progress and New Challenges

The clustered regularly interspaced short palindromic repeats (CRISPR) and CRISPR-associated protein (CRISPR-Cas) system has undergone substantial and transformative progress. Simultaneously, a spectrum of derivative technologies has emerged, spanning both conventional and non-conventional yeast strains. Non-conventional yeasts, distinguished by their robust metabolic pathways, formidable resilience against diverse stressors, and distinctive regulatory mechanisms, have emerged as a highly promising alternative for diverse industrial applications. This comprehensive review serves to encapsulate the prevailing gene editing methodologies and their associated applications within the traditional industrial microorganism, Saccharomyces cerevisiae. Additionally, it delineates the current panorama of non-conventional yeast strains, accentuating their latent potential in the realm of industrial and biotechnological utilization. Within this discourse, we also contemplate the potential value these tools offer alongside the attendant challenges they pose.

Large-Scale Purification and Characterization of Recombinant Receptor-Binding Domain (RBD) of SARS-CoV-2 Spike Protein Expressed in Yeast

SARS-CoV-2 spike protein is an essential component of numerous protein-based vaccines for COVID-19. The receptor-binding domain of this spike protein is a promising antigen with ease of expression in microbial hosts and scalability at comparatively low production costs. This study describes the production, purification, and characterization of RBD of SARS-CoV-2 protein, which is currently in clinical trials, from a commercialization perspective. The protein was expressed in Pichia pastoris in a large-scale bioreactor of 1200 L capacity. Protein capture and purification are conducted through mixed-mode chromatography followed by hydrophobic interaction chromatography. This two-step purification process produced RBD with an overall productivity of ~21 mg/L at >99% purity. The protein's primary, secondary, and tertiary structures were also verified using LCMS-based peptide mapping, circular dichroism, and fluorescence spectroscopy, respectively. The glycoprotein was further characterized for quality attributes such as glycosylation, molecular weight, purity, di-sulfide bonding, etc. Through structural analysis, it was confirmed that the product maintained a consistent quality across different batches during the large-scale production process. The binding capacity of RBD of spike protein was also assessed using human angiotensin-converting enzyme 2 receptor. A low binding constant range of KD values, ranging between 3.63 × 10-8 to 6.67 × 10-8, demonstrated a high affinity for the ACE2 receptor, revealing this protein as a promising candidate to prevent the entry of COVID-19 virus.

An improved CRISPRi system in Pichia pastoris

CRISPR interference (CRISPRi) has been developed and widely used for gene repression in various hosts. Here we report an improved CRISPRi system in Pichia pastoris by fusing dCas9 with endogenous transcriptional repressor domains. The CRISPRi system shows strong repression of eGFP, with the highest efficiency of 85%. Repression of native genes is demonstrated by targeting AOX1 promoter. AOX1 is efficiently repressed and the mutant strains show much slower growth in methanol medium. Effects of gRNA expression and processing on CRISPRi efficiency is also investigated. It is found that gRNA processing by HH/HDV ribozymes or Csy4 endoribonuclease generating clean gRNA is critical to achieve strong repression, and Csy4 cleavage shows higher repression efficiency. However, gRNA expression using native tRNA transcription and processing systems results in relatively weaker repression of eGFP. By expression of two gRNAs targeting promoters of eGFP and AOX1 in an array together with Cys4 recognition sites, both genes can be repressed simultaneously. Cys4-mediated gRNA array processing is further applied to repress fatty acyl-CoA synthetase genes (FAA1 and FAA2). Both genes are efficiently repressed, demonstrating that Cys4 endoribonuclease has the ability to cleave gRNAs array and can be can be used for multiplexed gene repression in P. pastoris.

Transcription regulation strategies in methylotrophs: progress and challenges

As a promising industrial microorganism, methylotroph is capable of using methane or methanol as the sole carbon source natively, which has been utilized in the biosynthesis of various bioproducts. However, the relatively low efficiency of carbon conversion has become a limiting factor throughout the development of methanotrophic cell factories due to the unclear genetic background. To better highlight their advantages in methane or methanol-based biomanufacturing, some metabolic engineering strategies, including upstream transcription regulation projects, are being popularized in methylotrophs. In this review, several strategies of transcription regulations applied in methylotrophs are summarized and their applications are discussed and prospected.

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.

Whole Genome Sequencing Analysis of Effects of CRISPR/Cas9 in Komagataella phaffii: A Budding Yeast in Distress

The industrially important non-conventional yeast Komagataella phaffii suffers from low rates of homologous recombination, making site specific genetic engineering tedious. Therefore, genome editing using CRISPR/Cas represents a simple and efficient alternative. To characterize on- and off-target mutations caused by CRISPR/Cas9 followed by non-homologous end joining repair, we chose a diverse set of CRISPR/Cas targets and conducted whole genome sequencing on 146 CRISPR/Cas9 engineered single colonies. We compared the outcomes of single target CRISPR transformations to double target experiments. Furthermore, we examined the extent of possible large deletions by targeting a large genomic region, which is likely to be non-essential. The analysis of on-target mutations showed an unexpectedly high number of large deletions and chromosomal rearrangements at the CRISPR target loci. We also observed an increase of on-target structural variants in double target experiments as compared to single target experiments. Targeting of two loci within a putatively non-essential region led to a truncation of chromosome 3 at the target locus in multiple cases, causing the deletion of 20 genes and several ribosomal DNA repeats. The identified de novo off-target mutations were rare and randomly distributed, with no apparent connection to unspecific CRISPR/Cas9 off-target binding sites.

Fusing an exonuclease with Cas9 enhances homologous recombination in Pichia pastoris

BACKGROUND

The methylotrophic yeast Pichia pastoris is considered as an ideal host for the production of recombinant proteins and chemicals. However, low homologous recombination (HR) efficiency hinders its precise and extensive genetic manipulation. To enhance the homology-directed repair over non-homologous end joining (NHEJ), we expressed five exonucleases that were fused with the Cas9 for enhancing end resection of double strand breaks (DSBs) of DNA cuts.

RESULTS

The endogenous exonuclease Mre11 and Exo1 showed the highest positive rates in seamless deletion of FAA1, and fusing the MRE11 to the C-terminal of CAS9 had the highest positive rate and relatively high number of clones. We observed that expression of CAS9-MRE11 significantly improved positive rates when simultaneously seamless deletion of double genes (from 76.7 to 86.7%) and three genes (from 10.8 to 16.7%) when overexpressing RAD52. Furthermore, MRE11 overexpression significantly improved the genomic integration of multi-fragments with higher positive rate and clone number.

CONCLUSIONS

Fusion expression of the endogenous exonuclease Mre11 with Cas9 enhances homologous recombination efficiency in P. pastoris. The strategy described here should facilitate the metabolic engineering of P. pastoris toward high-level production of value-added compounds.

Steric accessibility of the N-terminus improves the titer and quality of recombinant proteins secreted from Komagataella phaffii

BACKGROUND

Komagataella phaffii is a commonly used alternative host for manufacturing therapeutic proteins, in part because of its ability to secrete recombinant proteins into the extracellular space. Incorrect processing of secreted proteins by cells can, however, cause non-functional product-related variants, which are expensive to remove in purification and lower overall process yields. The secretion signal peptide, attached to the N-terminus of the recombinant protein, is a major determinant of the quality of the protein sequence and yield. In K. phaffii, the signal peptide from the Saccharomyces cerevisiae alpha mating factor often yields the highest secreted titer of recombinant proteins, but the quality of secreted protein can vary highly.

RESULTS

We determined that an aggregated product-related variant of the SARS-CoV-2 receptor binding domain is caused by N-terminal extension from incomplete cleavage of the signal peptide. We eliminated this variant and improved secreted protein titer up to 76% by extension of the N-terminus with a short, functional peptide moiety or with the EAEA residues from the native signal peptide. We then applied this strategy to three other recombinant subunit vaccine antigens and observed consistent elimination of the same aggregated product-related variant. Finally, we demonstrated that this benefit in quality and secreted titer can be achieved with addition of a single amino acid to the N-terminus of the recombinant protein.

CONCLUSIONS

Our observations suggest that steric hindrance of proteases in the Golgi that cleave the signal peptide can cause unwanted N-terminal extension and related product variants. We demonstrated that this phenomenon occurs for multiple recombinant proteins, and can be addressed by minimal modification of the N-terminus to improve steric accessibility. This strategy may enable consistent secretion of a broad range of recombinant proteins with the highly productive alpha mating factor secretion signal peptide.

Development of a gRNA Expression and Processing Platform for Efficient CRISPR-Cas9-Based Gene Editing and Gene Silencing in Candida tropicalis

Candida tropicalis, a nonmodel diploid microbe, has been applied in industry as a chassis cell. Metabolic engineering of C. tropicalis is challenging due to a lack of gene editing and regulation tools. Here, we report a tRNA:guide RNA (gRNA) platform for boosting gene editing and silencing efficiency in C. tropicalis. As the endogenous tRNA-processing system enables autocleavage for producing a large number of mature gRNAs, a tRNAGly sequence from the genome of C. tropicalis ATCC 20336 was selected for constructing the tRNA:gRNA platform. In the CRISPR-Cas9 system, the tRNA:gRNA platform proved to be efficient in single-gene and multi-gene editing. Furthermore, based on the tRNA:gRNA platform, a CRISPR interference (CRISPRi) system was developed to construct an efficient dCas9-mediated gene expression regulation system for C. tropicalis. The CRISPRi system was employed to regulate the expression of the exogenous gene GFP3 (green fluorescent protein) and the endogenous gene ADE2 (phosphoribosylaminoimidazole carboxylase). Different regions of GFP3 and ADE2 were targeted with the gRNAs processed by the tRNAGly, and the transcription levels of GFP3 and ADE2 were successfully downregulated to 23.9% ± 4.1% and 38.0% ± 7.4%, respectively. The effects of the target regions on gene regulation were also investigated. Additionally, the regulation system was applied to silence ERG9 (squalene synthase) to enhance β-carotene biosynthesis in a metabolically modified C. tropicalis strain. The results suggest that the endogenous tRNAGly and the CRISPRi system have great potential for metabolic engineering of C. tropicalis. IMPORTANCE In the nonmodel yeast Candida tropicalis, a lack of available RNA polymerase type III (Pol III) promoters hindered the development of guide RNA (gRNA) expression platforms for the establishment of CRISPR-Cas-mediated genome editing and silencing strategies. Here, a tRNA:gRNA platform was constructed. We show that this platform allows efficient and precise expression and processing of different gRNAs from a single polycistronic gene capable of mediating multi-gene editing in combination with CRISPR-Cas9. Furthermore, in combination with dCas9, the tRNA:gRNA platform was efficiently used for silencing of exogenous and endogenous genes, representing the first CRISPR interference tool (CRISPRi) in C. tropicalis. Importantly, the established CRISPRi-tRNA:gRNA tool was also used for metabolic engineering by regulating β-carotene biosynthesis in C. tropicalis. The results suggest that the tRNA:gRNA platform and the CRISPRi system will further advance the application of the CRISPR-Cas-based editing and CRISPRi systems for metabolic engineering in C. tropicalis.

Expanding the promoter toolbox for metabolic engineering of methylotrophic yeasts

Methylotrophic yeasts have been widely recognized as a promising host for production of recombinant proteins and value-added chemicals. Promoters for controlled gene expression are critical for construction of efficient methylotrophic yeasts cell factories. Here, we summarized recent advances in characterizing and engineering promoters in methylotrophic yeasts, such as Komagataella phaffii and Ogataea polymorpha. Constitutive and inducible promoters controlled by methanol or other inducers/repressors were introduced to demonstrate their applications in production of proteins and chemicals. Furthermore, efforts of promoter engineering, including site-directed mutagenesis, hybrid promoter, and transcription factor regulation to expand the promoter toolbox were also summarized. This mini-review also provides useful information on promoters for the application of metabolic engineering in methylotrophic yeasts. KEY POINTS: • The characteristics of six methylotrophic yeasts and their promoters are described. • The applications of Komagataella phaffii and Ogataea polymorpha in metabolic engineeringare expounded. • Three promoter engineering strategies are introduced in order to expand the promoter toolbox.

Enhancing Homologous Recombination Efficiency in Pichia pastoris for Multiplex Genome Integration Using Short Homology Arms

There is a growing interest in establishing the methylotrophic yeast Pichia pastoris as microbial cell factories for producing fuels, chemicals, and natural products, particularly with methanol as the feedstock. Although CRISPR (Clustered Regularly Interspaced Short Palindromic Repeats) based genome editing technology has been established for the integration of multigene biosynthetic pathways, long (500-1000 bp) homology arms are generally required, probably due to low homologous recombination (HR) efficiency in P. pastoris. To achieve efficient genome integration of heterologous genes with short homology arms, we aimed to enhance HR efficiency by introducing the recombination machinery from Saccharomyces cerevisiae. First, we overexpressed HR related genes, including RAD52, RAD59, MRE11, and SAE2, and evaluated their effects on genome integration efficiency. Then, we constructed HR efficiency enhanced P. pastoris, which enabled single-, two-, and three-loci integration of heterologous gene expression cassettes with ∼40 bp homology arms with efficiencies as high as 100%, ∼98%, and ∼81%, respectively. Finally, we demonstrated the construction of β-carotene producing strain and the optimization of betaxanthin producing strain in a single step. The HR efficiency enhanced P. pastoris strains can be used for the construction of robust cell factories, and our machinery engineering strategy can be employed for the modification of other nonconventional yeasts.

Genome-wide functional screens enable the prediction of high activity CRISPR-Cas9 and -Cas12a guides in Yarrowia lipolytica

Genome-wide functional genetic screens have been successful in discovering genotype-phenotype relationships and in engineering new phenotypes. While broadly applied in mammalian cell lines and in E. coli, use in non-conventional microorganisms has been limited, in part, due to the inability to accurately design high activity CRISPR guides in such species. Here, we develop an experimental-computational approach to sgRNA design that is specific to an organism of choice, in this case the oleaginous yeast Yarrowia lipolytica. A negative selection screen in the absence of non-homologous end-joining, the dominant DNA repair mechanism, was used to generate single guide RNA (sgRNA) activity profiles for both SpCas9 and LbCas12a. This genome-wide data served as input to a deep learning algorithm, DeepGuide, that is able to accurately predict guide activity. DeepGuide uses unsupervised learning to obtain a compressed representation of the genome, followed by supervised learning to map sgRNA sequence, genomic context, and epigenetic features with guide activity. Experimental validation, both genome-wide and with a subset of selected genes, confirms DeepGuide's ability to accurately predict high activity sgRNAs. DeepGuide provides an organism specific predictor of CRISPR guide activity that with retraining could be applied to other fungal species, prokaryotes, and other non-conventional organisms.

Scalable, methanol-free manufacturing of the SARS-CoV-2 receptor-binding domain in engineered Komagataella phaffii

Prevention of COVID-19 on a global scale will require the continued development of high-volume, low-cost platforms for the manufacturing of vaccines to supply ongoing demand. Vaccine candidates based on recombinant protein subunits remain important because they can be manufactured at low costs in existing large-scale production facilities that use microbial hosts like Komagataella phaffii (Pichia pastoris). Here, we report an improved and scalable manufacturing approach for the SARS-CoV-2 spike protein receptor-binding domain (RBD); this protein is a key antigen for several reported vaccine candidates. We genetically engineered a manufacturing strain of K. phaffii to obviate the requirement for methanol induction of the recombinant gene. Methanol-free production improved the secreted titer of the RBD protein by >5X by alleviating protein folding stress. Removal of methanol from the production process enabled to scale up to a 1200 L pre-existing production facility. This engineered strain is now used to produce an RBD-based vaccine antigen that is currently in clinical trials and could be used to produce other variants of RBD as needed for future vaccines.

Simplified Gene Knockout by CRISPR-Cas9-Induced Homologous Recombination

Genetic engineering of industrial cell lines often requires knockout of multiple endogenous genes. Tools like CRISPR-Cas9 have enabled serial or parallelized gene disruption in a wide range of industrial organisms, but common practices for the screening and validation of genome edits are lacking. For gene disruption, DNA repair by homologous recombination offers several advantages over nonhomologous end joining, including more efficient screening for knockout clones and improved genomic stability. Here we designed and characterized a knockout fragment intended to repair Cas9-induced gene disruptions by homologous recombination. We identified knockout clones of Komagataella phaffii with high fidelity by PCR, removing the need for Sanger sequencing. Short overlap sequences for homologous recombination (30 bp) enabled the generation of gene-specific knockout fragments by PCR, removing the need for subcloning. Finally, we demonstrated that the genotype conferred by the knockout fragment is stable under common cultivation conditions.

A Novel and Efficient Genome Editing Tool Assisted by CRISPR-Cas12a/Cpf1 for Pichia pastoris

Pichia pastoris has been widely exploited for the 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 chemicals and pharmaceuticals. Effective synthetic biology tools for genetic engineering are essential for industrial and biotechnological research in this yeast. Here, we describe a novel and efficient genome editing method mediated by the CRISPR-Cpf1 system, which could facilitate the deletion of large DNA fragments and integration of multiplexed gene fragments. The CRISPR-Cpf1 system exhibited a precise and high editing efficiency for single-gene disruption (99 ± 0.8%), duplex genome editing (65 ± 2.5% to 80 ± 3%), and triplex genome editing (30 ± 2.5%). In addition, the deletion of large DNA fragments of 20kb and one-step integration of multiple genes were first achieved using the developed CRISPR-Cpf1 system. Taken together, this study provides an efficient and simple gene editing tool for P. pastoris. The novel multiloci gene integration method mediated by CRISPR-Cpf1 may accelerate the ability to engineer this methylotrophic yeast for metabolic engineering and genome evolution in both biotechnological and biomedical applications.

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.

Advances and Opportunities of CRISPR/Cas Technology in Bioengineering Non-conventional Yeasts

Non-conventional yeasts have attracted a growing interest on account of their excellent characteristics. In recent years, the emerging of CRISPR/Cas technology has improved the efficiency and accuracy of genome editing. Utilizing the advantages of CRISPR/Cas in bioengineering of non-conventional yeasts, quite a few advancements have been made. Due to the diversity in their genetic background, the ways for building a functional CRISPR/Cas system of various species non-conventional yeasts were also species-specific. Herein, we have summarized the different strategies for optimizing CRISPR/Cas systems in different non-conventional yeasts and their biotechnological applications in the construction of cell factories. In addition, we have proposed some potential directions for broadening and improving the application of CRISPR/Cas technology in non-conventional yeasts.

Engineered SARS-CoV-2 receptor binding domain improves manufacturability in yeast and immunogenicity in mice

Global containment of COVID-19 still requires accessible and affordable vaccines for low- and middle-income countries (LMICs). Recently approved vaccines provide needed interventions, albeit at prices that may limit their global access. Subunit vaccines based on recombinant proteins are suited for large-volume microbial manufacturing to yield billions of doses annually, minimizing their manufacturing cost. These types of vaccines are well-established, proven interventions with multiple safe and efficacious commercial examples. Many vaccine candidates of this type for SARS-CoV-2 rely on sequences containing the receptor-binding domain (RBD), which mediates viral entry to cells via ACE2. Here we report an engineered sequence variant of RBD that exhibits high-yield manufacturability, high-affinity binding to ACE2, and enhanced immunogenicity after a single dose in mice compared to the Wuhan-Hu-1 variant used in current vaccines. Antibodies raised against the engineered protein exhibited heterotypic binding to the RBD from two recently reported SARS-CoV-2 variants of concern (501Y.V1/V2). Presentation of the engineered RBD on a designed virus-like particle (VLP) also reduced weight loss in hamsters upon viral challenge.

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.

Development of synthetic biology tools to engineer Pichia pastoris as a chassis for the production of natural products

The methylotrophic yeast Pichia pastoris (a.k.a. Komagataella phaffii) is one of the most commonly used hosts for industrial production of recombinant proteins. As a non-conventional yeast, P. pastoris has unique biological characteristics and its expression system has been well developed. With the advances in synthetic biology, more efforts have been devoted to developing P. pastoris into a chassis for the production of various high-value compounds, such as natural products. This review begins with the introduction of synthetic biology tools for the engineering of P. pastoris, including vectors, promoters, and terminators for heterologous gene expression as well as Clustered Regularly Interspaced Short Palindromic Repeats/CRISPR-associated System (CRISPR/Cas) for genome editing. This review is then followed by examples of the production of value-added natural products in metabolically engineered P. pastoris strains. Finally, challenges and outlooks in developing P. pastoris as a synthetic biology chassis are prospected.

Scalable, methanol-free manufacturing of the SARS-CoV-2 receptor binding domain in engineered Komagataella phaffii

Prevention of COVID-19 on a global scale will require the continued development of high-volume, low-cost platforms for the manufacturing of vaccines to supply on-going demand. Vaccine candidates based on recombinant protein subunits remain important because they can be manufactured at low costs in existing large-scale production facilities that use microbial hosts like Komagataella phaffii ( Pichia pastoris ). Here, we report an improved and scalable manufacturing approach for the SARS-CoV-2 spike protein receptor binding domain (RBD); this protein is a key antigen for several reported vaccine candidates. We genetically engineered a manufacturing strain of K. phaffii to obviate the requirement for methanol-induction of the recombinant gene. Methanol-free production improved the secreted titer of the RBD protein by >5x by alleviating protein folding stress. Removal of methanol from the production process enabled scale up to a 1,200 L pre-existing production facility. This engineered strain is now used to produce an RBD-based vaccine antigen that is currently in clinical trials and could be used to produce other variants of RBD as needed for future vaccines.

Engineered SARS-CoV-2 receptor binding domain improves immunogenicity in mice and elicits protective immunity in hamsters

Global containment of COVID-19 still requires accessible and affordable vaccines for low- and middle-income countries (LMICs).1 Recently approved vaccines provide needed interventions, albeit at prices that may limit their global access.2 Subunit vaccines based on recombinant proteins are suited for large-volume microbial manufacturing to yield billions of doses annually, minimizing their manufacturing costs.3 These types of vaccines are well-established, proven interventions with multiple safe and efficacious commercial examples.4-6 Many vaccine candidates of this type for SARS-CoV-2 rely on sequences containing the receptor-binding domain (RBD), which mediates viral entry to cells via ACE2.7,8 Here we report an engineered sequence variant of RBD that exhibits high-yield manufacturability, high-affinity binding to ACE2, and enhanced immunogenicity after a single dose in mice compared to the Wuhan-Hu-1 variant used in current vaccines. Antibodies raised against the engineered protein exhibited heterotypic binding to the RBD from two recently reported SARS-CoV-2 variants of concern (501Y.V1/V2). Presentation of the engineered RBD on a designed virus-like particle (VLP) also reduced weight loss in hamsters upon viral challenge.

Recent advances in systems and synthetic biology approaches for developing novel cell-factories in non-conventional yeasts

Microbial bioproduction of chemicals, proteins, and primary metabolites from cheap carbon sources is currently an advancing area in industrial research. The model yeast, Saccharomyces cerevisiae, is a well-established biorefinery host that has been used extensively for commercial manufacturing of bioethanol from myriad carbon sources. However, its Crabtree-positive nature often limits the use of this organism for the biosynthesis of commercial molecules that do not belong in the fermentative pathway. To avoid extensive strain engineering of S. cerevisiae for the production of metabolites other than ethanol, non-conventional yeasts can be selected as hosts based on their natural capacity to produce desired commodity chemicals. Non-conventional yeasts like Kluyveromyces marxianus, K. lactis, Yarrowia lipolytica, Pichia pastoris, Scheffersomyces stipitis, Hansenula polymorpha, and Rhodotorula toruloides have been considered as potential industrial eukaryotic hosts owing to their desirable phenotypes such as thermotolerance, assimilation of a wide range of carbon sources, as well as ability to secrete high titers of protein and lipid. However, the advanced metabolic engineering efforts in these organisms are still lacking due to the limited availability of systems and synthetic biology methods like in silico models, well-characterised genetic parts, and optimized genome engineering tools. This review provides an insight into the recent advances and challenges of systems and synthetic biology as well as metabolic engineering endeavours towards the commercial usage of non-conventional yeasts. Particularly, the approaches in emerging non-conventional yeasts for the production of enzymes, therapeutic proteins, lipids, and metabolites for commercial applications are extensively discussed here. Various attempts to address current limitations in designing novel cell factories have been highlighted that include the advances in the fields of genome-scale metabolic model reconstruction, flux balance analysis, 'omics'-data integration into models, genome-editing toolkit development, and rewiring of cellular metabolisms for desired chemical production. Additionally, the understanding of metabolic networks using ¹³C-labelling experiments as well as the utilization of metabolomics in deciphering intracellular fluxes and reactions have also been discussed here. Application of cutting-edge nuclease-based genome editing platforms like CRISPR/Cas9, and its optimization towards efficient strain engineering in non-conventional yeasts have also been described. Additionally, the impact of the advances in promising non-conventional yeasts for efficient commercial molecule synthesis has been meticulously reviewed. In the future, a cohesive approach involving systems and synthetic biology will help in widening the horizon of the use of unexplored non-conventional yeast species towards industrial biotechnology.

CRISPR-mediated multigene integration enables Shikimate pathway refactoring for enhanced 2-phenylethanol biosynthesis in Kluyveromyces marxianus

BACKGROUND

2-phenylethanol (2-PE) is a rose-scented flavor and fragrance compound that is used in food, beverages, and personal care products. Compatibility with gasoline also makes it a potential biofuel or fuel additive. A biochemical process converting glucose or other fermentable sugars to 2-PE can potentially provide a more sustainable and economical production route than current methods that use chemical synthesis and/or isolation from plant material.

RESULTS

We work toward this goal by engineering the Shikimate and Ehrlich pathways in the stress-tolerant yeast Kluyveromyces marxianus. First, we develop a multigene integration tool that uses CRISPR-Cas9 induced breaks on the genome as a selection for the one-step integration of an insert that encodes one, two, or three gene expression cassettes. Integration of a 5-kbp insert containing three overexpression cassettes successfully occurs with an efficiency of 51 ± 9% at the ABZ1 locus and was used to create a library of K. marxianus CBS 6556 strains with refactored Shikimate pathway genes. The 3³-factorial library includes all combinations of KmARO4, KmARO7, and KmPHA2, each driven by three different promoters that span a wide expression range. Analysis of the refactored pathway library reveals that high expression of the tyrosine-deregulated KmARO4^K221L and native KmPHA2, with the medium expression of feedback insensitive KmARO7^G141S, results in the highest increase in 2-PE biosynthesis, producing 684 ± 73 mg/L. Ehrlich pathway engineering by overexpression of KmARO10 and disruption of KmEAT1 further increases 2-PE production to 766 ± 6 mg/L. The best strain achieves 1943 ± 63 mg/L 2-PE after 120 h fed-batch operation in shake flask cultures.

CONCLUSIONS

The CRISPR-mediated multigene integration system expands the genome-editing toolset for K. marxianus, a promising multi-stress tolerant host for the biosynthesis of 2-PE and other aromatic compounds derived from the Shikimate pathway.

Fine-Tuning of Transcription in Pichia pastoris Using dCas9 and RNA Scaffolds

For metabolic engineering approaches, fast and reliable tools are required to precisely manipulate the expression of target genes. dCas9 can be fused via RNA scaffolds to trans-activator domains and thus regulate the gene expression when targeted to the promoter region of a gene. In this work we show that this strategy can be successfully implemented for the methylotrophic yeast Pichia pastoris. It is shown that the thiamine repressible promoter of THI11 can be activated under repression conditions using a scgRNA/dCas9 construct. Furthermore, the RIB1 gene required for riboflavin production was activated, leading to increased riboflavin production exceeding the riboflavin titers of a conventional RIB1 overexpression with a pGAP promoter.

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.

Identifying Improved Sites for Heterologous Gene Integration Using ATAC-seq

Constructing efficient cellular factories often requires integration of heterologous pathways for synthesis of novel compounds and improved cellular productivity. Few genomic sites are routinely used, however, for efficient integration and expression of heterologous genes, especially in nonmodel hosts. Here, a data-guided framework for informing suitable integration sites for heterologous genes based on ATAC-seq was developed in the nonmodel yeast Komagataella phaffii. Single-copy GFP constructs were integrated using CRISPR/Cas9 into 38 intergenic regions (IGRs) to evaluate the effects of IGR size, intensity of ATAC-seq peaks, and orientation and expression of adjacent genes. Only the intensity of accessibility peaks was observed to have a significant effect, with higher expression observed from IGRs with low- to moderate-intensity peaks than from high-intensity peaks. This effect diminished for tandem, multicopy integrations, suggesting that the additional copies of exogenous sequence buffered the transcriptional unit of the transgene against effects from endogenous sequence context. The approach developed from these results should provide a basis for nominating suitable IGRs in other eukaryotic hosts from an annotated genome and ATAC-seq data.

High efficiency CRISPR/Cas9 genome editing system with an eliminable episomal sgRNA plasmid in Pichia pastoris

Pichia pastoris is a methylotrophic yeast in which host heterologous expression of proteins has been developed owing to the strong inducible alcohol oxidase promoter (PAOX1). However, it is difficult to manipulate the genome in P. pastoris. Based on previous attempts to apply the CRISPR/Cas9 system in P. pastoris, a CRISPR/Cas9 system with episomal sgRNA plasmid was developed and 100 % genome editing efficiency, high multicopy gene editing and stable multigene editing were obtained without a sharp decline caused by multi-sgRNA. And 28/34 (∼82 %) sgRNAs tested were effective. The CGG may have a slightly higher and more stable cleavage efficiency than the other three NGG motifs, and a low GC content may be preferable for higher cleavage efficiency. This provides researchers with a stable genome editing tool that shows a high editing efficiency, shortening the experimentation period. Furthermore, we introduced dCas9 into P. pastoris and achieved target gene interference, expanding the CRISPR/Cas9 toolbox in P. pastoris.