PCR & Go: A Pre-installed Expression Chassis for Facile Integration of Multi-Gene Biosynthetic Pathways

Multiplex CRISPR-Cas Genome Editing: Next-Generation Microbial Strain Engineering

Genome editing is a crucial technology for obtaining desired phenotypes in a variety of species, ranging from microbes to plants, animals, and humans. With the advent of CRISPR-Cas technology, it has become possible to edit the intended sequence by modifying the target recognition sequence in guide RNA (gRNA). By expressing multiple gRNAs simultaneously, it is possible to edit multiple targets at the same time, allowing for the simultaneous introduction of various functions into the cell. This can significantly reduce the time and cost of obtaining engineered microbial strains for specific traits. In this review, we investigate the resolution of multiplex genome editing and its application in engineering microorganisms, including bacteria and yeast. Furthermore, we examine how recent advancements in artificial intelligence technology could assist in microbial genome editing and engineering. Based on these insights, we present our perspectives on the future evolution and potential impact of multiplex genome editing technologies in the agriculture and food industry.

Engineering yeast for the production of plant terpenoids using synthetic biology approaches

Covering: 2011-2022The low amounts of terpenoids produced in plants and the difficulty in synthesizing these complex structures have stimulated the production of terpenoid compounds in microbial hosts by metabolic engineering and synthetic biology approaches. Advances in engineering yeast for terpenoid production will be covered in this review focusing on four directions: (1) manipulation of host metabolism, (2) rewiring and reconstructing metabolic pathways, (3) engineering the catalytic activity, substrate selectivity and product specificity of biosynthetic enzymes, and (4) localizing terpenoid production via enzymatic fusions and scaffolds, or subcellular compartmentalization.

CRAPS: Chromosomal-Repair-Assisted Pathway Shuffling in Yeast

A fundamental challenge of metabolic engineering involves assembling and screening vast combinations of orthologous enzymes across a multistep biochemical pathway. Current pathway assembly workflows involve combining genetic parts ex vivo and assembling one pathway configuration per tube or well. Here, we present CRAPS, Chromosomal-Repair-Assisted Pathway Shuffling, an in vivo pathway engineering technique that enables the self-assembly of one pathway configuration per cell. CRAPS leverages the yeast chromosomal repair pathway and utilizes a pool of inactive, chromosomally integrated orthologous gene variants corresponding to a target multistep pathway. Supplying gRNAs to the CRAPS host activates the expression of one gene variant per pathway step, resulting in a unique pathway configuration in each cell. We deployed CRAPS to build more than 1000 theoretical combinations of a four-step carotenoid biosynthesis network. Sampling the CRAPS pathway space yielded strains with distinct color phenotypes and carotenoid product profiles. We anticipate that CRAPS will expedite strain engineering campaigns by enabling the generation and sampling of vast biochemical spaces.

Practical Approaches for the Yeast Saccharomyces cerevisiae Genome Modification

The yeast S. cerevisiae is a unique genetic object for which a wide range of relatively simple, inexpensive, and non-time-consuming methods have been developed that allow the performing of a wide variety of genome modifications. Among the latter, one can mention point mutations, disruptions and deletions of particular genes and regions of chromosomes, insertion of cassettes for the expression of heterologous genes, targeted chromosomal rearrangements such as translocations and inversions, directed changes in the karyotype (loss or duplication of particular chromosomes, changes in the level of ploidy), mating-type changes, etc. Classical yeast genome manipulations have been advanced with CRISPR/Cas9 technology in recent years that allow for the generation of multiple simultaneous changes in the yeast genome. In this review we discuss practical applications of both the classical yeast genome modification methods as well as CRISPR/Cas9 technology. In addition, we review methods for ploidy changes, including aneuploid generation, methods for mating type switching and directed DSB. Combined with a description of useful selective markers and transformation techniques, this work represents a nearly complete guide to yeast genome modification.

High fidelity one-pot DNA assembly using orthogonal serine integrases

BACKGROUND

Large serine integrases (LSIs, derived from temperate phages) have been adapted for use in a multipart DNA assembly process in vitro, called serine integrase recombinational assembly (SIRA). The versatility, efficiency, and fidelity of SIRA is limited by lack of a sufficient number of LSIs whose activities have been characterized in vitro.

METHODS AND MAJOR RESULTS

In this report, we compared the activities in vitro of 10 orthogonal LSIs to explore their suitability for multiplex SIRA reactions. We found that Bxb1, ϕR4, and TG1 integrases were the most active among the set we studied, but several others were also usable. As proof of principle, we demonstrated high-efficiency one-pot assembly of six DNA fragments (made by PCR) into a 7.5 kb plasmid that expresses the enzymes of the β-carotenoid pathway in Escherichia coli, using six different LSIs. We further showed that a combined approach using a few highly active LSIs, each acting on multiple pairs of att sites with distinct central dinucleotides, can be used to scale up "poly-part" gene assembly and editing.

CONCLUSIONS AND IMPLICATIONS

We conclude that use of multiple orthogonal integrases may be the most predictable, efficient, and programmable approach for SIRA and other in vitro applications.

Synthetic Biology Toolkit for Marker-Less Integration of Multigene Pathways into Pichia pastoris via CRISPR/Cas9

Pichia pastoris, an important methylotrophic yeast, is currently mainly used for the expression of recombinant proteins and has great potential applications in the production of value-added compounds (e.g., chemical and natural products). However, the construction of P. pastoris cell factories is largely hindered by the lack of genetic tools for the manipulation of multigene biosynthetic pathways. Therefore, the present study aimed to establish a CRISPR-based synthetic biology toolkit for the integration and assembly of multigene biosynthetic pathways into the chromosome of P. pastoris. First, 23 intergenic regions were selected and characterized as potential integration sites, with a focus on the integration efficiency and heterologous gene expression levels. In addition, a panel of constitutive and methanol-inducible promoters with different strengths (weak, medium, and strong promoters) were characterized to control the expression of biosynthetic pathway genes to the desirable levels. With a series of gRNA plasmids (for single-locus, two-loci, and three-loci integration) and donor plasmids (containing homology arms for integration and promoters and terminators for driving heterologous gene expression) as major components, a CRISPR-based synthetic biology toolkit was established, which enabled the integration of one locus, two loci, and three loci with efficiencies as high as ∼100, ∼93, and ∼75%, respectively, in P. pastoris GS115 strain. Finally, the application of the toolkit was demonstrated by the construction of a series of P. pastoris cell factories, which could produce 2,3-butanediol, β-carotene, zeaxanthin, and astaxanthin with methanol as the sole carbon and energy source. The P. pastoris synthetic biology toolkit is highly standardized and can be employed to construct P. pastoris cell factories with high efficiency.

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.

Random Base Editing for Genome Evolution in Saccharomyces cerevisiae

Because of the limited understanding of cellular metabolism and regulatory networks, the rational engineering of complex industrial traits remains a grand challenge for the construction of microbial cell factories. Thus the development of simple, efficient, and programmable genome evolution techniques is still in high demanded for industrial biotechnology. In the present study, we established a random base editing (rBE) system for genome evolution in Saccharomyces cerevisiae. By fusing an unspecific single-stranded DNA (ssDNA)-binding protein to a cytidine deaminase, rBE introduced C to T mutations in a genome-wide manner. Specifically, we chose DNA-replication-related proteins, including replication factor A (RFA1, RFA2, and RFA3), DNA primase (PRI1), DNA helicase A (HCS1), and topoisomerase I (TOP1), to mediate the deamination of genomic ssDNA. As a proof of concept, we roughly estimated the rBE-mediated yeast genome mutation rate using the CAN1 mutation/canavanine resistance reporter system. We then evaluated the performance of these rBEs in improving the resistance against isobutanol and acetate and increasing the production of β-carotene. Finally, we employed the optimal rBE for the continuous genome evolution of a yeast cell factory resistant to 9% isobutanol. Owing to the conservation of DNA replication mechanisms, rBE is generally applicable and theoretically can be adopted for the continuous genome evolution of all organisms.

Efficient production of vindoline from tabersonine by metabolically engineered Saccharomyces cerevisiae

Vindoline is a plant derived monoterpene indole alkaloid (MIA) with potential therapeutic applications and more importantly serves as the precursor to vinblastine and vincristine. To obtain a yeast strain for high yield production of vindoline from tabersonine, multiple metabolic engineering strategies were employed via the CRISPR/Cas9 mediated multiplex genome integration technology in the present study. Through increasing and tuning the copy numbers of the pathway genes, pairing cytochrome P450 enzymes (CYPs) with appropriate cytochrome P450 reductases (CPRs), engineering the microenvironment for functional expression of CYPs, enhancing cofactor supply, and optimizing fermentation conditions, the production of vindoline was increased to a final titer as high as ∼16.5 mg/L, which is more than 3,800,000-fold higher than the parent strain and the highest tabersonine to vindoline conversion yield ever reported. This work represents a key step of the engineering efforts to establish de novo biosynthetic pathways for vindoline, vinblastine, and vincristine.

Multiplex Genome Editing in Yeast by CRISPR/Cas9 - A Potent and Agile Tool to Reconstruct Complex Metabolic Pathways

Numerous important pharmaceuticals and nutraceuticals originate from plant specialized metabolites, most of which are synthesized via complex biosynthetic pathways. The elucidation of these pathways is critical for the applicable uses of these compounds. Although the rapid progress of the omics technology has revolutionized the identification of candidate genes involved in these pathways, the functional characterization of these genes remains a major bottleneck. Baker's yeast (Saccharomyces cerevisiae) has been used as a microbial platform for characterizing newly discovered metabolic genes in plant specialized metabolism. Using yeast for the investigation of numerous plant enzymes is a streamlined process because of yeast's efficient transformation, limited endogenous specialized metabolism, partially sharing its primary metabolism with plants, and its capability of post-translational modification. Despite these advantages, reconstructing complex plant biosynthetic pathways in yeast can be time intensive. Since its discovery, CRISPR/Cas9 has greatly stimulated metabolic engineering in yeast. Yeast is a popular system for genome editing due to its efficient homology-directed repair mechanism, which allows precise integration of heterologous genes into its genome. One practical use of CRISPR/Cas9 in yeast is multiplex genome editing aimed at reconstructing complex metabolic pathways. This system has the capability of integrating multiple genes of interest in a single transformation, simplifying the reconstruction of complex pathways. As plant specialized metabolites usually have complex multigene biosynthetic pathways, the multiplex CRISPR/Cas9 system in yeast is suited well for functional genomics research in plant specialized metabolism. Here, we review the most advanced methods to achieve efficient multiplex CRISPR/Cas9 editing in yeast. We will also discuss how this powerful tool has been applied to benefit the study of plant specialized metabolism.