Bug mapping and fitness testing of chemically synthesized chromosome X

Design and structure of overlapping regions in PCA via deep learning

Polymerase cycling assembly (PCA) stands out as the predominant method in the synthesis of kilobase-length DNA fragments. The design of overlapping regions is the core factor affecting the success rate of synthesis. However, there still exists DNA sequences that are challenging to design and construct in the genome synthesis. Here we proposed a deep learning model based on extensive synthesis data to discern latent sequence representations in overlapping regions with an AUPR of 0.805. Utilizing the model, we developed the SmartCut algorithm aimed at designing oligonucleotides and enhancing the success rate of PCA experiments. This algorithm was successfully applied to sequences with diverse synthesis constraints, 80.4 % of which were synthesized in a single round. We further discovered structure differences represented by major groove width, stagger, slide, and centroid distance between overlapping and non-overlapping regions, which elucidated the model's reasonableness through the lens of physical chemistry. This comprehensive approach facilitates streamlined and efficient investigations into the genome synthesis.

The design and engineering of synthetic genomes

Synthetic genomics seeks to design and construct entire genomes to mechanistically dissect fundamental questions of genome function and to engineer organisms for diverse applications, including bioproduction of high-value chemicals and biologics, advanced cell therapies, and stress-tolerant crops. Recent progress has been fuelled by advancements in DNA synthesis, assembly, delivery and editing. Computational innovations, such as the use of artificial intelligence to provide prediction of function, also provide increasing capabilities to guide synthetic genome design and construction. However, translating synthetic genome-scale projects from idea to implementation remains highly complex. Here, we aim to streamline this implementation process by comprehensively reviewing the strategies for design, construction, delivery, debugging and tailoring of synthetic genomes as well as their potential applications.

Efficient De Novo Assembly of 100 kb-Scale Human Functional Immunoglobulin Heavy Variable (IGHV) Gene Fragments In Vitro

Synthetic biology provides a powerful approach to functional studies of viral and microbial genomes. However, in vitro, efficient and scarless DNA manipulation on large and complex human genomes remains an inevitable challenge. Here, we de novo design and successfully assemble human functional immunoglobulin heavy variable (IGHV) gene fragments up to hundred-kilobase (kb)-sized, using an iterative in vitro assembly via Escherichia coli (E. coli) based on Gibson isothermal assembly. We describe an efficient method for "scarless" (without leaving any non-native sequences) engineering of the assembled ordered functional IGHV gene fragments, which contain complex and highly repetitive regions. Our method provides a suitable way to construct bacterial artificial chromosomes (BACs) (30-100 kb) with common materials, easy manipulations, and low cost. The construction of ordered functional IGHV gene BACs expands the synthetic biologist's chassis repertoire. It is essential for the adaptive immune response and constructing immunity humanized animal models.

Pragmatic soft-decision data readout of encoded large DNA

The encoded large DNA can be cloned and stored in vivo, capable of write-once and stable replication for multiple retrievals, offering potential in economic data archiving. Nanopore sequencing is advantageous in data access of large DNA due to its rapidity and long-read sequencing capability. However, the data readout is commonly limited by insertion and deletion (indel) errors and sequence assembly complexity. Here, a pragmatic soft-decision data readout is presented, achieving assembly-free sequence reconstruction, indel error correction, and ultra-low coverage data readout. Specifically, the watermark is cleverly embedded within large DNA fragments, allowing for the direct localization of raw reads via watermark alignment to avoid complex read assembly. A soft-decision forward-backward algorithm is proposed, which can identify indel errors and provide probability information to the error correction code, enabling error-free data recovery. Additionally, a minimum state transition is maintained, and a read segmentation is incorporated to achieve fast information reading. The readout assays for two circular plasmids (~51 kb) with different coding rates were demonstrated and achieved error-free recovery directly from noisy reads (error rate ~1%) at coverage of 1-4×. Simulations conducted on large-scale datasets across various error rates further confirm the scalability of the method and its robust performance under extreme conditions. This readout method enables nearly single-molecule recovery of large DNA, particularly suitable for rapid readout of DNA storage.

Creation of a eukaryotic multiplexed site-specific inversion system and its application for metabolic engineering

The site-specific recombination system is a versatile tool in genome engineering, enabling controlled DNA inversion or deletion at specific sites to generate genetic diversity. The multiplexed inversion system, which preferentially facilitates inversion at reverse-oriented sites rather than deletion at same-oriented sites, has not been found in eukaryotes. Here, we establish a multiplexed site-specific inversion system, Rci51-5/multi-sfxa101, in yeast. Firstly, we develop a high-throughput screening system based on the on/off transcriptional control of multiple markers by DNA inversion. After two rounds of progressively stringent directed evolution, a mutant Rci51-5 shows an ability of multisite inversion and a ~ 1000-fold increase in total inversion efficiency against the wild-type Rci derived from Salmonella typhimurium. Subsequently, we demonstrate that the Rci51-5/multi-sfxa101 system exhibits significantly lower deletion rate than the Cre/multi-loxP system. Using the synthetic metabolic pathway of β-carotene as an example, we illustrate that the system can effectively facilitate promoter substitution in the metabolic pathway, resulting in a more than 7-fold increase in the yield of β-carotene. In summary, we develop a multiplexed site-specific inversion system in eukaryotes, providing an approach to metabolic engineering and a tool for eukaryotic genome manipulation.

Artificial design of the genome: from sequences to the 3D structure of chromosomes

Genome design is the foundation of genome synthesis, which provides a new platform for deepening our understanding of biological systems by exploring the fundamental components and structure of the genome. Artificial genome designs can endow unnatural genomes with desired functions. We provide a comprehensive overview of genome design principles ranging from DNA sequences to the 3D structure of chromosomes. Furthermore, we highlight applications of genome design in gene expression, genome structure, genome function, and biocontainment, and discuss the potential of artificial intelligence (AI) in genome design.

Chromosome Engineering: Technologies, Applications, and Challenges

Chromosome engineering is a transformative field at the cutting edge of biological science, offering unprecedented precision in manipulating large-scale genomic DNA within cells. This discipline is central to deciphering how the multifaceted roles of chromosomes-guarding genetic information, encoding sequence positional information, and influencing organismal traits-shape the genetic blueprint of life. This review comprehensively examines the technological advancements in chromosome engineering, which center on engineering chromosomal rearrangements, generating artificial chromosomes, de novo synthesizing chromosomes, and transferring chromosomes. Additionally, we introduce the application progress of chromosome engineering in basic and applied research fields, showcasing its capacity to deepen our knowledge of genetics and catalyze breakthroughs in therapeutic strategies. Finally, we conclude with a discussion of the challenges the field faces and highlight the profound implications that chromosome engineering holds for the future of modern biology and medical applications.

Construction and iterative redesign of synXVI a 903 kb synthetic Saccharomyces cerevisiae chromosome

The Sc2.0 global consortium to design and construct a synthetic genome based on the Saccharomyces cerevisiae genome commenced in 2006, comprising 16 synthetic chromosomes and a new-to-nature tRNA neochromosome. In this paper we describe assembly and debugging of the 902,994-bp synthetic Saccharomyces cerevisiae chromosome synXVI of the Sc2.0 project. Application of the CRISPR D-BUGS protocol identified defective loci, which were modified to improve sporulation and recover wild-type like growth when grown on glycerol as a sole carbon source when grown at 37˚C. LoxPsym sites inserted downstream of dubious open reading frames impacted the 5' UTR of genes required for optimal growth and were identified as a systematic cause of defective growth. Based on lessons learned from analysis of Sc2.0 defects and synXVI, an in-silico redesign of the synXVI chromosome was performed, which can be used as a blueprint for future synthetic yeast genome designs. The in-silico redesign of synXVI includes reduced PCR tag frequency, modified chunk and megachunk termini, and adjustments to allocation of loxPsym sites and TAA stop codons to dubious ORFs. This redesign provides a roadmap into applications of Sc2.0 strategies in non-yeast organisms.

Evolutionary engineering of Saccharomyces cerevisiae: Crafting a synthetic methylotroph via self-reprogramming

Methanol, as a non-edible feedstock, offers a promising sustainable alternative to sugar-based substrates in biochemical production. Despite progress in engineering methanol assimilation in nonmethylotrophs, the full transformation into methanol-dependent synthetic methylotrophs remains a formidable challenge. Here, moving beyond the conventional rational design principle, we engineered a synthetic methylotrophic Saccharomyces cerevisiae through genome rearrangement and adaptive laboratory evolution. This evolutionarily advanced strain unexpectedly shed the heterologous methanol assimilation pathway and demonstrated the robust growth on sole methanol. We discovered that the evolved strain likely realized methanol assimilation through a previously unidentified Adh2-Sfa1-rGly (ASrG) pathway, facilitating the concurrent assimilation of formate and CO2. Furthermore, the incorporation of electron transfer material C3N4 quantum dots obviously enhanced methanol-dependent growth, emphasizing the role of energy availability in the ASrG pathway. This breakthrough introduces a previously unidentified C1 utilization pathway and highlights the exceptional adaptability and self-evolving capacity of the S. cerevisiae metabolic network.

Towards the first synthetic eukaryotic cell

With the rapid advance in synthetic biology and the expanding field of synthetic genomics, the realization of a redesigned yeast genome has become an achievable milestone. Multiple eukaryotic chromosomes, meticulously designed and synthesized, are now being systematically integrated to create an entirely synthetic eukaryotic cell. This comprehensive review examines the fundamental design principles and construction strategies, highlighting critical technological breakthroughs in pursuing the first synthetic eukaryotic cell. Additionally, it underscores the critical contributions of the Sc2.0 project, which has provided essential tools and engineered cellular platforms that have significantly accelerated research and industrial progress. The ethical and legal implications arising from synthetic eukaryotic life are also explored, offering insights into future research directions for synthetic eukaryotic genomes. The remarkable advances in deoxyribonucleic acid synthesis hold immense potential, promising to unlock new opportunities across medicine, industry, agriculture, and research.

The de novo design and synthesis of yeast chromosome XIII facilitates investigations on aging

In the era of synthetic biology, design, construction, and utilization of synthetic chromosomes with unique features provide a strategy to study complex cellular processes such as aging. Herein, we successfully construct the 884 Kb synXIII of Saccharomyces cerevisiae to investigate replicative aging using these synthetic strains. We verify that up-regulation of a rRNA-related transcriptional factor, RRN9, positively influence replicative lifespan. Using SCRaMbLE system that enables inducible whole-genome rearrangement on synXIII, we obtain 20 SCRaMbLEd synXIII strains with extended lifespan. Transcriptome analysis reveal the expression of genes involve in global protein synthesis is up-regulated in longer-lived strains. We establish causal links between genotypic change and the long-lived phenotype via reconstruction of some key structural variations observed in post-SCRaMbLE strains and further demonstrate combinatorial effects of multiple aging regulators on lifespan extension. Our findings underscore the potential of synthetic yeasts in unveiling the function of aging-related genes.

Reaching New Heights in Genetic Code Manipulation with High Throughput Screening

The chemical and physical properties of proteins are limited by the 20 canonical amino acids. Genetic code manipulation allows for the incorporation of noncanonical amino acids (ncAAs) that enhance or alter protein functionality. This review explores advances in the three main strategies for introducing ncAAs into biosynthesized proteins, focusing on the role of high throughput screening in these advancements. The first section discusses engineering aminoacyl-tRNA synthetases (aaRSs) and tRNAs, emphasizing how novel selection methods improve characteristics including ncAA incorporation efficiency and selectivity. The second section examines high-throughput techniques for improving protein translation machinery, enabling accommodation of alternative genetic codes. This includes opportunities to enhance ncAA incorporation through engineering cellular components unrelated to translation. The final section highlights various discovery platforms for high-throughput screening of ncAA-containing proteins, showcasing innovative binding ligands and enzymes that are challenging to create with only canonical amino acids. These advances have led to promising drug leads and biocatalysts. Overall, the ability to discover unexpected functionalities through high-throughput methods significantly influences ncAA incorporation and its applications. Future innovations in experimental techniques, along with advancements in computational protein design and machine learning, are poised to further elevate this field.

Advances on transfer and maintenance of large DNA in bacteria, fungi, and mammalian cells

Advances in synthetic biology allow the design and manipulation of DNA from the scale of genes to genomes, enabling the engineering of complex genetic information for application in biomanufacturing, biomedicine and other areas. The transfer and subsequent maintenance of large DNA are two core steps in large scale genome rewriting. Compared to small DNA, the high molecular weight and fragility of large DNA make its transfer and maintenance a challenging process. This review outlines the methods currently available for transferring and maintaining large DNA in bacteria, fungi, and mammalian cells. It highlights their mechanisms, capabilities and applications. The transfer methods are categorized into general methods (e.g., electroporation, conjugative transfer, induced cell fusion-mediated transfer, and chemical transformation) and specialized methods (e.g., natural transformation, mating-based transfer, virus-mediated transfection) based on their applicability to recipient cells. The maintenance methods are classified into genomic integration (e.g., CRISPR/Cas-assisted insertion) and episomal maintenance (e.g., artificial chromosomes). Additionally, this review identifies the major technological advantages and disadvantages of each method and discusses the development for large DNA transfer and maintenance technologies.

Designing a synthetic moss genome using GenoDesigner

The de novo synthesis of genomes has made unprecedented progress and achieved milestones, particularly in bacteria and yeast. However, the process of synthesizing a multicellular plant genome has not progressed at the same pace, due to the complexity of multicellular plant genomes, technical difficulties associated with large genome size and structure, and the intricacies of gene regulation and expression in plants. Here we outline the bottom-up design principles for the de novo synthesis of the Physcomitrium patens (that is, earthmoss) genome. To facilitate international collaboration and accessibility, we have developed and launched a public online design platform called GenoDesigner. This platform offers an intuitive graphical interface enabling users to efficiently manipulate extensive genome sequences, even up to the gigabase level. This tool is poised to greatly expedite the synthesis of the P. patens genome, offering an essential reference and roadmap for the synthesis of plant genomes.

Genomic deletions in Aureobasidium pullulans by an AMA1 plasmid for gRNA and CRISPR/Cas9 expression

BACKGROUND

Aureobasidium pullulans is a generalist polyextremotolerant black yeast fungus. It tolerates temperatures below 0 °C or salt concentrations up to 18%, among other stresses. A. pullulans genome sequencing revealed a high potential for producing bioactive metabolites. Only few molecular tools exist to edit the genome of A. pullulans, hence it is important to make full use of its potential. Two CRISPR/Cas9 methods have been proposed for the protoplast-based transformation of A. pullulans. These methods require the integration of a marker gene into the locus of the gene to be deleted, when the deletion of this gene does not yield a selectable phenotype. We present the adaptation of a plasmid-based CRISPR/Cas9 system developed in Aspergillus niger for A. pullulans to create deletion strains.

RESULTS

The A. niger CRISPR/Cas9 plasmid led to efficient genomic deletions in A. pullulans. In this study, strains with deletions ranging from 30 to 862 bp were obtained by using an AMA1 plasmid-based genome editing strategy.

CONCLUSION

The CRISPR/Cas9 transformation system presented in this study provides new opportunities for strain engineering of A. pullulans. This system allows expression of Cas9 and antibiotic resistance while being easy to adapt. This strategy could open the path to intensive genomic engineering in A. pullulans.

Research Progress on the Assembly of Large DNA Fragments

Synthetic biology, a newly and rapidly developing interdisciplinary field, has demonstrated increasing potential for extensive applications in the wide areas of biomedicine, biofuels, and novel materials. DNA assembly is a key enabling technology of synthetic biology and a central point for realizing fully synthetic artificial life. While the assembly of small DNA fragments has been successfully commercialized, the assembly of large DNA fragments remains a challenge due to their high molecular weight and susceptibility to breakage. This article provides an overview of the development and current state of DNA assembly technology, with a focus on recent advancements in the assembly of large DNA fragments in Escherichia coli, Bacillus subtilis, and Saccharomyces cerevisiae. In particular, the methods and challenges associated with the assembly of large DNA fragment in different hosts are highlighted. The advancements in DNA assembly have the potential to facilitate the construction of customized genomes, giving us the ability to modify cellular functions and even create artificial life. It is also contributing to our ability to understand, predict, and manipulate living organisms.

Recent advances in the synthesis of single-stranded DNA in vitro

Single-stranded DNA (ssDNA) is the foundation of modern biology, with wide applications in gene editing, sequencing, DNA information storage, and materials science. However, synthesizing ssDNA with high efficiency, high throughput, and low error rate in vitro remains a major challenge. Various methods have been developed for ssDNA synthesis, and some significant results have been achieved. In this review, six main methods were introduced, including solid-phase oligonucleotide synthesis, terminal deoxynucleotidyl transferase-based ssDNA synthesis, reverse transcription, primer exchange reaction, asymmetric polymerase chain reaction, and rolling circle amplification. The advantages and limitations of each method were compared, as well as illustrate their representative achievements and applications. Especially, rolling circle amplification has received significant attention, including ssDNA synthesis, assembly, and application based on recent work. Finally, the future challenges and opportunities of ssDNA synthesis were summarized and discussed. Envisioning the development of new methods and significant progress will be made in the near future with the efforts of scientists around the world.

Convenient synthesis and delivery of a megabase-scale designer accessory chromosome empower biosynthetic capacity

Synthetic biology confers new functions to hosts by introducing exogenous genetic elements, yet rebuilding complex traits that are based on large-scale genetic information remains challenging. Here, we developed a CRISPR/Cas9-mediated haploidization method that bypasses the natural process of meiosis. Based on the programmed haploidization in yeast, we further developed an easy-to-use method designated HAnDy (Haploidization-based DNA Assembly and Delivery in yeast) that enables efficient assembly and delivery of large DNA, with no need for any fussy in vitro manipulations. Using HAnDy, a de novo designed 1.024 Mb synthetic accessory chromosome (synAC) encoding 542 exogenous genes was parallelly assembled and then directly transferred to six phylogenetically diverse yeasts. The synAC significantly promotes hosts' adaptations and increases the scope of the metabolic network, which allows the emergence of valuable compounds. Our approach should facilitate the assembly and delivery of large-scale DNA for expanding and deciphering complex biological functions.

Automated high-throughput DNA synthesis and assembly

DNA synthesis and assembly primarily revolve around the innovation and refinement of tools that facilitate the creation of specific genes and the manipulation of entire genomes. This multifaceted process encompasses two fundamental steps: the synthesis of lengthy oligonucleotides and the seamless assembly of numerous DNA fragments. With the advent of automated pipetting workstations and integrated experimental equipment, a substantial portion of repetitive tasks in the field of synthetic biology can now be efficiently accomplished through integrated liquid handling workstations. This not only reduces the need for manual labor but also enhances overall efficiency. This review explores the ongoing advancements in the oligonucleotide synthesis platform, automated DNA assembly techniques, and biofoundries. The development of accurate and high-throughput DNA synthesis and assembly technologies presents both challenges and opportunities.

Building synthetic chromosomes from natural DNA

De novo chromosome synthesis is costly and time-consuming, limiting its use in research and biotechnology. Building synthetic chromosomes from natural components is an unexplored alternative with many potential applications. In this paper, we report CReATiNG (Cloning, Reprogramming, and Assembling Tiled Natural Genomic DNA), a method for constructing synthetic chromosomes from natural components in yeast. CReATiNG entails cloning segments of natural chromosomes and then programmably assembling them into synthetic chromosomes that can replace the native chromosomes in cells. We use CReATiNG to synthetically recombine chromosomes between strains and species, to modify chromosome structure, and to delete many linked, non-adjacent regions totaling 39% of a chromosome. The multiplex deletion experiment reveals that CReATiNG also enables recovery from flaws in synthetic chromosome design via recombination between a synthetic chromosome and its native counterpart. CReATiNG facilitates the application of chromosome synthesis to diverse biological problems.

Application and Technical Challenges in Design, Cloning, and Transfer of Large DNA

In the field of synthetic biology, rapid advancements in DNA assembly and editing have made it possible to manipulate large DNA, even entire genomes. These advancements have facilitated the introduction of long metabolic pathways, the creation of large-scale disease models, and the design and assembly of synthetic mega-chromosomes. Generally, the introduction of large DNA in host cells encompasses three critical steps: design-cloning-transfer. This review provides a comprehensive overview of the three key steps involved in large DNA transfer to advance the field of synthetic genomics and large DNA engineering.

Manipulating the 3D organization of the largest synthetic yeast chromosome

Whether synthetic genomes can power life has attracted broad interest in the synthetic biology field. Here, we report de novo synthesis of the largest eukaryotic chromosome thus far, synIV, a 1,454,621-bp yeast chromosome resulting from extensive genome streamlining and modification. We developed megachunk assembly combined with a hierarchical integration strategy, which significantly increased the accuracy and flexibility of synthetic chromosome construction. Besides the drastic sequence changes, we further manipulated the 3D structure of synIV to explore spatial gene regulation. Surprisingly, we found few gene expression changes, suggesting that positioning inside the yeast nucleoplasm plays a minor role in gene regulation. Lastly, we tethered synIV to the inner nuclear membrane via its hundreds of loxPsym sites and observed transcriptional repression of the entire chromosome, demonstrating chromosome-wide transcription manipulation without changing the DNA sequences. Our manipulation of the spatial structure of synIV sheds light on higher-order architectural design of the synthetic genomes.

Debugging and consolidating multiple synthetic chromosomes reveals combinatorial genetic interactions

The Sc2.0 project is building a eukaryotic synthetic genome from scratch. A major milestone has been achieved with all individual Sc2.0 chromosomes assembled. Here, we describe the consolidation of multiple synthetic chromosomes using advanced endoreduplication intercrossing with tRNA expression cassettes to generate a strain with 6.5 synthetic chromosomes. The 3D chromosome organization and transcript isoform profiles were evaluated using Hi-C and long-read direct RNA sequencing. We developed CRISPR Directed Biallelic URA3-assisted Genome Scan, or "CRISPR D-BUGS," to map phenotypic variants caused by specific designer modifications, known as "bugs." We first fine-mapped a bug in synthetic chromosome II (synII) and then discovered a combinatorial interaction associated with synIII and synX, revealing an unexpected genetic interaction that links transcriptional regulation, inositol metabolism, and tRNASerCGA abundance. Finally, to expedite consolidation, we employed chromosome substitution to incorporate the largest chromosome (synIV), thereby consolidating >50% of the Sc2.0 genome in one strain.

Consequences of a telomerase-related fitness defect and chromosome substitution technology in yeast synIX strains

We describe the complete synthesis, assembly, debugging, and characterization of a synthetic 404,963 bp chromosome, synIX (synthetic chromosome IX). Combined chromosome construction methods were used to synthesize and integrate its left arm (synIXL) into a strain containing previously described synIXR. We identified and resolved a bug affecting expression of EST3, a crucial gene for telomerase function, producing a synIX strain with near wild-type fitness. To facilitate future synthetic chromosome consolidation and increase flexibility of chromosome transfer between distinct strains, we combined chromoduction, a method to transfer a whole chromosome between two strains, with conditional centromere destabilization to substitute a chromosome of interest for its native counterpart. Both steps of this chromosome substitution method were efficient. We observed that wild-type II tended to co-transfer with synIX and was co-destabilized with wild-type IX, suggesting a potential gene dosage compensation relationship between these chromosomes.

Establishing chromosomal design-build-test-learn through a synthetic chromosome and its combinatorial reconfiguration

Chromosome-level design-build-test-learn cycles (chrDBTLs) allow systematic combinatorial reconfiguration of chromosomes with ease. Here, we established chrDBTL with a redesigned synthetic Saccharomyces cerevisiae chromosome XV, synXV. We designed and built synXV to harbor strategically inserted features, modified elements, and synonymously recoded genes throughout the chromosome. Based on the recoded chromosome, we developed a method to enable chrDBTL: CRISPR-Cas9-mediated mitotic recombination with endoreduplication (CRIMiRE). CRIMiRE allowed the creation of customized wild-type/synthetic combinations, accelerating genotype-phenotype mapping and synthetic chromosome redesign. We also leveraged synXV as a "build-to-learn" model organism for translation studies by ribosome profiling. We conducted a locus-to-locus comparison of ribosome occupancy between synXV and the wild-type chromosome, providing insight into the effects of codon changes and redesigned features on translation dynamics in vivo. Overall, we established synXV as a versatile reconfigurable system that advances chrDBTL for understanding biological mechanisms and engineering strains.

Parallel laboratory evolution and rational debugging reveal genomic plasticity to S. cerevisiae synthetic chromosome XIV defects

Synthetic chromosome engineering is a complex process due to the need to identify and repair growth defects and deal with combinatorial gene essentiality when rearranging chromosomes. To alleviate these issues, we have demonstrated novel approaches for repairing and rearranging synthetic Saccharomyces cerevisiae genomes. We have designed, constructed, and restored wild-type fitness to a synthetic 753,096-bp version of S. cerevisiae chromosome XIV as part of the Synthetic Yeast Genome project. In parallel to the use of rational engineering approaches to restore wild-type fitness, we used adaptive laboratory evolution to generate a general growth-defect-suppressor rearrangement in the form of increased TAR1 copy number. We also extended the utility of the synthetic chromosome recombination and modification by loxPsym-mediated evolution (SCRaMbLE) system by engineering synthetic-wild-type tetraploid hybrid strains that buffer against essential gene loss, highlighting the plasticity of the S. cerevisiae genome in the presence of rational and non-rational modifications.

Synthetic yeast chromosome XI design provides a testbed for the study of extrachromosomal circular DNA dynamics

We describe construction of the synthetic yeast chromosome XI (synXI) and reveal the effects of redesign at non-coding DNA elements. The 660-kb synthetic yeast genome project (Sc2.0) chromosome was assembled from synthesized DNA fragments before CRISPR-based methods were used in a process of bug discovery, redesign, and chromosome repair, including precise compaction of 200 kb of repeat sequence. Repaired defects were related to poor centromere function and mitochondrial health and were associated with modifications to non-coding regions. As part of the Sc2.0 design, loxPsym sequences for Cre-mediated recombination are inserted between most genes. Using the GAP1 locus from chromosome XI, we show that these sites can facilitate induced extrachromosomal circular DNA (eccDNA) formation, allowing direct study of the effects and propagation of these important molecules. Construction and characterization of synXI contributes to our understanding of non-coding DNA elements, provides a useful tool for eccDNA study, and will inform future synthetic genome design.

Synthetic chromosome fusion: Effects on mitotic and meiotic genome structure and function

We designed and synthesized synI, which is ∼21.6% shorter than native chrI, the smallest chromosome in Saccharomyces cerevisiae. SynI was designed for attachment to another synthetic chromosome due to concerns surrounding potential instability and karyotype imbalance and is now attached to synIII, yielding the first synthetic yeast fusion chromosome. Additional fusion chromosomes were constructed to study nuclear function. ChrIII-I and chrIX-III-I fusion chromosomes have twisted structures, which depend on silencing protein Sir3. As a smaller chromosome, chrI also faces special challenges in assuring meiotic crossovers required for efficient homolog disjunction. Centromere deletions into fusion chromosomes revealed opposing effects of core centromeres and pericentromeres in modulating deposition of the crossover-promoting protein Red1. These effects extend over 100 kb and promote disproportionate Red1 enrichment, and thus crossover potential, on small chromosomes like chrI. These findings reveal the power of synthetic genomics to uncover new biology and deconvolute complex biological systems.

Context-dependent neocentromere activity in synthetic yeast chromosome VIII

Pioneering advances in genome engineering, and specifically in genome writing, have revolutionized the field of synthetic biology, propelling us toward the creation of synthetic genomes. The Sc2.0 project aims to build the first fully synthetic eukaryotic organism by assembling the genome of Saccharomyces cerevisiae. With the completion of synthetic chromosome VIII (synVIII) described here, this goal is within reach. In addition to writing the yeast genome, we sought to manipulate an essential functional element: the point centromere. By relocating the native centromere sequence to various positions along chromosome VIII, we discovered that the minimal 118-bp CEN8 sequence is insufficient for conferring chromosomal stability at ectopic locations. Expanding the transplanted sequence to include a small segment (∼500 bp) of the CDEIII-proximal pericentromere improved chromosome stability, demonstrating that minimal centromeres display context-dependent functionality.

Dissecting aneuploidy phenotypes by constructing Sc2.0 chromosome VII and SCRaMbLEing synthetic disomic yeast

Aneuploidy compromises genomic stability, often leading to embryo inviability, and is frequently associated with tumorigenesis and aging. Different aneuploid chromosome stoichiometries lead to distinct transcriptomic and phenotypic changes, making it helpful to study aneuploidy in tightly controlled genetic backgrounds. By deploying the engineered SCRaMbLE (synthetic chromosome rearrangement and modification by loxP-mediated evolution) system to the newly synthesized megabase Sc2.0 chromosome VII (synVII), we constructed a synthetic disomic yeast and screened hundreds of SCRaMbLEd derivatives with diverse chromosomal rearrangements. Phenotypic characterization and multi-omics analysis revealed that fitness defects associated with aneuploidy could be restored by (1) removing most of the chromosome content or (2) modifying specific regions in the duplicated chromosome. These findings indicate that both chromosome copy number and specific chromosomal regions contribute to the aneuploidy-related phenotypes, and the synthetic chromosome resource opens new paradigms in studying aneuploidy.

YLC-assembly: large DNA assembly via yeast life cycle

As an enabling technique of synthetic biology, the scale of DNA assembly largely determines the scale of genetic manipulation. However, large DNA assembly technologies are generally cumbersome and inefficient. Here, we developed a YLC (yeast life cycle)-assembly method that enables in vivo iterative assembly of large DNA by nesting cell-cell transfer of assembled DNA in the cycle of yeast mating and sporulation. Using this method, we successfully assembled a hundred-kilobase (kb)-sized endogenous yeast DNA and a megabase (Mb)-sized exogenous DNA. For each round, over 104 positive colonies per 107 cells could be obtained, with an accuracy ranging from 67% to 100%. Compared with other Mb-sized DNA assembly methods, this method exhibits a higher success rate with an easy-to-operate workflow that avoid in vitro operations of large DNA. YLC-assembly lowers the technical difficulty of Mb-sized DNA assembly and could be a valuable tool for large-scale genome engineering and synthetic genomics.

Building synthetic chromosomes from natural DNA

De novo chromosome synthesis is costly and time-consuming, limiting its use in research and biotechnology. Building synthetic chromosomes from natural components is an unexplored alternative with many potential applications. In this paper, we report CReATiNG (Cloning, Reprogramming, and Assembling Tiled Natural Genomic DNA), a method for constructing synthetic chromosomes from natural components in yeast. CReATiNG entails cloning segments of natural chromosomes and then programmably assembling them into synthetic chromosomes that can replace the native chromosomes in cells. We used CReATiNG to synthetically recombine chromosomes between strains and species, to modify chromosome structure, and to delete many linked, non-adjacent regions totaling 39% of a chromosome. The multiplex deletion experiment revealed that CReATiNG also enables recovery from flaws in synthetic chromosome design via recombination between a synthetic chromosome and its native counterpart. CReATiNG facilitates the application of chromosome synthesis to diverse biological problems.

Dynamics of synthetic yeast chromosome evolution shaped by hierarchical chromatin organization

Synthetic genome evolution provides a dynamic approach for systematically and straightforwardly exploring evolutionary processes. Synthetic Chromosome Rearrangement and Modification by LoxP-mediated Evolution (SCRaMbLE) is an evolutionary system intrinsic to the synthetic yeast genome that can rapidly drive structural variations. Here, we detect over 260 000 rearrangement events after the SCRaMbLEing of a yeast strain harboring 5.5 synthetic yeast chromosomes (synII, synIII, synV, circular synVI, synIXR and synX). Remarkably, we find that the rearrangement events exhibit a specific landscape of frequency. We further reveal that the landscape is shaped by the combined effects of chromatin accessibility and spatial contact probability. The rearrangements tend to occur in 3D spatially proximal and chromatin-accessible regions. The enormous numbers of rearrangements mediated by SCRaMbLE provide a driving force to potentiate directed genome evolution, and the investigation of the rearrangement landscape offers mechanistic insights into the dynamics of genome evolution.

Trimming the genomic fat: minimising and re-functionalising genomes using synthetic biology

Naturally evolved organisms typically have large genomes that enable their survival and growth under various conditions. However, the complexity of genomes often precludes our complete understanding of them, and limits the success of biotechnological designs. In contrast, minimal genomes have reduced complexity and therefore improved engineerability, increased biosynthetic capacity through the removal of unnecessary genetic elements, and less recalcitrance to complete characterisation. Here, we review the past and current genome minimisation and re-functionalisation efforts, with an emphasis on the latest advances facilitated by synthetic genomics, and provide a critical appraisal of their potential for industrial applications.

Structural Variations and Adaptations of Synthetic Chromosome Ends Driven by SCRaMbLE in Haploid and Diploid Yeasts

Variations and adaptations of chromosome ends play an important role in eukaryotic karyotype evolution. Traditional experimental studies of the adaptations of chromosome ends mainly rely on the strategy of introducing defects; thus, the adaptation methods of survivors may vary depending on the initial defects. Here, using the SCRaMbLE strategy, we obtained a library of haploid and diploid synthetic strains with variations in chromosome ends. Analysis of the SCRaMbLEd survivors revealed four routes of adaptation: homologous recombination between nonhomologous chromosome arms (haploids) or homologous chromosome arms (diploids), site-specific recombination between intra- or interchromosomal ends, circularization of chromosomes, and loss of whole chromosomes (diploids). We also found that circularization of synthetic chromosomes can be generated by SCRaMbLE. Our study of various adaptation routes of chromosome ends provides insight into eukaryotic karyotype evolution from the viewpoint of synthetic genomics.

Machine learning-aided scoring of synthesis difficulties for designer chromosomes

Designer chromosomes are artificially synthesized chromosomes. Nowadays, these chromosomes have numerous applications ranging from medical research to the development of biofuels. However, some chromosome fragments can interfere with the chemical synthesis of designer chromosomes and eventually limit the widespread use of this technology. To address this issue, this study aimed to develop an interpretable machine learning framework to predict and quantify the synthesis difficulties of designer chromosomes in advance. Through the use of this framework, six key sequence features leading to synthesis difficulties were identified, and an eXtreme Gradient Boosting model was established to integrate these features. The predictive model achieved high-quality performance with an AUC of 0.895 in cross-validation and an AUC of 0.885 on an independent test set. Based on these results, the synthesis difficulty index (S-index) was proposed as a means of scoring and interpreting synthesis difficulties of chromosomes from prokaryotes to eukaryotes. The findings of this study emphasize the significant variability in synthesis difficulties between chromosomes and demonstrate the potential of the proposed model to predict and mitigate these difficulties through the optimization of the synthesis process and genome rewriting.

Rational search of genetic design space for a heterologous terpene metabolic pathway in Streptomyces

Modern tools in DNA synthesis and assembly give genetic engineers control over the nucleotide-level design of complex, multi-gene systems. Systematic approaches to explore genetic design space and optimize the performance of genetic constructs are lacking. Here we explore the application of a five-level Plackett-Burman fractional factorial design to improve the titer of a heterologous terpene biosynthetic pathway in Streptomyces. A library of 125 engineered gene clusters encoding the production of diterpenoid ent-atiserenoic acid (eAA) via the methylerythritol phosphate pathway was constructed and introduced into Streptomyces albidoflavus J1047 for heterologous expression. The eAA production titer varied within the library by over two orders of magnitude and host strains showed unexpected and reproducible colony morphology phenotypes. Analysis of Plackett-Burman design identified expression of dxs, the gene encoding the first and the flux-controlling enzyme, having the strongest impact on eAA titer, but with a counter-intuitive negative correlation between dxs expression and eAA production. Finally, simulation modeling was performed to determine how several plausible sources of experimental error/noise and non-linearity impact the utility of Plackett-Burman analyses.

Investigation of Genome Biology by Synthetic Genome Engineering

Synthetic genomes were designed based on an understanding of natural genomic information, offering an opportunity to engineer and investigate biological systems on a genome-wide scale. Currently, the designer version of the M. mycoides genome and the E. coli genome, as well as most of the S. cerevisiae genome, have been synthesized, and through the cycles of design-build-test and the following engineering of synthetic genomes, many fundamental questions of genome biology have been investigated. In this review, we summarize the use of synthetic genome engineering to explore the structure and function of genomes, and highlight the unique values of synthetic genomics.

Centromeres: From chromosome biology to biotechnology applications and synthetic genomes in plants

Centromeres are the genomic regions that organize and regulate chromosome behaviours during cell cycle, and their variations are associated with genome instability, karyotype evolution and speciation in eukaryotes. The highly repetitive and epigenetic nature of centromeres were documented during the past half century. With the aid of rapid expansion in genomic biotechnology tools, the complete sequence and structural organization of several plant and human centromeres were revealed recently. Here, we systematically summarize the current knowledge of centromere biology with regard to the DNA compositions and the histone H3 variant (CENH3)-dependent centromere establishment and identity. We discuss the roles of centromere to ensure cell division and to maintain the three-dimensional (3D) genomic architecture in different species. We further highlight the potential applications of manipulating centromeres to generate haploids or to induce polyploids offspring in plant for breeding programs, and of targeting centromeres with CRISPR/Cas for chromosome engineering and speciation. Finally, we also assess the challenges and strategies for de novo design and synthesis of centromeres in plant artificial chromosomes. The biotechnology applications of plant centromeres will be of great potential for the genetic improvement of crops and precise synthetic breeding in the future.

Direct Transfer and Consolidation of Synthetic Yeast Chromosomes by Abortive Mating and Chromosome Elimination

Large DNA transfer technology has been challenged with the rapid development of large DNA assembly technology. The research and application of synthetic yeast chromosomes have been mostly limited in the assembled host itself. The mutant of KAR1 prevents nuclear fusion during yeast mating, and occasionally single chromosome can be transferred from one parental nucleus to another. Using the kar1 mutant method, four synthetic yeast chromosomes of Sc2.0 (synIII, synV, synX, synXII) were transferred to wild-type yeasts separately. SynIII was also transferred into an industrial strain Y12, resulting in an improvement of thermotolerance. Moreover, by combining abortive mating and chromosome elimination by CRISPR-Cas9, which has been reported in our previous study, we developed a strategy for consolidation of multiple synthetic yeast chromosomes. Compared to the previous pyramidal strategy using endoreduplication backcross, our method is a linear process independent of meiosis, providing a convenient path for accelerating consolidation of Sc2.0 chromosomes. Overall, the method of transfer and consolidation of synthetic yeast chromosomes by abortive mating and chromosome elimination enables a novel route that large DNA was assembled in donor yeast and then in vivo directly transferred to receptor yeasts, enriching the manipulation tools for synthetic genomics.

DNA-assisted selective electrofusion (DASE) of Escherichia coli and giant lipid vesicles

Synthetic biology and cellular engineering require chemical and physical alterations, which are typically achieved by fusing target cells with each other or with payload-carrying vectors. On one hand, electrofusion can efficiently induce the merging of biological cells and/or synthetic analogues via the application of intense DC pulses, but it lacks selectivity and often leads to uncontrolled fusion. On the other hand, synthetic DNA-based constructs, inspired by natural fusogenic proteins, have been shown to induce a selective fusion between membranes, albeit with low efficiency. Here we introduce DNA-assisted selective electrofusion (DASE) which relies on membrane-anchored DNA constructs to bring together the objects one seeks to merge, and applying an electric impulse to trigger their fusion. The DASE process combines the efficiency of standard electrofusion and the selectivity of fusogenic nanostructures, as we demonstrate by inducing and characterizing the fusion of spheroplasts derived from Escherichia coli bacteria with cargo-carrying giant lipid vesicles.

Systematic dissection of key factors governing recombination outcomes by GCE-SCRaMbLE

With the completion of Sc2.0 chromosomes, synthetic chromosome rearrangement and modification by loxP-mediated evolution (SCRaMbLE) becomes more critical for in-depth investigation of fundamental biological questions and screening of industrially valuable characteristics. Further applications, however, are hindered due to the lack of facile and tight regulation of the SCRaMbLE process, and limited understanding of key factors that may affect the rearrangement outcomes. Here we propose an approach to precisely regulate SCRaMbLE recombination in a dose-dependent manner using genetic code expansion (GCE) technology with low basal activity. By systematically analyzing 1380 derived strains and six yeast pools subjected to GCE-SCRaMbLE, we find that Cre enzyme abundance, genome ploidy and chromosome conformation play key roles in recombination frequencies and determine the SCRaMbLE outcomes. With these insights, the GCE-SCRaMbLE system will serve as a powerful tool in the future exploitation and optimization of the Sc2.0-related technologies.

Enhancement and mapping of tolerance to salt stress and 5-fluorocytosine in synthetic yeast strains via SCRaMbLE

Varied environmental stress can affect cell growth and activity of the cellular catalyst. Traditional path of adaptive evolution generally takes a long time to achieve a tolerance phenotype, meanwhile, it is a challenge to dissect the underlying genetic mechanism. Here, using SCRaMbLE, a genome scale tool to generate random structural variations, a total of 222 evolved yeast strains with enhanced environmental tolerances were obtained in haploid or diploid yeasts containing six synthetic chromosomes. Whole genome sequencing of the evolved strains revealed that these strains generated different structural variants. Notably, by phenotypic-genotypic analysis of the SCRaMbLEd strains, we find that a deletion of gene YFR009W (GCN20) can improve salt tolerance of Saccharomyces cerevisiae, and a deletion of gene YER056C can improve 5-flucytosine tolerance of Saccharomyces cerevisiae. This study shows applications of SCRaMbLE to accelerate phenotypic evolution for varied environmental stress and to explore relationships between structural variations and evolved phenotypes.

The Synthesis and Assembly of a Truncated Cyanophage Genome and Its Expression in a Heterogenous Host

Cyanophages play an important role in regulating the dynamics of cyanobacteria communities in the hydrosphere, representing a promising biological control strategy for cyanobacterial blooms. Nevertheless, most cyanophages are host-specific, making it difficult to control blooming cyanobacteria via single or multiple cyanophages. In order to address the issue, we explore the interaction between cyanophages and their heterologous hosts, with the aim of revealing the principles of designing and constructing an artificial cyanophage genome towards multiple cyanobacterial hosts. In the present study, we use synthetic biological approaches to assess the impact of introducing a fragment of cyanophage genome into a heterologous cyanobacterium under a variety of environmental conditions. Based on a natural cyanophage A-4L genome (41,750 bp), a truncated cyanophage genome Syn-A-4-8 is synthesized and assembled in Saccharomyces cerevisiae. We found that a 351-15,930 bp area of the A-4L genome has a fragment that is lethal to Escherichia coli during the process of attempting to assemble the full-length A-4L genome. Syn-A-4-8 was successfully introduced into E. coli and then transferred into the model cyanobacterium Synechococcus elongatus PCC 7942 (Syn7942) via conjugation. Although no significant phenotypes of Syn7942 carrying Syn-A-4-8 (LS-02) could be observed under normal conditions, its growth exhibited a prolonged lag phase compared to that of the control strain under 290-millimolar NaCl stress. Finally, the mechanisms of altered salt tolerance in LS-02 were revealed through comparative transcriptomics, and ORF25 and ORF26 on Syn-A-4-8 turned out to be the key genes causing the phenotype. Our research represents an important attempt in designing artificial cyanophages towards multiple hosts, and offers new future insights into the control of cyanobacterial blooms.

Synthetic chromosomes, genomes, viruses, and cells

Synthetic genomics is the construction of viruses, bacteria, and eukaryotic cells with synthetic genomes. It involves two basic processes: synthesis of complete genomes or chromosomes and booting up of those synthetic nucleic acids to make viruses or living cells. The first synthetic genomics efforts resulted in the construction of viruses. This led to a revolution in viral reverse genetics and improvements in vaccine design and manufacture. The first bacterium with a synthetic genome led to construction of a minimal bacterial cell and recoded Escherichia coli strains able to incorporate multiple non-standard amino acids in proteins and resistant to phage infection. Further advances led to a yeast strain with a synthetic genome and new approaches for animal and plant artificial chromosomes. On the horizon there are dramatic advances in DNA synthesis that will enable extraordinary new opportunities in medicine, industry, agriculture, and research.

Efficient de novo assembly and modification of large DNA fragments

Synthetic genomics has provided new bottom-up platforms for the functional study of viral and microbial genomes. The construction of the large, gigabase (Gb)-sized genomes of higher organisms will deepen our understanding of genetic blueprints significantly. But for the synthesis and assembly of such large-scale genomes, the development of new or expanded methods is required. In this study, we develop an efficient pipeline for the construction of large DNA fragments sized 100 kilobases (kb) or above from scratches and describe an efficient method for "scar-free" engineering of the assembled sequences. Our method, therefore, should provide a standard framework for producing long DNA molecules, which are critical materials for synthetic genomics and metabolic engineering.