Directed genome evolution driven by structural rearrangement techniques.

Directed genome evolution simulates the process of natural evolution at the genomic level in the laboratory to generate desired phenotypes. Here we review the applications of recent technological advances in genome writing and editing to directed genome evolution, with a focus on structural rearrangement techniques. We highlight how these techniques can be used to generate diverse genotypes, and to accelerate the evolution of phenotypic traits. We also discuss the perspectives of directed genome evolution.

Discovering and genotyping genomic structural variations by yeast genome synthesis and inducible evolution.

Genomic structural variations (SVs) promote the evolution of Saccharomyces cerevisiae, and play an important role in phenotypic diversities. Yeast genomic structures can be remodeled by design and bottom-up synthesis. The synthesis of yeast genome creates novel copy number variations (CNVs) and SVs and develops new strategies to discover gene functions. Further, an inducible evolution system SCRaMbLE, consisted of 3,932 loxPsym sites, was incorporated on synthetic yeast genome. SCRaMbLE enables genomic rearrangements at will and rapidly generates chromosomal number variations, and massive SVs under customized conditions. The impacts of genetic variations on phenotypes can be revealed by genome analysis and chromosome restructuring. Yeast genome synthesis and SCRaMbLE provide a new research paradigm to explore the genotypic mechanisms of phenotype diversities, and can be used to improve biological traits and optimize industrial chassis.

Design and chemical synthesis of eukaryotic chromosomes.

Following the discovery of the DNA double helix structure and the advancement of genome sequencing, we have entered a promising stage with regard to genome writing. Recently, a milestone breakthrough was achieved in the chemical synthesis of designer yeast chromosomes. Here, we review the systematic approaches to the de novo synthesis of designer eukaryotic chromosomes, with an emphasis on technologies and methodologies that enable design, building, testing and debugging. The achievement of chemically synthesized genomes with customized genetic features offers an opportunity to rebuild genome organization, remold biological functions and promote life evolution, which will be of great benefit for application in medicine and industrial manufacturing.

Design and synthesis of yeast chromosomes.

With the rapid growth and development of synthetic biology, research in the genomics is advancing from genome sequencing to genome synthesis. In 2009, Professor Jef D. Boeke proposed the Synthetic Yeast Genome Project (Sc2.0), which aims to synthesize the world's first eukaryotic genome. With the efforts of scientists from the United States, China, Britain, France, Australia, Singapore and other countries, a third of the Saccharomyces cerevisiae chromosomes has now been synthesized. In the perspectives of synthetic genomics, we here review the recent progress in the Sc2.0 project, including discussion on the right arm of chromosome 9, and chromosomes 2, 5, 6, 10, 12, in terms of their designs and synthetic strategy as well as the biological significance, thereby providing a reference for further research in synthetic genomics.

Heterologous biosynthesis and manipulation of alkanes in Escherichia coli.

Biosynthesis of alkanes in microbial foundries offers a sustainable and green supplement to traditional fossil fuels. The dynamic equilibrium of fatty aldehydes, key intermediates, played a critical role in microbial alkanes production, due to the poor catalytic capability of aldehyde deformylating oxygenase (ADO). In our study, exploration of competitive pathway together with multi-modular optimization was utilized to improve fatty aldehydes balance and consequently enhance alkanes formation in Escherichia coli. Endogenous fatty alcohol formation was supposed to be competitive with alkane production, since both of the two routes consumed the same intermediate-fatty aldehyde. Nevertheless, in our case, alkanes production in E. coli was enhanced from trace amount to 58.8mg/L by the facilitation of moderate fatty alcohol biosynthesis, which was validated by deletion of endogenous aldehyde reductase (AHR), overexpression of fatty alcohol oxidase (FAO) and consequent transcriptional assay of aar, ado and adhP genes. Moreover, alkanes production was further improved to 81.8mg/L, 86.6mg/L or 101.7mg/L by manipulation of fatty acid biosynthesis, lipids degradation or electron transfer system modules, which directly referenced to fatty aldehydes dynamic pools. A titer of 1.31g/L alkanes was achieved in 2.5L fed-batch fermentation, which was the highest reported titer in E. coli. Our research has offered a reference for chemical overproduction in microbial cell factories facilitated by exploring competitive pathway.

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.

Genomic Variation-Mediating Fluconazole Resistance in Yeast.

Fungal infections pose a serious and growing threat to public health. These infections can be treated with antifungal drugs by killing hazardous fungi in the body. However, the resistance can develop over time when fungi are exposed to antifungal drugs by generating genomic variations, including mutation, aneuploidy, and loss of heterozygosity. The variations could reduce the binding affinity of a drug to its target or block the pathway through which drugs exert their activity. Here, we review genomic variation-mediating fluconazole resistance in the yeast Candida, with the hope of highlighting the functional consequences of genomic variations for the antifungal resistance.

Debugging: putting the synthetic yeast chromosome to work.

Synthetic genomics aims to de novo synthesize a functional genome redesigned from natural sequences with custom features. Designed genomes provide new toolkits for better understanding organisms, evolution and the construction of cellular factories. Currently maintaining the fitness of cells with synthetic genomes is particularly challenging as defective designs and unanticipated assembly errors frequently occur. Mapping and correcting bugs that arise during the synthetic process are imperative for the successful construction of a synthetic genome that can sustain a desired cellular function. Here, we review recently developed methods used to map and fix various bugs which arise during yeast genome synthesis with the hope of providing guidance for putting the synthetic yeast chromosome to work.

Genomic markers on synthetic genomes.

Genome synthesis endows scientists the ability of de novo creating genomes absent in nature, by thorough redesigning DNA sequences and introducing numerous custom features. However, the genome synthesis is a labor- and time-consuming work, and thus it is a challenge to verify and quantify the synthetic genome rapidly and precisely. Thus, specific DNA sequences different from native genomic sequences are designed into synthetic genomes during synthesis, namely genomic markers. Genomic markers can be easily detected by PCR reaction, whole-genome sequencing (WGS) and a variety of methods to identify the synthetic genome from native one. Here, we review types and applications of genomic markers utilized in synthetic genomes, with the hope of providing a guidance for future works.

Author Correction: Precise control of SCRaMbLE in synthetic haploid and diploid yeast.

The original version of this Article omitted a declaration from the Competing Interests statement, which should have included the following: 'J.D.B. is a founder and Director of the following: Neochromosome, Inc., the Center of Excellence for Engineering Biology, and CDI Labs, Inc. and serves on the Scientific Advisory Board of the following: Modern Meadow, Inc., Recombinetics, Inc., and Sample6, Inc.'. This has now been corrected in both the PDF and HTML versions of the Article.

Ring synthetic chromosome V SCRaMbLE.

Structural variations (SVs) exert important functional impacts on biological phenotypic diversity. Here we show a ring synthetic yeast chromosome V (ring_synV) can be used to continuously generate complex genomic variations and improve the production of prodeoxyviolacein (PDV) by applying Synthetic Chromosome Recombination and Modification by LoxP-mediated Evolution (SCRaMbLE) in haploid yeast cells. The SCRaMbLE of ring_synV generates aneuploid yeast strains with increased PDV productivity, and we identify aneuploid chromosome I, III, VI, XII, XIII, and ring_synV. The neochromosome of SCRaMbLEd ring_synV generated more unbalanced forms of variations, including duplication, insertions, and balanced forms of translocations and inversions than its linear form. Furthermore, of the 29 novel SVs detected, 11 prompted the PDV biosynthesis; and the deletion of uncharacterized gene YER182W is related to the improvement of the PDV. Overall, the SCRaMbLEing ring_synV embraces the evolution of the genome by modifying the chromosome number, structure, and organization, identifying targets for phenotypic comprehension.

Rapid and Efficient CRISPR/Cas9-Based Mating-Type Switching of Saccharomyces cerevisiae.

Rapid and highly efficient mating-type switching of Saccharomyces cerevisiae enables a wide variety of genetic manipulations, such as the construction of strains, for instance, isogenic haploid pairs of both mating-types, diploids and polyploids. We used the CRISPR/Cas9 system to generate a double-strand break at the MAT locus and, in a single cotransformation, both haploid and diploid cells were switched to the specified mating-type at ∼80% efficiency. The mating-type of strains carrying either rod or ring chromosome III were switched, including those lacking HMLα and HMRa cryptic mating loci. Furthermore, we transplanted the synthetic yeast chromosome V to build a haploid polysynthetic chromosome strain by using this method together with an endoreduplication intercross strategy. The CRISPR/Cas9 mating-type switching method will be useful in building the complete synthetic yeast (Sc2.0) genome. Importantly, it is a generally useful method to build polyploids of a defined genotype and generally expedites strain construction, for example, in the construction of fully a/a/α/α isogenic tetraploids.

Precise control of SCRaMbLE in synthetic haploid and diploid yeast.

Compatibility between host cells and heterologous pathways is a challenge for constructing organisms with high productivity or gain of function. Designer yeast cells incorporating the Synthetic Chromosome Rearrangement and Modification by LoxP-mediated Evolution (SCRaMbLE) system provide a platform for generating genotype diversity. Here we construct a genetic AND gate to enable precise control of the SCRaMbLE method to generate synthetic haploid and diploid yeast with desired phenotypes. The yield of carotenoids is increased to 1.5-fold by SCRaMbLEing haploid strains and we determine that the deletion of YEL013W is responsible for the increase. Based on the SCRaMbLEing in diploid strains, we develop a strategy called Multiplex SCRaMbLE Iterative Cycling (MuSIC) to increase the production of carotenoids up to 38.8-fold through 5 iterative cycles of SCRaMbLE. This strategy is potentially a powerful tool for increasing the production of bio-based chemicals and for mining deep knowledge.

3D organization of synthetic and scrambled chromosomes.

Although the design of the synthetic yeast genome Sc2.0 is highly conservative with respect to gene content, the deletion of several classes of repeated sequences and the introduction of thousands of designer changes may affect genome organization and potentially alter cellular functions. We report here the Hi-C-determined three-dimensional (3D) conformations of Sc2.0 chromosomes. The absence of repeats leads to a smoother contact pattern and more precisely tractable chromosome conformations, and the large-scale genomic organization is globally unaffected by the presence of synthetic chromosome(s). Two exceptions are synIII, which lacks the silent mating-type cassettes, and synXII, specifically when the ribosomal DNA is moved to another chromosome. We also exploit the contact maps to detect rearrangements induced in SCRaMbLE (synthetic chromosome rearrangement and modification by loxP-mediated evolution) strains.

Bug mapping and fitness testing of chemically synthesized chromosome X.

Debugging a genome sequence is imperative for successfully building a synthetic genome. As part of the effort to build a designer eukaryotic genome, yeast synthetic chromosome X (synX), designed as 707,459 base pairs, was synthesized chemically. SynX exhibited good fitness under a wide variety of conditions. A highly efficient mapping strategy called pooled PCRTag mapping (PoPM), which can be generalized to any watermarked synthetic chromosome, was developed to identify genetic alterations that affect cell fitness ("bugs"). A series of bugs were corrected that included a large region bearing complex amplifications, a growth defect mapping to a recoded sequence in FIP1, and a loxPsym site affecting promoter function of ATP2 PoPM is a powerful tool for synthetic yeast genome debugging and an efficient strategy for phenotype-genotype mapping.

"Perfect" designer chromosome V and behavior of a ring derivative.

Perfect matching of an assembled physical sequence to a specified designed sequence is crucial to verify design principles in genome synthesis. We designed and de novo synthesized 536,024-base pair chromosome synV in the "Build-A-Genome China" course. We corrected an initial isolate of synV to perfectly match the designed sequence using integrative cotransformation and clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9)-mediated editing in 22 steps; synV strains exhibit high fitness under a variety of culture conditions, compared with that of wild-type V strains. A ring synV derivative was constructed, which is fully functional in Saccharomyces cerevisiae under all conditions tested and exhibits lower spore viability during meiosis. Ring synV chromosome can extends Sc2.0 design principles and provides a model with which to study genomic rearrangement, ring chromosome evolution, and human ring chromosome disorders.