DNA Assembly Strategies Research Articles

Toolbox of Characterized Genetic Parts for Staphylococcus aureus.

Staphylococcus aureus is an important clinical bacterium prevalent in human-associated microbiomes and the cause of many diseases. However, S. aureus has been intractable to synthetic biology approaches due to limited characterized genetic parts for this nonmodel Gram-positive bacterium. Moreover, genetic manipulation of S. aureus has relied on cumbersome and inefficient cloning strategies. Here, we report the first standardized genetic parts toolbox for S. aureus, which includes characterized promoters, ribosome binding sites, terminators, and plasmid replicons from a variety of bacteria for precise control of gene expression. We established a standard relative expression unit (REU) for S. aureus using a plasmid reference and characterized genetic parts in standardized REUs using S. aureus ATCC 12600. We constructed promoter and terminator part plasmids that are compatible with an efficient Type IIS DNA assembly strategy to effectively build multipart DNA constructs. A library of 24 constitutive promoters was built and characterized in S. aureus, which showed a 380-fold activity range. This promoter library was also assayed in Bacillus subtilis (122-fold activity range) to demonstrate the transferability of the constitutive promoters between these Gram-positive bacteria. By applying an iterative design-build-test-learn cycle, we demonstrated the use of our toolbox for the rational design and engineering of a tetracycline sensor in S. aureus using the PXyl-TetO aTc-inducible promoter that achieved 25.8-fold induction. This toolbox greatly expands the growing number of genetic parts for Gram-positive bacteria and will allow researchers to leverage synthetic biology approaches to study and engineer cellular processes in S. aureus.

The Molecular Toolset and Techniques Required to Build Cyanobacterial Cell Factories.

Cyanobacteria are the only prokaryotes performing oxygenic photosynthesis, a solar-driven process which allows them to obtain electrons from water to reduce and finally assimilate carbon dioxide. Consequently, they are in the spotlight of biotechnology as photoautotrophic cell factories to generate a large variety of chemicals and biofuels in a sustainable way. Recent progress in synthetic biology has enlarged the molecular toolset to genetically engineer the metabolism of cyanobacteria, mainly targeting common model strains, such as Synechocystis sp. PCC 6803, Synechococcus elongatus PCC 7942, Synechococcus sp. PCC 7002, or Anabaena sp. PCC 7120. Nevertheless, the accessibility and flexibility of engineering cyanobacteria is still somewhat limited and less predictable compared to other biotechnologically employed microorganisms.This chapter gives a broad overview of currently available methods for the genetic modification of cyanobacterial model strains as well as more recently discovered and promising species, such as Synechococcus elongatus PCC 11801. It comprises approaches based on homologous recombination, replicative broad-host-range or strain-specific plasmids, CRISPR/Cas, as well as markerless selection. Furthermore, common and newly introduced molecular tools for gene expression regulation are presented, comprising promoters, regulatory RNAs, genetic insulators like transcription terminators, ribosome binding sites, CRISPR interference, and the utilization of heterologous RNA polymerases. Additionally, potential DNA assembly strategies, like modular cloning, are described. Finally, considerations about post-translational control via protein degradation tags and heterologous proteases, as well as small proteins working as enzyme effectors are briefly discussed.

PH-Responsive Reversible DNA Self-assembly Mediated by Zwitterion

pH-Responsive DNA assembles have drawn growing attentions owing to their great potential in diverse areas. However, pH-responsive motifs are limited to specific DNA sequences and annealing is usually needed for DNA assemblies; therefore, sequence-independent pH-responsive DNA assembly at room temperature is highly desired as a more general way. Here, we propose a reversible pH-responsive DNA assembly strategy at room-temperature using zwitterion, glycine betaine(GB), as charge-regulation molecules. The reversible assembly and disassembly of DNA nanostructures could be achieved by alternatively regulating the acidic and basic environments in the presence of GB, respectively. In an acidic environment, carboxylate group in GB was protonated and GB was positively charged, which facilitated to shield the inherent electrostatic repulsion of DNA strands. Molecular simulation showed that the newly formed carboxyl group in protonated GB could form hydrogen bonds with bases in DNA to promote the assembly of DNA strands. In a basic solution, carboxylate group in GB was deprotonated and GB was neutral, thus inducing the dissociation of DNA assembly.

Seamless insert-plasmid assembly at sub-terminal homologous sequences

The engineering of fusion proteins for structural biology and protein nanotechnology often requires seamless DNA assembly with slight variations in the domain boundaries. To improve the molecular biology workflow for such projects, we evaluated the use of sub-terminal homologous sequences (HS) for co-transformation cloning and for T5 exonuclease / Phusion DNA polymerase mediated in vitro assembly. To quantify the effects of different HS-to-ends distances on cloning efficiency, we designed a blue-white-pink screening system that allowed us to easily identify positive clones (blue colonies), negative clones resulting from circular template plasmid (pink colonies) and negative colonies originating from linearized plasmids that have recircularized without an insert (white colonies). Our experiments show that both methods are feasible with HS-to-ends distances up to at least 10 base pairs. Using a combination of co-transformation cloning at sub-terminal HS and nucleotide insertions in non-annealing primer 5′-overhangs, we integrated a fusion protein into the third intracellular loop (ICL) of a G-protein-coupled receptor (GPCR) with nine different linker boundaries, using only a single plasmid linearization reaction. This molecular cloning approach is an invaluable tool for protein engineering, protein nanotechnology and synthetic biology that extends the range of applications of DNA assembly strategies.

Peripheral infrastructure vectors and an extended set of plant parts for the Modular Cloning system.

Standardized DNA assembly strategies facilitate the generation of multigene constructs from collections of building blocks in plant synthetic biology. A common syntax for hierarchical DNA assembly following the Golden Gate principle employing Type IIs restriction endonucleases was recently developed, and underlies the Modular Cloning and GoldenBraid systems. In these systems, transcriptional units and/or multigene constructs are assembled from libraries of standardized building blocks, also referred to as phytobricks, in several hierarchical levels and by iterative Golden Gate reactions. Here, a toolkit containing further modules for the novel DNA assembly standards was developed. Intended for use with Modular Cloning, most modules are also compatible with GoldenBraid. Firstly, a collection of approximately 80 additional phytobricks is provided, comprising e.g. modules for inducible expression systems, promoters or epitope tags. Furthermore, DNA modules were developed for connecting Modular Cloning and Gateway cloning, either for toggling between systems or for standardized Gateway destination vector assembly. Finally, first instances of a “peripheral infrastructure” around Modular Cloning are presented: While available toolkits are designed for the assembly of plant transformation constructs, vectors were created to also use coding sequence-containing phytobricks directly in yeast two hybrid interaction or bacterial infection assays. The presented material will further enhance versatility of hierarchical DNA assembly strategies.

Universal Dynamic DNA Assembly-Programmed Surface Hybridization Effect for Single-Step, Reusable, and Amplified Electrochemical Nucleic Acid Biosensing.

The traditional sensitive electrochemical biosensors are commonly confronted with the cumbersome interface operation and washing procedures and the inclusion of extra exogenous reagents, which impose the challenge on the detection simplicity, reliability, and reusability. Herein, we present the proof-of-principle of a unique biosensor architecture based on dynamic DNA assembly programmed surface hybridization, which confers the single-step, reusable, and enzyme-free amplified electrochemical nucleic acid analysis. To demonstrate the fabrication universality three dynamic DNA assembly strategies including DNA-fueled target recycling, catalytic hairpin DNA assembly, and hybridization chain reaction were flexibly harnessed to convey the homogeneous target recognition and amplification events into various DNA scaffolds for the autonomous proximity-based surface hybridization. The current biosensor architecture features generalizability, simplicity, low cost, high sensitivity, and specificity over the traditional nucleic acid-related amplified biosensors. The lowest detection limit of 50 aM toward target DNA could be achieved by hybridization chain reaction-programmed surface hybridization. The reliable working ability for both homogeneous solution and heterogeneous inteface facilitates the target analysis with a robust reliability and reproducibility, also making it to be readily extended for the integration with the kinds of detecting platforms. Thus, it may hold great potential for the biosensor fabrication served for the point-of-care applications in resource constrained regions.

Flexible and Versatile Strategy for the Construction of Large Biochemical Pathways.

Synthetic pathways and circuits have been increasingly used for microbial production of fuels and chemicals. Here, we report a flexible and versatile DNA assembly strategy that allows rapid, modular, and reliable construction of biological pathways and circuits from basic genetic parts. This strategy combines the automation-friendly ligase cycling reaction (LCR) method and the high-fidelity in vivo yeast-based DNA assembly method, DNA assembler. Briefly, LCR is used to preassemble basic genetic parts into gene expression cassettes or to preassemble small parts into larger parts to reduce the number of parts, in which many basic genetic parts can be reused. With the help of specially designed unique linkers, all preassembled parts will then be directly assembled using DNA assembler to build the target constructs. As proof of concept, three plasmids with varying sizes of 13.4, 24, and 44 kb were rapidly constructed with fidelities of 100, 88, and 71%, respectively. The yeast strain harboring the constructed 44 kb plasmid was confirmed to be capable of utilizing xylose, cellobiose, and glucose to produce zeaxanthin. This strategy should be generally applicable to any custom-designed pathways, circuits, or plasmids.

DNA assembly techniques for next-generation combinatorial biosynthesis of natural products

Natural product scaffolds remain important leads for pharmaceutical development. However, transforming a natural product into a drug entity often requires derivatization to enhance the compound's therapeutic properties. A powerful method by which to perform this derivatization is combinatorial biosynthesis, the manipulation of the genes in the corresponding pathway to divert synthesis towards novel derivatives. While these manipulations have traditionally been carried out via restriction digestion/ligation-based cloning, the shortcomings of such techniques limit their throughput and thus the scope of corresponding combinatorial biosynthesis experiments. In the burgeoning field of synthetic biology, the demand for facile DNA assembly techniques has promoted the development of a host of novel DNA assembly strategies. Here we describe the advantages of these recently developed tools for rapid, efficient synthesis of large DNA constructs. We also discuss their potential to facilitate the simultaneous assembly of complete libraries of natural product biosynthetic pathways, ushering in the next generation of combinatorial biosynthesis.

2ab assembly: a methodology for automatable, high-throughput assembly of standard biological parts

There is growing demand for robust DNA assembly strategies to quickly and accurately fabricate genetic circuits for synthetic biology. One application of this technology is reconstitution of multi-gene assemblies. Here, we integrate a new software tool chain with 2ab assembly and show that it is robust enough to generate 528 distinct composite parts with an error-free success rate of 96%. Finally, we discuss our findings in the context of its implications for biosafety and biosecurity.

DNA assembly for synthetic biology: from parts to pathways and beyond

The assembly of DNA from small fragments into large constructs has seen significant recent development, becoming a pivotal technology in the ability to implement the vision of synthetic biology. As the cost of whole gene synthesis is decreasing, whole genome synthesis at the other end of the spectrum has expanded our horizons to the prospect of fully engineered synthetic cells. However, the recently proven ability to synthesise genome-scale DNA is at odds with our ability to rationally engineer biological devices, which lags significantly behind. Most work in synthetic biology takes place on an intermediate scale with the combinatorial construction of networks and metabolic pathways from registries of modular biopart components. Implementation for rapid prototyping of engineered biological circuits requires quick and reliable DNA assembly according to specific architectures. It is apparent that DNA assembly is now a limiting technology in advancing synthetic biology. Current techniques employ standardised restriction enzyme assembly protocols such as BioBricks™, BglBricks and Golden Gate methods. Alternatively, sequence-independent overlap techniques, such as In-Fusion™, SLIC and Gibson isothermal assembly are becoming popular for larger assemblies, and in vivo DNA assembly in yeast and bacillus appears adept for chromosome fabrication. It is important to consider how the use of different technologies may impact the outcome of a construction, since the assembly technique can direct the architecture and diversity of systems that can be made. This review provides a critical examination of recent DNA assembly strategies and considers how this important facilitating aspect of synthetic biology may proceed in the future.