Synthetic biology. Genomically encoded analog memory with precise in vivo DNA writing in living cell populations

Simultaneous multi-site editing of individual genomes using retron arrays

During recent years, the use of libraries-scale genomic manipulations scaffolded on CRISPR guide RNAs have been transformative. However, these existing approaches are typically multiplexed across genomes. Unfortunately, building cells with multiple, nonadjacent precise mutations remains a laborious cycle of editing, isolating an edited cell and editing again. The use of bacterial retrons can overcome this limitation. Retrons are genetic systems composed of a reverse transcriptase and a noncoding RNA that contains an multicopy single-stranded DNA, which is reverse transcribed to produce multiple copies of single-stranded DNA. Here we describe a technology-termed a multitron-for precisely modifying multiple sites on a single genome simultaneously using retron arrays, in which multiple donor-encoding DNAs are produced from a single transcript. The multitron architecture is compatible with both recombineering in prokaryotic cells and CRISPR editing in eukaryotic cells. We demonstrate applications for this approach in molecular recording, genetic element minimization and metabolic engineering.

High-throughput DNA synthesis for data storage

With the explosion of digital world, the dramatically increasing data volume is expected to reach 175 ZB (1 ZB = 1012 GB) in 2025. Storing such huge global data would consume tons of resources. Fortunately, it has been found that the deoxyribonucleic acid (DNA) molecule is the most compact and durable information storage medium in the world so far. Its high coding density and long-term preservation properties make itself one of the best data storage carriers for the future. High-throughput DNA synthesis is a key technology for "DNA data storage", which encodes binary data stream (0/1) into quaternary long DNA sequences consisting of four bases (A/G/C/T). In this review, the workflow of DNA data storage and the basic methods of artificial DNA synthesis technology are outlined first. Then, the technical characteristics of different synthesis methods and the state-of-the-art of representative commercial companies, with a primary focus on silicon chip microarray-based synthesis and novel enzymatic DNA synthesis are presented. Finally, the recent status of DNA storage and new opportunities for future development in the field of high-throughput, large-scale DNA synthesis technology are summarized.

Mobile genetic element-based gene editing and genome engineering: Recent advances and applications

Genome engineering has revolutionized several scientific fields, ranging from biochemistry and fundamental research to therapeutic uses and crop development. Diverse engineering toolkits have been developed and used to effectively modify the genome sequences of organisms. However, there is a lack of extensive reviews on genome engineering technologies based on mobile genetic elements (MGEs), which induce genetic diversity within host cells by changing their locations in the genome. This review provides a comprehensive update on the versatility of MGEs as powerful genome engineering tools that offers efficient solutions to challenges associated with genome engineering. MGEs, including DNA transposons, retrotransposons, retrons, and CRISPR-associated transposons, offer various advantages, such as a broad host range, genome-wide mutagenesis, efficient large-size DNA integration, multiplexing capabilities, and in situ single-stranded DNA generation. We focused on the components, mechanisms, and features of each MGE-based tool to highlight their cellular applications. Finally, we discussed the current challenges of MGE-based genome engineering and provided insights into the evolving landscape of this transformative technology. In conclusion, the combination of genome engineering with MGE demonstrates remarkable potential for addressing various challenges and advancing the field of genetic manipulation, and promises to revolutionize our ability to engineer and understand the genomes of diverse organisms.

A DNA circuit that records molecular events

Characterizing the relative onset time, strength, and duration of molecular signals is critical for understanding the operation of signal transduction and genetic regulatory networks. However, detecting multiple such molecules as they are produced and then quickly consumed is challenging. A MER can encode information about transient molecular events as stable DNA sequences and are amenable to downstream sequencing or other analysis. Here, we report the development of a de novo molecular event recorder that processes information using a strand displacement reaction network and encodes the information using the primer exchange reaction, which can be decoded and quantified by DNA sequencing. The event recorder was able to classify the order at which different molecular signals appeared in time with 88% accuracy, the concentrations with 100% accuracy, and the duration with 75% accuracy. This simultaneous and highly programmable multiparameter recording could enable the large-scale deciphering of molecular events such as within dynamic reaction environments, living cells, or tissues.

Engineering intelligent chassis cells via recombinase-based MEMORY circuits

Synthetic biologists seek to engineer intelligent living systems capable of decision-making, communication, and memory. Separate technologies exist for each tenet of intelligence; however, the unification of all three properties in a living system has not been achieved. Here, we engineer completely intelligent Escherichia coli strains that harbor six orthogonal and inducible genome-integrated recombinases, forming Molecularly Encoded Memory via an Orthogonal Recombinase arraY (MEMORY). MEMORY chassis cells facilitate intelligence via the discrete multi-input regulation of recombinase functions enabling inheritable DNA inversions, deletions, and genomic insertions. MEMORY cells can achieve programmable and permanent gain (or loss) of functions extrachromosomally or from a specific genomic locus, without the loss or modification of the MEMORY platform - enabling the sequential programming and reprogramming of DNA circuits within the cell. We demonstrate all three tenets of intelligence via a probiotic (Nissle 1917) MEMORY strain capable of information exchange with the gastrointestinal commensal Bacteroides thetaiotaomicron.

Resolving cellular dynamics using single-cell temporal transcriptomics

Cellular dynamics, the transition of a cell from one state to another, is central to understanding developmental processes and disease progression. Single-cell transcriptomics has been pushing the frontiers of cellular dynamics studies into a genome-wide and single-cell level. While most single-cell RNA sequencing approaches are disruptive and only provide a snapshot of cell states, the dynamics of a cell could be reconstructed by either exploiting temporal information hiding in the transcriptomics data or integrating additional information. In this review, we describe these approaches, highlighting their underlying principles, key assumptions, and the rationality to interpret the results as models. We also discuss the recently emerging nondisruptive live-cell transcriptomics methods, which are highly complementary to the computational models for their assumption-free nature.

Data Storage Using DNA

The exponential growth of global data has outpaced the storage capacities of current technologies, necessitating innovative storage strategies. DNA, as a natural medium for preserving genetic information, has emerged as a highly promising candidate for next-generation storage medium. Storing data in DNA offers several advantages, including ultrahigh physical density and exceptional durability. Facilitated by significant advancements in various technologies, such as DNA synthesis, DNA sequencing, and DNA nanotechnology, remarkable progress has been made in the field of DNA data storage over the past decade. However, several challenges still need to be addressed to realize practical applications of DNA data storage. In this review, the processes and strategies of in vitro DNA data storage are first introduced, highlighting recent advancements. Next, a brief overview of in vivo DNA data storage is provided, with a focus on the various writing strategies developed to date. At last, the challenges encountered in each step of DNA data storage are summarized and promising techniques are discussed that hold great promise in overcoming these obstacles.

Sentinel cells programmed to respond to environmental DNA including human sequences

Monitoring environmental DNA can track the presence of organisms, from viruses to animals, but requires continuous sampling of transient sequences from a complex milieu. Here we designed living sentinels using Bacillus subtilis to report the uptake of a DNA sequence after matching it to a preencoded target. Overexpression of ComK increased DNA uptake 3,000-fold, allowing for femtomolar detection in samples dominated by background DNA. This capability was demonstrated using human sequences containing single-nucleotide polymorphisms (SNPs) associated with facial features. Sequences were recorded with high efficiency and were protected from nucleases for weeks. The SNP could be determined by sequencing or in vivo using CRISPR interference to turn on reporter expression in response to a specific base. Multiple SNPs were recorded by one cell or through a consortium in which each member recorded a different sequence. Sentinel cells could surveil for specific sequences over long periods of time for applications spanning forensics, ecology and epidemiology.

An experimental census of retrons for DNA production and genome editing

Retrons are bacterial immune systems that use reverse transcribed DNA as a detector of phage infection. They are also increasingly deployed as a component of biotechnology. For genome editing, for instance, retrons are modified so that the reverse transcribed DNA (RT-DNA) encodes an editing donor. Retrons are commonly found in bacterial genomes; thousands of unique retrons have now been predicted bioinformatically. However, only a small number have been characterized experimentally. Here, we add substantially to the corpus of experimentally studied retrons. We synthesized >100 previously untested retrons to identify the natural sequence of RT-DNA they produce, quantify their RT-DNA production, and test the relative efficacy of editing using retron-derived donors to edit bacterial genomes, phage genomes, and human genomes. We add 62 new empirically determined, natural RT-DNAs, which are not predictable from the retron sequence alone. We report a large diversity in RT-DNA production and editing rates across retrons, finding that top performing editors outperform those used in previous studies, and are drawn from a subset of the retron phylogeny.

Generation of DNAzyme in Bacterial Cells by a Bacterial Retron System

DNAzymes are catalytically active single-stranded DNAs in which DNAzyme 10-23 (Dz 10-23) consists of a catalytic core and a substrate-binding arm that reduces gene expression through sequence-specific mRNA cleavage. However, the in vivo application of Dz 10-23 depends on exogenous delivery, which leads to its inability to be synthesized and stabilized in vivo, thus limiting its application. As a unique reverse transcription system, the bacterial retron system can synthesize single-stranded DNA in vivo using ncRNA msr/msd as a template. The objective of this work is to reduce target gene expression using Dz 10-23 generated in vivo by the retron system. In this regard, we successfully generated Dz 10-23 by cloning the Dz 10-23 coding sequence into the retron msd gene and tested its ability to reduce specific gene expression by examining the mRNA levels of cfp encoding cyan fluorescence protein and other functional genes such as mreB and ftsZ. We found that Dz had different repressive effects when targeting different mRNA regions, and in general, the repressive effect was stronger when targeting downstream of mRNAs. Our results also suggested that the reduction effect was due to cleavage of the substrate mRNA by Dz 10-23 rather than the antisense effect of the substrate-binding arm. Therefore, this study not only provided a retron-based method for the intracellular generation of Dz 10-23 but also demonstrated that Dz 10-23 could reduce gene expression by cleaving target mRNAs in cells. We believe that this new strategy would have great potential in the regulation of gene expression.

DNA Barcoding Technology for Lineage Recording and Tracing to Resolve Cell Fate Determination

In various biological contexts, cells receive signals and stimuli that prompt them to change their current state, leading to transitions into a future state. This change underlies the processes of development, tissue maintenance, immune response, and the pathogenesis of various diseases. Following the path of cells from their initial identity to their current state reveals how cells adapt to their surroundings and undergo transformations to attain adjusted cellular states. DNA-based molecular barcoding technology enables the documentation of a phylogenetic tree and the deterministic events of cell lineages, providing the mechanisms and timing of cell lineage commitment that can either promote homeostasis or lead to cellular dysregulation. This review comprehensively presents recently emerging molecular recording technologies that utilize CRISPR/Cas systems, base editing, recombination, and innate variable sequences in the genome. Detailing their underlying principles, applications, and constraints paves the way for the lineage tracing of every cell within complex biological systems, encompassing the hidden steps and intermediate states of organism development and disease progression.

Metagenomics harvested genus-specific single-stranded DNA-annealing proteins improve and expand recombineering in Pseudomonas species

The widespread Pseudomonas genus comprises a collection of related species with remarkable abilities to degrade plastics and polluted wastes and to produce a broad set of valuable compounds, ranging from bulk chemicals to pharmaceuticals. Pseudomonas possess characteristics of tolerance and stress resistance making them valuable hosts for industrial and environmental biotechnology. However, efficient and high-throughput genetic engineering tools have limited metabolic engineering efforts and applications. To improve their genome editing capabilities, we first employed a computational biology workflow to generate a genus-specific library of potential single-stranded DNA-annealing proteins (SSAPs). Assessment of the library was performed in different Pseudomonas using a high-throughput pooled recombinase screen followed by Oxford Nanopore NGS analysis. Among different active variants with variable levels of allelic replacement frequency (ARF), efficient SSAPs were found and characterized for mediating recombineering in the four tested species. New variants yielded higher ARFs than existing ones in Pseudomonas putida and Pseudomonas aeruginosa, and expanded the field of recombineering in Pseudomonas taiwanensisand Pseudomonas fluorescens. These findings will enhance the mutagenesis capabilities of these members of the Pseudomonas genus, increasing the possibilities for biotransformation and enhancing their potential for synthetic biology applications.  .

Digital data storage on DNA tape using CRISPR base editors

While the archival digital memory industry approaches its physical limits, the demand is significantly increasing, therefore alternatives emerge. Recent efforts have demonstrated DNA's enormous potential as a digital storage medium with superior information durability, capacity, and energy consumption. However, the majority of the proposed systems require on-demand de-novo DNA synthesis techniques that produce a large amount of toxic waste and therefore are not industrially scalable and environmentally friendly. Inspired by the architecture of semiconductor memory devices and recent developments in gene editing, we created a molecular digital data storage system called "DNA Mutational Overwriting Storage" (DMOS) that stores information by leveraging combinatorial, addressable, orthogonal, and independent in vitro CRISPR base-editing reactions to write data on a blank pool of greenly synthesized DNA tapes. As a proof of concept, this work illustrates writing and accurately reading of both a bitmap representation of our school's logo and the title of this study on the DNA tapes.

Advancements in synthetic biology-based bacterial cancer therapy: A modular design approach

Synthetic biology aims to program living bacteria cells with artificial genetic circuits for user-defined functions, transforming them into powerful tools with numerous applications in various fields, including oncology. Cancer treatments have serious side effects on patients due to the systemic action of the drugs involved. To address this, new systems that provide localized antitumoral action while minimizing damage to healthy tissues are required. Bacteria, often considered pathogenic agents, have been used as cancer treatments since the early 20th century. Advances in genetic engineering, synthetic biology, microbiology, and oncology have improved bacterial therapies, making them safer and more effective. Here we propose six modules for a successful synthetic biology-based bacterial cancer therapy, the modules include Payload, Release, Tumor-targeting, Biocontainment, Memory, and Genetic Circuit Stability Module. These will ensure antitumor activity, safety for the environment and patient, prevent bacterial colonization, maintain cell stability, and prevent loss or defunctionalization of the genetic circuit.

Algorithmic Programming of Sequential Logic and Genetic Circuits for Recording Biochemical Concentration in a Probiotic Bacterium

Through the implementation of designable genetic circuits, engineered probiotic microorganisms could be used as noninvasive diagnostic tools for the gastrointestinal tract. For these living cells to report detected biomarkers or signals after exiting the gut, the genetic circuits must be able to record these signals by using genetically encoded memory. Complex memory register circuits could enable multiplex interrogation of biomarkers and signals. A theory-based approach to create genetic circuits containing memory, known as sequential logic circuits, was previously established for a model laboratory strain of Escherichia coli, yet how circuit component performance varies for nonmodel and clinically relevant bacterial strains is poorly understood. Here, we develop a scalable computational approach to design robust sequential logic circuits in probiotic strain Escherichia coli Nissle 1917 (EcN). In this work, we used TetR-family transcriptional repressors to build genetic logic gates that can be composed into sequential logic circuits, along with a set of engineered sensors relevant for use in the gut environment. Using standard methods, 16 genetic NOT gates and nine sensors were experimentally characterized in EcN. These data were used to design and predict the performance of circuit designs. We present a set of genetic circuits encoding both combinational logic and sequential logic and show that the circuit outputs are in close agreement with our quantitative predictions from the design algorithm. Furthermore, we demonstrate an analog-like concentration recording circuit that detects and reports three input concentration ranges of a biochemical signal using sequential logic.

Synthetic circuits based on split Cas9 to detect cellular events

Synthetic biology involves the engineering of logic circuit gates that process different inputs to produce specific outputs, enabling the creation or control of biological functions. While CRISPR has become the tool of choice in molecular biology due to its RNA-guided targetability to other nucleic acids, it has not been frequently applied to logic gates beyond those controlling the guide RNA (gRNA). In this study, we present an adaptation of split Cas9 to generate logic gates capable of sensing biological events, leveraging a Cas9 reporter (EGxxFP) to detect occurrences such as cancer cell origin, epithelial to mesenchymal transition (EMT), and cell-cell fusion. First, we positioned the complementing halves of split Cas9 under different promoters-one specific to cancer cells of epithelial origin (phCEA) and the other a universal promoter. The use of self-assembling inteins facilitated the reconstitution of the Cas9 halves. Consequently, only cancer cells with an epithelial origin activated the reporter, exhibiting green fluorescence. Subsequently, we explored whether this system could detect biological processes such as epithelial to mesenchymal transition (EMT). To achieve this, we designed a logic gate where one half of Cas9 is expressed under the phCEA, while the other is activated by TWIST1. The results showed that cells undergoing EMT effectively activated the reporter. Next, we combined the two inputs (epithelial origin and EMT) to create a new logic gate, where only cancer epithelial cells undergoing EMT activated the reporter. Lastly, we applied the split-Cas9 logic gate as a sensor of cell-cell fusion, both in induced and naturally occurring scenarios. Each cell type expressed one half of split Cas9, and the induction of fusion resulted in the appearance of multinucleated syncytia and the fluorescent reporter. The simplicity of the split Cas9 system presented here allows for its integration into various cellular processes, not only as a sensor but also as an actuator.

Retron-mediated multiplex genome editing and continuous evolution in Escherichia coli

While there are several genome editing techniques available, few are suitable for dynamic and simultaneous mutagenesis of arbitrary targeted sequences in prokaryotes. Here, to address these limitations, we present a versatile and multiplex retron-mediated genome editing system (REGES). First, through systematic optimization of REGES, we achieve efficiency of ∼100%, 85 ± 3%, 69 ± 14% and 25 ± 14% for single-, double-, triple- and quadruple-locus genome editing, respectively. In addition, we employ REGES to generate pooled and barcoded variant libraries with degenerate RBS sequences to fine-tune the expression level of endogenous and exogenous genes, such as transcriptional factors to improve ethanol tolerance and biotin biosynthesis. Finally, we demonstrate REGES-mediated continuous in vivo protein evolution, by combining retron, polymerase-mediated base editing and error-prone transcription. By these case studies, we demonstrate REGES as a powerful multiplex genome editing and continuous evolution tool with broad applications in synthetic biology and metabolic engineering.

RNA recording in single bacterial cells using reprogrammed tracrRNAs

Capturing an individual cell's transcriptional history is a challenge exacerbated by the functional heterogeneity of cellular communities. Here, we leverage reprogrammed tracrRNAs (Rptrs) to record selected cellular transcripts as stored DNA edits in single living bacterial cells. Rptrs are designed to base pair with sensed transcripts, converting them into guide RNAs. The guide RNAs then direct a Cas9 base editor to target an introduced DNA target. The extent of base editing can then be read in the future by sequencing. We use this approach, called TIGER (transcribed RNAs inferred by genetically encoded records), to record heterologous and endogenous transcripts in individual bacterial cells. TIGER can quantify relative expression, distinguish single-nucleotide differences, record multiple transcripts simultaneously and read out single-cell phenomena. We further apply TIGER to record metabolic bet hedging and antibiotic resistance mobilization in Escherichia coli as well as host cell invasion by Salmonella. Through RNA recording, TIGER connects current cellular states with past transcriptional states to decipher complex cellular responses in single cells.

Beyond natural biology: rewiring cellular networks to study innate immunity

Within immune cells, microbial and self-ligands trigger pattern recognition receptors (PRRs) to nucleate and activate the signaling organelles of the immune system. Much work in this area has derived from observational biology of natural innate immune signaling. More recently, synthetic biology approaches have been used to rewire and study innate immune networks. By utilizing controllable chemical or optogenetic inputs, rearranging protein building blocks, or engineering signal recording circuits, synthetic biology-based techniques complement and inform studies of natural immune pathway operation. In this review, we describe recent synthetic biology-based approaches that have uncovered new insights into PRR signaling, virus-host interactions, and systemic cytokine responses.

Simultaneous multi-site editing of individual genomes using retron arrays

Our understanding of genomics is limited by the scale of our genomic technologies. While libraries of genomic manipulations scaffolded on CRISPR gRNAs have been transformative, these existing approaches are typically multiplexed across genomes. Yet much of the complexity of real genomes is encoded within a genome across sites. Unfortunately, building cells with multiple, non-adjacent precise mutations remains a laborious cycle of editing, isolating an edited cell, and editing again. Here, we describe a technology for precisely modifying multiple sites on a single genome simultaneously. This technology - termed a multitron - is built from a heavily modified retron, in which multiple donor-encoding msds are produced from a single transcript. The multitron architecture is compatible with both recombineering in prokaryotic cells and CRISPR editing in eukaryotic cells. We demonstrate applications for this approach in molecular recording, genetic element minimization, and metabolic engineering.

Generation of DNA Aptamers with Functional Activity in Mammalian Cells by Mimicking Retroviruses

DNA aptamers are single-stranded DNA oligonucleotide sequences that bind to specific targets with high affinity. Currently, DNA aptamers can be produced only by in vitro synthesis. It is difficult for DNA aptamers to have a sustained impact on intracellular protein activity, which limits their clinical application. In this study, we developed a DNA aptamer expression system to generate DNA aptamers with functional activity in mammalian cells by mimicking retroviruses. Using this system, DNA aptamers targeting intracellular Ras (Ra1) and membrane-bound CD71 (XQ2) were successfully generated in cells. In particular, the expressed Ra1 not only specifically bound to the intracellular Ras protein but also inhibited the phosphorylation of downstream ERK1/2 and AKT. Furthermore, by inserting the DNA aptamer expression system for Ra1 into a lentivirus vector, the system can be delivered into cells and stably produce Ra1 over time, resulting in the inhibition of lung cancer cell proliferation. Therefore, our study provides a novel strategy for the intracellular generation of DNA aptamers with functional activity and opens a new avenue for the clinical application of intracellular DNA aptamers in disease treatment.

Temporally resolved transcriptional recording in E. coli DNA using a Retro-Cascorder

Biological signals occur over time in living cells. Yet most current approaches to interrogate biology, particularly gene expression, use destructive techniques that quantify signals only at a single point in time. A recent technological advance, termed the Retro-Cascorder, overcomes this limitation by molecularly logging a record of gene expression events in a temporally organized genomic ledger. The Retro-Cascorder works by converting a transcriptional event into a DNA barcode using a retron reverse transcriptase and then storing that event in a unidirectionally expanding clustered regularly interspaced short palindromic repeats (CRISPR) array via acquisition by CRISPR-Cas integrases. This CRISPR array-based ledger of gene expression can be retrieved at a later point in time by sequencing. Here we describe an implementation of the Retro-Cascorder in which the relative timing of transcriptional events from multiple promoters of interest is recorded chronologically in Escherichia coli populations over multiple days. We detail the molecular components required for this technology, provide a step-by-step guide to generate the recording and retrieve the data by Illumina sequencing, and give instructions for how to use custom software to infer the relative transcriptional timing from the sequencing data. The example recording is generated in 2 d, preparation of sequencing libraries and sequencing can be accomplished in 2-3 d, and analysis of data takes up to several hours. This protocol can be implemented by someone familiar with basic bacterial culture, molecular biology and bioinformatics. Analysis can be minimally run on a personal computer.

Recording of cellular physiological histories along optically readable self-assembling protein chains

Observing cellular physiological histories is key to understanding normal and disease-related processes. Here we describe expression recording islands-a fully genetically encoded approach that enables both continual digital recording of biological information within cells and subsequent high-throughput readout in fixed cells. The information is stored in growing intracellular protein chains made of self-assembling subunits, human-designed filament-forming proteins bearing different epitope tags that each correspond to a different cellular state or function (for example, gene expression downstream of neural activity or pharmacological exposure), allowing the physiological history to be read out along the ordered subunits of protein chains with conventional optical microscopy. We use expression recording islands to record gene expression timecourse downstream of specific pharmacological and physiological stimuli in cultured neurons and in living mouse brain, with a time resolution of a fraction of a day, over periods of days to weeks.

Continuous Multiplexed Phage Genome Editing Using Recombitrons

Bacteriophages, which naturally shape bacterial communities, can be co-opted as a biological technology to help eliminate pathogenic bacteria from our bodies and food supply1. Phage genome editing is a critical tool to engineer more effective phage technologies. However, editing phage genomes has traditionally been a low efficiency process that requires laborious screening, counter selection, or in vitro construction of modified genomes2. These requirements impose limitations on the type and throughput of phage modifications, which in turn limit our knowledge and potential for innovation. Here, we present a scalable approach for engineering phage genomes using recombitrons: modified bacterial retrons3 that generate recombineering donor DNA paired with single stranded binding and annealing proteins to integrate those donors into phage genomes. This system can efficiently create genome modifications in multiple phages without the need for counterselection. Moreover, the process is continuous, with edits accumulating in the phage genome the longer the phage is cultured with the host, and multiplexable, with different editing hosts contributing distinct mutations along the genome of a phage in a mixed culture. In lambda phage, as an example, recombitrons yield single-base substitutions at up to 99% efficiency and up to 5 distinct mutations installed on a single phage genome, all without counterselection and only a few hours of hands-on time.

Application of CRISPR Cas systems in DNA recorders and writers

The necessity to record and store biological data is increasing in due course of time. However, it is quite difficult to understand biological mechanisms and keep a track of these events in some storage mediums. DNA (deoxyribonucleic acid) is the best candidate for the storage of cellular events in the biological system. It is energy efficient as well as stable at the same time. DNA-based writers and memory devices are continually evolving and finding new avenues in terms of their wide range of applications. Among all the DNA-based storage devices that employ enzymes like recombinases, nucleases, integrases, and polymerases, one of the most popular tools used for these devices is the emerging and versatile CRISPR Cas technology. CRISPR Cas is a prokaryotic immune system that keeps a memory of viral attacks and protects prokaryotes from potential future infections. The main aim of this short review is to study such molecular recorders and writers that employ CRISPR Cas technologies and obtain an in-depth overview of the mechanisms involved and the applications of these molecular devices.

Molecular recording: transcriptional data collection into the genome

Advances in regenerative medicine depend upon understanding the complex transcriptional choreography that guides cellular development. Transcriptional molecular recorders, tools that record different transcriptional events into the genome of cells, hold promise to elucidate both the intensity and timing of transcriptional activity at single-cell resolution without requiring destructive multitime point assays. These technologies are dependent on DNA writers, which translate transcriptional signals into stable genomic mutations that encode the duration, intensity, and order of transcriptional events. In this review, we highlight recent progress toward more informative and multiplexable transcriptional recording through the use of three different types of DNA writing - recombineering, Cas1-Cas2 acquisition, and prime editing - and the architecture of the genomic data generated.

Emerging Approaches to DNA Data Storage: Challenges and Prospects

With the total amount of worldwide data skyrocketing, the global data storage demand is predicted to grow to 1.75 × 10¹⁴ GB by 2025. Traditional storage methods have difficulties keeping pace given that current storage media have a maximum density of 10³ GB/mm³. As such, data production will far exceed the capacity of currently available storage methods. The costs of maintaining and transferring data, as well as the limited lifespans and significant data losses associated with current technologies also demand advanced solutions for information storage. Nature offers a powerful alternative through the storage of information that defines living organisms in unique orders of four bases (A, T, C, G) located in molecules called deoxyribonucleic acid (DNA). DNA molecules as information carriers have many advantages over traditional storage media. Their high storage density, potentially low maintenance cost, ease of synthesis, and chemical modification make them an ideal alternative for information storage. To this end, rapid progress has been made over the past decade by exploiting user-defined DNA materials to encode information. In this review, we discuss the most recent advances of DNA-based data storage with a major focus on the challenges that remain in this promising field, including the current intrinsic low speed in data writing and reading and the high cost per byte stored. Alternatively, data storage relying on DNA nanostructures (as opposed to DNA sequence) as well as on other combinations of nanomaterials and biomolecules are proposed with promising technological and economic advantages. In summarizing the advances that have been made and underlining the challenges that remain, we provide a roadmap for the ongoing research in this rapidly growing field, which will enable the development of technological solutions to the global demand for superior storage methodologies.

Multiplexed engineering and precision gene editing in cellular immunotherapy

The advent of cellular immunotherapy in the clinic has entirely redrawn the treatment landscape for a growing number of human cancers. Genetically reprogrammed immune cells, including chimeric antigen receptor (CAR)-modified immune effector cells as well as T cell receptor (TCR) therapy, have demonstrated remarkable responses across different hard-to-treat patient populations. While these novel treatment options have had tremendous success in providing long-term remissions for a considerable fraction of treated patients, a number of challenges remain. Limited in vivo persistence and functional exhaustion of infused immune cells as well as tumor immune escape and on-target off-tumor toxicities are just some examples of the challenges which restrain the potency of today's genetically engineered cell products. Multiple engineering strategies are being explored to tackle these challenges.The advent of multiplexed precision genome editing has in recent years provided a flexible and highly modular toolkit to specifically address some of these challenges by targeted genetic interventions. This class of next-generation cellular therapeutics aims to endow engineered immune cells with enhanced functionality and shield them from immunosuppressive cues arising from intrinsic immune checkpoints as well as the hostile tumor microenvironment (TME). Previous efforts to introduce additional genetic modifications into immune cells have in large parts focused on nuclease-based tools like the CRISPR/Cas9 system or TALEN. However, nuclease-inactive platforms including base and prime editors have recently emerged and promise a potentially safer route to rewriting genetic sequences and introducing large segments of transgenic DNA without inducing double-strand breaks (DSBs). In this review, we discuss how these two exciting and emerging fields-cellular immunotherapy and precision genome editing-have co-evolved to enable a dramatic expansion in the possibilities to engineer personalized anti-cancer treatments. We will lay out how various engineering strategies in addition to nuclease-dependent and nuclease-inactive precision genome editing toolkits are increasingly being applied to overcome today's limitations to build more potent cellular therapeutics. We will reflect on how novel information-rich unbiased discovery approaches are continuously deepening our understanding of fundamental mechanisms governing tumor biology. We will conclude with a perspective of how multiplexed-engineered and gene edited cell products may upend today's treatment paradigms as they evolve into the next generation of more potent cellular immunotherapies.

Biomolecule-Based Optical Metamaterials: Design and Applications

Metamaterials are broadly defined as artificial, electromagnetically homogeneous structures that exhibit unusual physical properties that are not present in nature. They possess extraordinary capabilities to bend electromagnetic waves. Their size, shape and composition can be engineered to modify their characteristics, such as iridescence, color shift, absorbance at different wavelengths, etc., and harness them as biosensors. Metamaterial construction from biological sources such as carbohydrates, proteins and nucleic acids represents a low-cost alternative, rendering high quantities and yields. In addition, the malleability of these biomaterials makes it possible to fabricate an endless number of structured materials such as composited nanoparticles, biofilms, nanofibers, quantum dots, and many others, with very specific, invaluable and tremendously useful optical characteristics. The intrinsic characteristics observed in biomaterials make them suitable for biomedical applications. This review addresses the optical characteristics of metamaterials obtained from the major macromolecules found in nature: carbohydrates, proteins and DNA, highlighting their biosensor field use, and pointing out their physical properties and production paths.

High-efficiency retron-mediated single-stranded DNA production in plants

Retrons are a class of retroelements that produce multicopy single-stranded DNA (ssDNA) and participate in anti-phage defenses in bacteria. Retrons have been harnessed for the overproduction of ssDNA, genome engineering and directed evolution in bacteria, yeast and mammalian cells. Retron-mediated ssDNA production in plants could unlock their potential applications in plant biotechnology. For example, ssDNA can be used as a template for homology-directed repair (HDR) in several organisms. However, current gene editing technologies rely on the physical delivery of synthetic ssDNA, which limits their applications. Here, we demonstrated retron-mediated overproduction of ssDNA in Nicotiana benthamiana. Additionally, we tested different retron architectures for improved ssDNA production and identified a new retron architecture that resulted in greater ssDNA abundance. Furthermore, co-expression of the gene encoding the ssDNA-protecting protein VirE2 from Agrobacterium tumefaciens with the retron systems resulted in a 10.7-fold increase in ssDNA production in vivo. We also demonstrated clustered regularly interspaced short palindromic repeats-retron-coupled ssDNA overproduction and targeted HDR in N. benthamiana. Overall, we present an efficient approach for in vivo ssDNA production in plants, which can be harnessed for biotechnological applications. Graphical Abstract.

Optogenetic Control of Bacterial Expression by Red Light

In optogenetics, as in nature, sensory photoreceptors serve to control cellular processes by light. Bacteriophytochrome (BphP) photoreceptors sense red and far-red light via a biliverdin chromophore and, in response, cycle between the spectroscopically, structurally, and functionally distinct Pr and Pfr states. BphPs commonly belong to two-component systems that control the phosphorylation of cognate response regulators and downstream gene expression through histidine kinase modules. We recently demonstrated that the paradigm BphP from Deinococcus radiodurans exclusively acts as a phosphatase but that its photosensory module can control the histidine kinase activity of homologous receptors. Here, we apply this insight to reprogram two widely used setups for bacterial gene expression from blue-light to red-light control. The resultant pREDusk and pREDawn systems allow gene expression to be regulated down and up, respectively, uniformly under red light by 100-fold or more. Both setups are realized as portable, single plasmids that encode all necessary components including the biliverdin-producing machinery. The triggering by red light affords high spatial resolution down to the single-cell level. As pREDusk and pREDawn respond sensitively to red light, they support multiplexing with optogenetic systems sensitive to other light colors. Owing to the superior tissue penetration of red light, the pREDawn system can be triggered at therapeutically safe light intensities through material layers, replicating the optical properties of the skin and skull. Given these advantages, pREDusk and pREDawn enable red-light-regulated expression for diverse use cases in bacteria.

Light-Dependent Control of Bacterial Expression at the mRNA Level

Sensory photoreceptors mediate numerous light-dependent adaptations across organisms. In optogenetics, photoreceptors achieve the reversible, non-invasive, and spatiotemporally precise control by light of gene expression and other cellular processes. The light-oxygen-voltage receptor PAL binds to small RNA aptamers with sequence specificity upon blue-light illumination. By embedding the responsive aptamer in the ribosome-binding sequence of genes of interest, their expression can be downregulated by light. We developed the pCrepusculo and pAurora optogenetic systems that are based on PAL and allow to down- and upregulate, respectively, bacterial gene expression using blue light. Both systems are realized as compact, single plasmids that exhibit stringent blue-light responses with low basal activity and up to several 10-fold dynamic range. As PAL exerts light-dependent control at the RNA level, it can be combined with other optogenetic circuits that control transcription initiation. By integrating regulatory mechanisms operating at the DNA and mRNA levels, optogenetic circuits with emergent properties can thus be devised. As a case in point, the pEnumbra setup permits to upregulate gene expression under moderate blue light whereas strong blue light shuts off expression again. Beyond providing novel signal-responsive expression systems for diverse applications in biotechnology and synthetic biology, our work also illustrates how the light-dependent PAL-aptamer interaction can be harnessed for the control and interrogation of RNA-based processes.

Light-regulated gene expression in Bacteria: Fundamentals, advances, and perspectives

Numerous photoreceptors and genetic circuits emerged over the past two decades and now enable the light-dependent i.e., optogenetic, regulation of gene expression in bacteria. Prompted by light cues in the near-ultraviolet to near-infrared region of the electromagnetic spectrum, gene expression can be up- or downregulated stringently, reversibly, non-invasively, and with precision in space and time. Here, we survey the underlying principles, available options, and prominent examples of optogenetically regulated gene expression in bacteria. While transcription initiation and elongation remain most important for optogenetic intervention, other processes e.g., translation and downstream events, were also rendered light-dependent. The optogenetic control of bacterial expression predominantly employs but three fundamental strategies: light-sensitive two-component systems, oligomerization reactions, and second-messenger signaling. Certain optogenetic circuits moved beyond the proof-of-principle and stood the test of practice. They enable unprecedented applications in three major areas. First, light-dependent expression underpins novel concepts and strategies for enhanced yields in microbial production processes. Second, light-responsive bacteria can be optogenetically stimulated while residing within the bodies of animals, thus prompting the secretion of compounds that grant health benefits to the animal host. Third, optogenetics allows the generation of precisely structured, novel biomaterials. These applications jointly testify to the maturity of the optogenetic approach and serve as blueprints bound to inspire and template innovative use cases of light-regulated gene expression in bacteria. Researchers pursuing these lines can choose from an ever-growing, versatile, and efficient toolkit of optogenetic circuits.

SURE editing: combining oligo-recombineering and programmable insertion/deletion of selection markers to efficiently edit the Mycoplasma pneumoniae genome

The development of advanced genetic tools is boosting microbial engineering which can potentially tackle wide-ranging challenges currently faced by our society. Here we present SURE editing, a multi-recombinase engineering rationale combining oligonucleotide recombineering with the selective capacity of antibiotic resistance via transient insertion of selector plasmids. We test this method in Mycoplasma pneumoniae, a bacterium with a very inefficient native recombination machinery. Using SURE editing, we can seamlessly generate, in a single step, a wide variety of genome modifications at high efficiencies, including the largest possible deletion of this genome (30 Kb) and the targeted complementation of essential genes in the deletion of a region of interest. Additional steps can be taken to remove the selector plasmid from the edited area, to obtain markerless or even scarless edits. Of note, SURE editing is compatible with different site-specific recombinases for mediating transient plasmid integration. This battery of selector plasmids can be used to select different edits, regardless of the target sequence, which significantly reduces the cloning load associated to genome engineering projects. Given the proven functionality in several microorganisms of the machinery behind the SURE editing logic, this method is likely to represent a valuable advance for the synthetic biology field.

Synthetic neuromorphic computing in living cells

Computational properties of neuronal networks have been applied to computing systems using simplified models comprising repeated connected nodes, e.g., perceptrons, with decision-making capabilities and flexible weighted links. Analogously to their revolutionary impact on computing, neuro-inspired models can transform synthetic gene circuit design in a manner that is reliable, efficient in resource utilization, and readily reconfigurable for different tasks. To this end, we introduce the perceptgene, a perceptron that computes in the logarithmic domain, which enables efficient implementation of artificial neural networks in Escherichia coli cells. We successfully modify perceptgene parameters to create devices that encode a minimum, maximum, and average of analog inputs. With these devices, we create multi-layer perceptgene circuits that compute a soft majority function, perform an analog-to-digital conversion, and implement a ternary switch. We also create a programmable perceptgene circuit whose computation can be modified from OR to AND logic using small molecule induction. Finally, we show that our approach enables circuit optimization via artificial intelligence algorithms.

Prebiotics-Controlled Disposable Engineered Bacteria for Intestinal Diseases

As a new method of diagnosis and treatment for intestinal diseases, intelligent engineered bacteria based on synthetic biology have been developed vigorously in recent years. However, how to deal with the engineered bacteria in vivo after completing the tasks is an urgent problem to be resolved. In this study, we constructed a thiosulfate (a biomarker of inflammatory bowel disease)-responsive engineered bacteria to generate two signals, sfGFP (monitoring) and gain-of-function (translation activation) mutation (ACG to ATG), in the initiation codon of lysisE (recording) via the CRISPR/Cas9-mediated base editing system. Once these two signals were detected, xylose could be added to induce lysis E expression, resulting in the destruction of the edited bacteria and the release of AvCystain simultaneously. Overall, our innovative engineered bacteria can record instant and historical information of the disease, and especially, the edited bacteria can be artificially attenuated and release drug in situ when needed, ultimately serving as a disposable and recyclable candidate for more types of diseases.

A Framework for the Systematic Selection of Biosensor Chassis for Environmental Synthetic Biology

Microbial biosensors sense and report exposures to stimuli, thereby facilitating our understanding of environmental processes. Successful design and deployment of biosensors hinge on the persistence of the microbial host of the genetic circuit, termed the chassis. However, model chassis organisms may persist poorly in environmental conditions. In contrast, non-model organisms persist better in environmental conditions but are limited by other challenges, such as genetic intractability and part unavailability. Here we identify ecological, metabolic, and genetic constraints for chassis development and propose a conceptual framework for the systematic selection of environmental biosensor chassis. We identify key challenges with using current model chassis and delineate major points of conflict in choosing the most suitable organisms as chassis for environmental biosensing. This framework provides a way forward in the selection of biosensor chassis for environmental synthetic biology.

Achieving spatially precise diagnosis and therapy in the mammalian gut using synthetic microbial gene circuits

The mammalian gut and its microbiome form a temporally dynamic and spatially heterogeneous environment. The inaccessibility of the gut and the spatially restricted nature of many gut diseases translate into difficulties in diagnosis and therapy for which novel tools are needed. Engineered bacterial whole-cell biosensors and therapeutics have shown early promise at addressing these challenges. Natural and engineered sensing systems can be repurposed in synthetic genetic circuits to detect spatially specific biomarkers during health and disease. Heat, light, and magnetic signals can also activate gene circuit function with externally directed spatial precision. The resulting engineered bacteria can report on conditions in situ within the complex gut environment or produce biotherapeutics that specifically target host or microbiome activity. Here, we review the current approaches to engineering spatial precision for in vivo bacterial diagnostics and therapeutics using synthetic circuits, and the challenges and opportunities this technology presents.

Cryo-EM structures of Escherichia coli Ec86 retron complexes reveal architecture and defence mechanism

First discovered in the 1980s, retrons are bacterial genetic elements consisting of a reverse transcriptase and a non-coding RNA (ncRNA). Retrons mediate antiphage defence in bacteria but their structure and defence mechanisms are unknown. Here, we investigate the Escherichia coli Ec86 retron and use cryo-electron microscopy to determine the structures of the Ec86 (3.1 Å) and cognate effector-bound Ec86 (2.5 Å) complexes. The Ec86 reverse transcriptase exhibits a characteristic right-hand-like fold consisting of finger, palm and thumb subdomains. Ec86 reverse transcriptase reverse-transcribes part of the ncRNA into satellite, multicopy single-stranded DNA (msDNA, a DNA-RNA hybrid) that we show wraps around the reverse transcriptase electropositive surface. In msDNA, both inverted repeats are present and the 3' sides of the DNA/RNA chains are close to the reverse transcriptase active site. The Ec86 effector adopts a two-lobe fold and directly binds reverse transcriptase and msDNA. These findings offer insights into the structure-function relationship of the retron-effector unit and provide a structural basis for the optimization of retron-based genome editing systems.

Live-seq enables temporal transcriptomic recording of single cells

Single-cell transcriptomics (scRNA-seq) has greatly advanced our ability to characterize cellular heterogeneity1. However, scRNA-seq requires lysing cells, which impedes further molecular or functional analyses on the same cells. Here, we established Live-seq, a single-cell transcriptome profiling approach that preserves cell viability during RNA extraction using fluidic force microscopy2,3, thus allowing to couple a cell's ground-state transcriptome to its downstream molecular or phenotypic behaviour. To benchmark Live-seq, we used cell growth, functional responses and whole-cell transcriptome read-outs to demonstrate that Live-seq can accurately stratify diverse cell types and states without inducing major cellular perturbations. As a proof of concept, we show that Live-seq can be used to directly map a cell's trajectory by sequentially profiling the transcriptomes of individual macrophages before and after lipopolysaccharide (LPS) stimulation, and of adipose stromal cells pre- and post-differentiation. In addition, we demonstrate that Live-seq can function as a transcriptomic recorder by preregistering the transcriptomes of individual macrophages that were subsequently monitored by time-lapse imaging after LPS exposure. This enabled the unsupervised, genome-wide ranking of genes on the basis of their ability to affect macrophage LPS response heterogeneity, revealing basal Nfkbia expression level and cell cycle state as important phenotypic determinants, which we experimentally validated. Thus, Live-seq can address a broad range of biological questions by transforming scRNA-seq from an end-point to a temporal analysis approach.

Active genetics comes alive: Exploring the broad applications of CRISPR-based selfish genetic elements (or gene-drives): Exploring the broad applications of CRISPR-based selfish genetic elements (or gene-drives)

Clustered Regularly Interspaced Short Palindromic Repeats (CRISPR)-based "active genetic" elements developed in 2015 bypassed the fundamental rules of traditional genetics. Inherited in a super-Mendelian fashion, such selfish genetic entities offered a variety of potential applications including: gene-drives to disseminate gene cassettes carrying desired traits throughout insect populations to control disease vectors or pest species, allelic drives biasing inheritance of preferred allelic variants, neutralizing genetic elements to delete and replace or to halt the spread of gene-drives, split-drives with the core constituent Cas9 endonuclease and guide RNA (gRNA) components inserted at separate genomic locations to accelerate assembly of complex arrays of genetic traits or to gain genetic entry into novel organisms (vertebrates, plants, bacteria), and interhomolog based copying systems in somatic cells to develop tools for treating inherited or infectious diseases. Here, we summarize the substantial advances that have been made on all of these fronts and look forward to the next phase of this rapidly expanding and impactful field.

A time-resolved, multi-symbol molecular recorder via sequential genome editing

DNA is naturally well suited to serve as a digital medium for in vivo molecular recording. However, contemporary DNA-based memory devices are constrained in terms of the number of distinct 'symbols' that can be concurrently recorded and/or by a failure to capture the order in which events occur1. Here we describe DNA Typewriter, a general system for in vivo molecular recording that overcomes these and other limitations. For DNA Typewriter, the blank recording medium ('DNA Tape') consists of a tandem array of partial CRISPR-Cas9 target sites, with all but the first site truncated at their 5' ends and therefore inactive. Short insertional edits serve as symbols that record the identity of the prime editing guide RNA2 mediating the edit while also shifting the position of the 'type guide' by one unit along the DNA Tape, that is, sequential genome editing. In this proof of concept of DNA Typewriter, we demonstrate recording and decoding of thousands of symbols, complex event histories and short text messages; evaluate the performance of dozens of orthogonal tapes; and construct 'long tape' potentially capable of recording as many as 20 serial events. Finally, we leverage DNA Typewriter in conjunction with single-cell RNA-seq to reconstruct a monophyletic lineage of 3,257 cells and find that the Poisson-like accumulation of sequential edits to multicopy DNA tape can be maintained across at least 20 generations and 25 days of in vitro clonal expansion.

Recording gene expression order in DNA by CRISPR addition of retron barcodes

Biological processes depend on the differential expression of genes over time, but methods to make physical recordings of these processes are limited. Here we report a molecular system for making time-ordered recordings of transcriptional events into living genomes. We do this through engineered RNA barcodes, based on prokaryotic retrons¹, that are reverse transcribed into DNA and integrated into the genome using the CRISPR-Cas system². The unidirectional integration of barcodes by CRISPR integrases enables reconstruction of transcriptional event timing based on a physical record through simple, logical rules rather than relying on pretrained classifiers or post hoc inferential methods. For disambiguation in the field, we will refer to this system as a Retro-Cascorder.

Preservation and Encryption in DNA Digital Data Storage

The exponential growth of the total amount of global data presents a huge challenge to mainstream storage media. The emergence of molecular digital storage inspires the development of the new-generation higher-density digital data storage. In particular, DNA with high storage density, reproducibility, and long recoverable lifetime behaves the ideal representative of molecular digital storage media. With the development of DNA synthesis and sequencing technologies and the reduction of cost, DNA digital storage has attracted more and more attention and achieved significant breakthroughs. Herein, this Review briefly describes the workflow of DNA storage, and highlights the storage step of DNA digital data storage. Then, according to different information storage forms, the current DNA information encryption methods are emphatically expounded. Finally, the brief perspectives on the current challenges and optimizing proposals in DNA information preservation and encryption are presented.

Engineered Living Hydrogels

Living biological systems, ranging from single cells to whole organisms, can sense, process information, and actuate in response to changing environmental conditions. Inspired by living biological systems, engineered living cells and nonliving matrices are brought together, which gives rise to the technology of engineered living materials. By designing the functionalities of living cells and the structures of nonliving matrices, engineered living materials can be created to detect variability in the surrounding environment and to adjust their functions accordingly, thereby enabling applications in health monitoring, disease treatment, and environmental remediation. Hydrogels, a class of soft, wet, and biocompatible materials, have been widely used as matrices for engineered living cells, leading to the nascent field of engineered living hydrogels. Here, the interactions between hydrogel matrices and engineered living cells are described, focusing on how hydrogels influence cell behaviors and how cells affect hydrogel properties. The interactions between engineered living hydrogels and their environments, and how these interactions enable versatile applications, are also discussed. Finally, current challenges facing the field of engineered living hydrogels for their applications in clinical and environmental settings are highlighted.

Scalable biological signal recording in mammalian cells using Cas12a base editors

Biological signal recording enables the study of molecular inputs experienced throughout cellular history. However, current methods are limited in their ability to scale up beyond a single signal in mammalian contexts. Here, we develop an approach using a hyper-efficient dCas12a base editor for multi-signal parallel recording in human cells. We link signals of interest to expression of guide RNAs to catalyze specific nucleotide conversions as a permanent record, enabled by Cas12's guide-processing abilities. We show this approach is plug-and-play with diverse biologically relevant inputs and extend it for more sophisticated applications, including recording of time-delimited events and history of chimeric antigen receptor T cells' antigen exposure. We also demonstrate efficient recording of up to four signals in parallel on an endogenous safe-harbor locus. This work provides a versatile platform for scalable recording of signals of interest for a variety of biological applications.

DNA nanotechnology-empowered finite state machines

A finite state machine (FSM, or automaton) is an abstract machine that can switch among a finite number of states in response to temporally ordered inputs, which allows storage and processing of information in an order-sensitive manner. In recent decades, DNA molecules have been actively exploited to develop information storage and nanoengineering materials, which hold great promise for smart nanodevices and nanorobotics under the framework of FSM. In this review, we summarize recent progress in utilizing DNA self-assembly and DNA nanostructures to implement FSMs. We describe basic principles for representative DNA FSM prototypes and highlight their advantages and potential in diverse applications. The challenges in this field and future directions have also been discussed.

Efficient and Iterative Retron-Mediated in vivo Recombineering in E. coli

Recombineering is an important tool in gene editing, enabling fast, precise and highly specific in vivo modification of microbial genomes. Oligonucleotide-mediated recombineering via the in vivo production of single-stranded DNA can overcome the limitations of traditional recombineering methods that rely on the exogenous delivery of editing templates. By modifying a previously reported plasmid-based system for fully in vivo single-stranded DNA recombineering, we demonstrate iterative editing of independent loci by utilizing a temperature-sensitive origin of replication for easy curing of the editing plasmid from recombinant cells. Optimization of the promoters driving the expression of the system’s functional components, combined with targeted counterselection against unedited cells with Cas9 nuclease, enabled editing efficiencies of 90–100%. The addition of a dominant-negative mutL allele to the system allowed single-nucleotide edits that were otherwise unachievable due to mismatch repair. Finally, we tested alternative recombinases and found that efficiency significantly increased for some targets. Requiring only a single cloning step for retargeting, our system provides an easy-to-use method for rapid, efficient construction of desired mutants.

Graphical Abstract

Dichotomous feedback: a signal sequestration-based feedback mechanism for biocontroller design

We introduce a new design framework for implementing negative feedback regulation in synthetic biology, which we term ‘dichotomous feedback’. Our approach is different from current methods, in that it sequesters existing fluxes in the process to be controlled, and in this way takes advantage of the process’s architecture to design the control law. This signal sequestration mechanism appears in many natural biological systems and can potentially be easier to realize than ‘molecular sequestration’ and other comparison motifs that are nowadays common in biomolecular feedback control design. The loop is closed by linking the strength of signal sequestration to the process output. Our feedback regulation mechanism is motivated by two-component signalling systems, where a second response regulator could be competing with the natural response regulator thus sequestering kinase activity. Here, dichotomous feedback is established by increasing the concentration of the second response regulator as the level of the output of the natural process increases. Extensive analysis demonstrates how this type of feedback shapes the signal response, attenuates intrinsic noise while increasing robustness and reducing crosstalk.

DNA Input Classification by a Riboregulator-Based Cell-Free Perceptron

The ability to recognize molecular patterns is essential for the continued survival of biological organisms, allowing them to sense and respond to their immediate environment. The design of synthetic gene-based classifiers has been explored previously; however, prior strategies have focused primarily on DNA strand-displacement reactions. Here, we present a synthetic in vitro transcription and translation (TXTL)-based perceptron consisting of a weighted sum operation (WSO) coupled to a downstream thresholding function. We demonstrate the application of toehold switch riboregulators to construct a TXTL-based WSO circuit that converts DNA inputs into a GFP output, the concentration of which correlates to the input pattern and the corresponding weights. We exploit the modular nature of the WSO circuit by changing the output protein to the Escherichia coli σ28-factor, facilitating the coupling of the WSO output to a downstream reporter network. The subsequent introduction of a σ28 inhibitor enabled thresholding of the WSO output such that the expression of the downstream reporter protein occurs only when the produced σ28 exceeds this threshold. In this manner, we demonstrate a genetically implemented perceptron capable of binary classification, i.e., the expression of a single output protein only when the desired minimum number of inputs is exceeded.