Multiplex navigation of global regulatory networks (MINR) in yeast for improved ethanol tolerance and production.

Multiplex navigation of global regulatory networks (MINR) is an approach for combinatorially reprogramming gene expression to manipulate complex phenotypes. We designed, constructed, and mapped MINR libraries containing 43,020 specific mutations in 25 regulatory genes expected to perturb the yeast regulatory network. We selected growth competition experiments for library mutants conferring increased ethanol and/or glucose tolerance. We identified specific mutants that not only possessed improved ethanol and/or glucose tolerance but also produced ethanol at concentrations up to 2-fold higher than those produced by the wild-type strain. We further determined that mutations increasing ethanol tolerance were transferable to a diploid industrial yeast strain. The facile construction and mapping of 43,020 designer regulatory mutations provide a roadmap for how to access and engineer complex phenotypes in future synthetic biology and broader efforts.

Deep scanning lysine metabolism in Escherichia coli.

Our limited ability to predict genotype-phenotype relationships has called for strategies that allow testing of thousands of hypotheses in parallel. Deep scanning mutagenesis has been successfully implemented to map genotype-phenotype relationships at a single-protein scale, allowing scientists to elucidate properties that are difficult to predict. However, most phenotypes are dictated by several proteins that are interconnected through complex and robust regulatory and metabolic networks. These sophisticated networks hinder our understanding of the phenotype of interest and limit our capabilities to rewire cellular functions. Here, we leveraged CRISPR-EnAbled Trackable genome Engineering to attempt a parallel and high-resolution interrogation of complex networks, deep scanning multiple proteins associated with lysine metabolism in Escherichia coli We designed over 16,000 mutations to perturb this pathway and mapped their contribution toward resistance to an amino acid analog. By doing so, we identified different routes that can alter pathway function and flux, uncovering mechanisms that would be difficult to rationally design. This approach sets a framework for forward investigation of complex multigenic phenotypes.

Directed combinatorial mutagenesis of Escherichia coli for complex phenotype engineering.

Strain engineering for industrial production requires a targeted improvement of multiple complex traits, which range from pathway flux to tolerance to mixed sugar utilization. Here, we report the use of an iterative CRISPR EnAbled Trackable genome Engineering (iCREATE) method to engineer rapid glucose and xylose co-consumption and tolerance to hydrolysate inhibitors in E. coli. Deep mutagenesis libraries were rationally designed, constructed, and screened to target ~40,000 mutations across 30 genes. These libraries included global and high-level regulators that regulate global gene expression, transcription factors that play important roles in genome-level transcription, enzymes that function in the sugar transport system, NAD(P)H metabolism, and the aldehyde reduction system. Specific mutants that conferred increased growth in mixed sugars and hydrolysate tolerance conditions were isolated, confirmed, and evaluated for changes in genome-wide expression levels. We tested the strain with positive combinatorial mutations for 3-hydroxypropionic acid (3HP) production under high furfural and high acetate hydrolysate fermentation, which demonstrated a 7- and 8-fold increase in 3HP productivity relative to the parent strain, respectively.

Iterative genome editing of Escherichia coli for 3-hydroxypropionic acid production.

Synthetic biology requires strategies for the targeted, efficient, and combinatorial engineering of biological sub-systems at the molecular level. Here, we report the use of the iterative CRISPR EnAbled Trackable genome Engineering (iCREATE) method for the rapid construction of combinatorially modified genomes. We coupled this genome engineering strategy with high-throughput phenotypic screening and selections to recursively engineer multiple traits in Escherichia coli for improved production of the platform chemical 3-hydroxypropionic acid (3HP). Specifically, we engineered i) central carbon metabolism, ii) 3HP synthesis, and (iii) 3HP tolerance through design, construction and testing of ~ 162,000 mutations across 115 genes spanning global regulators, transcription factors, and enzymes involved in 3HP synthesis and tolerance. The iCREATE process required ~ 1 month to perform 13 rounds of combinatorial genome modifications with targeted gene knockouts, expression modification by ribosomal binding site (RBS) engineering, and genome-level site-saturation mutagenesis. Specific mutants conferring increased 3HP titer, yield, and productivity were identified and then combined to produce 3HP at a yield and concentration ~ 60-fold higher than the wild-type strain.

CRISPR EnAbled Trackable genome Engineering for isopropanol production in Escherichia coli.

Isopropanol is an important target molecule for sustainable production of fuels and chemicals. Increases in DNA synthesis and synthetic biology capabilities have resulted in the development of a range of new strategies for the more rapid design, construction, and testing of production strains. Here, we report on the use of such capabilities to construct and test 903 different variants of the isopropanol production pathway in Escherichia coli. We first constructed variants to explore the effect of codon optimization, copy number, and translation initiation rates on isopropanol production. The best strain (PA06) produced isopropanol at titers of 17.5g/L, with a yield of 0.62 (mol/mol), and maximum productivity of 0.40g/L/h. We next integrated the isopropanol synthetic pathway into the genome and then used the CRISPR EnAbled Trackable genome Engineering (CREATE) strategy to generate an additional 640 individual RBS library variants for further evaluation. After testing each of these variants, we constructed a combinatorial library containing 256 total variants from four different RBS levels for each gene. The best producing variant, PA14, produced isopropanol at titers of 7.1g/L at 24h, with a yield of 0.75 (mol/mol), and maximum productivity of 0.62g/L/h (which was 0.22g/L/h above the parent strain PA07). We demonstrate the ability to rapidly construct and test close to ~1000 designer strains and identify superior performers.

Single-molecule studies of the lysine riboswitch reveal effector-dependent conformational dynamics of the aptamer domain.

The lysine riboswitch is a cis-acting RNA genetic regulatory element found in the leader sequence of bacterial mRNAs coding for proteins related to biosynthesis or transport of lysine. Structural analysis of the lysine-binding aptamer domain of this RNA has revealed that it completely encapsulates the ligand and therefore must undergo a structural opening/closing upon interaction with lysine. In this work, single-molecule fluorescence resonance energy transfer (FRET) methods are used to monitor these ligand-induced structural transitions that are central to lysine riboswitch function. Specifically, a model FRET system has been developed for characterizing the lysine dissociation constant as well as the opening/closing rate constants for the Bacillus subtilis lysC aptamer domain. These techniques permit measurement of the dissociation constant (K(D)) for lysine binding of 1.7(5) mM and opening/closing rate constants of 1.4(3) s(-1) and 0.203(7) s(-1), respectively. These rates predict an apparent dissociation constant for lysine binding (K(D,apparent)) of 0.25(9) mM at near physiological ionic strength, which differs markedly from previous reports.

A switch in time: detailing the life of a riboswitch.

Riboswitches are non-protein coding RNA elements typically found in the 5' untranslated region (5'-UTR) of mRNAs that utilize metabolite binding to control expression of their own transcript. The RNA-ligand interaction causes conformational changes in the RNA that direct the cotranscriptional folding of a downstream secondary structural switch that interfaces with the expression machinery. This review describes the structural themes common to the different RNA-metabolite complexes studied to date and conclusions that can be made regarding how these RNAs efficiently couple metabolite binding to gene regulation. Emphasis is placed on the temporal aspects of riboswitch regulation that are central to the function of these RNAs and the need to augment the wealth of data on metabolite receptor domains with further studies on the full regulatory element, particularly in the context of transcription.

Small molecule regulated sgRNAs enable control of genome editing in E. coli by Cas9.

CRISPR-Cas9 has led to great advances in gene editing for a broad spectrum of applications. To further the utility of Cas9 there have been efforts to achieve temporal control over its nuclease activity. While different approaches have focused on regulation of CRISPR interference or editing in mammalian cells, none of the reported methods enable control of the nuclease activity in bacteria. Here, we develop RNA linkers to combine theophylline- and 3-methylxanthine (3MX)-binding aptamers with the sgRNA, enabling small molecule-dependent editing in Escherichia coli. These activatable guide RNAs enable temporal and post-transcriptional control of in vivo gene editing. Further, they reduce the death of host cells caused by cuts in the genome, a major limitation of CRISPR-mediated bacterial recombineering.

Integrating CRISPR-Enabled Trackable Genome Engineering and Transcriptomic Analysis of Global Regulators for Antibiotic Resistance Selection and Identification in Escherichia coli.

It is important to expedite our understanding of antibiotic resistance to address the increasing numbers of fatalities and environmental pollution due to the emergence of antibiotic resistance and multidrug-resistant strains. Here, we combined the CRISPR-enabled trackable genome engineering (CREATE) technology and transcriptomic analysis to investigate antibiotic tolerance in Escherichia coli We developed rationally designed site saturation mutagenesis libraries targeting 23 global regulators to identify fitness-conferring mutations in response to diverse antibiotic stresses. We identified seven novel mutations that confer resistance to the ribosome-targeting antibiotics doxycycline, thiamphenicol, and gentamicin in E. coli To the best of our knowledge, these mutations that we identified have not been reported previously during treatment with the indicated antibiotics. Transcriptome sequencing-based transcriptome analysis was further employed to evaluate the genome-wide changes in gene expression in E. coli for SoxR G121P and cAMP receptor protein (CRP) V140W reconstructions, and improved fitness in response to doxycycline and gentamicin was seen. In the case of doxycycline, we speculated that SoxR G121P significantly increased the expression of genes involved in carbohydrate metabolism and energy metabolism to promote cell growth for improved adaptation. In the CRP V140W mutant with improved gentamicin tolerance, the expression of several amino acid biosynthesis genes and fatty acid degradation genes was significantly changed, and these changes probably altered the cellular energy state to improve adaptation. These findings have important significance for understanding such nonspecific mechanisms of antibiotic resistance and developing new antibacterial drugs.IMPORTANCE The growing threat of antimicrobial resistance poses a serious threat to public health care and motivates efforts to understand the means by which resistance acquisition occurs and how this can be combatted. To address these challenges, we expedited the identification of novel mutations that enable complex phenotypic changes that result in improved tolerance to antibiotics by integrating CREATE and transcriptomic analysis of global regulators. The results give us a better understanding of the mechanisms of resistance to tetracycline antibiotics and aminoglycoside antibiotics and also indicate that the method may be used for quickly identifying resistance-related mutations.

Determining structures of RNA aptamers and riboswitches by X-ray crystallography.

Structural biology plays a central role in gaining a full understanding of the myriad roles of RNA in biology. In recent years, innovative approaches in RNA purification and crystallographic methods have lead to the visualization of an increasing number of unique structures, providing new insights into its function at the atomic level. This article presents general protocols which have streamlined the process of obtaining a homogeneous sample of properly folded and active RNA in high concentrations that crystallizes well in the presence of a suitable heavy-atom for phasing. Of particular importance are approaches toward RNA crystallography that include exploring "construct space" as opposed to "condition space". Moreover, development of a highly flexible method for experimentally phasing RNA crystals may open the door to a relatively simple means of solving these structures.

Genomic Deoxyxylulose Phosphate Reductoisomerase (DXR) Mutations Conferring Resistance to the Antimalarial Drug Fosmidomycin in E. coli.

Sequence to activity mapping technologies are rapidly developing, enabling the generation and isolation of mutations conferring novel phenotypes. Here we used the CRISPR enabled trackable genome engineering (CREATE) technology to investigate the inhibition of the essential ispC gene in its native genomic context in Escherichia coli. We created a full saturation library of 33 sites proximal to the ligand binding pocket and challenged this library with the antimalarial drug fosmidomycin, which targets the ispC gene product, DXR. This selection is especially challenging since it is relatively weak in E. coli, with multiple naturally occurring pathways for resistance. We identified several previously unreported mutations that confer fosmidomycin resistance, in highly conserved sites that also exist in pathogens including the malaria-inducing Plasmodium falciparum. This approach may have implications for the isolation of resistance-conferring mutations and may affect the design of future generations of fosmidomycin-based drugs.

Genome-wide mapping of mutations at single-nucleotide resolution for protein, metabolic and genome engineering.

Improvements in DNA synthesis and sequencing have underpinned comprehensive assessment of gene function in bacteria and eukaryotes. Genome-wide analyses require high-throughput methods to generate mutations and analyze their phenotypes, but approaches to date have been unable to efficiently link the effects of mutations in coding regions or promoter elements in a highly parallel fashion. We report that CRISPR-Cas9 gene editing in combination with massively parallel oligomer synthesis can enable trackable editing on a genome-wide scale. Our method, CRISPR-enabled trackable genome engineering (CREATE), links each guide RNA to homologous repair cassettes that both edit loci and function as barcodes to track genotype-phenotype relationships. We apply CREATE to site saturation mutagenesis for protein engineering, reconstruction of adaptive laboratory evolution experiments, and identification of stress tolerance and antibiotic resistance genes in bacteria. We provide preliminary evidence that CREATE will work in yeast. We also provide a webtool to design multiplex CREATE libraries.

Rapid and Efficient One-Step Metabolic Pathway Integration in E. coli.

Methods for importing heterologous genes into genetically tractable hosts are among the most desired tools of synthetic biology. Easy plug-and-play construction methods to rapidly test genes and pathways stably in the host genome would expedite synthetic biology and metabolic engineering applications. Here, we describe a CRISPR-based strategy that allows highly efficient, single step integration of large pathways in Escherichia coli. This strategy allows high efficiency integration in a broad range of homology arm sizes and genomic positions, with efficiencies ranging from 70 to 100% in 7 distinct loci. To demonstrate the large size capability, we integrated a 10 kb construct to implement isobutanol production in a single day. The ability to efficiently integrate entire metabolic pathways in a rapid and markerless manner will facilitate testing and engineering of novel pathways using the E. coli genome as a stable testing platform.

The emergence of commodity-scale genetic manipulation.

Since the 1970s technological advancements in the fields of synthetic biology and metabolic engineering have led to a dramatic reduction in both time and cost required for generating genomic mutations in a variety of organisms. The union of genomic editing machinery, DNA inkjet printers, and bioinformatics algorithms allows engineers to design a library of thousands of unique oligos as well as build and test these designs on a ∼2 months time-scale and at a cost of roughly ∼0.3 cents per base pair. The implications of these capabilities for a variety of fields are far-reaching, with potential impacts in defense, agricultural, human health, and environmental research. The explosion of synthetic biology applications over the past two decades have led many to draw parallels between biological engineering and the computer sciences. In this review, we highlight some important parallels between these fields and emphasize the importance of engineering design strategies.

Codon compression algorithms for saturation mutagenesis.

Saturation mutagenesis is employed in protein engineering and genome-editing efforts to generate libraries that span amino acid design space. Traditionally, this is accomplished by using degenerate/compressed codons such as NNK (N = A/C/G/T, K = G/T), which covers all amino acids and one stop codon. These solutions suffer from two types of redundancy: (a) different codons for the same amino acid lead to bias, and (b) wild type amino acid is included within the library. These redundancies increase library size and downstream screening efforts. Here, we present a dynamic approach to compress codons for any desired list of amino acids, taking into account codon usage. This results in a unique codon collection for every amino acid to be mutated, with the desired redundancy level. Finally, we demonstrate that this approach can be used to design precise oligo libraries amendable to recombineering and CRISPR-based genome editing to obtain a diverse population with high efficiency.

Genome-Wide Tuning of Protein Expression Levels to Rapidly Engineer Microbial Traits.

The reliable engineering of biological systems requires quantitative mapping of predictable and context-independent expression over a broad range of protein expression levels. However, current techniques for modifying expression levels are cumbersome and are not amenable to high-throughput approaches. Here we present major improvements to current techniques through the design and construction of E. coli genome-wide libraries using synthetic DNA cassettes that can tune expression over a ∼10(4) range. The cassettes also contain molecular barcodes that are optimized for next-generation sequencing, enabling rapid and quantitative tracking of alleles that have the highest fitness advantage. We show these libraries can be used to determine which genes and expression levels confer greater fitness to E. coli under different growth conditions.

Multiplexed tracking of combinatorial genomic mutations in engineered cell populations.

Multiplexed genome engineering approaches can be used to generate targeted genetic diversity in cell populations on laboratory timescales, but methods to track mutations and link them to phenotypes have been lacking. We present an approach for tracking combinatorial engineered libraries (TRACE) through the simultaneous mapping of millions of combinatorially engineered genomes at single-cell resolution. Distal genomic sites are assembled into individual DNA constructs that are compatible with next-generation sequencing strategies. We used TRACE to map growth selection dynamics for Escherichia coli combinatorial libraries created by recursive multiplex recombineering at a depth 10(4)-fold greater than before. TRACE was used to identify genotype-to-phenotype correlations and to map the evolutionary trajectory of two individual combinatorial mutants in E. coli. Combinatorial mutations in the human ES2 ovarian carcinoma cell line were also assessed with TRACE. TRACE completes the combinatorial engineering cycle and enables more sophisticated approaches to genome engineering in both bacteria and eukaryotic cells than are currently possible.

Modularity of select riboswitch expression platforms enables facile engineering of novel genetic regulatory devices.

RNA-based biosensors and regulatory devices have received significant attention for their potential in a broad array of synthetic biology applications. One of the primary difficulties in engineering these molecules is the lack of facile methods to link sensory modules, or aptamers, to readout domains. Such efforts typically require extensive screening or selection of sequences that facilitate interdomain communication. Bacteria have evolved a widespread form of gene regulation known as riboswitches that perform this task with sufficient fidelity to control expression of biosynthetic and transport proteins essential for normal cellular homeostasis. In this work, we demonstrate that select riboswitch readout domains, called expression platforms, are modular in that they can host a variety of natural and synthetic aptamers to create novel chimeric RNAs that regulate transcription both in vitro and in vivo. Importantly, this technique does not require selection of device-specific "communication modules" required to transmit ligand binding to the regulatory domain, enabling rapid engineering of novel functional RNAs.

Insights into the regulatory landscape of the lysine riboswitch.

A prevalent means of regulating gene expression in bacteria is by riboswitches found within mRNA leader sequences. Like protein repressors, these RNA elements must bind an effector molecule with high specificity against a background of other cellular metabolites of similar chemical structure to elicit the appropriate regulatory response. Current crystal structures of the lysine riboswitch do not provide a complete understanding of selectivity as recognition is substantially mediated through main-chain atoms of the amino acid. Using a directed set of lysine analogs and other amino acids, we have determined the relative contributions of the polar functional groups to binding affinity and the regulatory response. Our results reveal that the lysine riboswitch has >1000-fold specificity for lysine over other amino acids. The aptamer is highly sensitive to the precise placement of the ε-amino group and relatively tolerant of alterations to the main-chain functional groups in order to achieve this specificity. At low nucleotide triphosphate (NTP) concentrations, we observe good agreement between the half-maximal regulatory activity (T(50)) and the affinity of the receptor for lysine (K(d)), as well as many of its analogs. However, above 400 µM [NTP], the concentration of lysine required to elicit transcription termination rises, moving into the riboswitch into a kinetic control regime. These data demonstrate that, under physiologically relevant conditions, riboswitches can integrate both effector and NTP concentrations to generate a regulatory response appropriate for global metabolic state of the cell.

Riboswitches: structures and mechanisms.

A critical feature of the hypothesized RNA world would have been the ability to control chemical processes in response to environmental cues. Riboswitches present themselves as viable candidates for a sophisticated mechanism of regulatory control in RNA-based life. These regulatory elements in the modern world are most commonly found in the 5'-untranslated regions of bacterial mRNAs, directly interacting with metabolites as a means of regulating expression of the coding region via a secondary structural switch. In this review, we focus on recent insights into how these RNAs fold into complex architectures capable of both recognizing a specific small molecule compound and exerting regulatory control over downstream sequences, with an emphasis on transcriptional regulation.