Artificial repressors for controlling gene expression in bacteria

When push comes to shove - RNA polymerase and DNA-bound protein roadblocks

In recent years, transcriptional roadblocking has emerged as a crucial regulatory mechanism in gene expression, whereby other DNA-bound obstacles can block the progression of transcribing RNA polymerase (RNAP), leading to RNAP pausing and ultimately dissociation from the DNA template. In this review, we discuss the mechanisms by which transcriptional roadblocks can impede RNAP progression, as well as how RNAP can overcome these obstacles to continue transcription. We examine different DNA-binding proteins involved in transcriptional roadblocking and their biophysical properties that determine their effectiveness in blocking RNAP progression. The catalytically dead CRISPR-Cas (dCas) protein is used as an example of an engineered programmable roadblock, and the current literature in understanding the polarity of dCas roadblocking is also discussed. Finally, we delve into a stochastic model of transcriptional roadblocking and highlight the importance of transcription factor binding kinetics and its resistance to dislodgement by an elongating RNAP in determining the strength of a roadblock.

Development of a Transcriptional Activator-Like Effector Protein to Accurately Discriminate Single Nucleotide Difference

Transcriptional activator-like effector (TALE), a DNA-binding protein, is widely used in genome editing. However, the recognition of the target sequence by the TALE is adversely affected by the number of mismatches. Therefore, the association constant of DNA-TALE complex formation can be controlled by appropriately introducing a mismatch into the TALE recognition sequence. This study aimed to construct a TALE that can distinguish a single nucleotide difference. Our results show that a single mismatch present in repeats 2 or 3 of TALE did not interfere with the complex formation with DNA, whereas continuous mismatches present in repeats 2 and 3 significantly reduced association with the target DNA. Based on these findings, we constructed a detection system of the one nucleotide difference in gene with high accuracy and constructed a TALE-nuclease (TALEN) that selectively cleaves DNA with a single mismatch.

Repressor-Like On-Off Regulation of Protein Expression by the DNA-Binding Transcription Activator-Like Effector in T7 Promoter-Based Cell-Free Protein Synthesis

The aim of this study was to develop a transcription activator-like effector (TALE)-based technology to regulate protein synthesis in cell-free systems. We attempted to regulate the T7 promoter system, which has no natural mechanism of expression control, and sought to arbitrarily induce protein expression through the formation and dissociation of TALE and target DNA complexes. Protein synthesis was performed in a cell-free system in the presence of TALE, which recognized and bound to a sequence upstream of the T7 promoter, and protein expression was suppressed by approximately 80 % compared to in the absence of TALE. This suggests that masking part of the promoter region strongly suppresses protein synthesis. Additionally, competitive inhibition of TALE binding to the target DNA template led to protein synthesis levels that were equivalent to the levels in the absence of TALE. Our results demonstrate that DNA recognition by TALE can regulate the expression of the T7 promoter system.

Universal Actuator for Efficient Silencing of Escherichia coli Genes Based on Convergent Transcription Resistant to Rho-Dependent Termination

Dynamic control is a distinguished strategy in modern metabolic engineering, in which inducible convergent transcription is an attractive approach for conditional gene silencing. Instead of a simple strong "reverse" (r-) promoter, a three-component actuator has been developed for constitutive genes silencing. These actuators, consisting of r-promoters with different strengths, the ribosomal transcription antitermination-inducing sequence rrnG-AT, and the RNase III processing site, were inserted into the 3'-UTR of three E. coli metabolic genes. Second and third actuator components were important to improve the effectiveness and robustness of the approach. The maximal silencing folds achieved for gltA, pgi, and ppc were approximately 7, 11, and >100, respectively. Data were analyzed using a simple model that considered RNA polymerase (RNAP) head-on collisions as the unique reason for gene silencing and continued transcription after collision with only one of two molecules. It was previously established that forward (f-) RNAP with a trailing ribosome was approximately 13-times more likely to continue transcription after head-on collision than untrailed r-RNAP which is sensitive to Rho-dependent transcription termination (RhoTT). According to the current results, this bias in complex stabilities decreased to no more than (3.0-5.7)-fold if r-RNAP became resistant to RhoTT. Therefore, the developed constitutive actuator could be considered as an improved tool for controlled gene expression mainly due to the transfer of r-transcription into a state that is resistant to potential termination and used as the basis for the design of tightly regulated actuators for the achievement of conditional silencing.

Growth-coupled bioconversion of levulinic acid to butanone

Common strategies for conversion of lignocellulosic biomass to chemical products center on deconstructing biomass polymers into fermentable sugars. Here, we demonstrate an alternative strategy, a growth-coupled, high-yield bioconversion, by feeding cells a non-sugar substrate, by-passing central metabolism, and linking a key metabolic step to generation of acetyl-CoA that is required for biomass and energy generation. Specifically, we converted levulinic acid (LA), an established degradation product of lignocellulosic biomass, to butanone (a.k.a. methyl-ethyl ketone - MEK), a widely used industrial solvent. Our strategy combines a catabolic pathway from Pseudomonas putida that enables conversion of LA to 3-ketovaleryl-CoA, a CoA transferase that generates 3-ketovalerate and acetyl-CoA, and a decarboxylase that generates 2-butanone. By removing the ability of E. coli to consume LA and supplying excess acetate as a carbon source, we built a strain of E. coli that could convert LA to butanone at high yields, but at the cost of significant acetate consumption. Using flux balance analysis as a guide, we built a strain of E. coli that linked acetate assimilation to production of butanone. This strain was capable of complete bioconversion of LA to butanone with a reduced acetate requirement and increased specific productivity. To demonstrate the bioconversion on real world feedstocks, we produced LA from furfuryl alcohol, a compound readily obtained from biomass. These LA feedstocks were found to contain inhibitors that prevented cell growth and bioconversion of LA to butanone. We used a combination of column chromatography and activated carbon to remove the toxic compounds from the feedstock, resulting in LA that could be completely converted to butanone. This work motivates continued collaboration between chemical and biological catalysis researchers to explore alternative conversion pathways and the technical hurdles that prevent their rapid deployment.

Guiding Biomolecular Interactions in Cells Using de Novo Protein-Protein Interfaces

An improved ability to direct and control biomolecular interactions in living cells would have an impact on synthetic biology. A key issue is the need to introduce interacting components that act orthogonally to endogenous proteomes and interactomes. Here, we show that low-complexity, de novo designed protein-protein interaction (PPI) domains can substitute for natural PPIs and guide engineered protein-DNA interactions in Escherichia coli. Specifically, we use de novo homo- and heterodimeric coiled coils to reconstitute a cytoplasmic split adenylate cyclase, recruit RNA polymerase to a promoter and activate gene expression, and oligomerize both natural and designed DNA-binding domains to repress transcription. Moreover, the stabilities of the heterodimeric coiled coils can be modulated by rational design and, thus, adjust the levels of gene activation and repression in vivo. These experiments demonstrate the possibilities for using designed proteins and interactions to control biomolecular systems such as enzyme cascades and circuits in cells.

Selective recognition of N4-methylcytosine in DNA by engineered transcription-activator-like effectors

The epigenetic DNA nucleobases 5-methylcytosine (5mC) and N4-methylcytosine (4mC) coexist in bacterial genomes and have important functions in host defence and transcription regulation. To better understand the individual biological roles of both methylated nucleobases, analytical strategies for distinguishing unmodified cytosine (C) from 4mC and 5mC are required. Transcription-activator-like effectors (TALEs) are programmable DNA-binding repeat proteins, which can be re-engineered for the direct detection of epigenetic nucleobases in user-defined DNA sequences. We here report the natural, cytosine-binding TALE repeat to not strongly differentiate between 5mC and 4mC. To engineer repeats with selectivity in the context of C, 5mC and 4mC, we developed a homogeneous fluorescence assay and screened a library of size-reduced TALE repeats for binding to all three nucleobases. This provided insights into the requirements of size-reduced TALE repeats for 4mC binding and revealed a single mutant repeat as a selective binder of 4mC. Employment of a TALE with this repeat in affinity enrichment enabled the isolation of a user-defined DNA sequence containing a single 4mC but not C or 5mC from the background of a bacterial genome. Comparative enrichments with TALEs bearing this or the natural C-binding repeat provides an approach for the complete, programmable decoding of all cytosine nucleobases found in bacterial genomes.This article is part of a discussion meeting issue 'Frontiers in epigenetic chemical biology'.

Exploring of the feature space of de novo developed post-transcriptional riboregulators

Metabolic engineering increasingly depends upon RNA technology to customly rewire the metabolism to maximize production. To this end, pure riboregulators allow dynamic gene repression without the need of a potentially burdensome coexpressed protein like typical Hfq binding small RNAs and clustered regularly interspaced short palindromic repeats technology. Despite this clear advantage, no clear general design principles are available to de novo develop repressing riboregulators, limiting the availability and the reliable development of these type of riboregulators. Here, to overcome this lack of knowledge on the functionality of repressing riboregulators, translation inhibiting RNAs are developed from scratch. These de novo developed riboregulators explore features related to thermodynamical and structural factors previously attributed to translation initiation modulation. In total, 12 structural and thermodynamic features were defined of which six features were retained after removing correlations from an in silico generated riboregulator library. From this translation inhibiting RNA library, 18 riboregulators were selected using a experimental design and subsequently constructed and co-expressed with two target untranslated regions to link the translation inhibiting RNA features to functionality. The pure riboregulators in the design of experiments showed repression down to 6% of the original protein expression levels, which could only be partially explained by a ordinary least squares regression model. To allow reliable forward engineering, a partial least squares regression model was constructed and validated to link the properties of translation inhibiting RNA riboregulators to gene repression. In this model both structural and thermodynamic features were important for efficient gene repression by pure riboregulators. This approach enables a more reliable de novo forward engineering of effective pure riboregulators, which further expands the RNA toolbox for gene expression modulation.

To allow reliable forward engineering of microbial cell factories, various metabolic engineering efforts rely on RNA-based technology. As such, programmable riboregulators allow dynamic control over gene expression. However, no clear design principles exist for de novo developed repressing riboregulators, which limits their applicability. Here, various engineering principles are identified and computationally explored. Subsequently, various design criteria are used in an experimental design, which were explored in an in vivo study. This resulted in a regression model that enables a more reliable computational design of repression small RNAs.

Distinct and redundant functions of three homologs of RNase III in the cyanobacterium Synechococcus sp. strain PCC 7002

RNase III is a ribonuclease that recognizes and cleaves double-stranded RNA. Across bacteria, RNase III is involved in rRNA maturation, CRISPR RNA maturation, controlling gene expression, and turnover of messenger RNAs. Many organisms have only one RNase III while others have both a full-length RNase III and another version that lacks a double-stranded RNA binding domain (mini-III). The genome of the cyanobacterium Synechococcus sp. strain PCC 7002 (PCC 7002) encodes three homologs of RNase III, two full-length and one mini-III, that are not essential even when deleted in combination. To discern if each enzyme had distinct responsibilities, we collected and sequenced global RNA samples from the wild type strain, the single, double, and triple RNase III mutants. Approximately 20% of genes were differentially expressed in various mutants with some operons and regulons showing complex changes in expression levels between mutants. Two RNase III's had a role in 23S rRNA maturation and the third was involved in copy number regulation one of six native plasmids. In vitro, purified RNase III enzymes were capable of cleaving some of the known Escherichia coli RNase III target sequences, highlighting the remarkably conserved substrate specificity between organisms yet complex regulation of gene expression.

The effect of increasing numbers of repeats on TAL effector DNA binding specificity

Transcription activator-like effectors (TALEs) recognize their DNA targets via tandem repeats, each specifying a single nucleotide base in a one-to-one sequential arrangement. Due to this modularity and their ability to bind long DNA sequences with high specificity, TALEs have been used in many applications. Contributions of individual repeat-nucleotide associations to affinity and specificity have been characterized. Here, using in vitro binding assays, we examined the relationship between the number of repeats in a TALE and its affinity, for both target and non-target DNA. Each additional repeat provides extra binding energy for the target DNA, with the gain decaying exponentially such that binding energy saturates. Affinity for non-target DNA also increases non-linearly with the number of repeats, but with a slower decay of gain. The difference between the effect of length on affinity for target versus non-target DNA manifests in specificity increasing then diminishing with increasing TALE length, peaking between 15 and 19 repeats. Modeling across different hypothetical saturation levels and rates of gain decay, reflecting different repeat compositions, yielded a similar range of specificity optima. This range encompasses the mean and median length of native TALEs, suggesting that these proteins as a group have evolved for maximum specificity.

Transcription control engineering and applications in synthetic biology

In synthetic biology, researchers assemble biological components in new ways to produce systems with practical applications. One of these practical applications is control of the flow of genetic information (from nucleic acid to protein), a.k.a. gene regulation. Regulation is critical for optimizing protein (and therefore activity) levels and the subsequent levels of metabolites and other cellular properties. The central dogma of molecular biology posits that information flow commences with transcription, and accordingly, regulatory tools targeting transcription have received the most attention in synthetic biology. In this mini-review, we highlight many past successes and summarize the lessons learned in developing tools for controlling transcription. In particular, we focus on engineering studies where promoters and transcription terminators (cis-factors) were directly engineered and/or isolated from DNA libraries. We also review several well-characterized transcription regulators (trans-factors), giving examples of how cis- and trans-acting factors have been combined to create digital and analogue switches for regulating transcription in response to various signals. Last, we provide examples of how engineered transcription control systems have been used in metabolic engineering and more complicated genetic circuits. While most of our mini-review focuses on the well-characterized bacterium Escherichia coli, we also provide several examples of the use of transcription control engineering in non-model organisms. Similar approaches have been applied outside the bacterial kingdom indicating that the lessons learned from bacterial studies may be generalized for other organisms.

A DNA Bubble-Mediated Gene Regulation System Based on Thrombin-Bound DNA Aptamers

We describe here a novel approach to enhance the transcription of a target gene in cell-free systems by symmetrically introducing duplex aptamers upstream to a T7 promoter in both the sense and antisense strands of double-stranded plasmids, which leads to the formation of a DNA bubble due to the none-complementary state of the ssDNA region harboring the aptamer sequences. With the presence of thrombins, the DNA bubble would be enlarged due to the binding of aptamers with thrombins. Consequently, the recognition region of the promoter contained in the DNA bubble can be more easily recognized and bound by RNA polymerases, and the separation efficiency of the unwinding region can also be significantly improved, leading to the enhanced expression of the target gene at the transcriptional level. The effectiveness of the proposed gene regulation system was demonstrated by enhancing the expression of gfp and ecaA genes in cell-free systems.

Exploiting the sequence diversity of TALE-like repeats to vary the strength of dTALE-promoter interactions

Designer transcription activator-like effectors (dTALEs) are programmable transcription factors used to regulate user-defined promoters. The TALE DNA-binding domain is a tandem series of amino acid repeats that each bind one DNA base. Each repeat is 33-35 amino acids long. A residue in the center of each repeat is responsible for defining DNA base specificity and is referred to as the base specificying residue (BSR). Other repeat residues are termed non-BSRs and can contribute to TALE DNA affinity in a non-base-specific manner. Previous dTALE engineering efforts have focused on BSRs. Non-BSRs have received less attention, perhaps because there is almost no non-BSR sequence diversity in natural TALEs. However, more sequence diverse, TALE-like proteins are found in diverse bacterial clades. Here, we show that natural non-BSR sequence diversity of TALEs and TALE-likes can be used to modify DNA-binding strength in a new form of dTALE repeat array that we term variable sequence TALEs (VarSeTALEs). We generated VarSeTALE repeat modules through random assembly of repeat sequences from different origins, while holding BSR composition, and thus base preference, constant. We used two different VarSeTALE design approaches combing either whole repeats from different TALE-like sources (inter-repeat VarSeTALEs) or repeat subunits corresponding to secondary structural elements (intra-repeat VarSeTALEs). VarSeTALE proteins were assayed in bacteria, plant protoplasts and leaf tissues. In each case, VarSeTALEs activated or repressed promoters with a range of activities. Our results indicate that natural non-BSR diversity can be used to diversify the binding strengths of dTALE repeat arrays while keeping target sequences constant.

A transcription activator-like effector (TALE) induction system mediated by proteolysis

Simple and predictable trans-acting regulatory tools are needed in the fields of synthetic biology and metabolic engineering to build complex genetic circuits and optimize the levels of native and heterologous gene products. Transcription activator-like effectors (TALEs) are bacterial virulence factors that have recently gained traction in biotechnology applications due to their customizable DNA binding specificity. In this work we expand the versatility of these transcription factors to create an inducible TALE system by inserting tobacco-etch virus (TEV) protease recognition sites into the TALE backbone. The resulting engineered TALEs maintain transcriptional repression of their target genes in Escherichia coli, but are degraded following the induction of the TEV protease, thereby promoting expression of the previously repressed target gene of interest. This TALE-TEV technology enables both repression and induction of plasmid or chromosomal target genes in a manner analogous to traditional repressor proteins but with the added flexibility of being operator agnostic.

DNA-binding proteins from marine bacteria expand the known sequence diversity of TALE-like repeats

Transcription Activator-Like Effectors (TALEs) of Xanthomonas bacteria are programmable DNA binding proteins with unprecedented target specificity. Comparative studies into TALE repeat structure and function are hindered by the limited sequence variation among TALE repeats. More sequence-diverse TALE-like proteins are known from Ralstonia solanacearum (RipTALs) and Burkholderia rhizoxinica (Bats), but RipTAL and Bat repeats are conserved with those of TALEs around the DNA-binding residue. We study two novel marine-organism TALE-like proteins (MOrTL1 and MOrTL2), the first to date of non-terrestrial origin. We have assessed their DNA-binding properties and modelled repeat structures. We found that repeats from these proteins mediate sequence specific DNA binding conforming to the TALE code, despite low sequence similarity to TALE repeats, and with novel residues around the BSR. However, MOrTL1 repeats show greater sequence discriminating power than MOrTL2 repeats. Sequence alignments show that there are only three residues conserved between repeats of all TALE-like proteins including the two new additions. This conserved motif could prove useful as an identifier for future TALE-likes. Additionally, comparing MOrTL repeats with those of other TALE-likes suggests a common evolutionary origin for the TALEs, RipTALs and Bats.

Control of Gene Expression in Leptospira spp. by Transcription Activator-Like Effectors Demonstrates a Potential Role for LigA and LigB in Leptospira interrogans Virulence

Leptospirosis is a zoonotic disease that affects ∼1 million people annually, with a mortality rate of >10%. Currently, there is an absence of effective genetic manipulation tools for targeted mutagenesis in pathogenic leptospires. Transcription activator-like effectors (TALEs) are a recently described group of repressors that modify transcriptional activity in prokaryotic and eukaryotic cells by directly binding to a targeted sequence within the host genome. To determine the applicability of TALEs within Leptospira spp., two TALE constructs were designed. First, a constitutively expressed TALE gene specific for the lacO-like region upstream of bgaL was trans inserted in the saprophyte Leptospira biflexa (the TALEβgal strain). Reverse transcriptase PCR (RT-PCR) analysis and enzymatic assays demonstrated that BgaL was not expressed in the TALEβgal strain. Second, to study the role of LigA and LigB in pathogenesis, a constitutively expressed TALE gene with specificity for the homologous promoter regions of ligA and ligB was cis inserted into the pathogen Leptospira interrogans (TALElig). LigA and LigB expression was studied by using three independent clones: TALElig1, TALElig2, and TALElig3. Immunoblot analysis of osmotically induced TALElig clones demonstrated 2- to 9-fold reductions in the expression levels of LigA and LigB, with the highest reductions being noted for TALElig1 and TALElig2, which were avirulent in vivo and nonrecoverable from animal tissues. This study reconfirms galactosidase activity in the saprophyte and suggests a role for LigA and LigB in pathogenesis. Collectively, this study demonstrates that TALEs are effective at reducing the expression of targeted genes within saprophytic and pathogenic strains of Leptospira spp., providing an additional genetic manipulation tool for this genus.

Putting RNA to work: Translating RNA fundamentals into biotechnological engineering practice

Synthetic biology, in close concert with systems biology, is revolutionizing the field of metabolic engineering by providing novel tools and technologies to rationally, in a standardized way, reroute metabolism with a view to optimally converting renewable resources into a broad range of bio-products, bio-materials and bio-energy. Increasingly, these novel synthetic biology tools are exploiting the extensive programmable nature of RNA, vis-à-vis DNA- and protein-based devices, to rationally design standardized, composable, and orthogonal parts, which can be scaled and tuned promptly and at will. This review gives an extensive overview of the recently developed parts and tools for i) modulating gene expression ii) building genetic circuits iii) detecting molecules, iv) reporting cellular processes and v) building RNA nanostructures. These parts and tools are becoming necessary armamentarium for contemporary metabolic engineering. Furthermore, the design criteria, technological challenges, and recent metabolic engineering success stories of the use of RNA devices are highlighted. Finally, the future trends in transforming metabolism through RNA engineering are critically evaluated and summarized.

DNA-binding proteins from marine bacteria expand the known sequence diversity of TALE-like repeats

Transcription Activator-Like Effectors (TALEs) of Xanthomonas bacteria are programmable DNA binding proteins with unprecedented target specificity. Comparative studies into TALE repeat structure and function are hindered by the limited sequence variation among TALE repeats. More sequence-diverse TALE-like proteins are known from Ralstonia solanacearum (RipTALs) and Burkholderia rhizoxinica (Bats), but RipTAL and Bat repeats are conserved with those of TALEs around the DNA-binding residue. We study two novel marine-organism TALE-like proteins (MOrTL1 and MOrTL2), the first to date of non-terrestrial origin. We have assessed their DNA-binding properties and modelled repeat structures. We found that repeats from these proteins mediate sequence specific DNA binding conforming to the TALE code, despite low sequence similarity to TALE repeats, and with novel residues around the BSR. However, MOrTL1 repeats show greater sequence discriminating power than MOrTL2 repeats. Sequence alignments show that there are only three residues conserved between repeats of all TALE-like proteins including the two new additions. This conserved motif could prove useful as an identifier for future TALE-likes. Additionally, comparing MOrTL repeats with those of other TALE-likes suggests a common evolutionary origin for the TALEs, RipTALs and Bats.

TAL effectors and activation of predicted host targets distinguish Asian from African strains of the rice pathogen Xanthomonas oryzae pv. oryzicola while strict conservation suggests universal importance of five TAL effectors

Xanthomonas oryzae pv. oryzicola (Xoc) causes the increasingly important disease bacterial leaf streak of rice (BLS) in part by type III delivery of repeat-rich transcription activator-like (TAL) effectors to upregulate host susceptibility genes. By pathogen whole genome, single molecule, real-time sequencing and host RNA sequencing, we compared TAL effector content and rice transcriptional responses across 10 geographically diverse Xoc strains. TAL effector content is surprisingly conserved overall, yet distinguishes Asian from African isolates. Five TAL effectors are conserved across all strains. In a prior laboratory assay in rice cv. Nipponbare, only two contributed to virulence in strain BLS256 but the strict conservation indicates all five may be important, in different rice genotypes or in the field. Concatenated and aligned, TAL effector content across strains largely reflects relationships based on housekeeping genes, suggesting predominantly vertical transmission. Rice transcriptional responses did not reflect these relationships, and on average, only 28% of genes upregulated and 22% of genes downregulated by a strain are up- and down- regulated (respectively) by all strains. However, when only known TAL effector targets were considered, the relationships resembled those of the TAL effectors. Toward identifying new targets, we used the TAL effector-DNA recognition code to predict effector binding elements in promoters of genes upregulated by each strain, but found that for every strain, all upregulated genes had at least one. Filtering with a classifier we developed previously decreases the number of predicted binding elements across the genome, suggesting that it may reduce false positives among upregulated genes. Applying this filter and eliminating genes for which upregulation did not strictly correlate with presence of the corresponding TAL effector, we generated testable numbers of candidate targets for four of the five strictly conserved TAL effectors.

RiboTALE: A modular, inducible system for accurate gene expression control

A limiting factor in synthetic gene circuit design is the number of independent control elements that can be combined together in a single system. Here, we present RiboTALEs, a new class of inducible repressors that combine the specificity of TALEs with the ability of riboswitches to recognize exogenous signals and differentially control protein abundance. We demonstrate the capacity of RiboTALEs, constructed through different combinations of TALE proteins and riboswitches, to rapidly and reproducibly control the expression of downstream targets with a dynamic range of 243.7 ± 17.6-fold, which is adequate for many biotechnological applications.

Modes of TAL effector-mediated repression

Engineered transcription activator-like effectors, or TALEs, have emerged as a new class of designer DNA-binding proteins. Their DNA recognition sites can be specified with great flexibility. When fused to appropriate transcriptional regulatory domains, they can serve as designer transcription factors, modulating the activity of targeted promoters. We created tet operator (tetO)-specific TALEs (tetTALEs), with an identical DNA-binding site as the Tet repressor (TetR) and the TetR-based transcription factors that are extensively used in eukaryotic transcriptional control systems. Different constellations of tetTALEs and tetO modified chromosomal transcription units were analyzed for their efficacy in mammalian cells. We find that tetTALE-silencers can entirely abrogate expression from the strong human EF1α promoter when binding upstream of the transcriptional control sequence. Remarkably, the DNA-binding domain of tetTALE alone can effectively counteract trans-activation mediated by the potent tettrans-activator and also directly interfere with RNA polymerase II transcription initiation from the strong CMV promoter. Our results demonstrate that TALEs can act as highly versatile tools in genetic engineering, serving as trans-activators, trans-silencers and also competitive repressors.

Regulation of endogenous human gene expression by ligand-inducible TALE transcription factors

The construction of increasingly sophisticated synthetic biological circuits is dependent on the development of extensible tools capable of providing specific control of gene expression in eukaryotic cells. Here, we describe a new class of synthetic transcription factors that activate gene expression in response to extracellular chemical stimuli. These inducible activators consist of customizable transcription activator-like effector (TALE) proteins combined with steroid hormone receptor ligand-binding domains. We demonstrate that these ligand-responsive TALE transcription factors allow for tunable and conditional control of gene activation and can be used to regulate the expression of endogenous genes in human cells. Since TALEs can be designed to recognize any contiguous DNA sequence, the conditional gene regulatory system described herein will enable the design of advanced synthetic gene networks.

Transcription activator-like effectors: a toolkit for synthetic biology

Transcription activator-like effectors (TALEs) are proteins secreted by Xanthomonas bacteria to aid the infection of plant species. TALEs assist infections by binding to specific DNA sequences and activating the expression of host genes. Recent results show that TALE proteins consist of a central repeat domain, which determines the DNA targeting specificity and can be rapidly synthesized de novo. Considering the highly modular nature of TALEs, their versatility, and the ease of constructing these proteins, this technology can have important implications for synthetic biology applications. Here, we review developments in the area with a particular focus on modifications for custom and controllable gene regulation.

Application of TALEs, CRISPR/Cas and sRNAs as trans-acting regulators in prokaryotes

The last several years have witnessed an explosion in the understanding and use of novel, versatile trans-acting elements. TALEs, CRISPR/Cas, and sRNAs can be easily fashioned to bind any specific sequence of DNA (TALEs, CRISPR/Cas) or RNA (sRNAs) because of the simple rules governing their interactions with nucleic acids. This unique property enables these tools to repress the expression of genes at the transcriptional or post-transcriptional levels, respectively, without prior manipulation of cis-acting and/or chromosomal target DNA sequences. These tools are now being harnessed by synthetic biologists, particularly those in the eukaryotic community, for genome-wide regulation, editing, or epigenetic studies. Here we discuss the exciting opportunities for using TALEs, CRISPR/Cas, and sRNAs as synthetic trans-acting regulators in prokaryotes.

Artificial transcription factor-mediated regulation of gene expression

The transcriptional regulation of endogenous genes with artificial transcription factors (TFs) can offer new tools for plant biotechnology. Three systems are available for mediating site-specific DNA recognition of artificial TFs: those based on zinc fingers, TALEs, and on the CRISPR/Cas9 technology. Artificial TFs require an effector domain that controls the frequency of transcription initiation at endogenous target genes. These effector domains can be transcriptional activators or repressors, but can also have enzymatic activities involved in chromatin remodeling or epigenetic regulation. Artificial TFs are able to regulate gene expression in trans, thus allowing them to evoke dominant mutant phenotypes. Large scale changes in transcriptional activity are induced when the DNA binding domain is deliberately designed to have lower binding specificity. This technique, known as genome interrogation, is a powerful tool for generating novel mutant phenotypes. Genome interrogation has clear mechanistic and practical advantages over activation tagging, which is the technique most closely resembling it. Most notably, genome interrogation can lead to the discovery of mutant phenotypes that are unlikely to be found when using more conventional single gene-based approaches.

TALEN-mediated genome editing: prospects and perspectives

Genome editing is the practice of making predetermined and precise changes to a genome by controlling the location of DNA DSBs (double-strand breaks) and manipulating the cell's repair mechanisms. This technology results from harnessing natural processes that have taken decades and multiple lines of inquiry to understand. Through many false starts and iterative technology advances, the goal of genome editing is just now falling under the control of human hands as a routine and broadly applicable method. The present review attempts to define the technique and capture the discovery process while following its evolution from meganucleases and zinc finger nucleases to the current state of the art: TALEN (transcription-activator-like effector nuclease) technology. We also discuss factors that influence success, technical challenges and future prospects of this quickly evolving area of study and application.

Principles and applications of TAL effectors for plant physiology and metabolism

Recent advances in DNA targeting allow unprecedented control over gene function and expression. Targeting based on TAL effectors is arguably the most promising for systems biology and metabolic engineering. Multiple, orthogonal TAL-effector reagents of different types can be used in the same cell. Furthermore, variation in base preferences of the individual structural repeats that make up the TAL effector DNA recognition domain makes targeting stringency tunable. Realized applications range from genome editing to epigenome modification to targeted gene regulation to chromatin labeling and capture. The principles that govern TAL effector DNA recognition make TAL effectors well suited for applications relevant to plant physiology and metabolism. TAL effector targeting has merits that are distinct from those of the RNA-based DNA targeting CRISPR/Cas9 system.

Precision genetic modifications: a new era in molecular biology and crop improvement

Recently, the use of programmable DNA-binding proteins such as ZFP/ZFNs, TALE/TALENs and CRISPR/Cas has produced unprecedented advances in gene targeting and genome editing in prokaryotes and eukaryotes. These advances allow researchers to specifically alter genes, reprogram epigenetic marks, generate site-specific deletions and potentially cure diseases. Unlike previous methods, these precision genetic modification techniques (PGMs) are specific, efficient, easy to use and economical. Here we discuss the capabilities and pitfalls of PGMs and highlight the recent, exciting applications of PGMs in molecular biology and crop genetic engineering. Further improvement of the efficiency and precision of PGM techniques will enable researchers to precisely alter gene expression and biological/chemical pathways, probe gene function, modify epigenetic marks and improve crops by increasing yield, quality and tolerance to limiting biotic and abiotic stress conditions.

A TAL effector repeat architecture for frameshift binding

Transcription activator-like effectors (TALEs) are important Xanthomonas virulence factors that bind DNA via a unique tandem 34-amino-acid repeat domain to induce expression of plant genes. So far, TALE repeats are described to bind as a consecutive array to a consecutive DNA sequence, in which each repeat independently recognizes a single DNA base. This modular protein architecture enables the design of any desired DNA-binding specificity for biotechnology applications. Here we report that natural TALE repeats of unusual amino-acid sequence length break the strict one repeat-to-one base pair binding mode and introduce a local flexibility to TALE-DNA binding. This flexibility allows TALEs and TALE nucleases to recognize target sequence variants with single nucleotide deletions. The flexibility also allows TALEs to activate transcription at allelic promoters that otherwise confer resistance to the host plant.

Genomic mining of prokaryotic repressors for orthogonal logic gates

Genetic circuits perform computational operations based on interactions between freely diffusing molecules within a cell. When transcription factors are combined to build a circuit, unintended interactions can disrupt its function. Here, we apply 'part mining' to build a library of 73 TetR-family repressors gleaned from prokaryotic genomes. The operators of a subset were determined using an in vitro method, and this information was used to build synthetic promoters. The promoters and repressors were screened for cross-reactions. Of these, 16 were identified that both strongly repress their cognate promoter (5- to 207-fold) and exhibit minimal interactions with other promoters. Each repressor-promoter pair was converted to a NOT gate and characterized. Used as a set of 16 NOT/NOR gates, there are >10(54) circuits that could be built by changing the pattern of input and output promoters. This represents a large set of compatible gates that can be used to construct user-defined circuits.

TALE activators regulate gene expression in a position- and strand-dependent manner in mammalian cells

Transcription activator-like effectors (TALEs) are a class of transcription factors that are readily programmable to regulate gene expression. Despite their growing popularity, little is known about binding site parameters that influence TALE-mediated gene activation in mammalian cells. We demonstrate that TALE activators modulate gene expression in mammalian cells in a position- and strand-dependent manner. To study the effects of binding site location, we engineered TALEs customized to recognize specific DNA sequences located in either the promoter or the transcribed region of reporter genes. We found that TALE activators robustly activated reporter genes when their binding sites were located within the promoter region. In contrast, TALE activators inhibited the expression of reporter genes when their binding sites were located on the sense strand of the transcribed region. Notably, this repression was independent of the effector domain utilized, suggesting a simple blockage mechanism. We conclude that TALE activators in mammalian cells regulate genes in a position- and strand-dependent manner that is substantially different from gene activation by native TALEs in plants. These findings have implications for optimizing the design of custom TALEs for genetic manipulation in mammalian cells.

Engineering plant disease resistance based on TAL effectors

Transcription activator-like (TAL) effectors are encoded by plant-pathogenic bacteria and induce expression of plant host genes. TAL effectors bind DNA on the basis of a unique code that specifies binding of amino acid residues in repeat units to particular DNA bases in a one-to-one correspondence. This code can be used to predict binding sites of natural TAL effectors and to design novel synthetic DNA-binding domains for targeted genome manipulation. Natural mechanisms of resistance in plants against TAL effector-containing pathogens have given insights into new strategies for disease control.