Conditional-replication, integration, excision, and retrieval plasmid-host systems for gene structure-function studies of bacteria

Programmable DNA looping using engineered bivalent dCas9 complexes

DNA looping is a ubiquitous and critical feature of gene regulation. Although DNA looping can be efficiently detected, tools to readily manipulate DNA looping are limited. Here we develop CRISPR-based DNA looping reagents for creation of programmable DNA loops. Cleavage-defective Cas9 proteins of different specificity are linked by heterodimerization or translational fusion to create bivalent complexes able to link two separate DNA regions. After model-directed optimization, the reagents are validated using a quantitative DNA looping assay in E. coli. Looping efficiency is ~15% for a 4.7 kb loop, but is significantly improved by loop multiplexing with additional guides. Bivalent dCas9 complexes are also used to activate endogenous norVW genes by rewiring chromosomal DNA to bring distal enhancer elements to the gene promoters. Such reagents should allow manipulation of DNA looping in a variety of cell types, aiding understanding of endogenous loops and enabling creation of new regulatory connections.

DNA loops are a ubiquitious feature of gene regulation across the kingdoms of life. Here the authors design a Cas9-based dimerization system for inducing DNA loops in E. coli, allowing activation and rewiring of gene expression.

Biosensor-aided high-throughput screening of hyper-producing cells for malonyl-CoA-derived products

Background

Malonyl-coenzyme A (CoA) is an important biosynthetic precursor in vivo. Although Escherichia coli is a useful organism for biosynthetic applications, its malonyl-CoA level is too low.

Results

To identify strains with the best potential for enhanced malonyl-CoA production, we developed a whole-cell biosensor for rapidly reporting intracellular malonyl-CoA concentrations. The biosensor was successfully applied as a high-throughput screening tool for identifying targets at a genome-wide scale that could be critical for improving the malonyl-CoA biosynthesis in vivo. The mutant strains selected synthesized significantly higher titers of the type III polyketide triacetic acid lactone (TAL), phloroglucinol, and free fatty acids compared to the wild-type strain, using malonyl-CoA as a precursor.

Conclusion

These results validated this novel whole-cell biosensor as a rapid and sensitive malonyl-CoA high-throughput screening tool. Further analysis of the mutant strains showed that the iron ion concentration is closely related to the intracellular malonyl-CoA biosynthesis.

Electronic supplementary material

The online version of this article (10.1186/s12934-017-0794-6) contains supplementary material, which is available to authorized users.

Circuitry Linking the Global Csr- and σE-Dependent Cell Envelope Stress Response Systems

CsrA of Escherichia coli is an RNA-binding protein that globally regulates a wide variety of cellular processes and behaviors, including carbon metabolism, motility, biofilm formation, and the stringent response. CsrB and CsrC are small RNAs (sRNAs) that sequester CsrA, thereby preventing CsrA-mRNA interaction. RpoE (σ^E) is the extracytoplasmic stress response sigma factor of E. coli Previous RNA sequencing (RNA-seq) studies identified rpoE mRNA as a CsrA target. Here, we explored the regulation of rpoE by CsrA and found that CsrA represses rpoE translation. Gel mobility shift, footprint, and toeprint studies identified three CsrA binding sites in the rpoE leader transcript, one of which overlaps the rpoE Shine-Dalgarno (SD) sequence, while another overlaps the rpoE translation initiation codon. Coupled in vitro transcription-translation experiments showed that CsrA represses rpoE translation by binding to these sites. We further demonstrate that σ^E indirectly activates the transcription of csrB and csrC, leading to increased sequestration of CsrA, such that repression of rpoE by CsrA is reduced. We propose that the Csr system fine-tunes the σ^E-dependent cell envelope stress response. We also identified a 51-amino-acid coding sequence whose stop codon overlaps the rpoE start codon and demonstrate that rpoE is translationally coupled with this upstream open reading frame (ORF51). The loss of coupling reduces rpoE translation by more than 50%. Identification of a translationally coupled ORF upstream of rpoE suggests that this previously unannotated protein may participate in the cell envelope stress response. In keeping with existing nomenclature, we named ORF51 rseD, resulting in an operon arrangement of rseD-rpoE-rseA-rseB-rseC IMPORTANCE CsrA posttranscriptionally represses genes required for bacterial stress responses, including the stringent response, catabolite repression, and the RpoS (σ^S)-mediated general stress response. We show that CsrA represses the translation of rpoE, encoding the extracytoplasmic stress response sigma factor, and that σ^E indirectly activates the transcription of csrB and csrC, resulting in reciprocal regulation of these two global regulatory systems. These findings suggest that extracytoplasmic stress leads to derepression of rpoE translation by CsrA, and CsrA-mediated repression helps reset RpoE abundance to prestress levels once envelope damage is repaired. The discovery of an ORF, rseD, translationally coupled with rpoE adds further complexity to translational control of rpoE.

Molecular characterization and verification of azido-3,8-dideoxy-d-manno-oct-2-ulosonic acid incorporation into bacterial lipopolysaccharide

3-Deoxy-d-manno-oct-2-ulosonic acid (Kdo) is an essential component of LPS in the outer leaflet of the Gram-negative bacterial outer membrane. Although labeling of Escherichia coli with the chemical reporter 8-azido-3,8-dideoxy-d-manno-oct-2-ulosonic acid (Kdo-N₃) has been reported, its incorporation into LPS has not been directly shown. We have now verified Kdo-N₃ incorporation into E. coli LPS at the molecular level. Using microscopy and PAGE analysis, we show that Kdo-N₃ is localized to the outer membrane and specifically incorporates into rough and deep-rough LPS. In an E. coli strain lacking endogenous Kdo biosynthesis, supplementation with exogenous Kdo restored full-length core-LPS, which suggests that the Kdo biosynthetic pathways might not be essential in vivo in the presence of sufficient exogenous Kdo. In contrast, exogenous Kdo-N₃ only restored a small fraction of core LPS with the majority incorporated into truncated LPS. The truncated LPS were identified as Kdo-N₃-lipid IV_A and (Kdo-N₃)₂-lipid IV_A by MS analysis. The low level of Kdo-N₃ incorporation could be partly explained by a 6-fold reduction in the specificity constant of the CMP-Kdo synthetase KdsB with Kdo-N₃ compared with Kdo. These results indicate that the azido moiety in Kdo-N₃ interferes with its utilization and may limit its utility as a tracer of LPS biosynthesis and transport in E. coli We propose that our findings will be helpful for researchers using Kdo and its chemical derivatives for investigating LPS biosynthesis, transport, and assembly in Gram-negative bacteria.

MazF activation promotes translational heterogeneity of the grcA mRNA in Escherichia coli populations

Bacteria adapt to adverse environmental conditions by altering gene expression patterns. Recently, a novel stress adaptation mechanism has been described that allows Escherichia coli to alter gene expression at the post-transcriptional level. The key player in this regulatory pathway is the endoribonuclease MazF, the toxin component of the toxin-antitoxin module mazEF that is triggered by various stressful conditions. In general, MazF degrades the majority of transcripts by cleaving at ACA sites, which results in the retardation of bacterial growth. Furthermore, MazF can process a small subset of mRNAs and render them leaderless by removing their ribosome binding site. MazF concomitantly modifies ribosomes, making them selective for the translation of leaderless mRNAs. In this study, we employed fluorescent reporter-systems to investigate mazEF expression during stressful conditions, and to infer consequences of the mRNA processing mediated by MazF on gene expression at the single-cell level. Our results suggest that mazEF transcription is maintained at low levels in single cells encountering adverse conditions, such as antibiotic stress or amino acid starvation. Moreover, using the grcA mRNA as a model for MazF-mediated mRNA processing, we found that MazF activation promotes heterogeneity in the grcA reporter expression, resulting in a subpopulation of cells with increased levels of GrcA reporter protein.

Translational Repression of the RpoS Antiadapter IraD by CsrA Is Mediated via Translational Coupling to a Short Upstream Open Reading Frame

CsrA is a global regulatory RNA binding protein that has important roles in regulating carbon metabolism, motility, biofilm formation, and numerous other cellular processes. IraD functions as an antiadapter protein that inhibits RssB-mediated degradation of RpoS, the general stress response and stationary-phase sigma factor of Escherichia coli. Here we identified a novel mechanism in which CsrA represses iraD translation via translational coupling. Expression studies with quantitative reverse transcriptase PCR, Western blotting, and lacZ fusions demonstrated that CsrA represses iraD expression. Gel mobility shift, footprint, and toeprint studies identified four CsrA binding sites in the iraD leader transcript, all of which are far upstream of the iraD ribosome binding site. Computational modeling and RNA structure mapping identified an RNA structure that sequesters the iraD Shine-Dalgarno (SD) sequence. Three open reading frames (ORFs), all of which are translated, were identified in the iraD leader region. Two of these ORFs do not affect iraD expression. However, the translation initiation region of the third ORF contains three of the CsrA binding sites, one of which overlaps its SD sequence. Furthermore, the ORF stop codon overlaps the iraD start codon, a sequence arrangement indicative of translational coupling. In vivo expression and in vitro translation studies with wild-type and mutant reporter fusions demonstrated that bound CsrA directly represses translation initiation of this ORF. We further established that CsrA-dependent repression of iraD translation occurs entirely via translational coupling with this ORF, leading to accelerated iraD mRNA decay.

CsrA posttranscriptionally represses gene expression associated with stationary-phase bacterial growth, often in opposition to the transcriptional effects of the stationary-phase sigma factor RpoS. We show that CsrA employs a novel regulatory mechanism to repress translation of iraD, which encodes an antiadapter protein that protects RpoS against proteolysis. CsrA binds to four sites in the iraD leader transcript but does not directly occlude ribosome binding to the iraD SD sequence. Instead, CsrA represses translation of a short open reading frame encoded upstream of iraD, causing repression of iraD translation via translational coupling. This finding offers a novel mechanism of gene regulation by the global regulator CsrA, and since RpoS can activate csrA transcription, this also highlights a new negative-feedback loop within the complex Csr and RpoS circuitry.

Complex chromosomal neighborhood effects determine the adaptive potential of a gene under selection

How the organization of genes on a chromosome shapes adaptation is essential for understanding evolutionary paths. Here, we investigate how adaptation to rapidly increasing levels of antibiotic depends on the chromosomal neighborhood of a drug-resistance gene inserted at different positions of the Escherichia coli chromosome. Using a dual-fluorescence reporter that allows us to distinguish gene amplifications from other up-mutations, we track in real-time adaptive changes in expression of the drug-resistance gene. We find that the relative contribution of several mutation types differs systematically between loci due to properties of neighboring genes: essentiality, expression, orientation, termination, and presence of duplicates. These properties determine rate and fitness effects of gene amplification, deletions, and mutations compromising transcriptional termination. Thus, the adaptive potential of a gene under selection is a system-property with a complex genetic basis that is specific for each chromosomal locus, and it can be inferred from detailed functional and genomic data.

DOI:http://dx.doi.org/10.7554/eLife.25100.001

MarRA, SoxSR, and Rob encode a signal dependent regulatory network in Escherichia coli

When exposed to low concentrations of toxic chemicals, bacteria modulate the expression of a number of cellular processes. Typically, these processes include those related to porin production, dismutases, and metabolic fluxes. In Escherichia coli (E. coli), the expression of these systems is largely controlled by three homologous transcriptional regulators: MarA, SoxS, and Rob. Each of the three regulators responds to distinct chemical signals (salicylate for MarA; paraquat for SoxS; and bipyridyl for Rob) and controls the expression of an overlapping set of downstream targets. In addition, the three systems autoregulate their own expression, and cross-regulate each other's expression. Specifically, MarA is known to activate SoxS expression, and Rob is known to activate MarA expression. In addition, a number of conflicting regulatory interactions are known to exist between the three loci. Thus, the three systems encode a complex regulatory topology with multiple feedback loops, the precise nature of whose interactions or their significance in cellular physiology is not well understood currently. In this work, we focus on understanding the details of this crosstalk between the Mar-Sox-Rob systems in E. coli, and the resulting control and dynamics of the expression of cellular processes by studying gene expression at the population level and at single-cell resolution in wild type and mutants. Our results indicate that the regulatory architecture between MarA, SoxS, and Rob is dependent on the signal (inducer) present in the environment. The regulators, in response to an inducer, form a Feed Forward Loop (FFL), which leads to faster and stronger induction of target genes in the cell, consequently resulting in better cellular growth. Through the FFL, the cell is able to integrate qualitatively different signals in the network, and consequently, control cellular physiology. In addition, we present two intriguing dynamic features of the Mar-Sox-Rob regulon. First, in the presence of salicylate, the activation of target genes via MarA and Rob, at single-cell resolution, is qualitatively different. Second, we report the synergistic activation of target and Mar/Sox systems in the presence of both salicylate and paraquat. These results strongly indicate that there exists a complex control of gene regulation in the Mar-Sox-Rob regulon. Mechanistic details of this control are likely quite complex, and may involve additional regulators.

NlpD links cell wall remodeling and outer membrane invagination during cytokinesis in Escherichia coli

Cytokinesis in gram-negative bacteria requires the constriction of all three cell envelope layers: the inner membrane (IM), the peptidoglycan (PG) cell wall and the outer membrane (OM). In order to avoid potentially lethal breaches in cell integrity, this dramatic reshaping of the cell surface requires tight coordination of the different envelope remodeling activities of the cytokinetic ring. However, the mechanisms responsible for this coordination remain poorly defined. One of the few characterized regulatory points in the envelope remodeling process is the activation of cell wall hydrolytic enzymes called amidases. These enzymes split cell wall material shared by developing daughter cells to facilitate their eventual separation. In Escherichia coli, amidase activity requires stimulation by one of two partially redundant activators: EnvC, which is associated with the IM, and NlpD, a lipoprotein anchored in the OM. Here, we investigate the regulation of amidase activation by NlpD. Structure-function analysis revealed that the OM localization of NlpD is critical for regulating its amidase activation activity. To identify additional factors involved in the NlpD cell separation pathway, we also developed a genetic screen using a flow cytometry-based enrichment procedure. This strategy allowed us to isolate mutants that form long chains of unseparated cells specifically when the redundant EnvC pathway is inactivated. The screen implicated the Tol-Pal system and YraP in NlpD activation. The Tol-Pal system is thought to promote OM invagination at the division site. YraP is a conserved protein of unknown function that we have identified as a new OM-localized component of the cytokinetic ring. Overall, our results support a model in which OM and PG remodeling events at the division site are coordinated in part through the coupling of NlpD activation with OM invagination.

Escherichia coli cytochrome c peroxidase is a respiratory oxidase that enables the use of hydrogen peroxide as a terminal electron acceptor

Microbial cytochrome c peroxidases (Ccp) have been studied for 75 years, but their physiological roles are unclear. Ccps are located in the periplasms of bacteria and the mitochondrial intermembrane spaces of fungi. In this study, Ccp is demonstrated to be a significant degrader of hydrogen peroxide in anoxic Escherichia coli Intriguingly, ccp transcription requires both the presence of H₂O₂ and the absence of O₂ Experiments show that Ccp lacks enough activity to shield the cytoplasm from exogenous H₂O₂ However, it receives electrons from the quinone pool, and its flux rate approximates flow to other anaerobic electron acceptors. Indeed, Ccp enabled E. coli to grow on a nonfermentable carbon source when H₂O₂ was supplied. Salmonella behaved similarly. This role rationalizes ccp repression in oxic environments. We speculate that micromolar H₂O₂ is created both biologically and abiotically at natural oxic/anoxic interfaces. The OxyR response appears to exploit this H₂O₂ as a terminal oxidant while simultaneously defending the cell against its toxicity.

Direct and convenient measurement of plasmid stability in lab and clinical isolates of E. coli

Plasmids are important mobile elements in bacteria, contributing to evolution, virulence, and antibiotic resistance. Natural plasmids are generally large and maintained at low copy number and thus prone to be lost. Therefore, dedicated plasmid maintenance systems have evolved, leading to plasmid loss rates as low as 1 per 10⁷ divisions. These low rates complicate studies of plasmid loss, as traditional techniques for measuring plasmid loss are laborious and not quantitative. To overcome these limitations, we leveraged a stringent negative selection system to develop a method for performing direct, quantitative measurements of plasmid loss in E. coli. We applied our method to gain mechanistic insights into a heterologously reconstituted segregation system in lab strains and clinical isolates of E. coli. We also performed direct stability studies of a currently circulating resistance plasmid in a clinical isolate, strain EC958, which is a member of the rapidly expanding global ST131 E. coli clone. Our results establish the foundational assays required to screen for small molecules targeting plasmid stability, which could complement current strategies for reducing the spread of antibiotic resistance, complementing other strategies for treating antibiotic resistant bacteria.

Co-utilization of hexoses by a microconsortium of sugar-specific E. coli strains

Escherichia coli is an important commercial species used for production of biofuels, biopolymers, organic acids, sugar alcohols, and natural compounds. Processed biomass and agroindustrial byproducts serve as low-cost nutrient sources and contain a variety of hexoses available for bioconversion. However, metabolism of hexose mixtures by E. coli is inefficient due to carbon catabolite repression (CCR), where the transport and catabolic activity of one or more carbon sources is repressed and/or inhibited by the transport and catabolism of another carbon source. In this work, we developed a microconsortium of different E. coli strains, each engineered to preferentially catabolize a different hexose-glucose, galactose, or mannose. We modified the specificity and preference of carbon source using a combination of rational strain design and adaptive evolution. The modifications ultimately resulted in strains that preferentially catabolized their specified sugar. Finally, comparative analysis in galactose- and mannose-rich sugar mixtures revealed that the consortium grew faster and to higher cell densities compared to the wild-type strain. Biotechnol. Bioeng. 2017;114: 2309-2318. © 2017 Wiley Periodicals, Inc.

Quantitative Kinetic Analyses of Shutting Off a Two-Component System

Cells rely on accurate control of signaling systems to adapt to environmental perturbations. System deactivation upon stimulus removal is as important as activation of signaling pathways. The two-component system (TCS) is one of the major bacterial signaling schemes. In many TCSs, phosphatase activity of the histidine kinase (HK) is believed to play an essential role in shutting off the pathway and resetting the system to the prestimulus state. Two basic challenges are to understand the dynamic behavior of system deactivation and to quantitatively evaluate the role of phosphatase activity under natural cellular conditions. Here we report a kinetic analysis of the response to shutting off the archetype Escherichia coli PhoR-PhoB TCS pathway using both transcription reporter assays and in vivo phosphorylation analyses. Upon removal of the stimulus, the pathway is shut off by rapid dephosphorylation of the PhoB response regulator (RR) while PhoB-regulated gene products gradually reset to prestimulus levels through growth dilution. We developed an approach combining experimentation and modeling to assess in vivo kinetic parameters of the phosphatase activity with kinetic data from multiple phosphatase-diminished mutants. This enabled an estimation of the PhoR phosphatase activity in vivo, which is much stronger than the phosphatase activity of PhoR cytoplasmic domains analyzed in vitro We quantitatively modeled how strong the phosphatase activity needs to be to suppress nonspecific phosphorylation in TCSs and discovered that strong phosphatase activity of PhoR is required for cross-phosphorylation suppression.IMPORTANCE Activation of TCSs has been extensively studied; however, the kinetics of shutting off TCS pathways is not well characterized. We present comprehensive analyses of the shutoff response for the PhoR-PhoB system that reveal the impact of phosphatase activity on shutoff kinetics. This allows development of a quantitative framework not only to characterize the phosphatase activity in the natural cellular environment but also to understand the requirement for specific strengths of phosphatase activity to suppress nonspecific phosphorylation. Our model suggests that the ratio of the phosphatase rate to the nonspecific phosphorylation rate correlates with TCS expression levels and the ratio of the RR to HK, which may contribute to the great diversity of enzyme levels and activities observed in different TCSs.

Complete nucleotide sequence and annotation of the temperate corynephage ϕ16 genome

The complete genome of ϕ16, a temperate corynephage from Corynebacterium glutamicum ATCC 21792, was sequenced and annotated (GenBank: KY250482). The electron microscopy study of ϕ16 virion confirmed that it belongs to the family Siphoviridae. The ϕ16 genome consists of a linear double-stranded DNA molecule of 58,200 bp (G+C = 52.2%) with protruding cohesive 3’-ends of 14 nt. Four major structural proteins were separated by SDS-PAGE and identified by peptide mass fingerprinting technique. Using bioinformatics analysis, 101 putative ORFs and 5 tRNA genes were predicted. Only 27 putative gene products could be assigned to known biological functions. The ϕ16 genome was divided into functional modules. Seven putative promoters and eight putative unidirectional intrinsic terminators were predicted. One site of putative «-1» programmed ribosomal frameshifting was proposed in the phage tail assembly genome region. C. glutamicum genetic tools could be broadened by exploiting the known integrase gene (gp33) and the newly identified excisionase gene (gp47), participating in site-specific recombination between ϕ16-attP/attB.

Development of a high efficiency integration system and promoter library for rapid modification of Pseudomonas putida KT2440

Pseudomonas putida strains are highly robust bacteria known for their ability to efficiently utilize a variety of carbon sources, including aliphatic and aromatic hydrocarbons. Recently, P. putida has been engineered to valorize the lignin stream of a lignocellulosic biomass pretreatment process. Nonetheless, when compared to platform organisms such as Escherichia coli, the toolkit for engineering P. putida is underdeveloped. Heterologous gene expression in particular is problematic. Plasmid instability and copy number variance provide challenges for replicative plasmids, while use of homologous recombination for insertion of DNA into the chromosome is slow and laborious. Further, most heterologous expression efforts to date typically rely on overexpression of exogenous pathways using a handful of poorly characterized promoters. To improve the P. putida toolkit, we developed a rapid genome integration system using the site-specific recombinase from bacteriophage Bxb1 to enable rapid, high efficiency integration of DNA into the P. putida chromosome. We also developed a library of synthetic promoters with various UP elements, −35 sequences, and −10 sequences, as well as different ribosomal binding sites. We tested these promoters using a fluorescent reporter gene, mNeonGreen, to characterize the strength of each promoter, and identified UP-element-promoter-ribosomal binding sites combinations capable of driving a ~150-fold range of protein expression levels. An additional integrating vector was developed that confers more robust kanamycin resistance when integrated at single copy into the chromosome. This genome integration and reporter systems are extensible for testing other genetic parts, such as examining terminator strength, and will allow rapid integration of heterologous pathways for metabolic engineering.

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BxB1 integrase catalyzes site-specific DNA integration into P. putida chromosome.

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Promoter library (−35/−10 variants) covers a 72-fold range of protein expression.

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Expression can be further tuned by 2-fold in P. putida with RBS and UP-elements.

Salmonella genomic island 1 (SGI1) reshapes the mating apparatus of IncC conjugative plasmids to promote self-propagation

IncC conjugative plasmids and Salmonella genomic island 1 (SGI1) and relatives are frequently associated with multidrug resistance of clinical isolates of pathogenic Enterobacteriaceae. SGI1 is specifically mobilized in trans by IncA and IncC plasmids (commonly referred to as A/C plasmids) following its excision from the chromosome, an event triggered by the transcriptional activator complex AcaCD encoded by these helper plasmids. Although SGI1 is not self-transmissible, it carries three genes, traN_S, traH_S and traG_S, coding for distant homologs of the predicted mating pore subunits TraN_C, TraH_C and TraG_C, respectively, encoded by A/C plasmids. Here we investigated the regulation of traN_S and traHG_S and the role of these three genes in the transmissibility of SGI1. Transcriptional fusion of the promoter sequences of traN_S and traHG_S to the reporter gene lacZ confirmed that expression of these genes is inducible by AcaCD. Mating experiments using combinations of deletion mutants of SGI1 and the helper IncC plasmid pVCR94 revealed complex interactions between these two mobile genetic elements. Whereas traN_C and traHG_C are essential for IncC plasmid transfer, SGI1 could rescue null mutants of each individual gene revealing that TraN_S, TraH_S and TraG_S are functional proteins. Complementation assays of individual tra_C and tra_S mutants showed that not only do TraN_S/H_S/G_S replace TraN_C/H_C/G_C in the mating pore encoded by IncC plasmids but also that traG_S and traH_S are both required for SGI1 optimal transfer. In fact, remodeling of the IncC-encoded mating pore by SGI1 was found to be essential to enhance transfer rate of SGI1 over the helper plasmid. Furthermore, traG_S was found to be crucial to allow DNA transfer between cells bearing IncC helper plasmids, thereby suggesting that by remodeling the mating pore SGI1 disables an IncC-encoded entry exclusion mechanism. Hence tra_S genes facilitate the invasion by SGI1 of cell populations bearing IncC plasmids.

Acquisition and dissemination of multidrug resistance genes among Enterobacteriaceae is in part driven by IncA and IncC (A/C) conjugative plasmids and Salmonella genomic island 1 (SGI1). Although unrelated, SGI1 relies on the self-transmissible A/C plasmids to disseminate within bacterial populations. The mechanisms allowing SGI1 to hijack the mating apparatus synthesized by A/C plasmids have not been previously established. Here, we show that IncC plasmids trigger the expression of three SGI1-borne genes that code for functional mating pore subunits distantly related to those encoded by the IncC helper plasmids. Our results indicate that these subunits alter the mating pore encoded by IncC plasmids to ensure optimal transfer of SGI1 and promote SGI1 dissemination in cell populations harboring IncC plasmids. Apart from SGI1 and relatives, documented mobilizable genomic islands are not known to code for mating pore components, possibly because of redundancy with those encoded by helper conjugative elements. Instead they usually code for mobilization proteins such as a relaxase and auxiliary factors involved in DNA recognition, processing and docking to the mating pore encoded by their helper conjugative element. From an ecological and epidemiological perspective, the strategy used by SGI1 likely confers a strong competitive advantage to SGI1 over IncC plasmids in clinical settings and could account for the high prevalence of SGI1 and relatives in multidrug-resistant Salmonella enterica and Proteus mirabilis.

ZapA and ZapB form an FtsZ-independent structure at midcell

Cell division in Escherichia coli begins with the polymerization of FtsZ into a ring-like structure, the Z-ring, at midcell. All other division proteins are thought to require the Z-ring for recruitment to the future division site. Here, it is reported that the Z-ring associated proteins ZapA and ZapB form FtsZ-independent structures at midcell. Upon Z-ring disruption by the FtsZ polymerization antagonist SulA, ZapA remained at midcell as a cloud-like accumulation. Using ZapA(N60Y), a variant defective for interaction with FtsZ, it was established that these ZapA structures form without a connection to the Z-ring. Furthermore, midcell accumulations of GFP-ZapA(N60Y) often preceded Z-rings at midcell and required ZapB to assemble, suggesting that ZapB polymers form the foundation of these structures. In the absence of MatP, a DNA-binding protein that links ZapB to the chromosomal terminus region, cloud-like ZapA structures still formed but failed to track with the chromosome terminus and did not consistently precede FtsZ at midcell. Taken together, the results suggest that FtsZ-independent structures of ZapA-ZapB provide additional positional cues for Z-ring formation and may help coordinate its assembly with chromosome replication and segregation.

Salmonella enterica Serovar Typhi Lipopolysaccharide O-Antigen Modification Impact on Serum Resistance and Antibody Recognition

Salmonella enterica serovar Typhi is a human-restricted Gram-negative bacterial pathogen responsible for causing an estimated 27 million cases of typhoid fever annually, leading to 217,000 deaths, and current vaccines do not offer full protection. The O-antigen side chain of the lipopolysaccharide is an immunodominant antigen, can define host-pathogen interactions, and is under consideration as a vaccine target for some Gram-negative species. The composition of the O-antigen can be modified by the activity of glycosyltransferase (gtr) operons acquired by horizontal gene transfer. Here we investigate the role of two gtr operons that we identified in the S. Typhi genome. Strains were engineered to express specific gtr operons. Full chemical analysis of the O-antigens of these strains identified gtr-dependent glucosylation and acetylation. The glucosylated form of the O-antigen mediated enhanced survival in human serum and decreased complement binding. A single nucleotide deviation from an epigenetic phase variation signature sequence rendered the expression of this glucosylating gtr operon uniform in the population. In contrast, the expression of the acetylating gtrC gene is controlled by epigenetic phase variation. Acetylation did not affect serum survival, but phase variation can be an immune evasion mechanism, and thus, this modification may contribute to persistence in a host. In murine immunization studies, both O-antigen modifications were generally immunodominant. Our results emphasize that natural O-antigen modifications should be taken into consideration when assessing responses to vaccines, especially O-antigen-based vaccines, and that the Salmonellagtr repertoire may confound the protective efficacy of broad-ranging Salmonella lipopolysaccharide conjugate vaccines.

Global analysis of translation termination in E. coli

Terminating protein translation accurately and efficiently is critical for both protein fidelity and ribosome recycling for continued translation. The three bacterial release factors (RFs) play key roles: RF1 and 2 recognize stop codons and terminate translation; and RF3 promotes disassociation of bound release factors. Probing release factors mutations with reporter constructs containing programmed frameshifting sequences or premature stop codons had revealed a propensity for readthrough or frameshifting at these specific sites, but their effects on translation genome-wide have not been examined. We performed ribosome profiling on a set of isogenic strains with well-characterized release factor mutations to determine how they alter translation globally. Consistent with their known defects, strains with increasingly severe release factor defects exhibit increasingly severe accumulation of ribosomes over stop codons, indicative of an increased duration of the termination/release phase of translation. Release factor mutant strains also exhibit increased occupancy in the region following the stop codon at a significant number of genes. Our global analysis revealed that, as expected, translation termination is generally efficient and accurate, but that at a significant number of genes (≥ 50) the ribosome signature after the stop codon is suggestive of translation past the stop codon. Even native E. coli K-12 exhibits the ribosome signature suggestive of protein extension, especially at UGA codons, which rely exclusively on the reduced function RF2 variant of the K-12 strain for termination. Deletion of RF3 increases the severity of the defect. We unambiguously demonstrate readthrough and frameshifting protein extensions and their further accumulation in mutant strains for a few select cases. In addition to enhancing recoding, ribosome accumulation over stop codons disrupts attenuation control of biosynthetic operons, and may alter expression of some overlapping genes. Together, these functional alterations may either augment the protein repertoire or produce deleterious proteins.

Proteins are the cellular workhorses, performing essentially all of the functions required for cell and organismal survival. But, it takes a great deal of energy to make proteins, making it critical that proteins are made accurately and in the proper time frame. After a ribosome synthesizes a protein, release factors catalyze the accurate and timely release of the finished protein from the ribosome, a process called termination. Ribosomes are then recycled and start the next protein. We utilized ribosome profiling, a method that allows us to follow the position of every ribosome that is making a protein, to globally investigate and strengthen insights on termination fidelity for cells with and without mutant release factors. We find that as we decrease release factor function, the time to terminate/release a protein increases across the genome. We observe that the accuracy of terminating a protein at the correct place decreases on a global scale. Using this metric we identify genes with inherently low termination efficiency and confirm two novel events resulting in extended protein products. In addition we find that beyond disrupting accurate protein synthesis, release factor mutations can alter expression of genes involved in the production of key amino acids.

Electrode-assisted acetoin production in a metabolically engineered Escherichia coli strain

BACKGROUND

This paper describes the metabolic engineering of Escherichia coli for the anaerobic fermentation of glucose to acetoin. Acetoin has well-established applications in industrial food production and was suggested to be a platform chemical for a bio-based economy. However, the biotechnological production is often hampered by the simultaneous formation of several end products in the absence of an electron acceptor. Moreover, typical production strains are often potentially pathogenic. The goal of this study was to overcome these limitations by establishing an electrode-assisted fermentation process in E. coli. Here, the surplus of electrons released in the production process is transferred to an electrode as anoxic and non-depletable electron acceptor.

RESULTS

In a first step, the central metabolism was steered towards the production of pyruvate from glucose by deletion of genes encoding for enzymes of central reactions of the anaerobic carbon metabolism (ΔfrdA-D ΔadhE ΔldhA Δpta-ack). Thereafter, the genes for the acetolactate synthase (alsS) and the acetolactate decarboxylase (alsD) were expressed in this strain from a plasmid. Addition of nitrate as electron acceptor led to an anaerobic acetoin production with a yield of up to 0.9 mol acetoin per mol of glucose consumed (90% of the theoretical maximum). In a second step, the electron acceptor nitrate was replaced by a carbon electrode. This interaction necessitated the further expression of c-type cytochromes from Shewanella oneidensis and the addition of the soluble redox shuttle methylene blue. The interaction with the non-depletable electron acceptor led to an acetoin formation with a yield of 79% of the theoretical maximum (0.79 mol acetoin per mol glucose).

CONCLUSION

Electrode-assisted fermentations are a new strategy to produce substances of biotechnological value that are more oxidized than the substrates. Here, we show for the first time a process in which the commonly used chassis strain E. coli was tailored for an electrode-assisted fermentation approach branching off from the central metabolite pyruvate. At this early stage, we see promising results regarding carbon and electron recovery and will use further strain development to increase the anaerobic metabolic turnover rate.

Genome-Wide Transcriptional Response to Varying RpoS Levels in Escherichia coli K-12

The alternative sigma factor RpoS is a central regulator of many stress responses in Escherichia coli The level of functional RpoS differs depending on the stress. The effect of these differing concentrations of RpoS on global transcriptional responses remains unclear. We investigated the effect of RpoS concentration on the transcriptome during stationary phase in rich media. We found that 23% of genes in the E. coli genome are regulated by RpoS, and we identified many RpoS-transcribed genes and promoters. We observed three distinct classes of response to RpoS by genes in the regulon: genes whose expression changes linearly with increasing RpoS level, genes whose expression changes dramatically with the production of only a little RpoS ("sensitive" genes), and genes whose expression changes very little with the production of a little RpoS ("insensitive"). We show that sequences outside the core promoter region determine whether an RpoS-regulated gene is sensitive or insensitive. Moreover, we show that sensitive and insensitive genes are enriched for specific functional classes and that the sensitivity of a gene to RpoS corresponds to the timing of induction as cells enter stationary phase. Thus, promoter sensitivity to RpoS is a mechanism to coordinate specific cellular processes with growth phase and may also contribute to the diversity of stress responses directed by RpoS.IMPORTANCE The sigma factor RpoS is a global regulator that controls the response to many stresses in Escherichia coli Different stresses result in different levels of RpoS production, but the consequences of this variation are unknown. We describe how changing the level of RpoS does not influence all RpoS-regulated genes equally. The cause of this variation is likely the action of transcription factors that bind the promoters of the genes. We show that the sensitivity of a gene to RpoS levels explains the timing of expression as cells enter stationary phase and that genes with different RpoS sensitivities are enriched for specific functional groups. Thus, promoter sensitivity to RpoS is a mechanism that coordinates specific cellular processes in response to stresses.

Mycobacterium tuberculosis and Mycobacterium marinum non-homologous end-joining proteins can function together to join DNA ends in Escherichia coli

Mycobacterium tuberculosis and Mycobacterium smegmatis express a Ku protein and a DNA ligase D and are able to repair DNA double strand breaks (DSBs) by non-homologous end-joining (NHEJ). This pathway protects against DNA damage when bacteria are in stationary phase. Mycobacterium marinum is a member of this mycobacterium family and like M. tuberculosis is pathogenic. M. marinum lives in water, forms biofilms and infects fish and frogs. M. marinum is a biosafety level 2 (BSL2) organism as it can infect humans, although infections are limited to the skin. M. marinum is accepted as a model to study mycobacterial pathogenesis, as M. marinum and M. tuberculosis are genetically closely related and have similar mechanisms of survival and persistence inside macrophage. The aim of this study was to determine whether M. marinum could be used as a model to understand M. tuberculosis NHEJ repair. We identified and cloned the M. marinum genes encoding NHEJ proteins and generated E. coli strains that express the M. marinum Ku (Mm-Ku) and ligase D (Mm-Lig) individually or together (LHmKumLig strain) from expression vectors integrated at phage attachment sites in the genome. We demonstrated that Mm-Ku and Mm-Lig are both required to re-circularize Cla I-linearized plasmid DNA in E. coli. We compared repair of strain LHmKumLig with that of an E. coli strain (BWKuLig#2) expressing the M. tuberculosis Ku (Mt-Ku) and ligase D (Mt-Lig), and found that LHmKumLig performed 3.5 times more repair and repair was more accurate than BWKuLig#2. By expressing the Mm-Ku with the Mt-Lig, or the Mt-Ku with the Mm-Lig in E. coli, we have shown that the NHEJ proteins from M. marinum and M. tuberculosis can function together to join DNA DSBs. NHEJ repair is therefore conserved between the two species. Consequently, M. marinum is a good model to study NHEJ repair during mycobacterial pathogenesis.

MISSA 2.0: an updated synthetic biology toolbox for assembly of orthogonal CRISPR/Cas systems

Efficient generation of plants carrying mutations in multiple genes remains a challenge. Using two or more orthogonal CRISPR/Cas systems can generate plants with multi-gene mutations, but assembly of these systems requires a robust, high-capacity toolkit. Here, we describe MISSA 2.0 (multiple-round in vivo site-specific assembly 2.0), an extensively updated toolkit for assembly of two or more CRISPR/Cas systems. We developed a novel suicide donor vector system based on plasmid RK2, which has much higher cloning capacity than the original, plasmid R6K-based system. We validated the utility of MISSA 2.0 by assembling multiple DNA fragments into the E. coli chromosome, and by creating transgenic Arabidopsis thaliana that constitutively or inducibly overexpress multiple genes. We then demonstrated that the higher cloning capacity of the RK2-derived MISSA 2.0 donor vectors facilitated the assembly of two orthogonal CRISPR/Cas systems including SpCas9 and SaCas9, and thus facilitated the creation of transgenic lines harboring these systems. We anticipate that MISSA 2.0 will enable substantial advancements in multiplex genome editing based on two or more orthogonal CRISPR/Cas9 systems, as well as in plant synthetic biology.

Clonetegration Using OSIP Plasmids: One-Step DNA Assembly and Site-Specific Genomic Integration in Bacteria

Clonetegration is a method for site-specific insertion of DNA into prokaryotic chromosomes, based on bacteriophage integrases. The method combines DNA cloning/assembly and chromosomal integration into a single step, providing a simple and rapid strategy for inserting DNA sequences into bacterial chromosomes.

Inhibition of CRISPR-Cas9 with Bacteriophage Proteins

Bacterial CRISPR-Cas systems utilize sequence-specific RNA-guided nucleases to defend against bacteriophage infection. As a countermeasure, numerous phages are known that produce proteins to block the function of class 1 CRISPR-Cas systems. However, currently no proteins are known to inhibit the widely used class 2 CRISPR-Cas9 system. To find these inhibitors, we searched cas9-containing bacterial genomes for the co-existence of a CRISPR spacer and its target, a potential indicator for CRISPR inhibition. This analysis led to the discovery of four unique type II-A CRISPR-Cas9 inhibitor proteins encoded by Listeria monocytogenes prophages. More than half of L. monocytogenes strains with cas9 contain at least one prophage-encoded inhibitor, suggesting widespread CRISPR-Cas9 inactivation. Two of these inhibitors also blocked the widely used Streptococcus pyogenes Cas9 when assayed in Escherichia coli and human cells. These natural Cas9-specific "anti-CRISPRs" present tools that can be used to regulate the genome engineering activities of CRISPR-Cas9.

Optimization and Characterization of the Synthetic Secondary Chromosome synVicII in Escherichia coli

Learning by building is one of the core ideas of synthetic biology research. Consequently, building synthetic chromosomes is the way to fully understand chromosome characteristics. The last years have seen exciting synthetic chromosome studies. We had previously introduced the synthetic secondary chromosome synVicII in Escherichia coli. It is based on the replication mechanism of the secondary chromosome in Vibrio cholerae. Here, we present a detailed analysis of its genetic characteristics and a selection approach to optimize replicon stability. We probe the origin diversity of secondary chromosomes from Vibrionaceae by construction of several new respective replicons. Finally, we present a synVicII version 2.0 with several innovations including its full compatibility with the popular modular cloning (MoClo) assembly system.

A Multiplexed Single-Cell CRISPR Screening Platform Enables Systematic Dissection of the Unfolded Protein Response

Functional genomics efforts face tradeoffs between number of perturbations examined and complexity of phenotypes measured. We bridge this gap with Perturb-seq, which combines droplet-based single-cell RNA-seq with a strategy for barcoding CRISPR-mediated perturbations, allowing many perturbations to be profiled in pooled format. We applied Perturb-seq to dissect the mammalian unfolded protein response (UPR) using single and combinatorial CRISPR perturbations. Two genome-scale CRISPR interference (CRISPRi) screens identified genes whose repression perturbs ER homeostasis. Subjecting ∼100 hits to Perturb-seq enabled high-precision functional clustering of genes. Single-cell analyses decoupled the three UPR branches, revealed bifurcated UPR branch activation among cells subject to the same perturbation, and uncovered differential activation of the branches across hits, including an isolated feedback loop between the translocon and IRE1α. These studies provide insight into how the three sensors of ER homeostasis monitor distinct types of stress and highlight the ability of Perturb-seq to dissect complex cellular responses.

The Virulence Regulator Rns Activates the Expression of CS14 Pili

Although many viral and bacterial pathogens cause diarrhea, enterotoxigenic E. coli (ETEC) is one of the most frequently encountered in impoverished regions where it is estimated to kill between 300,000 and 700,000 children and infants annually. Critical ETEC virulence factors include pili which mediate the attachment of the pathogen to receptors in the intestinal lumen. In this study we show that the ETEC virulence regulator Rns positively regulates the expression of CS14 pili. Three Rns binding sites were identified upstream of the CS14 pilus promoter centered at -34.5, -80.5, and -155.5 relative to the Rns-dependent transcription start site. Mutagenesis of the promoter proximal site significantly decreased expression from the CS14 promoter. In contrast, the contribution of Rns bound at the promoter distal site was negligible and largely masked by occupancy of the promoter proximal site. Unexpectedly, Rns bound at the site centered at -80.5 had a slight but statistically significant inhibitory effect upon the pilin promoter. Nevertheless, this weak inhibitory effect was not sufficient to overcome the substantial promoter activation from Rns bound to the promoter proximal site. Thus, CS14 pili belong to a group of pili that depend upon Rns for their expression.

Long-Term Stable and Tightly Controlled Expression of Recombinant Proteins in Antibiotics-Free Conditions

Plasmid-based gene expression is a fundamental tool in the field of biotechnology. However, overexpression of genes of interest with multi-copy plasmids often causes detrimental effects on host cells. To overcome this problem, chromosomal integration of target genes has been used for decades; however, insufficient protein expression occurred with this method. In this study, we developed a novel cloning and expression system named the chromosomal vector (ChroV) system, that has features of stable and high expression of target genes on the F' plasmid in the Escherichia coli JM109(DE3) strain. We used an RMT cluster (KCTC 11994BP) containing a silent cat gene from a previous study to clone a gene into the F' plasmid. The ChroV system was applied to clone two model targets, GFPuv and carotenoids gene clusters (4 kb), and successfully used to prove the inducible tightly regulated protein expression in the F' plasmid. In addition, we verified that the expression of heterologous genes in ChroV system maintained stably in the absence of antibiotics for 1 week, indicating ChroV system is applicable to antibiotics-free production of valuable proteins. This protocol can be widely applied to recombinant protein expression for antibiotics-free, stable, and genome-based expression, providing a new platform for recombinant protein synthesis in E. coli. Overall, our approach can be widely used for the economical and industrial production of proteins in E. coli.

Novel Technologies for Optimal Strain Breeding

The implementation of a knowledge-based bioeconomy requires the rapid development of highly efficient microbial production strains that are able to convert renewable carbon sources to value-added products, such as bulk and fine chemicals, pharmaceuticals, or proteins at industrial scale. Starting from classical strain breeding by random mutagenesis and screening in the 1950s via rational design by metabolic engineering initiated in the 1970s, a range of powerful new technologies have been developed in the past two decades that can revolutionize future strain engineering. In particular, next-generation sequencing technologies combined with new methods of genome engineering and high-throughput screening based on genetically encoded biosensors have allowed for new concepts. In this chapter, selected new technologies relevant for breeding microbial production strains with a special emphasis on amino acid producers will be summarized.

Circuitry Linking the Catabolite Repression and Csr Global Regulatory Systems of Escherichia coli

Cyclic AMP (cAMP) and the cAMP receptor protein (cAMP-CRP) and CsrA are the principal regulators of the catabolite repression and carbon storage global regulatory systems, respectively. cAMP-CRP controls the transcription of genes for carbohydrate metabolism and other processes in response to carbon nutritional status, while CsrA binds to diverse mRNAs and regulates translation, RNA stability, and/or transcription elongation. CsrA also binds to the regulatory small RNAs (sRNAs) CsrB and CsrC, which antagonize its activity. The BarA-UvrY two-component signal transduction system (TCS) directly activates csrB and csrC (csrB/C) transcription, while CsrA does so indirectly. We show that cAMP-CRP inhibits csrB/C transcription without negatively regulating phosphorylated UvrY (P-UvrY) or CsrA levels. A crp deletion caused an elevation in CsrB/C levels in the stationary phase of growth and increased the expression of csrB-lacZ and csrC-lacZ transcriptional fusions, although modest stimulation of CsrB/C turnover by the crp deletion partially masked the former effects. DNase I footprinting and other studies demonstrated that cAMP-CRP bound specifically to three sites located upstream from the csrC promoter, two of which overlapped the P-UvrY binding site. These two proteins competed for binding at the overlapping sites. In vitro transcription-translation experiments confirmed direct repression of csrC-lacZ expression by cAMP-CRP. In contrast, cAMP-CRP effects on csrB transcription may be mediated indirectly, as it bound nonspecifically to csrB DNA. In the reciprocal direction, CsrA bound to crp mRNA with high affinity and specificity and yet exhibited only modest, conditional effects on expression. Our findings are incorporated into an emerging model for the response of Csr circuitry to carbon nutritional status.

IMPORTANCE

Csr (Rsm) noncoding small RNAs (sRNAs) CsrB and CsrC of Escherichia coli use molecular mimicry to sequester the RNA binding protein CsrA (RsmA) away from lower-affinity mRNA targets, thus eliciting major shifts in the bacterial lifestyle. CsrB/C transcription and turnover are activated by carbon metabolism products (e.g., formate and acetate) and by a preferred carbon source (glucose), respectively. We show that cAMP-CRP, a mediator of classical catabolite repression, inhibits csrC transcription by binding to the upstream region of this gene and also inhibits csrB transcription, apparently indirectly. We propose that glucose availability activates pathways for both synthesis and turnover of CsrB/C, thus shaping the dynamics of global signaling in response to the nutritional environment by poising CsrB/C sRNA levels for rapid response.

The small membrane protein MgrB regulates PhoQ bifunctionality to control PhoP target gene expression dynamics

In Escherichia coli and other γ-proteobacteria, the PhoQ-PhoP two-component signaling system responds to low extracellular Mg⁺⁺ and cationic antimicrobial peptides. On transition to inducing conditions, the expression of PhoP-dependent genes increases rapidly, but then decays to a new, intermediate steady-state level, a phenomenon often referred to as partial adaptation. The molecular basis for this partial adaptation has been unclear. Here, using time-lapse fluorescence microscopy to examine PhoP-dependent gene expression in individual E. coli cells we show that partial adaptation arises through a negative feedback loop involving the small protein MgrB. When E. coli cells are shifted to low Mg⁺⁺ , PhoQ engages in multiple rounds of autophosphorylation and phosphotransfer to PhoP, which, in turn, drives the expression of mgrB. MgrB then feeds back to inhibit the kinase activity of PhoQ. PhoQ is bifunctional such that, when not active as a kinase, it can stimulate the dephosphorylation of PhoP. Thus, MgrB drives the inactivation of PhoP and the observed adaptation in PhoP-dependent gene expression. Our results clarify the source of feedback inhibition in the E. coli PhoQ-PhoP system and reveal how exogenous factors, such as MgrB, can combine with a canonical two-component signaling pathway to produce complex temporal dynamics in target gene expression.

Design, synthesis, and testing toward a 57-codon genome

Recoding--the repurposing of genetic codons--is a powerful strategy for enhancing genomes with functions not commonly found in nature. Here, we report computational design, synthesis, and progress toward assembly of a 3.97-megabase, 57-codon Escherichia coli genome in which all 62,214 instances of seven codons were replaced with synonymous alternatives across all protein-coding genes. We have validated 63% of recoded genes by individually testing 55 segments of 50 kilobases each. We observed that 91% of tested essential genes retained functionality with limited fitness effect. We demonstrate identification and correction of lethal design exceptions, only 13 of which were found in 2229 genes. This work underscores the feasibility of rewriting genomes and establishes a framework for large-scale design, assembly, troubleshooting, and phenotypic analysis of synthetic organisms.

Conjugative DNA Transfer Is Enhanced by Plasmid R1 Partitioning Proteins

Bacterial conjugation is a form of type IV secretion used to transport protein and DNA directly to recipient bacteria. The process is cell contact-dependent, yet the mechanisms enabling extracellular events to trigger plasmid transfer to begin inside the cell remain obscure. In this study of plasmid R1 we investigated the role of plasmid proteins in the initiation of gene transfer. We find that TraI, the central regulator of conjugative DNA processing, interacts physically, and functionally with the plasmid partitioning proteins ParM and ParR. These interactions stimulate TraI catalyzed relaxation of plasmid DNA in vivo and in vitro and increase ParM ATPase activity. ParM also binds the coupling protein TraD and VirB4-like channel ATPase TraC. Together, these protein-protein interactions probably act to co-localize the transfer components intracellularly and promote assembly of the conjugation machinery. Importantly these data also indicate that the continued association of ParM and ParR at the conjugative pore is necessary for plasmid transfer to start efficiently. Moreover, the conjugative pilus and underlying secretion machinery assembled in the absence of Par proteins mediate poor biofilm formation and are completely dysfunctional for pilus specific R17 bacteriophage uptake. Thus, functional integration of Par components at the interface of relaxosome, coupling protein, and channel ATPases appears important for an optimal conformation and effective activation of the transfer machinery. We conclude that low copy plasmid R1 has evolved an active segregation system that optimizes both its vertical and lateral modes of dissemination.

Creating Single-Copy Genetic Circuits

Synthetic biology is increasingly used to develop sophisticated living devices for basic and applied research. Many of these genetic devices are engineered using multi-copy plasmids, but as the field progresses from proof-of-principle demonstrations to practical applications, it is important to develop single-copy synthetic modules that minimize consumption of cellular resources and can be stably maintained as genomic integrants. Here we use empirical design, mathematical modeling, and iterative construction and testing to build single-copy, bistable toggle switches with improved performance and reduced metabolic load that can be stably integrated into the host genome. Deterministic and stochastic models led us to focus on basal transcription to optimize circuit performance and helped to explain the resulting circuit robustness across a large range of component expression levels. The design parameters developed here provide important guidance for future efforts to convert functional multi-copy gene circuits into optimized single-copy circuits for practical, real-world use.

The role of repressor kinetics in relief of transcriptional interference between convergent promoters

Transcriptional interference (TI), where transcription from a promoter is inhibited by the activity of other promoters in its vicinity on the same DNA, enables transcription factors to regulate a target promoter indirectly, inducing or relieving TI by controlling the interfering promoter. For convergent promoters, stochastic simulations indicate that relief of TI can be inhibited if the repressor at the interfering promoter has slow binding kinetics, making it either sensitive to frequent dislodgement by elongating RNA polymerases (RNAPs) from the target promoter, or able to be a strong roadblock to these RNAPs. In vivo measurements of relief of TI by CI or Cro repressors in the bacteriophage λ PR-PRE system show strong relief of TI and a lack of dislodgement and roadblocking effects, indicative of rapid CI and Cro binding kinetics. However, repression of the same λ promoter by a catalytically dead CRISPR Cas9 protein gave either compromised or no relief of TI depending on the orientation at which it binds DNA, consistent with dCas9 being a slow kinetics repressor. This analysis shows how the intrinsic properties of a repressor can be evolutionarily tuned to set the magnitude of relief of TI.

A series of template plasmids for Escherichia coli genome engineering

Metabolic engineering strategies often employ multi-copy episomal vectors to overexpress genes. However, chromosome-based overexpression is preferred as it avoids the use of selective pressure and reduces metabolic burden on the cell. We have constructed a series of template plasmids for λ Red-mediated Escherichia coli genome engineering. The template plasmids allow construction of genome integrating cassettes that can be used to integrate single copies of DNA sequences at predetermined sites or replace promoter regions. The constructed cassettes provide flexibility in terms of expression levels achieved and antibiotics used for selection, as well as allowing construction of marker-free strains. The modular design of the template plasmids allows replacement of genetic parts to construct new templates. Gene integration and promoter replacement using the template plasmids are illustrated.

Antagonistic control of the turnover pathway for the global regulatory sRNA CsrB by the CsrA and CsrD proteins

The widely conserved protein CsrA (carbon storage regulator A) globally regulates bacterial gene expression at the post-transcriptional level. In many species, CsrA activity is governed by untranslated sRNAs, CsrB and CsrC in Escherichia coli, which bind to multiple CsrA dimers, sequestering them from lower affinity mRNA targets. Both the synthesis and turnover of CsrB/C are regulated. Their turnover requires the housekeeping endonuclease RNase E and is activated by the presence of a preferred carbon source via the binding of EIIA^Glc of the glucose transport system to the GGDEF-EAL domain protein CsrD. We demonstrate that the CsrB 3′ segment contains the features necessary for CsrD-mediated decay. RNase E cleavage in an unstructured segment located immediately upstream from the intrinsic terminator is necessary for subsequent degradation to occur. CsrA stabilizes CsrB against RNase E cleavage by binding to two canonical sites adjacent to the necessary cleavage site, while CsrD acts by overcoming CsrA-mediated protection. Our genetic, biochemical and structural studies establish a molecular framework for sRNA turnover by the CsrD-RNase E pathway. We propose that CsrD evolution was driven by the selective advantage of decoupling Csr sRNA decay from CsrA binding, connecting it instead to the availability of a preferred carbon source.

A population-based temporal logic gate for timing and recording chemical events

Engineered bacterial sensors have potential applications in human health monitoring, environmental chemical detection, and materials biosynthesis. While such bacterial devices have long been engineered to differentiate between combinations of inputs, their potential to process signal timing and duration has been overlooked. In this work, we present a two‐input temporal logic gate that can sense and record the order of the inputs, the timing between inputs, and the duration of input pulses. Our temporal logic gate design relies on unidirectional DNA recombination mediated by bacteriophage integrases to detect and encode sequences of input events. For an E. coli strain engineered to contain our temporal logic gate, we compare predictions of Markov model simulations with laboratory measurements of final population distributions for both step and pulse inputs. Although single cells were engineered to have digital outputs, stochastic noise created heterogeneous single‐cell responses that translated into analog population responses. Furthermore, when single‐cell genetic states were aggregated into population‐level distributions, these distributions contained unique information not encoded in individual cells. Thus, final differentiated sub‐populations could be used to deduce order, timing, and duration of transient chemical events.

Molecular Cloning Designer Simulator (MCDS): All-in-one molecular cloning and genetic engineering design, simulation and management software for complex synthetic biology and metabolic engineering projects

Molecular Cloning Designer Simulator (MCDS) is a powerful new all-in-one cloning and genetic engineering design, simulation and management software platform developed for complex synthetic biology and metabolic engineering projects. In addition to standard functions, it has a number of features that are either unique, or are not found in combination in any one software package: (1) it has a novel interactive flow-chart user interface for complex multi-step processes, allowing an integrated overview of the whole project; (2) it can perform a user-defined workflow of cloning steps in a single execution of the software; (3) it can handle multiple types of genetic recombineering, a technique that is rapidly replacing classical cloning for many applications; (4) it includes experimental information to conveniently guide wet lab work; and (5) it can store results and comments to allow the tracking and management of the whole project in one platform. MCDS is freely available from https://mcds.codeplex.com.

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MCDS is an all-in-one in silico design, simulation and management platform.

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MCDS supports the design, simulation management of most cloning and recombineering technologies.

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MCDS has a novel interactive flowchart that allows simpler and more precise transactions.

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MCDS enables complete information integrity and consistency by keeping all details in one file.

λ Recombination and Recombineering

The bacteriophage λ Red homologous recombination system has been studied over the past 50 years as a model system to define the mechanistic details of how organisms exchange DNA segments that share extended regions of homology. The λ Red system proved useful as a system to study because recombinants could be easily generated by co-infection of genetically marked phages. What emerged from these studies was the recognition that replication of phage DNA was required for substantial Red-promoted recombination in vivo, and the critical role that double-stranded DNA ends play in allowing the Red proteins access to the phage DNA chromosomes. In the past 16 years, however, the λ Red recombination system has gained a new notoriety. When expressed independently of other λ functions, the Red system is able to promote recombination of linear DNA containing limited regions of homology (∼50 bp) with the Escherichia coli chromosome, a process known as recombineering. This review explains how the Red system works during a phage infection, and how it is utilized to make chromosomal modifications of E. coli with such efficiency that it changed the nature and number of genetic manipulations possible, leading to advances in bacterial genomics, metabolic engineering, and eukaryotic genetics.

Transcriptional Repression of the VC2105 Protein by Vibrio FadR Suggests that It Is a New Auxiliary Member of the fad Regulon

Recently, our group along with others reported that the Vibrio FadR regulatory protein is unusual in that, unlike the prototypical fadR product of Escherichia coli, which has only one ligand-binding site, Vibrio FadR has two ligand-binding sites and represents a new mechanism for fatty acid sensing. The promoter region of the vc2105 gene, encoding a putative thioesterase, was mapped, and a putative FadR-binding site (AA CTG GTA AGA GCA CTT) was proposed. Different versions of the FadR regulatory proteins were prepared and purified to homogeneity. Both electrophoretic mobility shift assay (EMSA) and surface plasmon resonance (SPR) determined the direct interaction of the vc2105 gene with FadR proteins of various origins. Further, EMSAs illustrated that the addition of long-chain acyl-coenzyme A (CoA) species efficiently dissociates the vc2105 promoter from the FadR regulator. The expression level of the Vibrio cholerae vc2105 gene was elevated 2- to 3-fold in a fadR null mutant strain, validating that FadR is a repressor for the vc2105 gene. The β-galactosidase activity of a vc2105-lacZ transcriptional fusion was increased over 2-fold upon supplementation of growth medium with oleic acid. Unlike the fadD gene, a member of the Vibrio fad regulon, the VC2105 protein played no role in bacterial growth and virulence-associated gene expression of ctxAB (cholera toxin A/B) and tcpA (toxin coregulated pilus A). Given that the transcriptional regulation of vc2105 fits the criteria for fatty acid degradation (fad) genes, we suggested that it is a new member of the Vibrio fad regulon.

IMPORTANCE

The Vibrio FadR regulator is unusual in that it has two ligand-binding sites. Different versions of the FadR regulatory proteins were prepared and characterized in vitro and in vivo. An auxiliary fad gene (vc2105) from Vibrio was proposed that encodes a putative thioesterase and has a predicted FadR-binding site (AAC TGG TA A GAG CAC TT). The function of this putative binding site was proved using both EMSA and SPR. Further in vitro and in vivo experiments revealed that the Vibrio FadR is a repressor for the vc2105 gene. Unlike fadD, a member of the Vibrio fad regulon, VC2105 played no role in bacterial growth and expression of the two virulence-associated genes (ctxAB and tcpA). Therefore, since transcriptional regulation of vc2105 fits the criteria for fad genes, it seems likely that vc2105 acts as a new auxiliary member of the Vibrio fad regulon.

A BioBrick™-Compatible Vector for Allelic Replacement Using the XylE Gene as Selection Marker

Background

Circular plasmid-mediated homologous recombination is commonly used for marker-less allelic replacement, exploiting the endogenous recombination machinery of the host. Common limitations of existing methods include high false positive rates due to mutations in counter-selection genes, and limited applicability to specific strains or growth media. Finally, solutions compatible with physical standards, such as the BioBrick™, are not currently available, although they proved to be successful in the design of other replicative or integrative plasmids.

Findings

We illustrate pBBknock, a novel BioBrick™-compatible vector for allelic replacement in Escherichia coli. It includes a temperature-sensitive replication origin and enables marker-less genome engineering via two homologous recombination events. Chloramphenicol resistance allows positive selection of clones after the first event, whereas a colorimetric assay based on the xylE gene provides a simple way to screen clones in which the second recombination event occurs. Here we successfully use pBBknock to delete the lactate dehydrogenase gene in E. coli W, a popular host used in metabolic engineering.

Conclusions

Compared with other plasmid-based solutions, pBBknock has a broader application range, not being limited to specific strains or media. We expect that pBBknock will represent a versatile solution both for practitioners, also among the iGEM competition teams, and for research laboratories that use BioBrick™-based assembly procedures.

Electronic supplementary material

The online version of this article (doi:10.1186/s12575-016-0036-z) contains supplementary material, which is available to authorized users.

Revisiting demand rules for gene regulation

Starting with Savageau's pioneering work regarding demand rules for gene regulation from the 1970s, here, we choose the simplest transcription network and ask: how does the cell choose a particular regulatory topology from all available possibilities?