Genetic Loci of Plant Pathogenic Dickeya solani IPO 2222 Expressed in Contact with Weed-Host Bittersweet Nightshade (Solanum dulcamara L.) Plants

Dickeya solani, belonging to the Soft Rot Pectobacteriaceae, are aggressive necrotrophs, exhibiting both a wide geographic distribution and a wide host range that includes many angiosperm orders, both dicot and monocot plants, cultivated under all climatic conditions. Little is known about the infection strategies D. solani employs to infect hosts other than potato (Solanum tuberosum L.). Our earlier study identified D. solani Tn5 mutants induced exclusively by the presence of the weed host S. dulcamara. The current study assessed the identity and virulence contribution of the selected genes mutated by the Tn5 insertions and induced by the presence of S. dulcamara. These genes encode proteins with functions linked to polyketide antibiotics and polysaccharide synthesis, membrane transport, stress response, and sugar and amino acid metabolism. Eight of these genes, encoding UvrY (GacA), tRNA guanosine transglycosylase Tgt, LPS-related WbeA, capsular biosynthesis protein VpsM, DltB alanine export protein, glycosyltransferase, putative transcription regulator YheO/PAS domain-containing protein, and a hypothetical protein, were required for virulence on S. dulcamara plants. The implications of D. solani interaction with a weed host, S. dulcamara, are discussed.

Selection of Flax Genotypes for Pan-Genomic Studies by Sequencing Tagmentation-Based Transcriptome Libraries

Flax (Linum usitatissimum L.) products are used in the food, pharmaceutical, textile, polymer, medical, and other industries. The creation of a pan-genome will be an important advance in flax research and breeding. The selection of flax genotypes that sufficiently cover the species diversity is a crucial step for the pan-genomic study. For this purpose, we have adapted a method based on Illumina sequencing of transcriptome libraries prepared using the Tn5 transposase (tagmentase). This approach reduces the cost of sample preparation compared to commercial kits and allows the generation of a large number of cDNA libraries in a short time. RNA-seq data were obtained for 192 flax plants (3-6 individual plants from 44 flax accessions of different morphology and geographical origin). Evaluation of the genetic relationship between flax plants based on the sequencing data revealed incorrect species identification for five accessions. Therefore, these accessions were excluded from the sample set for the pan-genomic study. For the remaining samples, typical genotypes were selected to provide the most comprehensive genetic diversity of flax for pan-genome construction. Thus, high-throughput sequencing of tagmentation-based transcriptome libraries showed high efficiency in assessing the genetic relationship of flax samples and allowed us to select genotypes for the flax pan-genomic analysis.

HiCAR: Analysis of Open Chromatin Associated Long-range Chromatin Interaction Using Low-Input Materials

Cis-regulatory elements (cREs) and their long-range interactions are crucial for spatial-temporal gene regulation. While cREs can be characterized as accessible chromatin sequences, comprehensively identifying their spatial interactions remains a challenge. We recently developed a method, HiCAR (Hi-C on Accessible Regulatory DNA), which combines Tn5 transposase and chromatin proximity ligation to analyze open chromatin-anchored interactions in low-input cells. Application of HiCAR in human embryonic stem cells and lymphoblastoid cells reveals high-resolution chromatin contacts with efficiency comparable to in situ Hi-C across various distance ranges. Moreover, HiCAR was successfully applied to 30,000 primary human muscle stem cells, showcasing its potential for analyzing chromatin accessibility and looping in low-input primary cells and clinical samples. Here, we provide a detailed step-by-step protocol to perform the updated HiCAR experiments. © 2023 Wiley Periodicals LLC. Basic Protocol 1: Tn5 Transposase Assembly Basic Protocol 2: HiCAR Library Preparation.

Biotinylated Tn5 transposase-mediated CUT&Tag efficiently profiles transcription factor-DNA interactions in plants

In contrast to CUT&Tag approaches for profiling bulk histone modifications, current CUT&Tag methods for analysing specific transcription factor (TF)-DNA interactions remain technically challenging due to TFs having relatively low abundance. Moreover, an efficient CUT&Tag strategy for plant TFs is not yet available. Here, we first applied biotinylated Tn5 transposase-mediated CUT&Tag (B-CUT&Tag) to produce high-quality libraries for interrogating TF-DNA interactions. B-CUT&Tag combines streptavidin-biotin-based DNA purification with routine CUT&Tag, optimizing the removal of large amounts of intact chromatin not targeted by specific TFs. The biotinylated chromatin fragments are then purified for construction of deep sequencing libraries or qPCR analysis. We applied B-CUT&Tag to probe genome-wide DNA targets of Squamosa promoter-binding-like protein 9 (SPL9), a well-established TF in Arabidopsis; the resulting profiles were efficient and consistent in demonstrating its well-established target genes in juvenile-adult transition/flowering, trichome development, flavonoid biosynthesis, wax synthesis and branching. Interestingly, our results indicate functions of AtSPL9 in modulating growth-defence trade-offs. In addition, we established a method for applying qPCR after CUT&Tag (B-CUT&Tag-qPCR) and successfully validated the binding of SPL9 in Arabidopsis and PHR2 in rice. Our study thus provides a convenient and highly efficient CUT&Tag strategy for profiling TF-chromatin interactions that is widely applicable to the annotation of cis-regulatory elements for crop improvement.

Tn5 Transposon-based Mutagenesis for Engineering Phage-resistant Strains of Escherichia coli BL21 (DE3)

Escherichia coli is a preferred strain for recombinant protein production, however, it is often plagued by phage infection during experimental studies and industrial fermentation. While the existing methods of obtaining phage-resistant strains by natural mutation are not efficient enough and time-consuming. Herein, a high-throughput method by combining Tn5 transposon mutation and phage screening was used to produce Escherichia coli BL21 (DE3) phage-resistant strains. Mutant strains PR281-7, PR338-8, PR339-3, PR340-8, and PR347-9 were obtained, and they could effectively resist phage infection. Meanwhile, they had good growth ability, did not contain pseudolysogenic strains, and were controllable. The resultant phage-resistant strains maintained the capabilities of producing recombinant proteins since no difference in mCherry red fluorescent protein expression was found in phage-resistant strains. Comparative genomics showed that PR281-7, PR338-8, PR339-3, and PR340-8 mutated in ecpE, nohD, nrdR, and livM genes, respectively. In this work, a strategy was successfully developed to obtain phage-resistant strains with excellent protein expression characteristics by Tn5 transposon mutation. This study provides a new reference to solve the phage contamination problem.

Phage resistance of Salmonella enterica obtained by transposon Tn5-mediated SefR gene silent mutation

Salmonella enterica contamination is a primary cause of global food poisoning. Using phages as bactericidal alternatives to antibiotics could confront the issue of drug resistance. However, the problem of phage resistance, especially mutant strains with multiple phage resistance, is a critical barrier to the practical application of phages. In this study, a library of EZ-Tn5 transposable mutants of susceptible host S. enterica B3-6 was constructed. After the infestation pressure of a broad-spectrum phage TP1, a mutant strain with resistance to eight phages was obtained. Analysis of the genome resequencing results revealed that the SefR gene was disrupted in the mutant strain. The mutant strain displayed a reduced adsorption rate of 42% and a significant decrease in swimming and swarming motility, as well as a significantly reduced expression of the flagellar-related FliL and FliO genes to 17% and 36%, respectively. An uninterrupted form of the SefR gene was cloned into vector pET-21a (+) and used for complementation of the mutant strain. The complemented mutant exhibited similar adsorption and motility as the wild-type control. These results suggest that the disrupted flagellar-mediated SefR gene causes an adsorption inhibition, which is responsible for the phage-resistant phenotype of the S. enterica transposition mutant.

A novel strand-specific RNA-sequencing protocol using dU-adaptor-assembled Tn5

Strand-specific RNA-seq is a powerful tool for the discovery of novel transcripts, annotation of genomes, and profiling of gene expression levels. Tn5 transposase has been successfully applied in massive-scale sequencing projects; in particular, Tn5 adaptor modification is used in epigenetics, genomic structure, and chromatin visualization. We developed a novel dU-adaptor-assembled Tn5-mediated strand-specific RNA-sequencing protocol and compared this method with the leading dUTP method in terms of experimental procedure and multiple quality metrics of the generated libraries. The results showed that the dU-Tn5 method is easy to operate and generates a strand-specific RNA-seq library of comparable quality considering library complexity, strand-specificity, evenness, and continuity of annotated transcript coverage. We also evaluated the performance of the dU-Tn5 method in identifying nitrogen-responsive protein-coding genes and long non-coding RNAs in soybean roots. The results indicated that ~62-70% of differentially expressed genes detected from conventional libraries were also detected in dU-Tn5 libraries, indicating good agreement of our method with the current standard; moreover, their fold-changes were highly correlated (R>0.9). Thus, our method provides a promising 'do-it-yourself' stranded RNA-seq procedure for gene expression profiling.

Analysis of Chromatin Interaction and Accessibility by Trac-Looping

Spatial organization of the genome modulates pivotal biological processes. The emerging new technologies have provided novel insights into genome structure and its role in regulating cell activities. To examine the genome-wide chromatin interactions at accessible chromatin regions, we developed a DNA transposase-mediated analysis of chromatin looping (Trac-looping) method for simultaneously detecting chromatin interactions and chromatin accessibility. Here, we describe a detailed protocol of generating Trac-looping libraries.

His-Ala-Phe-Lys peptide from Burkholderia arboris possesses antifungal activity

Burkholderia arboris, which belongs to the Burkholderia cepacia complex, has been shown to possess antifungal activity against several plant fungal pathogens; however, the antifungal compounds are yet to be identified. Here, we identified the antifungal compounds produced by B. arboris using genetic and metabolomic approaches. We generated a Tn5 transposon mutation library of 3,000 B. arboris mutants and isolated three mutants with reduced antifungal activity against the plant fungal pathogen Fusarium oxysporum. Among the mutants, the M464 mutant exhibited the weakest antifungal activity. In the M464 genome, the transposon was inserted into the cobA gene, encoding uroporphyrin-III methyltransferase. Deletion of the cobA gene also resulted in reduced antifungal activity, indicating that the cobA gene contributed to the antifungal activity of B. arboris. Furthermore, a comparison of the differential metabolites between wild type B. arboris and the ∆cobA mutant showed a significantly decreased level of tetrapeptide His-Ala-Phe-Lys (Hafk) in the ∆cobA mutant. Therefore, a Hafk peptide with D-amino acid residues was synthesized and its antifungal activity was evaluated. Notably, the Hafk peptide displayed significant antifungal activity against F. oxysporum and Botrytis cinerea, two plant pathogens that cause destructive fungal diseases. Overall, a novel antifungal compound (Hafk) that can be used for the biocontrol of fungal diseases in plants was identified in B. arboris.

Identification of novel biofilm genes in avian pathogenic Escherichia coli by Tn5 transposon mutant library

Avian pathogenic Escherichia coli (APEC) is the main pathogens that inflict the poultry industry. Biofilm as the pathogenic factors of APEC, which can enhance the anti-host immune system of APEC and improve its survival in the environment. In order to screen for new genes related to APEC biofilm. The APEC strain APEC81 was used to construct a mutant library by Tn5 insertion mutagenesis. Moreover the 28 mutant strains with severely weakened biofilm were successfully screened from 1500 mutant strains by crystal violet staining, in which 17 genes were obtained by high-efficiency thermal asymmetric interlaced PCR. The reported genes include 3 flagella genes (fliS, fliD, and fliR), 4 curli fimbriae genes (csgD, csgA, csgF, and csgG) and 3 type 1 fimbriae genes (fimA, fimD, and fimC). The novel genes include 3 coenzyme genes (gltA, bglX, and mltF) and 4 putative protein genes (yehE, 07045, 11735, 11255). To investigate whether these 17 genes co-regulate the biofilm, the 17 identified genes were deleted from APEC strain APEC81. The results showed that except for the 11735 and 11255 genes, the deletion of 15 genes significantly reduced the biofilm formation ability of APEC81 (P < 0.05). The result of rdar (red, dry and rough) colony morphology showed that curli fimbriae genes (csgD, csgA, csgF, and csgG) and other functional genes (fimC, glxK, yehE, 07045, and 11255) affected the colony morphology. In particular, the hypothetical protein YehE had the greatest influence on the biofilm. It was predicted to have the same structure as the type 1 fimbria protein. When yehE was deleted, the fimE transcription was up-regulated, and the fimA and fimB transcription were down-regulated, resulting in a decrease in type 1 fimbriae. Hence, the yehE mutant significantly reduced the biofilm and the adhesion and invasion ability to cells (P < 0.05). This study identified 5 novel genes (gltA, bglX, mltF, yehE, and 07045) related to biofilm formation and confirmed that yehE affects biofilm formation by type 1 fimbriae, which will benefit further study of the mechanism of biofilm regulation in APEC.

Extensive evaluation of ATAC-seq protocols for native or formaldehyde-fixed nuclei

BACKGROUND

The "Assay for Transposase Accessible Chromatin sequencing" (ATAC-seq) is an efficient and easy to implement protocol to measure chromatin accessibility that has been widely used in multiple applications studying gene regulation. While several modifications or variants of the protocol have been published since it was first described, there has not yet been an extensive evaluation of the effects of specific protocol choices head-to-head in a consistent experimental setting. In this study, we tested multiple protocol options for major ATAC-seq components (including three reaction buffers, two reaction temperatures, two enzyme sources, and the use of either native or fixed nuclei) in a well-characterized cell line. With all possible combinations of components, we created 24 experimental conditions with four replicates for each (a total of 96 samples). In addition, we tested the 12 native conditions in a primary sample type (mouse lung tissue) with two different input amounts. Through these extensive comparisons, we were able to observe the effect of different ATAC-seq conditions on data quality and to examine the utility and potential redundancy of various quality metrics.

RESULTS

In general, native samples yielded more peaks (particularly at loci not overlapping transcription start sites) than fixed samples, and the temperature at which the enzymatic reaction was carried out had a major impact on data quality metrics for both fixed and native nuclei. However, the effect of various conditions tested was not always consistent between the native and fixed samples. For example, the Nextera and Omni buffers were largely interchangeable across all other conditions, while the THS buffer resulted in markedly different profiles in native samples. In-house and commercial enzymes performed similarly.

CONCLUSIONS

We found that the relationship between commonly used measures of library quality differed across temperature and fixation, and so evaluating multiple metrics in assessing the quality of a sample is recommended. Notably, we also found that these choices can bias the functional class of elements profiled and so we recommend evaluating several formulations in any new experiments. Finally, we hope the ATAC-seq workflow formulated in this study on crosslinked samples will help to profile archival clinical specimens.

Fast and inexpensive whole-genome sequencing library preparation from intact yeast cells

Through the increase in the capacity of sequencing machines massively parallel sequencing of thousands of samples in a single run is now possible. With the improved throughput and resulting drop in the price of sequencing, the cost and time for preparation of sequencing libraries have become the major bottleneck in large-scale experiments. Methods using a hyperactive variant of the Tn5 transposase efficiently generate libraries starting from cDNA or genomic DNA in a few hours and are highly scalable. For genome sequencing, however, the time and effort spent on genomic DNA isolation limit the practicability of sequencing large numbers of samples. Here, we describe a highly scalable method for preparing high-quality whole-genome sequencing libraries directly from Saccharomyces cerevisiae cultures in less than 3 h at 34 cents per sample. We skip the rate-limiting step of genomic DNA extraction by directly tagmenting lysed yeast spheroplasts and add a nucleosome release step prior to enrichment PCR to improve the evenness of genomic coverage. Resulting libraries do not show any GC bias and are comparable in quality to libraries processed from genomic DNA with a commercially available Tn5-based kit. We use our protocol to investigate CRISPR/Cas9 on- and off-target edits and reliably detect edited variants and shared polymorphisms between strains. Our protocol enables rapid preparation of unbiased and high-quality, sequencing-ready indexed libraries for hundreds of yeast strains in a single day at a low price. By adjusting individual steps of our workflow, we expect that our protocol can be adapted to other organisms.

Tn5 Transposase Applied in Genomics Research

The development of high-throughput sequencing (next-generation sequencing technology (NGS)) and the continuous increase in experimental throughput require the upstream sample processing steps of NGS to be as simple as possible to improve the efficiency of the entire NGS process. The transposition system has fast “cut and paste” and “copy and paste” functions, and has been innovatively applied to the NGS field. For example, the Assay for Transposase-Accessible Chromatin with high throughput sequencing (ATAC-Seq) uses high-throughput sequencing to detect chromatin regions accessible by Tn5 transposase. Linear Amplification via Transposon Insertion (LIANTI) uses Tn5 transposase for linear amplification, haploid typing, and structural variation detection. Not only is it efficient and simple, it effectively shortens the time for NGS sample library construction, realizes large-scale and rapid sequencing, improves sequencing resolution, and can be flexibly modified for more technological innovation.

Identification of Genes Involved in Antifungal Activity of Burkholderia seminalis Against Rhizoctonia solani Using Tn5 Transposon Mutation Method

Rhizoctonia solani is the causative agent of rice sheath blight disease. In a previous study, we found that the growth of R. solani was inhibited by Burkholderia seminalis strain R456. Therefore, the present study was conducted to identify the genes involved in the antifungal activity of B. seminalis strain R456 by using a Tn5 transposon mutation method. Firstly, we constructed a random insertion transposon library of 997 mutants, out of which 11 mutants showed the defective antifungal activity against R. solani. Furthermore, the 10 antagonism-related genes were successfully identified based on analysis of the Tn5 transposon insertion site. Indeed, this result indicated that three mutants were inserted on an indigenous plasmid in which the same insertion site was observed in two mutants. In addition, the remaining eight mutants were inserted on different genes encoding glycosyl transferase, histone H1, nonribosomal peptide synthetase, methyltransferase, MnmG, sulfate export transporter, catalase/peroxidase HPI and CysD, respectively. Compared to the wild type, the 11 mutants showed a differential effect in bacteriological characteristics such as cell growth, biofilm formation and response to H₂O₂ stress, revealing the complexity of action mode of these antagonism-related genes. However, a significant reduction of cell motility was observed in the 11 mutants compared to the wild type. Therefore, it can be inferred that the antifungal mechanism of the 10 above-mentioned genes may be, at least partially, due to the weakness of cell motility. Overall, the result of this study will be helpful for us to understand the biocontrol mechanism of this bacterium.

Generation of Random luxCDABE Transcriptional Fusions in the Genome of Salmonella enterica

The luxCDABE operon of Photorhabdus luminescens can be used as a bioluminescent reporter to measure gene transcription nondestructively. Here we describe protocols to (1) generate random transcriptional fusions of the lux operon to genes of the Salmonella genome, (2) screen for specific fusions with constitutive expression, Salmonella pathogenicity island 1-related expression, or Salmonella pathogenicity island 2-related expression, and (3) determine the site of luxCDABE integration.

Genome-Wide Identification of Dickeya solani Transcriptional Units Up-Regulated in Response to Plant Tissues From a Crop-Host Solanum tuberosum and a Weed-Host Solanum dulcamara

Dickeya solani is a Gram-negative bacterium able to cause disease symptoms on a variety of crop and ornamental plants worldwide. Weeds including Solanum dulcamara (bittersweet nightshade) growing near agricultural fields have been reported to support populations of soft rot bacteria in natural settings. However, little is known about the specific interaction of D. solani with such weed plants that may contribute to its success as an agricultural pathogen. The aim of this work was to assess the interaction of D. solani with its crop plant (Solanum tuberosum) and an alternative (S. dulcamara) host plant. From a collection of 10,000 Tn5 transposon mutants of D. solani IPO2222 carrying an inducible, promotorless gusA reporter gene, 210 were identified that exhibited plant tissue-dependent expression of the gene/operon into which the Tn5 insertion had occurred. Thirteen Tn5 mutants exhibiting the greatest plant tissue induction of such transcriptional units in S. tuberosum or S. dulcamara as measured by qRT-PCR were assessed for plant host colonization, virulence, and ability to macerate plant tissue, as well as phenotypes likely to contribute to the ecological fitness of D. solani, including growth rate, carbon and nitrogen source utilization, motility, chemotaxis toward plant extracts, biofilm formation, growth under anaerobic conditions and quorum sensing. These 13 transcriptional units encode proteins involved in bacterial interactions with plants, with functions linked to cell envelope structure, chemotaxis and carbon metabolism. The selected 13 genes/operons were differentially expressed in, and thus contributed preferentially to D. solani fitness in potato and/or S. dulcamara stem, leaf, and root tissues.

Transposase-assisted tagmentation of RNA/DNA hybrid duplexes

Tn5-mediated transposition of double-strand DNA has been widely utilized in various high-throughput sequencing applications. Here, we report that the Tn5 transposase is also capable of direct tagmentation of RNA/DNA hybrids in vitro. As a proof-of-concept application, we utilized this activity to replace the traditional library construction procedure of RNA sequencing, which contains many laborious and time-consuming processes. Results of Transposase-assisted RNA/DNA hybrids Co-tagmEntation (termed ‘TRACE-seq’) are compared to traditional RNA-seq methods in terms of detected gene number, gene body coverage, gene expression measurement, library complexity, and differential expression analysis. At the meantime, TRACE-seq enables a cost-effective one-tube library construction protocol and hence is more rapid (within 6 hr) and convenient. We expect this tagmentation activity on RNA/DNA hybrids to have broad potentials on RNA biology and chromatin research.

ATAC-seq on Sorted Adult Mouse Neurons

Transcription regulation is a key aspect of cellular identity established during development and maintained into adulthood. Molecular and biochemical assays that probe the genome are critical tools in exploring mechanisms of transcription regulation and cell type identity. The mammalian brain is composed of a huge diversity of cell types with distinct properties and functions. To understand these specific roles, it is necessary to selectively target cell populations for study. However, the need to selectively study restricted cell populations poses a challenge in neurobiology. It is often difficult to collect sufficient cellular input for many standard biochemical and molecular assays. Recently, important advances have been made to scale assays down, opening up new frontiers to explore molecular mechanisms in neurons. Concurrently, methodologies for preparing neurons for such assays has advanced taking into consideration specific methods to preserve the cell biology meant to be assayed. Here we describe a method for preparing live neurons from adult brain tissue for the Assay for Transposase Accessible Chromatin (ATAC).

Methods for Tn-Seq Analysis in Acinetobacter baumannii

Transposon insertion sequencing (Tn-seq) is a powerful method for identifying genes required for virtually any growth or survival trait in bacteria. The technology employs next-generation DNA sequencing to identify and quantify the relative abundances of individual transposon mutants within complex pools of such mutants. When applied to pools of thousands to millions of random transposon mutants grown under selective pressure, the technique can rapidly identify, at genome scale, the mutants and corresponding genes negatively or positively selected. This chapter presents core protocols for Tn-seq analysis of Acinetobacter baumannii: generation of a high-saturation random transposon mutant pool, and isolation and sequencing of transposon-genome junctions from such a pool for identifying and quantifying the individual mutants. With these tools, the researcher can address diverse biological questions by carrying out selective growth of a mutant pool followed by Tn-seq analysis to identify genotype-phenotype associations.

Implementation of Transposon Mutagenesis in Bifidobacterium

Random transposon mutagenesis allows for relatively rapid, genome-wide surveys to detect genes involved in functional traits, by performing screens of mutant libraries. This approach has been widely applied to identify genes responsible for activities of interest in multiple eukaryote and prokaryote organisms, although most studies on microorganisms have focused on pathogenic and clinically relevant bacteria. In this chapter we describe the implementation of an in vitro Tn5-based transposome strategy to generate a large collection of random mutants in the gut commensal Bifidobacterium breve UCC2003, and discuss considerations when applying this mutagenesis system to other Bifidobacterium species or strains of interest.

Genome-wide analysis of chromatin accessibility using ATAC-seq

Programs of gene transcription are controlled by cis-acting DNA elements, including enhancers, silencers, and promoters. Local accessibility of chromatin has proven to be a highly informative structural feature for identifying such regulatory elements, which tend to be relatively open due to their interactions with proteins. Recently, ATAC-seq (assay for transposase-accessible chromatin using sequencing) has emerged as one of the most powerful approaches for genome-wide chromatin accessibility profiling. This method assesses DNA accessibility using hyperactive Tn5 transposase, which simultaneously cuts DNA and inserts sequencing adaptors, preferentially in regions of open chromatin. ATAC-seq is a relatively simple procedure which can be applied to only a few thousand cells. It is well-suited to developing embryos of sea urchins and other echinoderms, which are a prominent experimental model for understanding the genomic control of animal development. In this chapter, we present a protocol for applying ATAC-seq to embryonic cells of sea urchins.

Chromatic Bacteria - A Broad Host-Range Plasmid and Chromosomal Insertion Toolbox for Fluorescent Protein Expression in Bacteria

Differential fluorescent labeling of bacteria has become instrumental for many aspects of microbiological research, such as the study of biofilm formation, bacterial individuality, evolution, and bacterial behavior in complex environments. We designed a variety of plasmids, each bearing one of eight unique, constitutively expressed fluorescent protein genes in conjunction with one of four different antibiotic resistance combinations. The fluorophores mTagBFP2, mTurquoise2, sGFP2, mClover3, sYFP2, mOrange2, mScarlet-I, and mCardinal, encoding for blue, cyan, green, green-yellow, yellow, orange, red, and far-red fluorescent proteins, respectively, were combined with selectable markers conferring tetracycline, gentamicin, kanamycin, and/or chloramphenicol resistance. These constructs were cloned into three different plasmid backbones: a broad host-range plasmid, a Tn5 transposon delivery plasmid, and a Tn7 transposon delivery plasmid. The utility of the plasmids and transposons was tested in bacteria from the phyla Actinobacteria, Proteobacteria, and Bacteroidetes. We were able to tag representatives from the phylum Proteobacteria at least via our Tn5 transposon delivery system. The present study enables labeling bacteria with a set of plasmids available to the community. One potential application of fluorescently-tagged bacterial species is the study of bacteria-bacteria, bacteria-host, and bacteria-environment interactions.

Dissecting the Acid Stress Response of Rhizobium tropici CIAT 899

Rhizobium tropici CIAT899 is a nodule-forming α-proteobacterium displaying intrinsic resistance to several abiotic stress conditions such as low pH and high temperatures, which are common in tropical environments. It is a good competitor for Phaseolus vulgaris (common bean) nodule occupancy at low pH values, however little is known about the genetic and physiological basis of the tolerance to acidic conditions. To identify genes in R. tropici involved in pH stress response we combined two different approaches: (1) A Tn5 mutant library of R. tropici CIAT899 was screened and 26 acid-sensitive mutants were identified. For 17 of these mutants, the transposon insertion sites could be identified. (2) We also studied the transcriptomes of cells grown under different pH conditions using RNA-Seq. RNA was extracted from cells grown for several generations in minimal medium at 6.8 or 4.5 (adapted cells). In addition, we acid-shocked cells pre-grown at pH 6.8 for 45 min at pH 4.5. Of the 6,289 protein-coding genes annotated in the genome of R. tropici CIAT 899, 383 were differentially expressed under acidic conditions (pH 4.5) vs. control condition (pH 6.8). Three hundred and fifty one genes were induced and 32 genes were repressed; only 11 genes were induced upon acid shock. The acid stress response of R. tropici CIAT899 is versatile: we found genes encoding response regulators and membrane transporters, enzymes involved in amino acid and carbohydrate metabolism and proton extrusion, in addition to several hypothetical genes. Our findings enhance our understanding of the core genes that are important during the acid stress response in R. tropici.

SALP, a new single-stranded DNA library preparation method especially useful for the high-throughput characterization of chromatin openness states

BACKGROUND

Next-generation sequencing (NGS) is fundamental to the current biological and biomedical research. Construction of sequencing library is a key step of NGS. Therefore, various library construction methods have been explored. However, the current methods are still limited by some shortcomings.

RESULTS

This study developed a new NGS library construction method, Single strand Adaptor Library Preparation (SALP), by using a novel single strand adaptor (SSA). SSA is a double-stranded oligonucleotide with a 3' overhang of 3 random nucleotides, which can be efficiently ligated to the 3' end of single strand DNA by T4 DNA ligase. SALP can be started with any denatured DNA fragments such as those sheared by Tn5 tagmentation, enzyme digestion and sonication. When started with Tn5-tagmented chromatin, SALP can overcome a key limitation of ATAC-seq and become a high-throughput NGS library construction method, SALP-seq, which can be used to comparatively characterize the chromatin openness state of multiple cells unbiasly. In this way, this study successfully characterized the comparative chromatin openness states of four different cell lines, including GM12878, HepG2, HeLa and 293T, with SALP-seq. Similarly, this study also successfully characterized the chromatin openness states of HepG2 cells with SALP-seq by using 10⁵ to 500 cells.

CONCLUSIONS

This study developed a new NGS library construction method, SALP, by using a novel kind of single strand adaptor (SSA), which should has wide applications in the future due to its unique performance.

Large-Scale Low-Cost NGS Library Preparation Using a Robust Tn5 Purification and Tagmentation Protocol

Efficient preparation of high-quality sequencing libraries that well represent the biological sample is a key step for using next-generation sequencing in research. Tn5 enables fast, robust, and highly efficient processing of limited input material while scaling to the parallel processing of hundreds of samples. Here, we present a robust Tn5 transposase purification strategy based on an N-terminal His₆-Sumo3 tag. We demonstrate that libraries prepared with our in-house Tn5 are of the same quality as those processed with a commercially available kit (Nextera XT), while they dramatically reduce the cost of large-scale experiments. We introduce improved purification strategies for two versions of the Tn5 enzyme. The first version carries the previously reported point mutations E54K and L372P, and stably produces libraries of constant fragment size distribution, even if the Tn5-to-input molecule ratio varies. The second Tn5 construct carries an additional point mutation (R27S) in the DNA-binding domain. This construct allows for adjustment of the fragment size distribution based on enzyme concentration during tagmentation, a feature that opens new opportunities for use of Tn5 in customized experimental designs. We demonstrate the versatility of our Tn5 enzymes in different experimental settings, including a novel single-cell polyadenylation site mapping protocol as well as ultralow input DNA sequencing.

Profiling of Single-Cell Transcriptomes

Complex biological systems are composed of multiple cell types whose transcriptional activity can vary due to differences in cell state, environmental stimulation, or intrinsic programs. Conventional bulk analysis methods capture the average transcriptional programs of the cell population, thus missing the unique cellular signature of each single cell. In recent years, the development of single-cell RNA-sequencing (scRNA-seq) technologies has provided a powerful approach to dissect the cellular heterogeneity of complex biological systems. However, such approaches require specialized equipment or are costly. In this article, we describe an improved Smart-seq2-based method to profile the transcriptome of hundreds of single cells simultaneously, without utilizing commercial kits or requiring any specialized single-cell capture/library preparation tools. Moreover, we introduce the Automated Single-cell Analysis Pipeline (ASAP), which allows researchers without strong computational expertise to explore scRNA-seq data using a wide range of commonly used algorithms and sophisticated visualization tools. © 2017 by John Wiley & Sons, Inc.

Modification of the Method of Receiving of Insertion Mutants with the EZ::TN5 System

We demonstrated the possibility of obtaining insertion mutants by a modified technique using EZ::TN5 system during culturing of the recipient strain on a dense nutrient medium and exclusion of the centrifugation stage. The frequency of transposon mutants of E. coli 10979/EZ::TN5 was 2×10^-6. Genetically modified strains were characterized by kanamycin resistance, inability to L-malate assimilation, changes in the expression of individual proteins of protein mass-spectra (5096.3, 6252.9, and 9067.7 Da), and the presence of fragments in genomic DNA amplified by specific forward and reverse primers that were homologous to Tn5 transposon insertion sites. The modified procedure for obtaining insertion mutants by using EZ::TN5 system was not inferior by the efficiency to the standard procedure, but shortens experiment duration, simplifies it, and reduces the risks related to working with group 2 pathogenicity microorganisms.

Identification of Pathogenicity-Related Genes in Biofilm-Defective Acidovorax citrulli by Transposon Tn5 Mutagenesis

Biofilm formation is important for virulence of a large number of plant pathogenic bacteria. Indeed, some virulence genes have been found to be involved in the formation of biofilm in bacterial fruit blotch pathogen Acidovorax citrulli. However, some virulent strains of A. citrulli were unable to format biofilm, indicating the complexity between biofilm formation and virulence. In this study, virulence-related genes were identified in the biofilm-defective strain A1 of A. citrulli by using Tn5 insertion, pathogenicity test, and high-efficiency thermal asymmetric interlaced PCR (hiTAIL-PCR). Results from this study indicated that 22 out of the obtained 301 mutants significantly decreased the virulence of strain A1 compared to the wild-type. Furthermore, sequence analysis indicated that the obtained 22 mutants were due to the insertion of Tn5 into eight genes, including Aave 4244 (cation diffusion facilitator family transporter), Aave 4286 (hypothetical protein), Aave 4189 (alpha/beta hydrolase fold), Aave 1911 (IMP dehydrogenase/GMP reductase domain), Aave 4383 (bacterial export proteins, family 1), Aave 4256 (Hsp70 protein), Aave 0003 (histidine kinase, DNA gyrase B, and HSP90-like ATPase), and Aave 2428 (pyridoxal-phosphate dependent enzyme). Furthermore, the growth of mutant Aave 2428 was unaffected and even increased by the change in incubation temperature, NaCl concentration and the pH of the LB broth, indicating that this gene may be directly involved in the bacterial virulence. Overall, the determination of the eight pathogenicity-related genes in strain A1 will be helpful to elucidate the pathogenesis of biofilm-defective A. citrulli.

Characterization of the hrpZ gene from Pseudomonas syringae pv. maculicola M2

Pseudomonas syringae pv. maculicola is a natural pathogen of members of the Brassicaceae plant family. Using a transposon-based mutagenesis strategy in Pseudomonas syringaepv. maculicola M2 (PsmM2), we conducted a genetic screen to identify mutants that were capable of growing in M9 medium supplemented with a crude extract from the leaves of Arabidopsis thaliana. A mutant containing a transposon insertion in the hrpZ gene (PsmMut8) was unable to infect adult plants from Arabidopsis thaliana or Brassica oleracea, suggesting a loss of pathogenicity. The promotorless cat reporter present in the gene trap was expressed if PsmMut8 was grown in minimal medium (M9) supplemented with the leaf extract but not if grown in normal rich medium (KB). We conducted phylogenetic analysis using hrpAZB genes, showing the classical 5-clade distribution, and nucleotide diversity analysis, showing the putative position for selective pressure in this operon. Our results indicate that the hrpAZB operon from Pseudomonas syringaepv. maculicola M2 is necessary for its pathogenicity and that its diversity would be under host-mediated diversifying selection.