Impact of Xenogeneic Silencing on Phage-Host Interactions

Bacterial chromatin proteins, transcription, and DNA topology: Inseparable partners in the control of gene expression

DNA in bacterial chromosomes is organized into higher-order structures by DNA-binding proteins called nucleoid-associated proteins (NAPs) or bacterial chromatin proteins (BCPs). BCPs often bind to or near DNA loci transcribed by RNA polymerase (RNAP) and can either increase or decrease gene expression. To understand the mechanisms by which BCPs alter transcription, one must consider both steric effects and the topological forces that arise when DNA deviates from its fully relaxed double-helical structure. Transcribing RNAP creates DNA negative (-) supercoils upstream and positive (+) supercoils downstream whenever RNAP and DNA are unable to rotate freely. This (-) and (+) supercoiling generates topological forces that resist forward translocation of DNA through RNAP unless the supercoiling is constrained by BCPs or relieved by topoisomerases. BCPs also may enhance topological stress and overall can either inhibit or aid transcription. Here, we review current understanding of how RNAP, BCPs, and DNA topology interplay to control gene expression.

Acetylation of xenogeneic silencer H-NS regulates biofilm development through the nitrogen homeostasis regulator in Shewanella

Adjusting intracellular metabolic pathways and adopting suitable live state such as biofilms, are crucial for bacteria to survive environmental changes. Although substantial progress has been made in understanding how the histone-like nucleoid-structuring (H-NS) protein modulates the expression of the genes involved in biofilm formation, the precise modification that the H-NS protein undergoes to alter its DNA binding activity is still largely uncharacterized. This study revealed that acetylation of H-NS at Lys19 inhibits biofilm development in Shewanella oneidensis MR-1 by downregulating the expression of glutamine synthetase, a critical enzyme in glutamine synthesis. We further found that nitrogen starvation, a likely condition in biofilm development, induces deacetylation of H-NS and the trimerization of nitrogen assimilation regulator GlnB. The acetylated H-NS strain exhibits significantly lower cellular glutamine concentration, emphasizing the requirement of H-NS deacetylation in Shewanella biofilm development. Moreover, we discovered in vivo that the activation of glutamine biosynthesis pathway and the concurrent suppression of the arginine synthesis pathway during both pellicle and attached biofilms development, further suggesting the importance of fine tune nitrogen assimilation by H-NS acetylation in Shewanella. In summary, posttranslational modification of H-NS endows Shewanella with the ability to respond to environmental needs by adjusting the intracellular metabolism pathways.

[Lsr2: A Nucleoid Associated Protein (NAP) and a transcription factor in mycobacteria]

Lsr2, a small protein mainly found in actinobacteria, plays a crucial role in the virulence and adaptation of mycobacteria to environmental conditions. As a member of the nucleoid-associated protein (NAPs) superfamily, Lsr2 influences DNA organization by facilitating the formation of chromosomal loops in vitro and, therefore, may be a major player in the three-dimensional folding of the genome. Additionally, Lsr2 also acts as a transcription factor, regulating the expression of numerous genes responsible for coordinating a myriad of cellular and molecular processes essential for the actinobacteria. Similar to the H-NS protein, its ortholog in enterobacteria, its role in transcriptional repression likely relies on oligomerization, rigidifying, and bridging of DNA, thereby disrupting RNA polymerase recruitment as well as the elongation of RNA transcripts.

The Ros/MucR Zinc-Finger Protein Family in Bacteria: Structure and Functions

Ros/MucR is a widespread family of bacterial zinc-finger-containing proteins that integrate multiple functions, such as symbiosis, virulence, transcription regulation, motility, production of surface components, and various other physiological processes in cells. This regulatory protein family is conserved in bacteria and is characterized by its zinc-finger motif, which has been proposed as the ancestral domain from which the eukaryotic C₂H₂ zinc-finger structure has evolved. The first prokaryotic zinc-finger domain found in the transcription regulator Ros was identified in Agrobacterium tumefaciens. In the past decades, a large body of evidence revealed Ros/MucR as pleiotropic transcriptional regulators that mainly act as repressors through oligomerization and binding to AT-rich target promoters. The N-terminal domain and the zinc-finger-bearing C-terminal region of these regulatory proteins are engaged in oligomerization and DNA binding, respectively. These properties of the Ros/MucR proteins are similar to those of xenogeneic silencers, such as H-NS, MvaT, and Lsr2, which are mainly found in other lineages. In fact, a novel functional model recently proposed for this protein family suggests that they act as H-NS-'like' gene silencers. The prokaryotic zinc-finger domain exhibits interesting structural and functional features that are different from that of its eukaryotic counterpart (a βββα topology), as it folds in a significantly larger zinc-binding globular domain (a βββαα topology). Phylogenetic analysis of Ros/MucR homologs suggests an ancestral origin of this type of protein in α-Proteobacteria. Furthermore, multiple duplications and lateral gene transfer events contributing to the diversity and phyletic distribution of these regulatory proteins were found in bacterial genomes.

When bacteria are phage playgrounds: interactions between viruses, cells, and mobile genetic elements

Studies of viral adaptation have focused on the selective pressures imposed by hosts. However, there is increasing evidence that interactions between viruses, cells, and other mobile genetic elements are determinant to the success of infections. These interactions are often associated with antagonism and competition, but sometimes involve cooperation or parasitism. We describe two key types of interactions - defense systems and genetic regulation - that allow the partners of the interaction to destroy or control the others. These interactions evolve rapidly by genetic exchanges, including among competing partners. They are sometimes followed by functional diversification. Gene exchanges also facilitate the emergence of cross-talk between elements in the same bacterium. In the end, these processes produce multilayered networks of interactions that shape the outcome of viral infections.

Influence of Staphylococcus aureus Strain Background on Sa3int Phage Life Cycle Switches

Staphylococcus aureus asymptomatically colonizes the nasal cavity of mammals, but it is also a leading cause of life-threatening infections. Most human nasal isolates carry Sa3 phages, which integrate into the bacterial hlb gene encoding a sphingomyelinase. The virulence factor-encoding genes carried by the Sa3-phages are highly human-specific, and most animal strains are Sa3 negative. Thus, both insertion and excision of the prophage could potentially confer a fitness advantage to S. aureus. Here, we analyzed the phage life cycle of two Sa3 phages, Φ13 and ΦN315, in different phage-cured S. aureus strains. Based on phage transfer experiments, strains could be classified into low (8325-4, SH1000, and USA300c) and high (MW2c and Newman-c) transfer strains. High-transfer strains promoted the replication of phages, whereas phage adsorption, integration, excision, or recA transcription was not significantly different between strains. RNASeq analyses of replication-deficient lysogens revealed no strain-specific differences in the CI/Mor regulatory switch. However, lytic genes were significantly upregulated in the high transfer strain MW2c Φ13 compared to strain 8325-4 Φ13. By transcriptional start site prediction, new promoter regions within the lytic modules were identified, which are likely targeted by specific host factors. Such host-phage interaction probably accounts for the strain-specific differences in phage replication and transfer frequency. Thus, the genetic makeup of the host strains may determine the rate of phage mobilization, a feature that might impact the speed at which certain strains can achieve host adaptation.

Isolation of Novel Xanthomonas Phages Infecting the Plant Pathogens X. translucens and X. campestris

The genus of Xanthomonas contains many well-known plant pathogens with the ability to infect some of the most important crop plants, thereby causing significant economic damage. Unfortunately, classical pest-control strategies are neither particularly efficient nor sustainable and we are, therefore, in demand of alternatives. Here, we present the isolation and characterization of seven novel phages infecting the plant-pathogenic species Xanthomonas translucens and Xanthomonas campestris. Transmission electron microscopy revealed that all phages show a siphovirion morphology. The analysis of genome sequences and plaque morphologies are in agreement with a lytic lifestyle of the phages making them suitable candidates for biocontrol. Moreover, three of the isolated phages form the new genus “Shirevirus”. All seven phages belong to four distinct clusters underpinning their phylogenetic diversity. Altogether, this study presents the first characterized isolates for the plant pathogen X. translucens and expands the number of available phages for plant biocontrol.

Phylogenetic Distribution of WhiB- and Lsr2-Type Regulators in Actinobacteriophage Genomes

Viruses that infect different actinobacterial host species are known as actinobacteriophages. They are composed of highly divergent and mosaic genomes due to frequent gene exchange between their bacterial hosts and related viral species. This is also reflected by the adaptive incorporation of host transcription factors (TFs) into phage regulatory networks. Previous studies discovered Lsr2-type and WhiB-type regulators encoded by actinobacteriophage genomes. However, limited information is available about their distribution, evolution, and impact on host species. In this study, we computationally screened the distribution of known bacterial and phage TFs inside 2951 complete actinobacteriophage genomes and identified 13 different TF domains. Among those, WhiB, Lsr2, MerR, and Cro/CI-like proteins were widespread and found in more than 10% of the analyzed actinobacteriophage genomes. Neighboring genomic context analysis of the whiB and lsr2 loci showed group-specific conservation of gene synteny and potential involvement of these genes in diverse regulatory functions. Both genes were significantly enriched in temperate phages, and the Lsr2-encoding genomes featured an overall lower GC content. Phylogenetic analysis of WhiB and Lsr2 proteins showed the grouping of phage sequences within bacterial clades, suggesting gene acquisition by phages from their bacterial host species or by multiple, independent acquisition events. Overall, our study reports the global distribution of actinobacteriophage regulatory proteins and sheds light on their origin and evolution.

IMPORTANCE Actinobacteriophages are viruses that infect bacterial species of the diverse phylum of Actinobacteria. Phages engage in a close relationship with their bacterial host. This is also reflected by the adoption of genetic material from their host and its incorporation into phage regulatory circuits. In this study, we systematically searched the genomes of actinobacteriophages for the presence of transcription factor domains. We show that proteins belonging to the regulator families of WhiB and Lsr2 belong to the most abundant regulatory proteins encoded by actinobacteriophages. Further phylogenetic analysis shed light on their origin and evolution. Altogether, this study provides an important basis for further experimental investigation of their role in the coordination of the phage life cycle and their interaction with the host regulatory network in this important bacterial phylum.

The horizontal transfer of Pseudomonas aeruginosa PA14 ICE PAPI-1 is controlled by a transcriptional triad between TprA, NdpA2 and MvaT

Pseudomonas aeruginosa is a major cause of nosocomial infections, particularly in immunocompromised patients or in individuals with cystic fibrosis. Genome sequences reveal that most P. aeruginosa strains contain a significant number of accessory genes gathered in genomic islands. Those genes are essential for P. aeruginosa to invade new ecological niches with high levels of antibiotic usage, like hospitals, or to survive during host infection by providing pathogenicity determinants. P. aeruginosa pathogenicity island 1 (PAPI-1), one of the largest genomic islands, encodes several putative virulence factors, including toxins, biofilm genes and antibiotic-resistance traits. The integrative and conjugative element (ICE) PAPI-1 is horizontally transferable by conjugation via a specialized GI-T4SS, but the mechanism regulating this transfer is currently unknown. Here, we show that this GI-T4SS conjugative machinery is directly induced by TprA, a regulator encoded within PAPI-1. Our data indicate that the nucleotide associated protein NdpA2 acts in synergy with TprA, removing a repressive mechanism exerted by MvaT. In addition, using a transcriptomic approach, we unravelled the regulon controlled by Ndpa2/TprA and showed that they act as major regulators on the genes belonging to PAPI-1. Moreover, TprA and NdpA2 trigger an atypical biofilm structure and enhance ICE PAPI-1 transfer.

Novel anti-repression mechanism of H-NS proteins by a phage protein

H-NS family proteins, bacterial xenogeneic silencers, play central roles in genome organization and in the regulation of foreign genes. It is thought that gene repression is directly dependent on the DNA binding modes of H-NS family proteins. These proteins form lateral protofilaments along DNA. Under specific environmental conditions they switch to bridging two DNA duplexes. This switching is a direct effect of environmental conditions on electrostatic interactions between the oppositely charged DNA binding and N-terminal domains of H-NS proteins. The Pseudomonas lytic phage LUZ24 encodes the protein gp4, which modulates the DNA binding and function of the H-NS family protein MvaT of Pseudomonas aeruginosa. However, the mechanism by which gp4 affects MvaT activity remains elusive. In this study, we show that gp4 specifically interferes with the formation and stability of the bridged MvaT-DNA complex. Structural investigations suggest that gp4 acts as an 'electrostatic zipper' between the oppositely charged domains of MvaT protomers, and stabilizes a structure resembling their 'half-open' conformation, resulting in relief of gene silencing and adverse effects on P. aeruginosa growth. The ability to control H-NS conformation and thereby its impact on global gene regulation and growth might open new avenues to fight Pseudomonas multidrug resistance.

H-NS family protein MvaU downregulates phenazine-1-carboxylic acid (PCA) biosynthesis via binding to an AT-rich region within the promoter of the phz2 gene cluster in the rhizobacterium Pseudomonas strain PA1201

Histone-like nucleoid-structuring (H-NS) proteins are key regulators in gene expression silencing and in nucleoid compaction. The H-NS family member proteins MvaU in Pseudomonas aeruginosa are thought to bind the same AT-rich regions of chromosomes and function to coordinate the control of a common set of genes. Here, we explored the molecular mechanism by which MvaU controls PCA biosynthesis in P. aeruginosa PA1201. We present evidence suggesting that MvaU is self-regulated. Deletion of mvaU significantly increased PCA production, and PCA production sharply decreased when mvaU was over-expressed. MvaU transcriptionally repressed phz2 cluster expression and consequently reduced PCA biosynthesis. β-galactosidase assays confirmed that base pairing near the −35 box is required when MvaU regulates PCA production in PA1201. Electrophoretic mobility shift assays (EMSA) and additional point mutation analysis demonstrated that MvaU directly bound to an AT-rich motif within the promoter of the phz2 cluster. Chromatin immunoprecipitation (ChIP) analysis also indicated that MvaU directly bound to the P5 region of the phz2 cluster promoter. MvaU repression of PCA biosynthesis was independent of QscR and OxyR in PA1201 and neither PCA or H₂O₂ were the environmental signals that induced mvaU expression. These findings detail a new MvaU-dependent regulatory pathway of PCA biosynthesis in PA1201 and provide a foundation to increase PCA fermentation titer by genetic engineering.

A phage-encoded nucleoid associated protein compacts both host and phage DNA and derepresses H-NS silencing

Nucleoid Associated Proteins (NAPs) organize the bacterial chromosome within the nucleoid. The interaction of the NAP H-NS with DNA also represses specific host and xenogeneic genes. Previously, we showed that the bacteriophage T4 early protein MotB binds to DNA, co-purifies with H-NS/DNA, and improves phage fitness. Here we demonstrate using atomic force microscopy that MotB compacts the DNA with multiple MotB proteins at the center of the complex. These complexes differ from those observed with H-NS and other NAPs, but resemble those formed by the NAP-like proteins CbpA/Dps and yeast condensin. Fluorescent microscopy indicates that expression of motB in vivo, at levels like that during T4 infection, yields a significantly compacted nucleoid containing MotB and H-NS. motB overexpression dysregulates hundreds of host genes; ∼70% are within the hns regulon. In infected cells overexpressing motB, 33 T4 late genes are expressed early, and the T4 early gene repEB, involved in replication initiation, is up ∼5-fold. We postulate that MotB represents a phage-encoded NAP that aids infection in a previously unrecognized way. We speculate that MotB-induced compaction may generate more room for T4 replication/assembly and/or leads to beneficial global changes in host gene expression, including derepression of much of the hns regulon.

Xenogeneic Silencing and Bacterial Genome Evolution: Mechanisms for DNA Recognition Imply Multifaceted Roles of Xenogeneic Silencers

Horizontal gene transfer (HGT) is a major driving force for bacterial evolution. To avoid the deleterious effects due to the unregulated expression of newly acquired foreign genes, bacteria have evolved specific proteins named xenogeneic silencers to recognize foreign DNA sequences and suppress their transcription. As there is considerable diversity in genomic base compositions among bacteria, how xenogeneic silencers distinguish self- from nonself DNA in different bacteria remains poorly understood. This review summarizes the progress in studying the DNA binding preferences and the underlying molecular mechanisms of known xenogeneic silencer families, represented by H-NS of Escherichia coli, Lsr2 of Mycobacterium, MvaT of Pseudomonas, and Rok of Bacillus. Comparative analyses of the published data indicate that the differences in DNA recognition mechanisms enable these xenogeneic silencers to have clear characteristics in DNA sequence preferences, which are further correlated with different host genomic features. These correlations provide insights into the mechanisms of how these xenogeneic silencers selectively target foreign DNA in different genomic backgrounds. Furthermore, it is revealed that the genomic AT contents of bacterial species with the same xenogeneic silencer family proteins are distributed in a limited range and are generally lower than those species without any known xenogeneic silencers in the same phylum/class/genus, indicating that xenogeneic silencers have multifaceted roles on bacterial genome evolution. In addition to regulating horizontal gene transfer, xenogeneic silencers also act as a selective force against the GC to AT mutational bias found in bacterial genomes and help the host genomic AT contents maintained at relatively low levels.

Xenogeneic silencing relies on temperature-dependent phosphorylation of the host H-NS protein in Shewanella

Lateral gene transfer (LGT) plays a key role in shaping the genome evolution and environmental adaptation of bacteria. Xenogeneic silencing is crucial to ensure the safe acquisition of LGT genes into host pre-existing regulatory networks. We previously found that the host nucleoid structuring protein (H-NS) silences prophage CP4So at warm temperatures yet enables this prophage to excise at cold temperatures in Shewanella oneidensis. However, whether H-NS silences other genes and how bacteria modulate H-NS to regulate the expression of genes have not been fully elucidated. In this study, we discovered that the H-NS silences many LGT genes and the xenogeneic silencing of H-NS relies on a temperature-dependent phosphorylation at warm temperatures in S. oneidensis. Specifically, phosphorylation of H-NS at Ser42 is critical for silencing the cold-inducible genes including the excisionase of CP4So prophage, a cold shock protein, and a stress-related chemosensory system. By contrast, nonphosphorylated H-NS derepresses the promoter activity of these genes/operons to enable their expression at cold temperatures. Taken together, our results reveal that the posttranslational modification of H-NS can function as a regulatory switch to control LGT gene expression in host genomes to enable the host bacterium to react and thrive when environmental temperature changes.

Genome Sequence of the Bacteriophage CL31 and Interaction with the Host Strain Corynebacterium glutamicum ATCC 13032

In this study, we provide a comprehensive analysis of the genomic features of the phage CL31 and the infection dynamics with the biotechnologically relevant host strain Corynebacterium glutamicum ATCC 13032. Genome sequencing and annotation of CL31 revealed a 45-kbp genome composed of 72 open reading frames, mimicking the GC content of its host strain (54.4%). An ANI-based distance matrix showed the highest similarity of CL31 to the temperate corynephage Φ16. While the C. glutamicum ATCC 13032 wild type strain showed only mild propagation of CL31, a strain lacking the cglIR-cglIIR-cglIM restriction-modification system was efficiently infected by this phage. Interestingly, the prophage-free strain C. glutamicum MB001 featured an even accelerated amplification of CL31 compared to the ∆resmod strain suggesting a role of cryptic prophage elements in phage defense. Proteome analysis of purified phage particles and transcriptome analysis provide important insights into structural components of the phage and the response of C. glutamicum to CL31 infection. Isolation and sequencing of CL31-resistant strains revealed SNPs in genes involved in mycolic acid biosynthesis suggesting a role of this cell envelope component in phage adsorption. Altogether, these results provide an important basis for further investigation of phage-host interactions in this important biotechnological model organism.

Live cell dynamics of production, explosive release and killing activity of phage tail-like weapons for Pseudomonas kin exclusion

Interference competition among bacteria requires a highly specialized, narrow-spectrum weaponry when targeting closely-related competitors while sparing individuals from the same clonal population. Here we investigated mechanisms by which environmentally important Pseudomonas bacteria with plant-beneficial activity perform kin interference competition. We show that killing between phylogenetically closely-related strains involves contractile phage tail-like devices called R-tailocins that puncture target cell membranes. Using live-cell imaging, we evidence that R-tailocins are produced at the cell center, transported to the cell poles and ejected by explosive cell lysis. This enables their dispersal over several tens of micrometers to reach targeted cells. We visualize R-tailocin-mediated competition dynamics between closely-related Pseudomonas strains at the single-cell level, both in non-induced condition and upon artificial induction. We document the fatal impact of cellular self-sacrifice coupled to deployment of phage tail-like weaponry in the microenvironment of kin bacterial competitors, emphasizing the necessity for microscale assessment of microbial competitions.

Overexpression of the Bacteriophage T4 motB Gene Alters H-NS Dependent Repression of Specific Host DNA

The bacteriophage T4 early gene product MotB binds tightly but nonspecifically to DNA, copurifies with the host Nucleoid Associated Protein (NAP) H-NS in the presence of DNA and improves T4 fitness. However, the T4 transcriptome is not significantly affected by a motB knockdown. Here we have investigated the phylogeny of MotB and its predicted domains, how MotB and H-NS together interact with DNA, and how heterologous overexpression of motB impacts host gene expression. We find that motB is highly conserved among Tevenvirinae. Although the MotB sequence has no homology to proteins of known function, predicted structure homology searches suggest that MotB is composed of an N-terminal Kyprides-Onzonis-Woese (KOW) motif and a C-terminal DNA-binding domain of oligonucleotide/oligosaccharide (OB)-fold; either of which could provide MotB’s ability to bind DNA. DNase I footprinting demonstrates that MotB dramatically alters the interaction of H-NS with DNA in vitro. RNA-seq analyses indicate that expression of plasmid-borne motB up-regulates 75 host genes; no host genes are down-regulated. Approximately 1/3 of the up-regulated genes have previously been shown to be part of the H-NS regulon. Our results indicate that MotB provides a conserved function for Tevenvirinae and suggest a model in which MotB functions to alter the host transcriptome, possibly by changing the association of H-NS with the host DNA, which then leads to conditions that are more favorable for infection.

Ancestral zinc-finger bearing protein MucR in alpha-proteobacteria: A novel xenogeneic silencer?

The MucR/Ros family protein is conserved in alpha-proteobacteria and characterized by its zinc-finger motif that has been proposed as the ancestral domain from which the eukaryotic C2H2 zinc-finger structure evolved. In the past decades, accumulated evidences have revealed MucR as a pleiotropic transcriptional regulator that integrating multiple functions such as virulence, symbiosis, cell cycle and various physiological processes. Scattered reports indicate that MucR mainly acts as a repressor, through oligomerization and binding to multiple sites of AT-rich target promoters. The N-terminal region and zinc-finger bearing C-terminal region of MucR mediate oligomerization and DNA-binding, respectively. These features are convergent to those of xenogeneic silencers such as H-NS, MvaT, Lsr2 and Rok, which are mainly found in other lineages. Phylogenetic analysis of MucR homologs suggests an ancestral origin of MucR in alpha- and delta-proteobacteria. Multiple independent duplication and lateral gene transfer events contribute to the diversity and phyletic distribution of MucR. Finally, we posed questions which remain unexplored regarding the putative roles of MucR as a xenogeneic silencer and a general manager in balancing adaptation and regulatory integration in the pangenome context.

From sequence to information

Today massive amounts of sequenced metagenomic and metatranscriptomic data from different ecological niches and environmental locations are available. Scientific progress depends critically on methods that allow extracting useful information from the various types of sequence data. Here, we will first discuss types of information contained in the various flavours of biological sequence data, and how this information can be interpreted to increase our scientific knowledge and understanding. We argue that a mechanistic understanding of biological systems analysed from different perspectives is required to consistently interpret experimental observations, and that this understanding is greatly facilitated by the generation and analysis of dynamic mathematical models. We conclude that, in order to construct mathematical models and to test mechanistic hypotheses, time-series data are of critical importance. We review diverse techniques to analyse time-series data and discuss various approaches by which time-series of biological sequence data have been successfully used to derive and test mechanistic hypotheses. Analysing the bottlenecks of current strategies in the extraction of knowledge and understanding from data, we conclude that combined experimental and theoretical efforts should be implemented as early as possible during the planning phase of individual experiments and scientific research projects.

This article is part of the theme issue ‘Integrative research perspectives on marine conservation’.

Genome Sequence and Characterization of Five Bacteriophages Infecting Streptomyces Coelicolor and Streptomyces Venezuelae: Alderaan, Coruscant, Dagobah, Endor1 and Endor2

Streptomyces are well-known antibiotic producers, also characterized by a complex morphological differentiation. Streptomyces, like all bacteria, are confronted with the constant threat of phage predation, which in turn shapes bacterial evolution. However, despite significant sequencing efforts recently, relatively few phages infecting Streptomyces have been characterized compared to other genera. Here, we present the isolation and characterization of five novel Streptomyces phages. All five phages belong to the Siphoviridae family, based on their morphology as determined by transmission electron microscopy. Genome sequencing and life style predictions suggested that four of them were temperate phages, while one had a lytic lifestyle. Moreover, one of the newly sequenced phages shows very little homology to already described phages, highlighting the still largely untapped viral diversity. Altogether, this study expands the number of characterized phages of Streptomyces and sheds light on phage evolution and phage-host dynamics in Streptomyces.

Inducible Expression Systems Based on Xenogeneic Silencing and Counter-Silencing and Design of a Metabolic Toggle Switch

Inducible expression systems represent key modules in regulatory circuit design and metabolic engineering approaches. However, established systems are often limited in terms of applications due to high background expression levels and inducer toxicity. In bacteria, xenogeneic silencing (XS) proteins are involved in the tight control of horizontally acquired, AT-rich DNA. The action of XS proteins may be opposed by interference with a specific transcription factor, resulting in the phenomenon of counter-silencing, thereby activating gene expression. In this study, we harnessed this principle for the construction of a synthetic promoter library consisting of phage promoters targeted by the Lsr2-like XS protein CgpS of Corynebacterium glutamicum. Counter-silencing was achieved by inserting the operator sequence of the gluconate-responsive transcription factor GntR. The GntR-dependent promoter library is comprised of 28 activated and 16 repressed regulatory elements featuring effector-dependent tunability. For selected candidates, background expression levels were confirmed to be significantly reduced in comparison to established heterologous expression systems. Finally, a GntR-dependent metabolic toggle switch was implemented in a C. glutamicum l-valine production strain allowing the dynamic redirection of carbon flux between biomass and product formation.

Genome and sequence determinants governing the expression of horizontally acquired DNA in bacteria

While horizontal gene transfer is prevalent across the biosphere, the regulatory features that enable expression and functionalization of foreign DNA remain poorly understood. Here, we combine high-throughput promoter activity measurements and large-scale genomic analysis of regulatory regions to investigate the cross-compatibility of regulatory elements (REs) in bacteria. Functional characterization of thousands of natural REs in three distinct bacterial species revealed distinct expression patterns according to RE and recipient phylogeny. Host capacity to activate foreign promoters was proportional to their genomic GC content, while many low GC regulatory elements were both broadly active and had more transcription start sites across hosts. The difference in expression capabilities could be explained by the influence of the host GC content on the stringency of the AT-rich canonical σ70 motif necessary for transcription initiation. We further confirm the generalizability of this model and find widespread GC content adaptation of the σ70 motif in a set of 1,545 genomes from all major bacterial phyla. Our analysis identifies a key mechanism by which the strength of the AT-rich σ70 motif relative to a host's genomic GC content governs the capacity for expression of acquired DNA. These findings shed light on regulatory adaptation in the context of evolving genomic composition.

Deciphering the Rules Underlying Xenogeneic Silencing and Counter-Silencing of Lsr2-like Proteins Using CgpS of Corynebacterium glutamicum as a Model

Lsr2-like nucleoid-associated proteins play an important role as xenogeneic silencers (XS) of horizontally acquired genomic regions in actinobacteria. In this study, we systematically analyzed the in vivo constraints underlying silencing and counter-silencing of the Lsr2-like protein CgpS in Corynebacterium glutamicum Genome-wide analysis revealed binding of CgpS to regions featuring a distinct drop in GC profile close to the transcription start site (TSS) but also identified an overrepresented motif with multiple A/T steps at the nucleation site of the nucleoprotein complex. Binding of specific transcription factors (TFs) may oppose XS activity, leading to counter-silencing. Following a synthetic counter-silencing approach, target gene activation was realized by inserting operator sites of an effector-responsive TF within various CgpS target promoters, resulting in increased promoter activity upon TF binding. Analysis of reporter constructs revealed maximal counter-silencing when the TF operator site was inserted at the position of maximal CgpS coverage. This principle was implemented in a synthetic toggle switch, which features a robust and reversible response to effector availability, highlighting the potential for biotechnological applications. Together, our results provide comprehensive insights into how Lsr2 silencing and counter-silencing shape evolutionary network expansion in this medically and biotechnologically relevant bacterial phylum.IMPORTANCE In actinobacteria, Lsr2-like nucleoid-associated proteins function as xenogeneic silencers (XS) of horizontally acquired genomic regions, including viral elements, virulence gene clusters in Mycobacterium tuberculosis, and genes involved in cryptic specialized metabolism in Streptomyces species. Consequently, a detailed mechanistic understanding of Lsr2 binding in vivo is relevant as a potential drug target and for the identification of novel bioactive compounds. Here, we followed an in vivo approach to investigate the rules underlying xenogeneic silencing and counter-silencing of the Lsr2-like XS CgpS from Corynebacterium glutamicum Our results demonstrated that CgpS distinguishes between self and foreign by recognizing a distinct drop in GC profile in combination with a short, sequence-specific motif at the nucleation site. Following a synthetic counter-silencer approach, we studied the potential and constraints of transcription factors to counteract CgpS silencing, thereby facilitating the integration of new genetic traits into host regulatory networks.

Horizontally Acquired Homologs of Xenogeneic Silencers: Modulators of Gene Expression Encoded by Plasmids, Phages and Genomic Islands

Acquisition of mobile elements by horizontal gene transfer can play a major role in bacterial adaptation and genome evolution by providing traits that contribute to bacterial fitness. However, gaining foreign DNA can also impose significant fitness costs to the host bacteria and can even produce detrimental effects. The efficiency of horizontal acquisition of DNA is thought to be improved by the activity of xenogeneic silencers. These molecules are a functionally related group of proteins that possess affinity for the acquired DNA. Binding of xenogeneic silencers suppresses the otherwise uncontrolled expression of genes from the newly acquired nucleic acid, facilitating their integration to the bacterial regulatory networks. Even when the genes encoding for xenogeneic silencers are part of the core genome, homologs encoded by horizontally acquired elements have also been identified and studied. In this article, we discuss the current knowledge about horizontally acquired xenogeneic silencer homologs, focusing on those encoded by genomic islands, highlighting their distribution and the major traits that allow these proteins to become part of the host regulatory networks.

Generation of a Prophage-Free Variant of the Fast-Growing Bacterium Vibrio natriegens

The fast-growing marine bacterium Vibrio natriegens represents an emerging strain for molecular biology and biotechnology. Genome sequencing and quantitative PCR analysis revealed that the first chromosome of V. natriegens ATCC 14048 contains two prophage regions (VNP1 and VNP2) that are both inducible by the DNA-damaging agent mitomycin C and exhibit spontaneous activation under standard cultivation conditions. Their activation was also confirmed by live cell imaging of an mCherry fusion to the major capsid proteins of VNP1 and VNP2. Transmission electron microscopy visualized the release of phage particles belonging to the Siphoviridae family into the culture supernatant. Freeing V. natriegens from its proviral load, followed by phenotypic characterization, revealed an improved robustness of the prophage-free variant toward DNA-damaging conditions, reduced cell lysis under hypo-osmotic conditions, and an increased pyruvate production compared to wild-type levels. Remarkably, the prophage-free strain outcompeted the wild type in a competitive growth experiment, emphasizing that this strain is a promising platform for future metabolic engineering approaches.IMPORTANCE The fast-growing marine bacterium Vibrio natriegens represents an emerging model host for molecular biology and biotechnology, featuring a reported doubling time of less than 10 minutes. In many bacterial species, viral DNA (prophage elements) may constitute a considerable fraction of the whole genome and may have detrimental effects on the growth and fitness of industrial strains. Genome analysis revealed the presence of two prophage regions in the V. natriegens genome that were shown to undergo spontaneous induction under standard cultivation conditions. In this study, we generated a prophage-free variant of V. natriegens Remarkably, the prophage-free strain exhibited a higher tolerance toward DNA damage and hypo-osmotic stress. Moreover, it was shown to outcompete the wild-type strain in a competitive growth experiment. In conclusion, our study presents the prophage-free variant of V. natriegens as a promising platform strain for future biotechnological applications.

The MarR-Type Regulator MalR Is Involved in Stress-Responsive Cell Envelope Remodeling in Corynebacterium glutamicum

It is the enormous adaptive capacity of microorganisms, which is key to their competitive success in nature, but also challenges antibiotic treatment of human diseases. To deal with a diverse set of stresses, bacteria are able to reprogram gene expression using a wide variety of transcription factors. Here, we focused on the MarR-type regulator MalR conserved in the Corynebacterineae, including the prominent pathogens Corynebacterium diphtheriae and Mycobacterium tuberculosis. In several corynebacterial species, the malR gene forms an operon with a gene encoding a universal stress protein (uspA). Chromatin affinity purification and sequencing (ChAP-Seq) analysis revealed that MalR binds more than 60 target promoters in the C. glutamicum genome as well as in the large cryptic prophage CGP3. Overproduction of MalR caused severe growth defects and an elongated cell morphology. ChAP-Seq data combined with a global transcriptome analysis of the malR overexpression strain emphasized a central role of MalR in cell envelope remodeling in response to environmental stresses. For example, prominent MalR targets are involved in peptidoglycan biosynthesis and synthesis of branched-chain fatty acids. Phenotypic microarrays suggested an altered sensitivity of a ΔmalR mutant toward several β-lactam antibiotics. Furthermore, we revealed MalR as a repressor of several prophage genes, suggesting that MalR may be involved in the control of stress-responsive induction of the large CGP3 element. In conclusion, our results emphasize MalR as a regulator involved in stress-responsive remodeling of the cell envelope of C. glutamicum and suggest a link between cell envelope stress and the control of phage gene expression.