Comparative genome analysis of Pseudomonas knackmussii B13, the first bacterium known to degrade chloroaromatic compounds

Triclosan Dioxygenase: A Novel Two-component Rieske Nonheme Iron Ring-hydroxylating Dioxygenase Initiates Triclosan Degradation

The emerging contaminant triclosan (TCS) is widely distributed both in surface water and in wastewater and poses a threat to aquatic organisms and human health due to its resistance to degradation. The dioxygenase enzyme TcsAB has been speculated to perform the initial degradation of TCS, but its precise catalytic mechanism remains unclear. In this study, the function of TcsAB was elucidated using multiple biochemical and molecular biology methods. Escherichia coli BL21(DE3) heterologously expressing tcsAB from Sphingomonas sp. RD1 converted TCS to 2,4-dichlorophenol. TcsAB belongs to the group IA family of two-component Rieske nonheme iron ring-hydroxylating dioxygenases. The highest amino acid identity of TcsA and the large subunits of other dioxygenases in the same family was only 35.50%, indicating that TcsAB is a novel dioxygenase. Mutagenesis of residues near the substrate binding pocket decreased the TCS-degrading activity and narrowed the substrate spectrum, except for the TcsAF343A mutant. A meta-analysis of 1492 samples from wastewater treatment systems worldwide revealed that tcsA genes are widely distributed. This study is the first to report that the TCS-specific dioxygenase TcsAB is responsible for the initial degradation of TCS. Studying the microbial degradation mechanism of TCS is crucial for removing this pollutant from the environment.

Diversity and evolution of an abundant ICEclc family of integrative and conjugative elements in Pseudomonas aeruginosa

IMPORTANCE

Microbial populations swiftly adapt to changing environments through horizontal gene transfer. While the mechanisms of gene transfer are well known, the impact of environmental conditions on the selection of transferred gene functions remains less clear. We investigated ICEs, specifically the ICEclc-type, in Pseudomonas aeruginosa clinical isolates. Our findings revealed co-evolution between ICEs and their hosts, with ICE transfers occurring within strains. Gene functions carried by ICEs are positively selected, including potential virulence factors and heavy metal resistance. Comparison to publicly available P. aeruginosa genomes unveiled widespread antibiotic-resistance determinants within ICEclc clades. Thus, the ubiquitous ICEclc family significantly contributes to P. aeruginosa's adaptation and fitness in diverse environments.

Evolution End Classification of tfd Gene Clusters Mediating Bacterial Degradation of 2,4-Dichlorophenoxyacetic Acid (2,4-D)

The tfd (tfdI and tfdII) are gene clusters originally discovered in plasmid pJP4 which are involved in the bacterial degradation of 2,4-dichlorophenoxyacetic acid (2,4-D) via the ortho-cleavage pathway of chlorinated catechols. They share this activity, with respect to substituted catechols, with clusters tcb and clc. Although great effort has been devoted over nearly forty years to exploring the structural diversity of these clusters, their evolution has been poorly resolved to date, and their classification is clearly obsolete. Employing comparative genomic and phylogenetic approaches has revealed that all tfd clusters can be classified as one of four different types. The following four-type classification and new nomenclature are proposed: tfdI, tfdII, tfdIII and tfdIV(A,B,C). Horizontal gene transfer between Burkholderiales and Sphingomonadales provides phenomenal linkage between tfdI, tfdII, tfdIII and tfdIV type clusters and their mosaic nature. It is hypothesized that the evolution of tfd gene clusters proceeded within first (tcb, clc and tfdI), second (tfdII and tfdIII) and third (tfdIV(A,B,C)) evolutionary lineages, in each of which, the genes were clustered in specific combinations. Their clustering is discussed through the prism of hot spots and driving forces of various models, theories, and hypotheses of cluster and operon formation. Two hypotheses about series of gene deletions and displacements are also proposed to explain the structural variations across members of clusters tfdII and tfdIII, respectively. Taking everything into account, these findings reconstruct the phylogeny of tfd clusters, have delineated their evolutionary trajectories, and allow the contribution of various evolutionary processes to be assessed.

Characterization of an atypical but widespread type IV secretion system for transfer of the integrative and conjugative element (ICEclc) in Pseudomonas putida

Conjugation of DNA relies on multicomponent protein complexes bridging two bacterial cytoplasmic compartments. Whereas plasmid conjugation systems have been well documented, those of integrative and conjugative elements (ICEs) have remained poorly studied. We characterize here the conjugation system of the ICEclc element in Pseudomonas putida UWC1 that is a model for a widely distributed family of ICEs. By in frame deletion and complementation, we show the importance on ICE transfer of 22 genes in a 20-kb conserved ICE region. Protein comparisons recognized seven homologs to plasmid type IV secretion system components, another six homologs to frequent accessory proteins, and the rest without detectable counterparts. Stationary phase imaging of P. putida ICEclc with in-frame fluorescent protein fusions to predicted type IV components showed transfer-competent cell subpopulations with multiple fluorescent foci, largely overlapping in dual-labeled subcomponents, which is suggestive for multiple conjugation complexes per cell. Cross-dependencies between subcomponents in ICE-type IV secretion system assembly were revealed by quantitative foci image analysis in a variety of ICEclc mutant backgrounds. In conclusion, the ICEclc family presents an evolutionary distinct type IV conjugative system with transfer competent cells specialized in efficient transfer.

Diversity and Evolution of Integrative and Conjugative Elements Involved in Bacterial Aromatic Compound Degradation and Their Utility in Environmental Remediation

Integrative and conjugative elements (ICEs) are mobile DNA molecules that can be transferred through excision, conjugation, and integration into chromosomes. They contribute to the horizontal transfer of genomic islands across bacterial species. ICEs carrying genes encoding aromatic compound degradation pathways are of interest because of their contribution to environmental remediation. Recent advances in DNA sequencing technology have increased the number of newly discovered ICEs in bacterial genomes and have enabled comparative analysis of their evolution. The two different families of ICEs carry various aromatic compound degradation pathway genes. ICEclc and its related ICEs contain a number of members with diverse catabolic capabilities. In addition, the Tn4371 family, which includes ICEs that carry the chlorinated biphenyl catabolic pathway, has been identified. It is apparent that they underwent evolution through the acquisition, deletion, or exchange of modules to adapt to an environmental niche. ICEs have the property of both stability and mobility in the chromosome. Perspectives on the use of ICEs in environmental remediation are also discussed.

In silico prediction of the enzymes involved in the degradation of the herbicide molinate by Gulosibacter molinativorax ON4T

Gulosibacter molinativorax ON4^T is the only known organism to produce molinate hydrolase (MolA), which catalyses the breakdown of the thiocarbamate herbicide into azepane-1-carboxylic acid (ACA) and ethanethiol. A combined genomic and transcriptomic strategy was used to fully characterize the strain ON4^T genome, particularly the molA genetic environment, to identify the potential genes encoding ACA degradation enzymes. Genomic data revealed that molA is the only catabolic gene of a novel composite transposon (Tn6311), located in a novel low copy number plasmid (pARLON1) harbouring a putative T4SS of the class FATA. pARLON1 had an ANI value of 88.2% with contig 18 from Agrococcus casei LMG 22410^T draft genome. Such results suggest that pARLON1 is related to genomic elements of other Actinobacteria, although Tn6311 was observed only in strain ON4^T. Furthermore, genomic and transcriptomic data demonstrated that the genes involved in ACA degradation are chromosomal. Based on their overexpression when growing in the presence of molinate, the enzymes potentially involved in the heterocyclic ring breakdown were predicted. Among these, the activity of a protein related to caprolactone hydrolase was demonstrated using heterologous expression. However, further studies are needed to confirm the role of the other putative enzymes.

Life Within a Contaminated Niche: Comparative Genomic Analyses of an Integrative Conjugative Element ICEnahCSV86 and Two Genomic Islands From Pseudomonas bharatica CSV86T Suggest Probable Role in Colonization and Adaptation

Comparative genomic and functional analyses revealed the presence of three genomic islands (GIs, >50 Kb size): ICEnahCSV86, Pseudomonas bharatica genomic island-1 (PBGI-1), and PBGI-2 in the preferentially aromatic-degrading soil bacterium, Pseudomonas bharatica CSV86^T. Site-specific genomic integration at or near specific transfer RNAs (tRNAs), near-syntenic structural modules, and phylogenetic relatedness indicated their evolutionary lineage to the type-4 secretion system (T4SS) ICEclc family, thus predicting these elements to be integrative conjugative elements (ICEs). These GIs were found to be present as a single copy in the genome and the encoded phenotypic traits were found to be stable, even in the absence of selection pressure. ICEnahCSV86 harbors naphthalene catabolic (nah-sal) cluster, while PBGI-1 harbors Co-Zn-Cd (czc) efflux genes as cargo modules, whereas PBGI-2 was attributed to as a mixed-function element. The ICEnahCSV86 has been reported to be conjugatively transferred (frequency of 7 × 10^–8/donor cell) to Stenotrophomonas maltophilia CSV89. Genome-wide comparative analyses of aromatic-degrading bacteria revealed nah-sal clusters from several Pseudomonas spp. as part of probable ICEs, syntenic to conjugatively transferable ICEnahCSV86 of strain CSV86^T, suggesting it to be a prototypical element for naphthalene degradation. It was observed that the plasmids harboring nah-sal clusters were phylogenetically incongruent with predicted ICEs, suggesting genetic divergence of naphthalene metabolic clusters in the Pseudomonas population. Gene synteny, divergence estimates, and codon-based Z-test indicated that ICEnahCSV86 is probably derived from PBGI-2, while multiple recombination events masked the ancestral lineage of PBGI-1. Diversifying selection pressure (dN-dS = 2.27–4.31) imposed by aromatics and heavy metals implied the modular exchange-fusion of various cargo clusters through events like recombination, rearrangement, domain reshuffling, and active site optimization, thus allowing the strain to evolve, adapt, and maximize the metabolic efficiency in a contaminated niche. The promoters (Pnah and Psal) of naphthalene cargo modules (nah, sal) on ICEnahCSV86 were proved to be efficient for heterologous protein expression in Escherichia coli. GI-based genomic plasticity expands the metabolic spectrum and versatility of CSV86^T, rendering efficient adaptation to the contaminated niche. Such isolate(s) are of utmost importance for their application in bioremediation and are the probable ideal host(s) for metabolic engineering.

Comparative Genome Analysis of Two Heterotrophic Nitrifying Pseudomonas putida Strains Isolated from Freshwater Shrimp Ponds in Soc Trang Province

Nitrogen compounds, especially ammonia, are widely produced in aquaculture systems during cultivation. Ammonia has been investigated as a model compound for use by heterotrophic nitrifying bacteria. Pseudomonas TT321 and Pseudomonas TT322, isolated from shrimp pond water in Soc Trang province, Vietnam, are identified by comparing them with 31 of the closest genomes sequences from the NCBI nucleotide database. The genome sizes of strains TT321 and TT322 were 5,566,241 bp and 5,563,644 bp, respectively. No plasmids were evident in these strains. Genome analysis revealed that TT321 and TT322 belonged to Pseudomonas putida and shared a common ancestor with 33 genomes. Analysis based on the comparison of genomes showed that three genes, carbamate kinase (arcC), glutamine synthetase (Glul), and aminomethyltransferase (amt), are involved in three metabolic pathways. These pathways are: (i) arginine and proline metabolism, (ii) alanine, aspartate and glutamate metabolism, and (iii) glycine, serine and threonine metabolism. These genes may play important roles in ammonia reduction and support bacterial growth via ammonia assimilation.

A bistable prokaryotic differentiation system underlying development of conjugative transfer competence

The mechanisms and impact of horizontal gene transfer processes to distribute gene functions with potential adaptive benefit among prokaryotes have been well documented. In contrast, little is known about the life-style of mobile elements mediating horizontal gene transfer, whereas this is the ultimate determinant for their transfer fitness. Here, we investigate the life-style of an integrative and conjugative element (ICE) within the genus Pseudomonas that is a model for a widespread family transmitting genes for xenobiotic compound metabolism and antibiotic resistances. Previous work showed bimodal ICE activation, but by using single cell time-lapse microscopy coupled to combinations of chromosomally integrated single copy ICE promoter-driven fluorescence reporters, RNA sequencing and mutant analysis, we now describe the complete regulon leading to the arisal of differentiated dedicated transfer competent cells. The regulon encompasses at least three regulatory nodes and five (possibly six) further conserved gene clusters on the ICE that all become expressed under stationary phase conditions. Time-lapse microscopy indicated expression of two regulatory nodes (i.e., bisR and alpA-bisDC) to precede that of the other clusters. Notably, expression of all clusters except of bisR was confined to the same cell subpopulation, and was dependent on the same key ICE regulatory factors. The ICE thus only transfers from a small fraction of cells in a population, with an estimated proportion of between 1.7–4%, which express various components of a dedicated transfer competence program imposed by the ICE, and form the centerpiece of ICE conjugation. The components mediating transfer competence are widely conserved, underscoring their selected fitness for efficient transfer of this class of mobile elements.

Horizontal gene transfer processes among prokaryotes have raised wide interest, which is attested by broad public health concern of rapid spread of antibiotic resistances. However, we typically take for granted that horizontal transfer is the result of some underlying spontaneous low frequency event, but this is not necessarily the case. As we show here, mobile genetic elements from the class of integrative and conjugative elements (ICEs) impose a coordinated program on the host cell in order to transfer, leading to an exclusive differentiated set of transfer competent cells. We base our conclusions on single cell microscopy studies to compare the rare activation of ICE promoters in individual cells in bacterial populations, and on mutant and RNA-seq analysis to show their dependency on ICE factors. This is an important finding because it implies that conjugation itself is subject to natural selection, which would lead to selection of fitter elements that transfer better or become more widespread.

A New ICEclc Subfamily Integrative and Conjugative Element Responsible for Horizontal Transfer of Biphenyl and Salicylic Acid Catabolic Pathway in the PCB-Degrading Strain Pseudomonas stutzeri KF716

Integrative and conjugative elements (ICEs) are chromosomally integrated self-transmissible mobile genetic elements. Although some ICEs are known to carry genes for the degradation of aromatic compounds, information on their genetic features is limited. We identified a new member of the ICEclc family carrying biphenyl catabolic bph genes and salicylic acid catabolic sal genes from the PCB-degrading strain Pseudomonas stutzeri KF716. The 117-kb ICE_bph-salKF716 contains common core regions exhibiting homology with those of degradative ICEclc from P. knackmussii B13 and ICE_XTD from Azoarcus sp. CIB. A comparison of the gene loci collected from the public database revealed that several putative ICEs from P. putida B6-2, P, alcaliphila JAB1, P. stutzeri AN10, and P. stutzeri 2A20 had highly conserved core regions with those of ICE_bph-salKF716, along with the variable region that encodes the catabolic genes for biphenyl, naphthalene, toluene, or phenol. These data indicate that this type of ICE subfamily is ubiquitously distributed within aromatic compound-degrading bacteria. ICE_bph-salKF716 was transferred from P. stutzeri KF716 to P. aeruginosa PAO1 via a circular extrachromosomal intermediate form. In this study, we describe the structure and genetic features of ICE_bph-salKF716 compared to other catabolic ICEs.

Combinatorial pathway balancing provides biosynthetic access to 2-fluoro-cis,cis-muconate in engineered Pseudomonas putida

The wealth of bio-based building blocks produced by engineered microorganisms seldom include halogen atoms. Muconate is a platform chemical with a number of industrial applications that could be broadened by introducing fluorine atoms to tune its physicochemical properties. The soil bacterium Pseudomonas putida naturally assimilates benzoate via the ortho-cleavage pathway with cis,cis-muconate as intermediate. Here, we harnessed the native enzymatic machinery (encoded within the ben and cat gene clusters) to provide catalytic access to 2-fluoro-cis,cis-muconate (2-FMA) from fluorinated benzoates. The reactions in this pathway are highly imbalanced, leading to accumulation of toxic intermediates and limited substrate conversion. By disentangling regulatory patterns of ben and cat in response to fluorinated effectors, metabolic activities were adjusted to favor 2-FMA biosynthesis. After implementing this combinatorial approach, engineered P. putida converted 3-fluorobenzoate to 2-FMA at the maximum theoretical yield. Hence, this study illustrates how synthetic biology can expand the diversity of nature's biochemical catalysis.

Eco-physiological portrait of a novel Pseudomonas sp. CSV86: an ideal host/candidate for metabolic engineering and bioremediation

Pseudomonas sp. CSV86, an Indian soil isolate, degrades wide range of aromatic compounds like naphthalene, benzoate and phenylpropanoids, amongst others. Isolate displays the unique and novel property of preferential utilization of aromatics over glucose and co-metabolizes them with organic acids. Interestingly, as compared to other Pseudomonads, strain CSV86 harbours only high-affinity glucokinase pathway (and absence of low-affinity oxidative route) for glucose metabolism. Such lack of gluconate loop might be responsible for the novel phenotype of preferential utilization of aromatics. The genome analysis and comparative functional mining indicated a large genome (6.79 Mb) with significant enrichment of regulators, transporters as well as presence of various secondary metabolite production clusters, suggesting its eco-physiological and metabolic versatility. Strain harbours various integrative conjugative elements (ICEs) and genomic islands, probably acquired through horizontal gene transfer events, leading to genome mosaicity and plasticity. Naphthalene degradation genes are arranged as regulonic clusters and found to be part of ICE_CSV86nah . Various eco-physiological properties and absence of major pathogenicity and virulence factors (risk group-1) in CSV86 suggest it to be an ideal candidate for bioremediation. Further, strain can serve as an ideal chassis for metabolic engineering to degrade various xenobiotics preferentially over simple carbon sources for efficient remediation.

Transcriptional regulation of organohalide pollutant utilisation in bacteria

Organohalides are organic molecules formed biotically and abiotically, both naturally and through industrial production. They are usually toxic and represent a health risk for living organisms, including humans. Bacteria capable of degrading organohalides for growth express dehalogenase genes encoding enzymes that cleave carbon-halogen bonds. Such bacteria are of potential high interest for bioremediation of contaminated sites. Dehalogenase genes are often part of gene clusters that may include regulators, accessory genes and genes for transporters and other enzymes of organohalide degradation pathways. Organohalides and their degradation products affect the activity of regulatory factors, and extensive genome-wide modulation of gene expression helps dehalogenating bacteria to cope with stresses associated with dehalogenation, such as intracellular increase of halides, dehalogenase-dependent acid production, organohalide toxicity and misrouting and bottlenecks in metabolic fluxes. This review focuses on transcriptional regulation of gene clusters for dehalogenation in bacteria, as studied in laboratory experiments and in situ. The diversity in gene content, organization and regulation of such gene clusters is highlighted for representative organohalide-degrading bacteria. Selected examples illustrate a key, overlooked role of regulatory processes, often strain-specific, for efficient dehalogenation and productive growth in presence of organohalides.

Whole Genome Sequencing and Tn5-Insertion Mutagenesis of Pseudomonas taiwanensis CMS to Probe Its Antagonistic Activity Against Rice Bacterial Blight Disease

The Gram-negative bacterium Pseudomonas taiwanensis is a novel bacterium that uses shrimp shell waste as its sole sources of carbon and nitrogen. It is a versatile bacterium with potential for use in biological control, with activities including toxicity toward insects, fungi, and the rice pathogen Xanthomonas oryzae pv.oryzae (Xoo). In this study, the complete 5.08-Mb genome sequence of P. taiwanensis CMS was determined by a combination of NGS/Sanger sequencing and optical mapping. Comparison of optical maps of seven Pseudomonas species showed that P. taiwanensis is most closely related to P. putida KT 2400. We screened a total of 11,646 individual Tn5-transponson tagged strains to identify genes that are involved in the production and regulation of the iron-chelator pyoverdine in P. taiwanensis, which is a key anti-Xoo factor. Our results indicated that the two-component system (TCS) EnvZ/OmpR plays a positive regulatory role in the production of pyoverdine, whereas the sigma factor RpoS functions as a repressor. The knowledge of the molecular basis of the regulation of pyoverdine by P. taiwanensis provided herein will be useful for its development for use in biological control, including as an anti-Xoo agent.

Genomic Features of a Food-Derived Pseudomonas aeruginosa Strain PAEM and Biofilm-Associated Gene Expression under a Marine Bacterial α-Galactosidase

The biofilm-producing strains of P. aeruginosa colonize various surfaces, including food products and industry equipment that can cause serious human and animal health problems. The biofilms enable microorganisms to evolve the resistance to antibiotics and disinfectants. Analysis of the P. aeruginosa strain (serotype O6, sequence type 2502), isolated from an environment of meat processing (PAEM) during a ready-to-cook product storage (−20 °C), showed both the mosaic similarity and differences between free-living and clinical strains by their coding DNA sequences. Therefore, a cold shock protein (CspA) has been suggested for consideration of the evolution probability of the cold-adapted P. aeruginosa strains. In addition, the study of the action of cold-active enzymes from marine bacteria against the food-derived pathogen could contribute to the methods for controlling P. aeruginosa biofilms. The genes responsible for bacterial biofilm regulation are predominantly controlled by quorum sensing, and they directly or indirectly participate in the synthesis of extracellular polysaccharides, which are the main element of the intercellular matrix. The levels of expression for 14 biofilm-associated genes of the food-derived P. aeruginosa strain PAEM in the presence of different concentrations of the glycoside hydrolase of family 36, α-galactosidase α-PsGal, from the marine bacterium Pseudoalteromonas sp. KMM 701 were determined. The real-time PCR data clustered these genes into five groups according to the pattern of positive or negative regulation of their expression in response to the action of α-galactosidase. The results revealed a dose-dependent mechanism of the enzymatic effect on the PAEM biofilm synthesis and dispersal genes.

Insights into Mobile Genetic Elements of the Biocide-Degrading Bacterium Pseudomonas nitroreducens HBP-1

The sewage sludge isolate Pseudomonas nitroreducens HBP-1 was the first bacterium known to completely degrade the fungicide 2-hydroxybiphenyl. PacBio and Illumina whole-genome sequencing revealed three circular DNA replicons: a chromosome and two plasmids. Plasmids were shown to code for putative adaptive functions such as heavy metal resistance, but with unclarified ability for self-transfer. About one-tenth of strain HBP-1's chromosomal genes are likely of recent horizontal influx, being part of genomic islands, prophages and integrative and conjugative elements (ICEs). P. nitroreducens carries two large ICEs with different functional specialization, but with homologous core structures to the well-known ICEclc of Pseudomonas knackmussii B13. The variable regions of ICEPni1 (96 kb) code for, among others, heavy metal resistances and formaldehyde detoxification, whereas those of ICEPni2 (171 kb) encodes complete meta-cleavage pathways for catabolism of 2-hydroxybiphenyl and salicylate, a protocatechuate pathway and peripheral enzymes for 4-hydroxybenzoate, ferulate, vanillin and vanillate transformation. Both ICEs transferred at frequencies of 10^-6-10^-8 per P. nitroreducens HBP-1 donor into Pseudomonas putida, where they integrated site specifically into tRNA^Gly-gene targets, as expected. Our study highlights the underlying determinants and mechanisms driving dissemination of adaptive properties allowing bacterial strains to cope with polluted environments.

An analog to digital converter controls bistable transfer competence development of a widespread bacterial integrative and conjugative element

Conjugative transfer of the integrative and conjugative element ICEclc in Pseudomonas requires development of a transfer competence state in stationary phase, which arises only in 3–5% of individual cells. The mechanisms controlling this bistable switch between non-active and transfer competent cells have long remained enigmatic. Using a variety of genetic tools and epistasis experiments in P. putida, we uncovered an ‘upstream’ cascade of three consecutive transcription factor-nodes, which controls transfer competence initiation. One of the uncovered transcription factors (named BisR) is representative for a new regulator family. Initiation activates a feedback loop, controlled by a second hitherto unrecognized heteromeric transcription factor named BisDC. Stochastic modelling and experimental data demonstrated the feedback loop to act as a scalable converter of unimodal (population-wide or ‘analog’) input to bistable (subpopulation-specific or ‘digital’) output. The feedback loop further enables prolonged production of BisDC, which ensures expression of the ‘downstream’ functions mediating ICE transfer competence in activated cells. Phylogenetic analyses showed that the ICEclc regulatory constellation with BisR and BisDC is widespread among Gamma- and Beta-proteobacteria, including various pathogenic strains, highlighting its evolutionary conservation and prime importance to control the behaviour of this wide family of conjugative elements.

Mobile DNA elements are pieces of genetic material that can jump from one bacterium to another, and even across species. They are often useful to their host, for example carrying genes that allow bacteria to resist antibiotics.

One example of bacterial mobile DNA is the ICEclc element. Usually, ICEclc sits passively within the bacterium’s own DNA, but in a small number of cells, it takes over, hijacking its host to multiply and to get transferred to other bacteria. Cells that can pass on the elements cannot divide, and so this ability is ultimately harmful to individual bacteria. Carrying ICEclc can therefore be positive for a bacterium but passing it on is not in the cell’s best interest. On the other hand, mobile DNAs like ICEclc have evolved to be disseminated as efficiently as possible. To shed more light on this tense relationship, Carraro et al. set out to identify the molecular mechanisms ICEclc deploys to control its host.

Experiments using mutant bacteria revealed that for ICEclc to successfully take over the cell, a number of proteins needed to be produced in the correct order. In particular, a protein called BisDC triggers a mechanism to make more of itself, creating a self-reinforcing ‘feedback loop’.

Mathematical simulations of the feedback loop showed that it could result in two potential outcomes for the cell. In most of the ‘virtual cells’, ICEclc ultimately remained passive; however, in a few, ICEclc managed to take over its hosts. In this case, the feedback loop ensured that there was always enough BisDC to maintain ICEclc’s control over the cell. Further analyses suggested that this feedback mechanism is also common in many other mobile DNA elements, including some that help bacteria to resist drugs.

These results are an important contribution to understand how mobile DNAs manipulate their bacterial host in order to propagate and disperse. In the future, this knowledge could help develop new strategies to combat the spread of antibiotic resistance.

Microbial genomic island discovery, visualization and analysis

Horizontal gene transfer (also called lateral gene transfer) is a major mechanism for microbial genome evolution, enabling rapid adaptation and survival in specific niches. Genomic islands (GIs), commonly defined as clusters of bacterial or archaeal genes of probable horizontal origin, are of particular medical, environmental and/or industrial interest, as they disproportionately encode virulence factors and some antimicrobial resistance genes and may harbor entire metabolic pathways that confer a specific adaptation (solvent resistance, symbiosis properties, etc). As large-scale analyses of microbial genomes increases, such as for genomic epidemiology investigations of infectious disease outbreaks in public health, there is increased appreciation of the need to accurately predict and track GIs. Over the past decade, numerous computational tools have been developed to tackle the challenges inherent in accurate GI prediction. We review here the main types of GI prediction methods and discuss their advantages and limitations for a routine analysis of microbial genomes in this era of rapid whole-genome sequencing. An assessment is provided of 20 GI prediction software methods that use sequence-composition bias to identify the GIs, using a reference GI data set from 104 genomes obtained using an independent comparative genomics approach. Finally, we present guidelines to assist researchers in effectively identifying these key genomic regions.

A genomic view of trophic and metabolic diversity in clade-specific Lamellodysidea sponge microbiomes

BACKGROUND

Marine sponges and their microbiomes contribute significantly to carbon and nutrient cycling in global reefs, processing and remineralizing dissolved and particulate organic matter. Lamellodysidea herbacea sponges obtain additional energy from abundant photosynthetic Hormoscilla cyanobacterial symbionts, which also produce polybrominated diphenyl ethers (PBDEs) chemically similar to anthropogenic pollutants of environmental concern. Potential contributions of non-Hormoscilla bacteria to Lamellodysidea microbiome metabolism and the synthesis and degradation of additional secondary metabolites are currently unknown.

RESULTS

This study has determined relative abundance, taxonomic novelty, metabolic capacities, and secondary metabolite potential in 21 previously uncharacterized, uncultured Lamellodysidea-associated microbial populations by reconstructing near-complete metagenome-assembled genomes (MAGs) to complement 16S rRNA gene amplicon studies. Microbial community compositions aligned with sponge host subgroup phylogeny in 16 samples from four host clades collected from multiple sites in Guam over a 3-year period, including representatives of Alphaproteobacteria, Gammaproteobacteria, Oligoflexia, and Bacteroidetes as well as Cyanobacteria (Hormoscilla). Unexpectedly, microbiomes from one host clade also included Cyanobacteria from the prolific secondary metabolite-producer genus Prochloron, a common tunicate symbiont. Two novel Alphaproteobacteria MAGs encoded pathways diagnostic for methylotrophic metabolism as well as type III secretion systems, and have been provisionally assigned to a new order, designated Candidatus Methylospongiales. MAGs from other taxonomic groups encoded light-driven energy production pathways using not only chlorophyll, but also bacteriochlorophyll and proteorhodopsin. Diverse heterotrophic capabilities favoring aerobic versus anaerobic conditions included pathways for degrading chitin, eukaryotic extracellular matrix polymers, phosphonates, dimethylsulfoniopropionate, trimethylamine, and benzoate. Genetic evidence identified an aerobic catabolic pathway for halogenated aromatics that may enable endogenous PBDEs to be used as a carbon and energy source.

CONCLUSIONS

The reconstruction of high-quality MAGs from all microbial taxa comprising greater than 0.1% of the sponge microbiome enabled species-specific assignment of unique metabolic features that could not have been predicted from taxonomic data alone. This information will promote more representative models of marine invertebrate microbiome contributions to host bioenergetics, the identification of potential new sponge parasites and pathogens based on conserved metabolic and physiological markers, and a better understanding of biosynthetic and degradative pathways for secondary metabolites and halogenated compounds in sponge-associated microbiota. Video Abstract.

Analysis of the biodegradative and adaptive potential of the novel polychlorinated biphenyl degrader Rhodococcus sp. WAY2 revealed by its complete genome sequence

The complete genome sequence of Rhodococcus sp. WAY2 (WAY2) consists of a circular chromosome, three linear replicons and a small circular plasmid. The linear replicons contain typical actinobacterial invertron-type telomeres with the central CGTXCGC motif. Comparative phylogenetic analysis of the 16S rRNA gene along with phylogenomic analysis based on the genome-to-genome blast distance phylogeny (GBDP) algorithm and digital DNA–DNA hybridization (dDDH) with other Rhodococcus type strains resulted in a clear differentiation of WAY2, which is likely a new species. The genome of WAY2 contains five distinct clusters of bph, etb and nah genes, putatively involved in the degradation of several aromatic compounds. These clusters are distributed throughout the linear plasmids. The high sequence homology of the ring-hydroxylating subunits of these systems with other known enzymes has allowed us to model the range of aromatic substrates they could degrade. Further functional characterization revealed that WAY2 was able to grow with biphenyl, naphthalene and xylene as sole carbon and energy sources, and could oxidize multiple aromatic compounds, including ethylbenzene, phenanthrene, dibenzofuran and toluene. In addition, WAY2 was able to co-metabolize 23 polychlorinated biphenyl congeners, consistent with the five different ring-hydroxylating systems encoded by its genome. WAY2 could also use n-alkanes of various chain-lengths as a sole carbon source, probably due to the presence of alkB and ladA gene copies, which are only found in its chromosome. These results show that WAY2 has a potential to be used for the biodegradation of multiple organic compounds.

Search for new cultured lipophilic bacteria in industrial fat-containing wastes

Fat-containing wastes that are generated as a result of industrial production of food products and are being accumulated in large quantities in wastewater and sewage treatment plants and present a serious environmental problem. Microorganisms that decompose various types of lipids may be potential candidates for creation of commercial bioformulations for fat destruction. The aim of the study was to obtain pure cultures of lipophilic bacteria from fat-containing wastes, to study their diversity and activity for the development of a biological product. As a result, 30 strains of different phylogenetic groups with lipolytic activity was obtained. The most isolated strains were represented by enterobacteria and pseudomonas members within the Gammaproteobacteria. Almost half of the isolated strains were closely related to conditionally pathogenic microorganisms such as Serratia, Klebsiella etc. Non-pathogenic strains and promising for biotechnology ones belonged to Pseudomonas citronellolis, P. nitroreducens, P. synxantha, P. extremaustralis, Bacillus subtilis, B. amyloliquefaciens, Brevibacillus brevis and Microvirgula sp.

A novel system of bacterial cell division arrest implicated in horizontal transmission of an integrative and conjugative element

Integrative and conjugative elements (ICEs) are widespread mobile DNA elements in the prokaryotic world. ICEs are usually retained within the bacterial chromosome, but can be excised and transferred from a donor to a new recipient cell, even of another species. Horizontal transmission of ICEclc, a prevalent ICE in proteobacteria, only occurs from developed specialized transfer competent (tc) cells in the donor population. tc cells become entirely dedicated to the ICE transmission at the cost of cell proliferation. The cell growth impairment is mediated by two ICEclc located genes, parA and shi, but the mechanistic and dynamic details of this process are unknown. To better understand the function of ParA and Shi, we followed their intracellular behavior from fluorescent protein fusions, and studied host cell division at single-cell level. Superresolution imaging revealed that ParA-mCherry colocalized with the host nucleoid while Shi-GFP was enriched at the membrane during the growth impairment. Despite being enriched at different cellular locations, the two proteins showed in vivo interactions, and mutations in the Walker A motif of ParA dislocalized both ParA and Shi. In addition, ParA mutations in the ATPase motif abolished the growth arrest on the host cell. Time-lapse microscopy revealed that ParA and Shi initially delay cell division, suggesting an extension of the S phase of cells, but eventually completely inhibit cell elongation. The parA-shi locus is highly conserved in other ICEclc-related elements, and expressing ParA-Shi from ICEclc in other proteobacterial species caused similar growth arrest, suggesting that the system functions similarly across hosts. The results of our study provide mechanistic insight into the novel and unique system on ICEs and help to understand such epistatic interaction between ICE genes and host physiology that entails efficient horizontal gene transfer.

IslandViewer 4: expanded prediction of genomic islands for larger-scale datasets

IslandViewer (http://www.pathogenomics.sfu.ca/islandviewer/) is a widely-used webserver for the prediction and interactive visualization of genomic islands (GIs, regions of probable horizontal origin) in bacterial and archaeal genomes. GIs disproportionately encode factors that enhance the adaptability and competitiveness of the microbe within a niche, including virulence factors and other medically or environmentally important adaptations. We report here the release of IslandViewer 4, with novel features to accommodate the needs of larger-scale microbial genomics analysis, while expanding GI predictions and improving its flexible visualization interface. A user management web interface as well as an HTTP API for batch analyses are now provided with a secured authentication to facilitate the submission of larger numbers of genomes and the retrieval of results. In addition, IslandViewer's integrated GI predictions from multiple methods have been improved and expanded by integrating the precise Islander method for pre-computed genomes, as well as an updated IslandPath-DIMOB for both pre-computed and user-supplied custom genome analysis. Finally, pre-computed predictions including virulence factors and antimicrobial resistance are now available for 6193 complete bacterial and archaeal strains publicly available in RefSeq. IslandViewer 4 provides key enhancements to facilitate the analysis of GIs and better understand their role in the evolution of successful environmental microbes and pathogens.

Pushing the limits of de novo genome assembly for complex prokaryotic genomes harboring very long, near identical repeats

Generating a complete, de novo genome assembly for prokaryotes is often considered a solved problem. However, we here show that Pseudomonas koreensis P19E3 harbors multiple, near identical repeat pairs up to 70 kilobase pairs in length, which contained several genes that may confer fitness advantages to the strain. Its complex genome, which also included a variable shufflon region, could not be de novo assembled with long reads produced by Pacific Biosciences’ technology, but required very long reads from Oxford Nanopore Technologies. Importantly, a repeat analysis, whose results we release for over 9600 prokaryotes, indicated that very complex bacterial genomes represent a general phenomenon beyond Pseudomonas. Roughly 10% of 9331 complete bacterial and a handful of 293 complete archaeal genomes represented this ‘dark matter’ for de novo genome assembly of prokaryotes. Several of these ‘dark matter’ genome assemblies contained repeats far beyond the resolution of the sequencing technology employed and likely contain errors, other genomes were closed employing labor-intense steps like cosmid libraries, primer walking or optical mapping. Using very long sequencing reads in combination with assembly algorithms capable of resolving long, near identical repeats will bring most prokaryotic genomes within reach of fast and complete de novo genome assembly.

Transient Replication in Specialized Cells Favors Transfer of an Integrative and Conjugative Element

Bacterial evolution is driven to a large extent by horizontal gene transfer (HGT)—the processes that distribute genetic material between species rather than by vertical descent. The different elements and processes mediating HGT have been characterized in great molecular detail. In contrast, very little is known on adaptive features selecting HGT evolvability and fitness optimization. By studying the molecular behavior of an integrated mobile DNA of the class of integrative and conjugative elements in individual Pseudomonas putida donor bacteria, we report here how transient replication of the element after its excision from the chromosome is favorable for its transfer success. Since successful transfer into a new recipient is a measure of the element’s fitness, transient replication may have been selected as an adaptive benefit for more-optimal transfer.

Integrative and conjugative elements (ICEs) are widespread mobile DNA within bacterial genomes, whose lifestyle is relatively poorly understood. ICEs transmit vertically through donor cell chromosome replication, but in order to transfer, they have to excise from the chromosome. The excision step makes ICEs prone to loss, in case the donor cell divides and the ICE is not replicated. By adapting the system of LacI-cyan fluorescent protein (CFP) binding to lacO operator arrays, we analyze here the process of excision and transfer of the ICE for 3-chlorobenzoate degradation (ICEclc) in individual cells of the bacterium Pseudomonas putida. We provide evidence that ICEclc excises exclusively in a subset of specialized transfer-competent cells. ICEclc copy numbers in transfer-competent cells were higher than in regular nontransferring cells but were reduced in mutants lacking the ICE oriT1 origin of transfer, the ICE DNA relaxase, or the excision recombination sites. Consistently, transfer-competent cells showed a higher proportion without any observable LacI-CFP foci, suggesting ICEclc loss, but this proportion was independent of the ICE relaxase or the ICE origins of transfer. Our results thus indicated that the excised ICE becomes transiently replicated in transfer-competent cells, with up to six observable copies from LacI-CFP fluorescent focus measurements. Most of the observed ICEclc transfer to ICE-free P. putida recipients occurred from donors displaying 3 to 4 ICE copies, which constitute a minority among all transfer-competent cells. This finding suggests, therefore, that replication of the excised ICEclc in donors is beneficial for transfer fitness to recipient cells.

The Integrative Conjugative Element clc (ICEclc) of Pseudomonas aeruginosa JB2

Integrative conjugative elements (ICE) are a diverse group of chromosomally integrated, self-transmissible mobile genetic elements (MGE) that are active in shaping the functions of bacteria and bacterial communities. Each type of ICE carries a characteristic set of core genes encoding functions essential for maintenance and self-transmission, and cargo genes that endow on hosts phenotypes beneficial for niche adaptation. An important area to which ICE can contribute beneficial functions is the biodegradation of xenobiotic compounds. In the biodegradation realm, the best-characterized ICE is ICEclc, which carries cargo genes encoding for ortho-cleavage of chlorocatechols (clc genes) and aminophenol metabolism (amn genes). The element was originally identified in the 3-chlorobenzoate-degrader Pseudomonas knackmussii B13, and the closest relative is a nearly identical element in Burkholderia xenovorans LB400 (designated ICEclc-B13 and ICEclc-LB400, respectively). In the present report, genome sequencing of the o-chlorobenzoate degrader Pseudomonas aeruginosa JB2 was used to identify a new member of the ICEclc family, ICEclc-JB2. The cargo of ICEclc-JB2 differs from that of ICEclc-B13 and ICEclc-LB400 in consisting of a unique combination of genes that encode for the utilization of o-halobenzoates and o-hydroxybenzoate as growth substrates (ohb genes and hyb genes, respectively) and which are duplicated in a tandem repeat. Also, ICEclc-JB2 lacks an operon of regulatory genes (tciR-marR-mfsR) that is present in the other two ICEclc, and which controls excision from the host. Thus, the mechanisms regulating intracellular behavior of ICEclc-JB2 may differ from that of its close relatives. The entire tandem repeat in ICEclc-JB2 can excise independently from the element in a process apparently involving transposases/insertion sequence associated with the repeats. Excision of the repeats removes important niche adaptation genes from ICEclc-JB2, rendering it less beneficial to the host. However, the reduced version of ICEclc-JB2 could now acquire new genes that might be beneficial to a future host and, consequently, to the survival of ICEclc-JB2. Collectively, the present identification and characterization of ICEclc-JB2 provides insights into roles of MGE in bacterial niche adaptation and the evolution of catabolic pathways for biodegradation of xenobiotic compounds.

Physiological and transcriptome changes induced by Pseudomonas putida acquisition of an integrative and conjugative element

Integrative and conjugative elements (ICEs) comprise ubiquitous large mobile regions in prokaryotic chromosomes that transmit vertically to daughter cells and transfer horizontally to distantly related lineages. Their evolutionary success originates in maximized combined ICE-host fitness trade-offs, but how the ICE impacts on the host metabolism and physiology is poorly understood. Here we investigate global changes in the host genetic network and physiology of Pseudomonas putida with or without an integrated ICEclc, a model ICE widely distributed in proteobacterial genomes. Genome-wide gene expression differences were analyzed by RNA-seq using exponentially growing or stationary phase-restimulated cultures on 3-chlorobenzoate, an aromatic compound metabolizable thanks to specific ICEclc-located genes. We found that the presence of ICEclc imposes a variety of changes in global pathways such as cell cycle and amino acid metabolism, which were more numerous in stationary-restimulated than exponential phase cells. Unexpectedly, ICEclc stimulates cellular motility and leads to more rapid growth on 3-chlorobenzoate than cells carrying only the integrated clc genes. ICEclc also concomitantly activates the P. putida Pspu28-prophage, but this in itself did not provoke measurable fitness effects. ICEclc thus interferes in a number of cellular pathways, inducing both direct benefits as well as indirect costs in P. putida.

Cloning and expression of the carbaryl hydrolase gene mcbA and the identification of a key amino acid necessary for carbaryl hydrolysis

Carbamate hydrolase is the initial and key enzyme for degradation of carbamate pesticides. In the present study, we report the isolation of a carbaryl-degrading strain Pseudomonas sp. XWY-1, the cloning of its carbaryl hydrolase gene (mcbA) and the characterization of McbA. Strain XWY-1 was able to utilize carbaryl as a sole carbon source and degrade it using 1-naphthol as an intermediate. Transposon mutagenesis identified a mutant of XWY-1M that was unable to hydrolyze carbaryl. The transposon-disrupted gene mcbA was cloned by self-formed adaptor PCR, then expressed in Escherichia coli BL21(DE3) and purified. McbA was able to hydrolyze carbamate pesticides including carbaryl, isoprocarb, fenobucarb, carbofuran efficiently, while it hydrolyzed aldicarb, and propoxur poorly. The optimal pH of McbA was 7.0 and the optimal temperature was 40°C. The apparent K_m and k_cat values of McbA for carbaryl were 77.67±12.31μM and 2.12±0.10s^-1, respectively. Three amino acid residues (His467, His477 and His504) in the predicted polymerase/histidinol phosphatase-like domain were shown to be closely related to the activity of McbA, with His504 being the most important, as a replacement of His504 led to the complete loss of activity. This is the first study to identify key amino acids in McbA.

The hidden life of integrative and conjugative elements

Integrative and conjugative elements (ICEs) are widespread mobile DNA that transmit both vertically, in a host-integrated state, and horizontally, through excision and transfer to new recipients. Different families of ICEs have been discovered with more or less restricted host ranges, which operate by similar mechanisms but differ in regulatory networks, evolutionary origin and the types of variable genes they contribute to the host. Based on reviewing recent experimental data, we propose a general model of ICE life style that explains the transition between vertical and horizontal transmission as a result of a bistable decision in the ICE-host partnership. In the large majority of cells, the ICE remains silent and integrated, but hidden at low to very low frequencies in the population specialized host cells appear in which the ICE starts its process of horizontal transmission. This bistable process leads to host cell differentiation, ICE excision and transfer, when suitable recipients are present. The ratio of ICE bistability (i.e. ratio of horizontal to vertical transmission) is the outcome of a balance between fitness costs imposed by the ICE horizontal transmission process on the host cell, and selection for ICE distribution (i.e. ICE 'fitness'). From this emerges a picture of ICEs as elements that have adapted to a mostly confined life style within their host, but with a very effective and dynamic transfer from a subpopulation of dedicated cells.

Complete genome sequence of Pseudomonas citronellolis P3B5, a candidate for microbial phyllo-remediation of hydrocarbon-contaminated sites

Pseudomonas citronellolis is a Gram negative, motile gammaproteobacterium belonging to the order Pseudomonadales and the family Pseudomonadaceae. We isolated strain P3B5 from the phyllosphere of basil plants (Ocimum basilicum L.). Here we describe the physiology of this microorganism, its full genome sequence, and detailed annotation. The 6.95 Mbp genome contains 6071 predicted protein coding sequences and 96 RNA coding sequences. P. citronellolis has been the subject of many studies including the investigation of long-chain aliphatic compounds and terpene degradation. Plant leaves are covered by long-chain aliphates making up a waxy layer that is associated with the leaf cuticle. In addition, basil leaves are known to contain high amounts of terpenoid substances, hinting to a potential nutrient niche that might be exploited by P. citronellolis. Furthermore, the isolated strain exhibited resistance to several antibiotics. To evaluate the potential of this strain as source of transferable antibiotic resistance genes on raw consumed herbs we therefore investigated if those resistances are encoded on mobile genetic elements. The availability of the genome will be helpful for comparative genomics of the phylogenetically broad pseudomonads, in particular with the sequence of the P. citronellolis type strain PRJDB205 not yet publicly available. The genome is discussed with respect to a phyllosphere related lifestyle, aliphate and terpenoid degradation, and antibiotic resistance.

The ICEXTD of Azoarcus sp. CIB, an integrative and conjugative element with aerobic and anaerobic catabolic properties

Integrative and conjugative elements (ICE) play a major role in aerobic degradation of aromatic compounds, but they have not yet been shown to be involved in anaerobic degradation. We have characterized here the ICE_XTD element which endows to the beta-proteobacterium Azoarcus sp. CIB with the ability to utilize aromatic hydrocarbons. The core region of ICE_XTD , which shows a remarkable synteny with that of ICEclc-like elements, allows its own intracellular and intercellular mobility. ICE_XTD integrates at the tRNAGly of the host chromosome, but it can also excise to produce a ready to transfer circular form. The adaptation modules of ICE_XTD represent a unique combination of gene clusters for aerobic (tod genes) and anaerobic (bss-bbs and mbd genes) degradation of certain aromatic hydrocarbons, e.g., toluene, m-xylene and cumene. Transfer of ICE_XTD to other Azoarcus strains, e.g., A. evansii, confers them the ability to degrade aromatic hydrocarbons both aerobically and anaerobically. Interestingly, ICE_XTD allows Cupriavidus pinatubonensis, a bacterium unable to degrade anaerobically aromatic compounds, to grow with m-xylene under anoxic conditions. Thus, ICE_XTD constitutes the first mobile genetic element able to expand the catabolic abilities of certain bacteria for the removal of aromatic hydrocarbons either in the presence or absence of oxygen.

Comparative genomic and functional analyses: unearthing the diversity and specificity of nematicidal factors in Pseudomonas putida strain 1A00316

We isolated Pseudomonas putida (P. putida) strain 1A00316 from Antarctica. This bacterium has a high efficiency against Meloidogyne incognita (M. incognita) in vitro and under greenhouse conditions. The complete genome of P. putida 1A00316 was sequenced using PacBio single molecule real-time (SMRT) technology. A comparative genomic analysis of 16 Pseudomonas strains revealed that although P. putida 1A00316 belonged to P. putida, it was phenotypically more similar to nematicidal Pseudomonas fluorescens (P. fluorescens) strains. We characterized the diversity and specificity of nematicidal factors in P. putida 1A00316 with comparative genomics and functional analysis, and found that P. putida 1A00316 has diverse nematicidal factors including protein alkaline metalloproteinase AprA and two secondary metabolites, hydrogen cyanide and cyclo-(l-isoleucyl-l-proline). We show for the first time that cyclo-(l-isoleucyl-l-proline) exhibit nematicidal activity in P. putida. Interestingly, our study had not detected common nematicidal factors such as 2,4-diacetylphloroglucinol (2,4-DAPG) and pyrrolnitrin in P. putida 1A00316. The results of the present study reveal the diversity and specificity of nematicidal factors in P. putida strain 1A00316.

A Comparison of 14 Erythrobacter Genomes Provides Insights into the Genomic Divergence and Scattered Distribution of Phototrophs

Aerobic anoxygenic phototrophic bacteria (AAPB) are bacteriochlorophyll a (Bchl a)-containing microbial functional population. Erythrobacter is the first genus that was identified to contain AAPB species. Here, we compared 14 Erythrobacter genomes: seven phototrophic strains and seven non- phototrophic strains. Interestingly, AAPB strains are scattered in this genus based on their phylogenetic relationships. All 14 strains could be clustered into three groups based on phylo-genomic analysis, average genomic nucleotide identity and the phylogeny of signature genes (16S rRNA and virB4 genes). The AAPB strains were distributed in three groups, and gain and loss of phototrophic genes co-occurred in the evolutionary history of the genus Erythrobacter. The organization and structure of photosynthesis gene clusters (PGCs) in seven AAPB genomes displayed high synteny of major regions except for few insertions. The 14 Erythrobacter genomes had a large range of genome sizes, from 2.72 to 3.60 M, and the sizes of the core and pan- genomes were 1231 and 8170 orthologous clusters, respectively. Integrative and conjugative elements (ICEs) were frequently identified in genomes we studied, which might play significant roles in shaping or contributing to the pan-genome of Erythrobacter. Our findings suggest the ongoing evolutionary divergence of Erythrobacter genomes and the scattered distribution characteristic of PGC.

Highly variable individual donor cell fates characterize robust horizontal gene transfer of an integrative and conjugative element

Horizontal gene transfer is an important evolutionary mechanism for bacterial adaptation. However, given the typical low transfer frequencies in a bacterial population, little is known about the fate and interplay of donor cells and the mobilized DNA during transfer. Here we study transfer of an integrative and conjugative element (ICE) among individual live bacterial cells. ICEs are widely distributed mobile DNA elements that are different than plasmids because they reside silent in the host chromosome and are maintained through vertical descent. Occasionally, ICEs become active, excise, and transmit their DNA to a new recipient, where it is reintegrated. We develop a fluorescent tool to differentiate excision, transfer, and reintegration of a model ICE named ICEclc (for carrying the clc genes for chlorocatechol metabolism) among single Pseudomonas cells by using time-lapse microscopy. We find that ICEclc activation is initiated in stationary phase cells, but excision and transfer predominantly occur only when such cells have been presented with new nutrients. Donors with activated ICE develop a number of different states, characterized by reduced cell division rates or growth arrest, persistence, or lysis, concomitant with ICE excision, and likely, ICE loss or replication. The donor cell state transitions can be described by using a stochastic model, which predicts that ICE fitness is optimal at low initiation rates in stationary phase. Despite highly variable donor cell fates, ICE transfer is remarkably robust overall, with 75% success after excision. Our results help to better understand ICE behavior and shed a new light on bacterial cellular differentiation during horizontal gene transfer.

Different Ancestries of R Tailocins in Rhizospheric Pseudomonas Isolates

Bacterial genomes accommodate a variety of mobile genetic elements, including bacteriophage-related clusters that encode phage tail-like protein complexes playing a role in interactions with eukaryotic or prokaryotic cells. Such tailocins are unable to replicate inside target cells due to the lack of a phage head with associated DNA. A subset of tailocins mediate antagonistic activities with bacteriocin-like specificity. Functional characterization of bactericidal tailocins of two Pseudomonas putida rhizosphere isolates revealed not only extensive similarity with the tail assembly module of the Pseudomonas aeruginosa R-type pyocins but also differences in genomic integration site, regulatory genes, and lytic release modules. Conversely, these three features are quite similar between strains of the P. putida and Pseudomonas fluorescens clades, although phylogenetic analysis of tail genes suggests them to have evolved separately. Unlike P. aeruginosa R pyocin elements, the tailocin gene clusters of other pseudomonads frequently carry cargo genes, including bacteriocins. Compared with P. aeruginosa, the tailocin tail fiber sequences that act as specificity determinants have diverged much more extensively among the other pseudomonad species, mostly isolates from soil and plant environments. Activity of the P. putida antibacterial particles requires a functional lipopolysaccharide layer on target cells, but contrary to R pyocins from P. aeruginosa, strain susceptibilities surpass species boundaries.

Sequencing and characterizing the genome of Estrella lausannensis as an undergraduate project: training students and biological insights

With the widespread availability of high-throughput sequencing technologies, sequencing projects have become pervasive in the molecular life sciences. The huge bulk of data generated daily must be analyzed further by biologists with skills in bioinformatics and by "embedded bioinformaticians," i.e., bioinformaticians integrated in wet lab research groups. Thus, students interested in molecular life sciences must be trained in the main steps of genomics: sequencing, assembly, annotation and analysis. To reach that goal, a practical course has been set up for master students at the University of Lausanne: the "Sequence a genome" class. At the beginning of the academic year, a few bacterial species whose genome is unknown are provided to the students, who sequence and assemble the genome(s) and perform manual annotation. Here, we report the progress of the first class from September 2010 to June 2011 and the results obtained by seven master students who specifically assembled and annotated the genome of Estrella lausannensis, an obligate intracellular bacterium related to Chlamydia. The draft genome of Estrella is composed of 29 scaffolds encompassing 2,819,825 bp that encode for 2233 putative proteins. Estrella also possesses a 9136 bp plasmid that encodes for 14 genes, among which we found an integrase and a toxin/antitoxin module. Like all other members of the Chlamydiales order, Estrella possesses a highly conserved type III secretion system, considered as a key virulence factor. The annotation of the Estrella genome also allowed the characterization of the metabolic abilities of this strictly intracellular bacterium. Altogether, the students provided the scientific community with the Estrella genome sequence and a preliminary understanding of the biology of this recently-discovered bacterial genus, while learning to use cutting-edge technologies for sequencing and to perform bioinformatics analyses.

The TetR-type MfsR protein of the integrative and conjugative element (ICE) ICEclc controls both a putative efflux system and initiation of ICE transfer

Integrative and conjugating elements (ICE) are self-transferable DNAs widely present in bacterial genomes, which often carry a variety of auxiliary genes of potential adaptive benefit. One of the model ICE is ICEclc, an element originally found in Pseudomonas knackmussii B13 and known for its propensity to provide its host with the capacity to metabolize chlorocatechols and 2-aminophenol. In this work, we studied the mechanism and target of regulation of MfsR, a TetR-type repressor previously found to exert global control on ICEclc horizontal transfer. By using a combination of ICEclc mutant and transcriptome analysis, gene reporter fusions, and DNA binding assays, we found that MfsR is a repressor of both its own expression and that of a gene cluster putatively coding for a major facilitator superfamily efflux system on ICEclc (named mfsABC). Phylogenetic analysis suggests that mfsR was originally located immediately adjacent to the efflux pump genes but became displaced from its original cis target DNA by a gene insertion. This resulted in divergence of the original bidirectional promoters into two separated individual regulatory units. Deletion of mfsABC did not result in a strong phenotype, and despite screening a large number of compounds and conditions, we were unable to define the precise current function or target of the putative efflux pump. Our data reconstruct how the separation of an ancestor mfsR-mfsABC system led to global control of ICEclc transfer by MfsR.