Distribution of IS91 family insertion sequences in bacterial genomes: evolutionary implications

The virulence plasmid associated with AHPND in shrimp appears to have originated from Vibrio owensii through a process of homologous recombination of parental plasmids and the transposable insertion of two large fragments

Acute hepatopancreatic necrosis disease (AHPND) is a highly contagious and lethal disease of shrimp caused by Vibrio strains carrying the virulence plasmid (pAHPND) containing the pirAB virulence genes. Through analysis of plasmid sequence similarity, clustering, and phylogeny, a horizontal transfer element similar to IS91 was discovered within the pAHPND plasmid. Additionally, two distinct clades of plasmids related to pAHPND (designated as pAHPND-r1 and pAHPND-r2) were identified, which may serve as potential parental plasmids for pAHPND. The available evidence, including the difference in G+C content between the plasmid and its host, codon usage preference, and plasmid recombination event prediction, suggests that the formation of the pAHPND plasmid in the Vibrio owensii strain was likely due to the synergistic effect of the recombinase RecA and the associated proteins RecBCD on the pAHPND-r1 and pAHPND-r2, resulting in the recombination and formation of the precursor plasmid for pAHPND (pre-pAHPND). The emergence of pAHPND was found to be a result of successive insertions of the horizontal transfer elements of pirAB-Tn903 and IS91-like segment, which led to the deletion of one third of the pre-pAHPND. This plasmid was then able to spread horizontally to other Vibrio strains, contributing to the epidemics of AHPND. These findings shed light on previously unknown mechanisms involved in the emergence of pAHPND and improve our understanding of the disease's spread.

Characterization of the soil resistome and mobilome in Namib Desert soils

The study of the soil resistome is important in understanding the evolution of antibiotic resistance and its dissemination between the clinic and the environment. However, very little is known about the soil resistome, especially of those from deserts. Here, we characterize the bacterial communities, using targeted sequencing of the 16S rRNA genes, and both the resistome and the mobilome in Namib Desert soils, using shotgun metagenomics. We detected a variety of antibiotic resistance genes (ARGs) that conferred resistance to antibiotics such as elfamycin, rifampicin, and fluoroquinolones, metal/biocide resistance genes (MRGs/BRGs) conferring resistance to metals such as arsenic and copper, and mobile genetic elements (MGEs) such as the ColE1-like plasmid. The presence of metal/biocide resistance genes in close proximity to ARGs indicated a potential for co-selection of resistance to antibiotics and metals/biocides. The co-existence of MGEs and horizontally acquired ARGs most likely contributed to a decoupling between bacterial community composition and ARG profiles. Overall, this study indicates that soil bacterial communities in Namib Desert soils host a diversity of resistance elements and that horizontal gene transfer, rather than host phylogeny, plays an essential role in their dynamics.

Capture of armA by a novel ISCR element, ISCR28

ISCR28 is a fully functional and active member of the IS91-like family of insertion sequences. ISCR28 is 1,708-bp long and contains a 1,293-bp long putative open reading frame that codes a transposase. Sixty ISCR28-containing sequences from GenBank generated 27 non-repeat genetic contexts, all of which represented naturally occurring biological events that had occurred in a wide range of gram-negative organisms. Insertion of ISCR28 into target DNA preferred the presence of a 5'-GXXT-3' sequence at its terIS (replication terminator) end. Loss of the first 4 bp of its oriIS (origin of replication) likely caused ISCR28 to be trapped in ISApl1-based transposons or similar structures. Loss of terIS and fusion with a mobile element upstream likely promoted co-transfer of ISCR28 and the downstream resistance genes. ArmA and its downstream intact ISCR28 can be excised from recombinant pKD46 plasmids forming circular intermediates, further elucidating its activity as a transposase.

Metagenomics reveals differences in the composition of bacterial antimicrobial resistance and antibiotic resistance genes in pasteurized yogurt and probiotic bacteria yogurt from China

Antimicrobial resistance has become a global public health concern, and antibiotic resistance genes (ARG) in food are a research focus. In China, probiotics and pasteurized yogurts are the 2 main types of commercially available yogurt, but the distribution and differences of antibiotic-resistant bacteria and gene types in these products are not well known. This study used a shotgun metagenomic approach to analyze 22 different types of yogurt collected from 9 main yogurt-producing areas in China; each type of yogurt included 8 different batches of samples. The abundance and diversity of bacteria identified in probiotic yogurt were significantly higher than those in pasteurized yogurt, with Acetobacter, Raoultella, and Burkholderia identified as unique and highly abundant genera in probiotic yogurt. Similarly, the abundance of Enterococcus faecium, Staphylococcus aureus, Klebsiella pneumoniae, Acinetobacter baumannii, Pseudomonas aeruginosa, and Enterobacter spp. was higher than that in pasteurized yogurt. A total of 1,149 ARG subtypes belonging to 16 ARG types were identified, with the highest abundance of rifampicin, multidrug efflux pumps, and quinolone resistance genes detected. Network analysis revealed significant nonrandom co-occurrence relationships between different types and subtypes of ARG in yogurt samples. A total of 44 ARG subtypes in pasteurized yogurt were potentially hosted by 36 bacterial genera, and in probiotic yogurt, 63 ARG were expected to be hosted by 86 bacterial species from 37 genera. These findings indicate potential safety issues in fermented dairy products and emphasize the need for a more hygienic environment when processing probiotic yogurt.

Metagenomic and Culturomics Analysis of Microbial Communities within Surface Sediments and the Prevalence of Antibiotic Resistance Genes in a Pristine River: The Zaqu River in the Lancang River Source Region, China

Microbial communities inhabiting sedimentary environments in river source regions serve as pivotal indicators of pristine river ecosystems. While the correlation between antibiotic resistome and pathogenicity with core gut bacteria in humans is well established, there exists a significant knowledge gap concerning the interaction of antibiotic resistance genes (ARGs) and human pathogenic bacteria (HPB) with specific microbes in river source basins, often referred to as "terrestrial gut". Understanding the microbial composition, including bacteria and resident genetic elements such as ARGs, HPB, Mobile Genetic Elements (MGEs), and Virulence Factors (VFs), within natural habitats against the backdrop of global change, is imperative. To address this gap, an enrichment-based culturomics complementary along with metagenomics was conducted in this study to characterize the microbial biobank and provide preliminary ecological insights into profiling the dissemination of ARGs in the Lancang River Source Basin. Based on our findings, in the main stream of the Lancang River Source Basin, 674 strains of bacteria, comprising 540 strains under anaerobic conditions and 124 under aerobic conditions, were successfully isolated. Among these, 98 species were identified as known species, while 4 were potential novel species. Of these 98 species, 30 were HPB relevant to human health. Additionally, bacA and bacitracin emerged as the most abundant ARGs and antibiotics in this river, respectively. Furthermore, the risk assessment of ARGs predominantly indicated the lowest risk rank (Rank Ⅳ) in terms of endangering human health. In summary, enrichment-based culturomics proved effective in isolating rare and unknown bacteria, particularly under anaerobic conditions. The emergence of ARGs showed limited correlation with MGEs, indicating minimal threats to human health within the main stream of the Lancang River Source Basin.

Characterization and Identification of a novel chromosome-encoded metallo-β-lactamase WUS-1 in Myroides albus P34

In this study, we identified and characterized a novel chromosomally-encoded class B metallo-β-lactamase (MBL) gene designated bla WUS-1 in a carbapenem-resistant isolate Myroides albus P34 isolated from sewage discharged from an animal farm. Comparative analysis of the deduced amino acid sequence revealed that WUS-1 shares the highest amino acid similarities with the function-characterized MBLs MUS-1 (AAN63647.1; 70.73%) and TUS-1 (AAN63648.1; 70.32%). The recombinant carrying bla WUS-1 exhibited increased MICs levels against a number of β-lactam antimicrobials such as carbenicillin, ampicillin and imipenem, and β-lactamase inhibitors (clavulanic acid and tazobactam). The metallo-β-lactamase WUS-1 could also hydrolyze these antimicrobials and the hydrolytic activities could be inhibited by EDTA. Genetic context analysis of bla WUS-1 revealed that no mobile genetic element was found in its surrounding region. The plasmid pMA84474 of Myroides albus P34 harbored 6 resistance genes (bla OXA-347, aadS, bla MYO-1, ereD, sul2 and ermF) within an approximately 17 kb multidrug resistance (MDR) region. These genes, however, were all related to mobile genetic elements.

Dissemination of metaldehyde catabolic pathways is driven by mobile genetic elements in Proteobacteria

Bioremediation of metaldehyde from drinking water using metaldehyde-degrading strains has recently emerged as a promising alternative. Whole-genome sequencing was used to obtain full genomes for metaldehyde degraders Acinetobacter calcoaceticus E1 and Sphingobium CMET-H. For the former, the genetic context of the metaldehyde-degrading genes had not been explored, while for the latter, none of the degrading genes themselves had been identified. In A. calcoaceticus E1, IS91 and IS6-family insertion sequences (ISs) were found surrounding the metaldehyde-degrading gene cluster located in plasmid pAME76. This cluster was located in closely-related plasmids and associated to identical ISs in most metaldehyde-degrading β- and γ-Proteobacteria, indicating horizontal gene transfer (HGT). For Sphingobium CMET-H, sequence analysis suggested a phytanoyl-CoA family oxygenase as a metaldehyde-degrading gene candidate due to its close homology to a previously identified metaldehyde-degrading gene known as mahX. Heterologous gene expression in Escherichia coli alongside degradation tests verified its functional significance and the degrading gene homolog was henceforth called mahS. It was found that mahS is hosted within the conjugative plasmid pSM1 and its genetic context suggested a crossover between the metaldehyde and acetoin degradation pathways. Here, specific replicons and ISs responsible for maintaining and dispersing metaldehyde-degrading genes in α, β and γ-Proteobacteria through HGT were identified and described. In addition, a homologous gene implicated in the first step of metaldehyde utilisation in an α-Proteobacteria was uncovered. Insights into specific steps of this possible degradation pathway are provided.

Metagenomic comparison of gut communities between hawksbills (Eretmochelys imbricata) and green sea turtles (Chelonia mydas)

The gut microbiota is closely linked to host nutrition, immunity, and health. Here, metagenomic analysis was conducted to elucidate the taxonomic and functional diversity of gut communities from hawksbills and green sea turtles. In terms of diversity and abundance, the gut microbiota of herbivorous green sea turtles showed a higher bacterial diversity and richness than that of hawksbills. Firmicutes dominated in all groups; however, the phylum Proteobacteria showed a higher relative abundance in hawksbills. Several metabolic pathways displayed broad prevalence and high relative abundances in the two sea turtle populations. Antibiotic resistance genes (ARGs) responsible for resistance to glycopeptide and tetracycline were the most abundant in all samples. In ARGs, the subtype macB was the most abundant in the two different sea turtle populations; however, evgS, bcrA, and efrA were more abundant in the green sea turtles, while in the hawksbills, tetT and tetB(P) were more abundant. Among mobile genetic elements (MGEs), the abundance of 16 MGE types showed a significant difference between the two sea turtle populations. MGE type transposase and plasmid were the most abundant in the two sea turtle populations. Additionally, gene functions were enriched in carbohydrate esterases, glycoside hydrolases, and polysaccharide lyases in the green sea turtles, whereas genes related to glycosyltransferases and auxiliary activities were highly abundant in hawksbills. These metagenomic profiles provide further insights into the microbial diversities of the two types of sea turtles and provide valuable information for future conservation efforts.

Swine growth promotion with antibiotics or alternatives can increase antibiotic resistance gene mobility potential

Even though the use of antibiotics for food-producing animals may contribute to the emergence of antimicrobial resistance, antibiotics are still used as growth promoters. Due to consumer and regulatory pressures, the use of alternatives to antibiotics as growth promoters is increasing, thus more information is needed on their capability to disseminate antimicrobial resistance compared to antibiotics. We investigated the impacts of carbadox (antibiotic), copper sulfate and zinc oxide (metals) and mushroom powder (natural product) on the pig fecal resistome and microbiome. Antibiotic resistance gene (ARG) and mobile genetic element (MGE) abundances were measured using a high-throughput qPCR array with 382 primer pairs. Bacterial community composition was determined by 16S rRNA gene sequencing. More ARGs co-occurred with MGEs in the growth promoter group samples than in the control group samples. Community composition could not be linked to resistome in the growth promoter group samples, indicating a potential decoupling of ARGs and phylogeny. Additionally, machine-learning methods aided in defining the community and resistome differences in response to treatments. Since increased ARG mobility potential was the primary response to the dietary additives used in this study, we suggest that ARG mobility should be considered when designing antimicrobial use policies and antimicrobial resistance surveillances.

Intra-species variation within Lactobacillus rhamnosus correlates to beneficial or harmful outcomes: lessons from the oral cavity

Background

The origin of most of the Lactobacillus rhamnosus genome sequences lodged in NCBI can be traced to food and faecal isolates followed by blood and tissue sites but with minimal representation from oral and vaginal isolates. However, on the L. rhamnosus phylogenetic tree no apparent clade is linked to the origin of isolation or to the relevant clinical source, except for a distinct clade exclusively shared by L. rhamnosus isolates from early stages of dental pulp infection (LRHMDP2 and LRHMDP3) and from bronchoalveolar lavage (699_LRHA and 708_LRHA) from a critical care patient. These L. rhamnosus strains, LRHMDP2, LRHMDP3, 699_LRHA and 708_LRHA isolated from different continents, display closest genome neighbour gapped identity of 99.95%. The aim of this study was to define a potentially unique complement of genes of clinical relevance shared between these L. rhamnosus clinical isolates in comparison to probiotic L. rhamnosus strains.

Results

In this analysis we used orthologous protein identification tools such as ProteinOrtho followed by tblastn alignments to identify a novel tyrosine protein phosphatase (wzb)-tyrosine-protein kinase modulator EpsC (wzd)- synteny exopolysaccharide (EPS) cluster. This EPS cluster was specifically conserved in a clade of 5 clinical isolates containing the four L. rhamnosus clinical isolates noted above and Lactobacillus spp. HMSC077C11, a clinical isolate from a neck abscess. The EPS cluster was shared with only two other strains, L. rhamnosus BPL5 and BPL15, which formed a distant clade on the L. rhamnosus phylogenetic tree, with a closest genome neighbour gapped identity of 97.51% with L. rhamnosus LRHMDP2 and LRHMDP3.

Exclusivity of this EPS cluster (from those identified before) was defined by five EPS genes, which were specifically conserved between the clade of 5 clinical isolates and L. rhamnosus BPL5 and BPL15 when compared to the remaining L. rhamnosus strains. Comparative genome analysis between the clade of 5 clinical isolates and L. rhamnosus BPL5 and BPL15 showed a set of 58 potentially unique genes characteristic of the clade of 5.

Conclusion

The potentially unique functional protein orthologs associated with the clade of 5 clinical isolates may provide understanding of fitness under selective pressure.

Plasmids of Shigella flexneri serotype 1c strain Y394 provide advantages to bacteria in the host

Background

Shigella flexneri has an extremely complex genome with a significant number of virulence traits acquired by mobile genetic elements including bacteriophages and plasmids. S. flexneri serotype 1c is an emerging etiological agent of bacillary dysentery in developing countries. In this study, the complete nucleotide sequence of two plasmids of S. flexneri serotype 1c strain Y394 was determined and analysed.

Results

The plasmid pINV-Y394 is an invasive or virulence plasmid of size 221,293 bp composed of a large number of insertion sequences (IS), virulence genes, regulatory and maintenance genes. Three hundred and twenty-eight open reading frames (ORFs) were identified in pINV-Y394, of which about a half (159 ORFs) were identified as IS elements. Ninety-seven ORFs were related to characterized genes (majority of which are associated with virulence and their regulons), and 72 ORFs were uncharacterized or hypothetical genes. The second plasmid pNV-Y394 is of size 10,866 bp and encodes genes conferring resistance against multiple antibiotics of clinical importance. The multidrug resistance gene cassette consists of tetracycline resistance gene tetA, streptomycin resistance gene strA-strB and sulfonamide-resistant dihydropteroate synthase gene sul2.

Conclusions

These two plasmids together play a key role in the fitness of Y394 in the host environment. The findings from this study indicate that the pathogenic S. flexneri is a highly niche adaptive pathogen which is able to co-evolve with its host and respond to the selection pressure in its environment.

Electronic supplementary material

The online version of this article (10.1186/s12866-019-1455-1) contains supplementary material, which is available to authorized users.

The toxic guardians - multiple toxin-antitoxin systems provide stability, avoid deletions and maintain virulence genes of Pseudomonas syringae virulence plasmids

Background

Pseudomonas syringae is a γ-proteobacterium causing economically relevant diseases in practically all cultivated plants. Most isolates of this pathogen contain native plasmids collectively carrying many pathogenicity and virulence genes. However, P. syringae is generally an opportunistic pathogen primarily inhabiting environmental reservoirs, which could exert a low selective pressure for virulence plasmids. Additionally, these plasmids usually contain a large proportion of repeated sequences, which could compromise plasmid integrity. Therefore, the identification of plasmid stability determinants and mechanisms to preserve virulence genes is essential to understand the evolution of this pathogen and its adaptability to agroecosystems.

Results

The three virulence plasmids of P. syringae pv. savastanoi NCPPB 3335 contain from one to seven functional stability determinants, including three highly active toxin-antitoxin systems (TA) in both pPsv48A and pPsv48C. The TA systems reduced loss frequency of pPsv48A by two orders of magnitude, whereas one of the two replicons of pPsv48C likely confers stable inheritance by itself. Notably, inactivation of the TA systems from pPsv48C exposed the plasmid to high-frequency deletions promoted by mobile genetic elements. Thus, recombination between two copies of MITEPsy2 caused the deletion of an 8.3 kb fragment, with a frequency of 3.8 ± 0.3 × 10^− 3. Likewise, one-ended transposition of IS801 generated plasmids containing deletions of variable size, with a frequency of 5.5 ± 2.1 × 10^− 4, of which 80% had lost virulence gene idi. These deletion derivatives were stably maintained in the population by replication mediated by repJ, which is adjacent to IS801. IS801 also promoted deletions in plasmid pPsv48A, either by recombination or one-ended transposition. In all cases, functional TA systems contributed significantly to reduce the occurrence of plasmid deletions in vivo.

Conclusions

Virulence plasmids from P. syringae harbour a diverse array of stability determinants with a variable contribution to plasmid persistence. Importantly, we showed that multiple plasmid-borne TA systems have a prominent role in preserving plasmid integrity and ensuring the maintenance of virulence genes in free-living conditions. This strategy is likely widespread amongst native plasmids of P. syringae and other bacteria.

Electronic supplementary material

The online version of this article (10.1186/s13100-019-0149-4) contains supplementary material, which is available to authorized users.

Evolution of Sphingomonad Gene Clusters Related to Pesticide Catabolism Revealed by Genome Sequence and Mobilomics of Sphingobium herbicidovorans MH

Bacterial degraders of chlorophenoxy herbicides have been isolated from various ecosystems, including pristine environments. Among these degraders, the sphingomonads constitute a prominent group that displays versatile xenobiotic-degradation capabilities. Four separate sequencing strategies were required to provide the complete sequence of the complex and plastic genome of the canonical chlorophenoxy herbicide-degrading Sphingobium herbicidovorans MH. The genome has an intricate organization of the chlorophenoxy-herbicide catabolic genes sdpA, rdpA, and cadABCD that encode the (R)- and (S)-enantiomer-specific 2,4-dichlorophenoxypropionate dioxygenases and four subunits of a Rieske non-heme iron oxygenase involved in 2-methyl-chlorophenoxyacetic acid degradation, respectively. Several major genomic rearrangements are proposed to help understand the evolution and mobility of these important genes and their genetic context. Single-strain mobilomic sequence analysis uncovered plasmids and insertion sequence-associated circular intermediates in this environmentally important bacterium and enabled the description of evolutionary models for pesticide degradation in strain MH and related organisms. The mobilome presented a complex mosaic of mobile genetic elements including four plasmids and several circular intermediate DNA molecules of insertion-sequence elements and transposons that are central to the evolution of xenobiotics degradation. Furthermore, two individual chromosomally integrated prophages were shown to excise and form free circular DNA molecules. This approach holds great potential for improving the understanding of genome plasticity, evolution, and microbial ecology.

The Different Faces of Rolling-Circle Replication and Its Multifunctional Initiator Proteins

Horizontal gene transfer (HGT) contributes greatly to the plasticity and evolution of prokaryotic and eukaryotic genomes. The main carriers of foreign DNA in HGT are mobile genetic elements (MGEs) that have extremely diverse genetic structures and properties. Various strategies are used for the maintenance and spread of MGEs, including (i) vegetative replication, (ii) transposition (and other types of recombination), and (iii) conjugal transfer. In many MGEs, all of these processes are dependent on rolling-circle replication (RCR). RCR is one of the most well characterized models of DNA replication. Although many studies have focused on describing its mechanism, the role of replication initiator proteins has only recently been subject to in-depth analysis, which indicates their involvement in multiple biological process associated with RCR. In this review, we present a general overview of RCR and its impact in HGT. We focus on the molecular characteristics of RCR initiator proteins belonging to the HUH and Rep_trans protein families. Despite analogous mechanisms of action these are distinct groups of proteins with different catalytic domain structures. This is the first review describing the multifunctional character of various types of RCR initiator proteins, including the latest discoveries in the field. Recent reports provide evidence that (i) proteins initiating vegetative replication (Rep) or mobilization for conjugal transfer (Mob) may also have integrase (Int) activity, (ii) some Mob proteins are capable of initiating vegetative replication (Rep activity), and (iii) some Rep proteins can act like Mob proteins to mobilize plasmid DNA for conjugal transfer. These findings have significant consequences for our understanding of the role of RCR, not only in DNA metabolism but also in the biology of many MGEs.

A Helitron transposon reconstructed from bats reveals a novel mechanism of genome shuffling in eukaryotes

Helitron transposons capture and mobilize gene fragments in eukaryotes, but experimental evidence for their transposition is lacking in the absence of an isolated active element. Here we reconstruct Helraiser, an ancient element from the bat genome, and use this transposon as an experimental tool to unravel the mechanism of Helitron transposition. A hairpin close to the 3′-end of the transposon functions as a transposition terminator. However, the 3′-end can be bypassed by the transposase, resulting in transduction of flanking sequences to new genomic locations. Helraiser transposition generates covalently closed circular intermediates, suggestive of a replicative transposition mechanism, which provides a powerful means to disseminate captured transcriptional regulatory signals across the genome. Indeed, we document the generation of novel transcripts by Helitron promoter capture both experimentally and by transcriptome analysis in bats. Our results provide mechanistic insight into Helitron transposition, and its impact on diversification of gene function by genome shuffling.

Helitron elements are proposed rolling-circle transposons in eukaryotic genomes, but experimental evidence for their transposition has been lacking. Here, Grabundzija et al. reconstruct an active Helitron from bats which they name Helraiser, and characterize its mechanism of transposition in cell-free reactions and in human cell cultures in vitro.

Comparison of Four Comamonas Catabolic Plasmids Reveals the Evolution of pBHB To Catabolize Haloaromatics

Comamonas plasmids play important roles in shaping the phenotypes of their hosts and the adaptation of these hosts to changing environments, and understanding the evolutionary strategy of these plasmids is thus of great concern. In this study, the sequence of the 119-kb 3,5-dibromo-4-hydroxybenzonitrile-catabolizing plasmid pBHB from Comamonas sp. strain 7D-2 was studied and compared with those of three other Comamonas haloaromatic catabolic plasmids. Incompatibility group determination based on a phylogenetic analysis of 24 backbone gene proteins, as well as TrfA, revealed that these four plasmids all belong to the IncP-1β subgroup. Comparison of the four plasmids revealed a conserved backbone region and diverse genetic-load regions. The four plasmids share a core genome consisting of 40 genes (>50% similarities) and contain 12 to 50 unique genes each, most of which are xenobiotic-catabolic genes. Two functional reductive dehalogenase gene clusters are specifically located on pBHB, showing distinctive evolution of pBHB for haloaromatics. The higher catabolic ability of the bhbA2B2 cluster than the bhbAB cluster may be due to the transcription levels and the character of the dehalogenase gene itself rather than that of its extracytoplasmic binding receptor gene. The plasmid pBHB is riddled with transposons and insertion sequence (IS) elements, and ISs play important roles in the evolution of pBHB. The analysis of the origin of the bhb genes on pBHB suggested that these accessory genes evolved independently. Our work provides insights into the evolutionary strategies of Comamonas plasmids, especially into the adaptation mechanism employed by pBHB for haloaromatics.

Contribution of the non-effector members of the HrpL regulon, iaaL and matE, to the virulence of Pseudomonas syringae pv. tomato DC3000 in tomato plants

BACKGROUND

The phytohormone indole-3-acetic acid (IAA) is widely distributed among plant-associated bacteria. Certain strains of the Pseudomonas syringae complex can further metabolize IAA into a less biologically active amino acid conjugate, 3-indole-acetyl-ε-L-lysine, through the action of the iaaL gene. In P. syringae and Pseudomonas savastanoi strains, the iaaL gene is found in synteny with an upstream gene, here called matE, encoding a putative MATE family transporter. In P. syringae pv. tomato (Pto) DC3000, a pathogen of tomato and Arabidopsis plants, the HrpL sigma factor controls the expression of a suite of virulence-associated genes via binding to hrp box promoters, including that of the iaaL gene. However, the significance of HrpL activation of the iaaL gene in the virulence of Pto DC3000 is still unclear.

RESULTS

A conserved hrp box motif is found upstream of the iaaL gene in the genomes of P. syringae strains. However, although the promoter region of matE is only conserved in genomospecies 3 of this bacterial group, we showed that this gene also belongs to the Pto DC3000 HrpL regulon. We also demonstrated that the iaaL gene is transcribed both independently and as part of an operon with matE in this pathogen. Deletion of either the iaaL or the matE gene resulted in reduced fitness and virulence of Pto DC3000 in tomato plants. In addition, we used multicolor fluorescence imaging to visualize the responses of tomato plants to wild-type Pto DC3000 and to its ΔmatE and ΔiaaL mutants. Activation of secondary metabolism prior to the development of visual symptoms was observed in tomato leaves after bacterial challenges with all strains. However, the observed changes were strongest in plants challenged by the wild-type strain, indicating lower activation of secondary metabolism in plants infected with the ΔmatE or ΔiaaL mutants.

CONCLUSIONS

Our results provide new evidence for the roles of non-type III effector genes belonging to the Pto DC3000 HrpL regulon in virulence.

Mechanisms of DNA Transposition

DNA transposases use a limited repertoire of structurally and mechanistically distinct nuclease domains to catalyze the DNA strand breaking and rejoining reactions that comprise DNA transposition. Here, we review the mechanisms of the four known types of transposition reactions catalyzed by (1) RNase H-like transposases (also known as DD(E/D) enzymes); (2) HUH single-stranded DNA transposases; (3) serine transposases; and (4) tyrosine transposases. The large body of accumulated biochemical and structural data, particularly for the RNase H-like transposases, has revealed not only the distinguishing features of each transposon family, but also some emerging themes that appear conserved across all families. The more-recently characterized single-stranded DNA transposases provide insight into how an ancient HUH domain fold has been adapted for transposition to accomplish excision and then site-specific integration. The serine and tyrosine transposases are structurally and mechanistically related to their cousins, the serine and tyrosine site-specific recombinases, but have to date been less intensively studied. These types of enzymes are particularly intriguing as in the context of site-specific recombination they require strict homology between recombining sites, yet for transposition can catalyze the joining of transposon ends to form an excised circle and then integration into a genomic site with much relaxed sequence specificity.

Bacterial insertion sequences: their genomic impact and diversity

Insertion sequences (ISs), arguably the smallest and most numerous autonomous transposable elements (TEs), are important players in shaping their host genomes. This review focuses on prokaryotic ISs. We discuss IS distribution and impact on genome evolution. We also examine their effects on gene expression, especially their role in activating neighbouring genes, a phenomenon of particular importance in the recent upsurge of bacterial antibiotic resistance. We explain how ISs are identified and classified into families by a combination of characteristics including their transposases (Tpases), their overall genetic organisation and the accessory genes which some ISs carry. We then describe the organisation of autonomous and nonautonomous IS‐related elements. This is used to illustrate the growing recognition that the boundaries between different types of mobile element are becoming increasingly difficult to define as more are being identified. We review the known Tpase types, their different catalytic activities used in cleaving and rejoining DNA strands during transposition, their organisation into functional domains and the role of this in regulation. Finally, we consider examples of prokaryotic IS domestication. In a more speculative section, we discuss the necessity of constructing more quantitative dynamic models to fully appreciate the continuing impact of TEs on prokaryotic populations.

Breaking and joining single-stranded DNA: the HUH endonuclease superfamily

HUH endonucleases are numerous and widespread in all three domains of life. The major function of these enzymes is processing a range of mobile genetic elements by catalysing cleavage and rejoining of single-stranded DNA using an active-site Tyr residue to make a transient 5'-phosphotyrosine bond with the DNA substrate. These enzymes have a key role in rolling-circle replication of plasmids and bacteriophages, in plasmid transfer, in the replication of several eukaryotic viruses and in various types of transposition. They have also been appropriated for cellular processes such as intron homing and the processing of bacterial repeated extragenic palindromes. Here, we provide an overview of these fascinating enzymes and their functions, using well-characterized examples of Rep proteins, relaxases and transposases, and we explore the molecular mechanisms used in their diverse activities.

Antibiotic resistance shaping multi-level population biology of bacteria

Antibiotics have natural functions, mostly involving cell-to-cell signaling networks. The anthropogenic production of antibiotics, and its release in the microbiosphere results in a disturbance of these networks, antibiotic resistance tending to preserve its integrity. The cost of such adaptation is the emergence and dissemination of antibiotic resistance genes, and of all genetic and cellular vehicles in which these genes are located. Selection of the combinations of the different evolutionary units (genes, integrons, transposons, plasmids, cells, communities and microbiomes, hosts) is highly asymmetrical. Each unit of selection is a self-interested entity, exploiting the higher hierarchical unit for its own benefit, but in doing so the higher hierarchical unit might acquire critical traits for its spread because of the exploitation of the lower hierarchical unit. This interactive trade-off shapes the population biology of antibiotic resistance, a composed-complex array of the independent "population biologies." Antibiotics modify the abundance and the interactive field of each of these units. Antibiotics increase the number and evolvability of "clinical" antibiotic resistance genes, but probably also many other genes with different primary functions but with a resistance phenotype present in the environmental resistome. Antibiotics influence the abundance, modularity, and spread of integrons, transposons, and plasmids, mostly acting on structures present before the antibiotic era. Antibiotics enrich particular bacterial lineages and clones and contribute to local clonalization processes. Antibiotics amplify particular genetic exchange communities sharing antibiotic resistance genes and platforms within microbiomes. In particular human or animal hosts, the microbiomic composition might facilitate the interactions between evolutionary units involved in antibiotic resistance. The understanding of antibiotic resistance implies expanding our knowledge on multi-level population biology of bacteria.

The processing of repetitive extragenic palindromes: the structure of a repetitive extragenic palindrome bound to its associated nuclease

Extragenic sequences in genomes, such as microRNA and CRISPR, are vital players in the cell. Repetitive extragenic palindromic sequences (REPs) are a class of extragenic sequences, which form nucleotide stem-loop structures. REPs are found in many bacterial species at a high copy number and are important in regulation of certain bacterial functions, such as Integration Host Factor recruitment and mRNA turnover. Although a new clade of putative transposases (RAYTs or TnpA_REP) is often associated with an increase in these repeats, it is not clear how these proteins might have directed amplification of REPs. We report here the structure to 2.6 Å of TnpA_REP from Escherichia coli MG1655 bound to a REP. Sequence analysis showed that TnpA_REP is highly related to the IS200/IS605 family, but in contrast to IS200/IS605 transposases, TnpA_REP is a monomer, is auto-inhibited and is active only in manganese. These features suggest that, relative to IS200/IS605 transposases, it has evolved a different mechanism for the movement of discrete segments of DNA and has been severely down-regulated, perhaps to prevent REPs from sweeping through genomes.

Antibiotics shaping bacterial genome: deletion of an IS91 flanked virulence determinant upon exposure to subinhibitory antibiotic concentrations

The nucleoid-associated proteins Hha and YdgT repress the expression of the toxin α-hemolysin. An Escherichia coli mutant lacking these proteins overexpresses the toxin α-hemolysin encoded in the multicopy recombinant plasmid pANN202-312R. Unexpectedly, we could observe that this mutant generated clones that no further produced hemolysin (Hly(-)). Generation of Hly(-) clones was dependent upon the presence in the culture medium of the antibiotic kanamycin (km), a marker of the hha allele (hha::Tn5). Detailed analysis of different Hly(-) clones evidenced that recombination between partial IS91 sequences that flank the hly operon had occurred. A fluctuation test evidenced that the presence of km in the culture medium was underlying the generation of these clones. A decrease of the km concentration from 25 mg/l to 12.5 mg/l abolished the appearance of Hly(-) derivatives. We considered as a working hypothesis that, when producing high levels of the toxin (combination of the hha ydgT mutations with the presence of the multicopy hemolytic plasmid pANN202-312R), the concentration of km of 25 mg/l resulted subinhibitory and stimulated the recombination between adjacent IS91 flanking sequences. To further test this hypothesis, we analyzed the effect of subinhibitory km concentrations in the wild type E. coli strain MG1655 harboring the parental low copy number plasmid pHly152. At a km concentration of 5 mg/l, subinhibitory for strain MG1655 (pHly152), generation of Hly(-) clones could be readily detected. Similar results were also obtained when, instead of km, ampicillin was used. IS91 is flanking several virulence determinants in different enteric bacterial pathogenic strains from E. coli and Shigella. The results presented here evidence that stress generated by exposure to subinhibitory antibiotic concentrations may result in rearrangements of the bacterial genome. Whereas some of these rearrangements may be deleterious, others may generate genotypes with increased virulence, which may resume infection.

Miniature transposable sequences are frequently mobilized in the bacterial plant pathogen Pseudomonas syringae pv. phaseolicola

Mobile genetic elements are widespread in Pseudomonas syringae, and often associate with virulence genes. Genome reannotation of the model bean pathogen P. syringae pv. phaseolicola 1448A identified seventeen types of insertion sequences and two miniature inverted-repeat transposable elements (MITEs) with a biased distribution, representing 2.8% of the chromosome, 25.8% of the 132-kb virulence plasmid and 2.7% of the 52-kb plasmid. Employing an entrapment vector containing sacB, we estimated that transposition frequency oscillated between 2.6×10⁻⁵ and 1.1×10⁻⁶, depending on the clone, although it was stable for each clone after consecutive transfers in culture media. Transposition frequency was similar for bacteria grown in rich or minimal media, and from cells recovered from compatible and incompatible plant hosts, indicating that growth conditions do not influence transposition in strain 1448A. Most of the entrapped insertions contained a full-length IS801 element, with the remaining insertions corresponding to sequences smaller than any transposable element identified in strain 1448A, and collectively identified as miniature sequences. From these, fragments of 229, 360 and 679-nt of the right end of IS801 ended in a consensus tetranucleotide and likely resulted from one-ended transposition of IS801. An average 0.7% of the insertions analyzed consisted of IS801 carrying a fragment of variable size from gene PSPPH_0008/PSPPH_0017, showing that IS801 can mobilize DNA in vivo. Retrospective analysis of complete plasmids and genomes of P. syringae suggests, however, that most fragments of IS801 are likely the result of reorganizations rather than one-ended transpositions, and that this element might preferentially contribute to genome flexibility by generating homologous regions of recombination. A further miniature sequence previously found to affect host range specificity and virulence, designated MITEPsy1 (100-nt), represented an average 2.4% of the total number of insertions entrapped in sacB, demonstrating for the first time the mobilization of a MITE in bacteria.

Structural characterization of ISCR8, ISCR22, and ISCR23, subgroups of IS91-like insertion elements

Analysis of ISCR8 (ISPps1) revealed that this group of insertion elements has to be subdivided into three subgroups: ISCR8, ISCR22, and ISCR23. The distinction of three subgroups is supported by phylogenetic analysis of the transposase open reading frames (ORFs). Comparison of over 20 complete and partial ISCR8/22/23 elements identified oriIS candidate sequences for all groups and a terIS candidate sequence for ISCR8. The oriIS sequences, their distance to the transposase ORFs, and the sequence of this intervening region are group specific, further supporting the definition of two new ISCR elements. ISCR8/22/23 have a very broad host range, including Gram-positive and Gram-negative bacteria, among which are several (opportunistic) pathogens. The IS often resides on plasmids or in the vicinity of other mobile elements and is mostly associated with genes for the degradation of halo- or nitro-aromatics. However, in one case ISCR8 was found in the neighborhood of an antibiotic resistance determinant in Klebsiella pneumoniae. ISCR8 resembles other IS91 family elements in mediating genetic rearrangements by homologous recombination between two copies. In Delftia acidovorans this led to the loss of the genes encoding dichlorprop cleavage. In conclusion, this study shows that ISCR8 could be a fully functional and active member of the IS91 family of insertion elements.

The diversity of conjugative relaxases and its application in plasmid classification

Bacterial conjugation is an efficient and sophisticated mechanism of DNA transfer among bacteria. While mobilizable plasmids only encode a minimal MOB machinery that allows them to be transported by other plasmids, conjugative plasmids encode a complete set of transfer genes (MOB1T4SS). The only essential ingredient of the MOB machinery is the relaxase, the protein that initiates and terminates conjugative DNA processing. In this review we compared the sequences and properties of the relaxase proteins contained in gene sequence databases. Proteins were arranged in families and phylogenetic trees constructed from the family alignments. This allowed the classification of conjugative transfer systems in six MOB families:MOB(F), MOB(H), MOB(Q), MOB(C), MOB(P) and MOB(V). The main characteristics of each family were reviewed. The phylogenetic relationships of the coupling proteins were also analysed and resulted in phylogenies congruent to those of the cognate relaxases. We propose that the sequences of plasmid relaxases can be used for plasmid classification. We hope our effort will provide researchers with a useful tool for further mining and analysing the plasmid universe both experimentally and in silico.

The rolling-circle plasmid pTN1 from the hyperthermophilic archaeon Thermococcus nautilus

The hyperthermophilic archaeon Thermococcus nautilus carries a plasmid, pTN1, which encodes a rolling-circle (RC) replication initiator protein of 74 kDa (Rep74) and an orphan protein of 24 kDa (p24). The Rep74 protein is homologous to the Rep75 protein encoded by the RC plasmid pGT5 from Pyrococcus abyssi. Comparative analysis of Rep74 and Rep75 sequences shows that these proteins correspond to a new family of RC initiators formed by the fusion of a Rep domain with an N-terminal domain of unknown function. Surprisingly, the Rep domain of Rep74/75 is more closely related to transposases encoded by IS elements than to Rep proteins of other RC plasmids. The p24 protein contains a hydrophobic segment, a highly charged region and a zinc finger motif. A recombinant p24 protein lacking the hydrophobic segment binds and condenses both single- and double-stranded DNA, and forms DNA aggregates with extreme compaction at high protein to DNA ratio. In addition to encoding proteins of significant interest, pTN1 is remarkable by being the only characterized plasmid isolated from a Thermococcus strain, thus being useful to develop genetic tools in Thermococcus kodakaraensis for which gene disruption methods became recently available.

Genomic insights into the contribution of phytopathogenic bacterial plasmids to the evolutionary history of their hosts

Plasmids are common residents of phytopathogenic bacteria and contribute significantly to host evolution in a multi-faceted manner. Plasmids tend to encode determinants of virulence and ecological fitness that can enhance adaptation to a specific niche or can influence niche expansion. Many of these determinants appear to have been acquired from other bacteria via horizontal transfer, illustrating an important function of plasmids in the acquisition of sequences that enable rapid evolution. These genes can ultimately be delivered to the host chromosome through plasmid integration events, thus stabilizing important acquired determinants within the genome. Most plasmids characterized in phytopathogenic bacteria are self-transmissible and possess suites of genes encoding type IV secretion systems. In addition, the phytopathogenic bacterial plasmid "mobilome" includes insertion sequence and other transposable elements that contribute to the movement of sequences within and between genomes. Possession of mosaic and ever-changing plasmids allows phytopathogenic bacteria to maintain a dynamic, flexible genome and possible advantage in host-pathogen and other environmental interactions that belies the concept of plasmids as apparently selfish genetic elements.

Localization and characterization of two novel genes encoding stereospecific dioxygenases catalyzing 2(2,4-dichlorophenoxy)propionate cleavage in Delftia acidovorans MC1

Two novel genes, rdpA and sdpA, encoding the enantiospecific alpha-ketoglutarate dependent dioxygenases catalyzing R,S-dichlorprop cleavage in Delftia acidovorans MC1 were identified. Significant similarities to other known genes were not detected, but their deduced amino acid sequences were similar to those of other alpha-ketoglutarate dioxygenases. RdpA showed 35% identity with TauD of Pseudomonas aeruginosa, and SdpA showed 37% identity with TfdA of Ralstonia eutropha JMP134. The functionally important amino acid sequence motif HX(D/E)X(23-26)(T/S)X(114-183)HX(10-13)R/K, which is highly conserved in group II alpha-ketoglutarate-dependent dioxygenases, was present in both dichlorprop-cleaving enzymes. Transposon mutagenesis of rdpA inactivated R-dichlorprop cleavage, indicating that it was a single-copy gene. Both rdpA and sdpA were located on the plasmid pMC1 that also carries the lower pathway genes. Sequencing of a 25.8-kb fragment showed that the dioxygenase genes were separated by a 13.6-kb region mainly comprising a Tn501-like transposon. Furthermore, two copies of a sequence similar to IS91-like elements were identified. Hybridization studies comparing the wild-type plasmid and that of the mutant unable to cleave dichlorprop showed that rdpA and sdpA were deleted, whereas the lower pathway genes were unaffected, and that deletion may be caused by genetic rearrangements of the IS91-like elements. Two other dichlorprop-degrading bacterial strains, Rhodoferax sp. strain P230 and Sphingobium herbicidovorans MH, were shown to carry rdpA genes of high similarity to rdpA from strain MC1, but sdpA was not detected. This suggested that rdpA gene products are involved in the degradation of R-dichlorprop in these strains.