Pilus assembly by Agrobacterium T-DNA transfer genes

Expanded geographic distribution for two Legionella pneumophila sequence types of clinical concern

Legionella pneumophila serogroup 1 sequence types (ST) 213 and 222, a single-locus variant of ST213, were first detected in the early 1990s in the Midwest United States (U.S.) and the late 1990s in the Northeast U.S. and Canada. Since 1992, these STs have increasingly been implicated in community-acquired sporadic and outbreak-associated Legionnaires' disease (LD) cases. We were interested in understanding the change in LD frequency due to these STs and identifying genetic features that differentiate these STs from one another. For the geographic area examined here (Mountain West to Northeast) and over the study period (1992-2020), ST213/222-associated LD cases identified by the Centers for Disease Control and Prevention increased by 0.15 cases per year, with ST213/222-associated LD cases concentrated in four states: Michigan (26%), New York (18%), Minnesota (16%), and Ohio (10%). Additionally, between 2002 and 2021, ST222 caused at least five LD outbreaks in the U.S.; no known outbreaks due to ST213 occurred in the U.S. during this time. We compared the genomes of 230 ST213/222 isolates and found that the mean of the average nucleotide identity (ANI) within each ST was high (99.92% for ST222 and 99.92% for ST213), with a minimum between ST ANI of 99.50% and a maximum of 99.87%, indicating low genetic diversity within and between these STs. While genomic features were identified (e.g., plasmids and CRISPR-Cas systems), no association explained the increasing geographic distribution and prevalence of ST213 and ST222. Yet, we provide evidence of the expanded geographical distribution of ST213 and ST222 in the U.S.IMPORTANCESince the 1990s, cases of Legionnaires' disease (LD) attributed to a pair of closely related Legionella pneumophila variants, ST213 and ST222, have increased in the U.S. Furthermore, between 2002 and 2021, ST222 caused at least five outbreaks of LD in the U.S., while ST213 has not been linked to any U.S. outbreak. We wanted to understand how the rate of LD cases attributed to these variants has changed over time and compare the genetic features of the two variants. Between 1992 and 2020, we determined an increase of 0.15 LD cases ascribed to ST213/222 per year in the geographic region studied. Our research shows that these STs are spreading within the U.S., yet most of the cases occurred in four states: Michigan, New York, Minnesota, and Ohio. Additionally, we found little genetic diversity within and between these STs nor could specific genetic features explain their geographic spread.

Impacts of microplastic type on the fate of antibiotic resistance genes and horizontal gene transfer mechanism during anaerobic digestion

Microplastics (MPs) and antibiotic resistance genes (ARGs) are important pollutants in waste activated sludge (WAS), but their interactions during anaerobic digestion (AD) still need to be further explored. This study investigated variations in ARGs, mobile genetic elements (MGEs), and host bacteria during AD under the pressure of polyamide (PA), polyethylene (PE), and polypropylene (PP). The results showed that the MPs increased methane production by 11.7-35.5%, and decreased ARG abundance by 5.6-24.6%. Correlation analysis showed that the decrease of MGEs (plasmid, prophage, etc.) promoted the decrease of the abundance of multidrug, aminoglycoside and tetracycline resistance genes. Metagenomic annotation revealed that the reduction of key host bacteria (Arenimonas, Lautropia, etc.) reduced the abundance of major ARGs (rsmA, rpoB2, etc.). Moreover, PP MPs contributed to a reduction in the abundance of functional genes related to the production of reactive oxygen species, ATP synthesis, and cell membrane permeability, which was conducive to reducing the potential for horizontal gene transfer of ARGs. These findings provide insights into the treatment of organic waste containing MPs.

Genomic consequences associated with Agrobacterium-mediated transformation of plants

Attenuated strains of the naturally occurring plant pathogen Agrobacterium tumefaciens can transfer virtually any DNA sequence of interest to model plants and crops. This has made Agrobacterium-mediated transformation (AMT) one of the most commonly used tools in agricultural biotechnology. Understanding AMT, and its functional consequences, is of fundamental importance given that it sits at the intersection of many fundamental fields of study, including plant-microbe interactions, DNA repair/genome stability, and epigenetic regulation of gene expression. Despite extensive research and use of AMT over the last 40 years, the extent of genomic disruption associated with integrating exogenous DNA into plant genomes using this method remains underappreciated. However, new technologies like long-read sequencing make this disruption more apparent, complementing previous findings from multiple research groups that have tackled this question in the past. In this review, we cover progress on the molecular mechanisms involved in Agrobacterium-mediated DNA integration into plant genomes. We also discuss localized mutations at the site of insertion and describe the structure of these DNA insertions, which can range from single copy insertions to large concatemers, consisting of complex DNA originating from different sources. Finally, we discuss the prevalence of large-scale genomic rearrangements associated with the integration of DNA during AMT with examples. Understanding the intended and unintended effects of AMT on genome stability is critical to all plant researchers who use this methodology to generate new genetic variants.

Soil Inoculation and Blocker-Mediated Sequencing Show Effects of the Antibacterial T6SS on Agrobacterial Tumorigenesis and Gallobiome

The type VI secretion system (T6SS) is deployed by many proteobacteria to secrete effector proteins into bacterial competitors for competition or eukaryotic cells for pathogenesis. Agrobacteria, a group of soilborne phytopathogens causing crown gall disease on various plant species, deploy the T6SS to attack closely and distantly related bacterial species in vitro and in planta. Current evidence suggests that the T6SS is not essential for pathogenesis under direct inoculation, but it remains unknown whether the T6SS influences natural disease incidence or the microbial community within crown galls (i.e., the gallobiome). To address these two key questions, we established a soil inoculation method on wounded tomato seedlings that mimics natural infections and developed a bacterial 16S rRNA gene amplicon enrichment sequencing platform. By comparing the Agrobacterium wild-type strain C58 with two T6SS mutants, we demonstrate that the T6SS influences both disease occurrence and gallobiome composition. Based on multiple inoculation trials across seasons, all three strains induced tumors, but the mutants had significantly lower disease incidences. The season of inoculation played a more important role than the T6SS in shaping the gallobiome. The influence of the T6SS was evident in summer, during which two Sphingomonadaceae species and the family Burkholderiaceae were enriched in the gallobiome induced by the mutants. Further in vitro competition and colonization assays demonstrated the T6SS-mediated antagonism to a Sphingomonas sp. R1 strain isolated from tomato rhizosphere in this study. In conclusion, this work demonstrates that the Agrobacterium T6SS promotes tumorigenesis in infection processes and provides competitive advantages in gall-associated microbiota. IMPORTANCE The T6SS is widespread among proteobacteria and used for interbacterial competition by agrobacteria, which are soil inhabitants and opportunistic bacterial pathogens causing crown gall disease in a wide range of plants. Current evidence indicates that the T6SS is not required for gall formation when agrobacteria are inoculated directly on plant wounding sites. However, in natural settings, agrobacteria may need to compete with other bacteria in bulk soil to gain access to plant wounds and influence the microbial community inside crown galls. The role of the T6SS in these critical aspects of disease ecology have remained largely unknown. In this study, we successfully developed a soil inoculation method coupled with blocker-mediated enrichment of bacterial 16S rRNA gene amplicon sequencing, named SI-BBacSeq, to address these two important questions. We provided evidence that the T6SS promotes disease occurrence and influences crown gall microbiota composition by interbacterial competition.

Effects of functional modules and bacterial clusters response on transmission performance of antibiotic resistance genes under antibiotic stress during anaerobic digestion of livestock wastewater

The formation and transmission of antibiotic resistance genes (ARGs) have attracted increasing attention. It is unclear whether the internal mechanisms by which antibiotics affect horizontal gene transfer (HGT) of ARGs during anaerobic digestion (AD) were influenced by dose and type. We investigated the effects of two major antibiotics (oxytetracycline, OTC, and sulfamethoxazole, SMX) on ARGs during AD according to antibiotic concentration in livestock wastewater influent. The low-dose antibiotic (0.5 mg/L) increased ROS and SOS responses, promoting the formation of ARGs. Meanwhile, low-dose antibiotics could also promote the spread of ARGs by promoting pili, communication responses, and the type IV secretion system (T4SS). However, different types and doses of antibiotics would lead to changes in the above functional modules and then affect the enrichment of ARGs. With the increasing dose of SMX, the advantages of pili and communication responses would gradually change. In the OTC system, low-dose has the strongest promoting ability in both pili and communication responses. Similarly, an increase in the dose of SMX would change T4SS from facilitation to inhibition, while OTC completely inhibits T4SS. Microbial and network analysis also revealed that low-dose antibiotics were more favorable for the growth of host bacteria.

Secretion of Pertussis Toxin from Bordetella pertussis

Production and secretion of pertussis toxin (PT) is essential for the virulence of Bordetella pertussis. Due to the large oligomeric structure of PT, transport of the toxin across bacterial membrane barriers represents a significant hurdle that the bacteria must overcome in order to maintain pathogenicity. During the secretion process, PT undergoes a two-step transport process. The first step involves transport of the individual polypeptide chains of PT across the inner membrane utilizing a generalized secretion pathway, most likely the bacterial Sec system. The second step involves the use of a specialized apparatus to transport the toxin across the outer membrane of the bacterial cell. This apparatus, which has been termed the Ptl transporter and which is unique to the PT secretion pathway, is a member of the type IV family of bacterial transporters. Here, the current understanding of the PT secretion process is reviewed including a description of the Ptl proteins that assemble to form the transporter, the general structure of type IV transporters, the known similarities and differences between canonical type IV substrate transport and Ptl-mediated transport of PT, as well as the known sequence of events in the assembly and secretion of PT.

Comparative Analysis of Genome of Ehrlichia sp. HF, a Model Bacterium to Study Fatal Human Ehrlichiosis

BACKGROUND

The genus Ehrlichia consists of tick-borne obligatory intracellular bacteria that can cause deadly diseases of medical and agricultural importance. Ehrlichia sp. HF, isolated from Ixodes ovatus ticks in Japan [also referred to as I. ovatus Ehrlichia (IOE) agent], causes acute fatal infection in laboratory mice that resembles acute fatal human monocytic ehrlichiosis caused by Ehrlichia chaffeensis. As there is no small laboratory animal model to study fatal human ehrlichiosis, Ehrlichia sp. HF provides a needed disease model. However, the inability to culture Ehrlichia sp. HF and the lack of genomic information have been a barrier to advance this animal model. In addition, Ehrlichia sp. HF has several designations in the literature as it lacks a taxonomically recognized name.

RESULTS

We stably cultured Ehrlichia sp. HF in canine histiocytic leukemia DH82 cells from the HF strain-infected mice, and determined its complete genome sequence. Ehrlichia sp. HF has a single double-stranded circular chromosome of 1,148,904 bp, which encodes 866 proteins with a similar metabolic potential as E. chaffeensis. Ehrlichia sp. HF encodes homologs of all virulence factors identified in E. chaffeensis, including 23 paralogs of P28/OMP-1 family outer membrane proteins, type IV secretion system apparatus and effector proteins, two-component systems, ankyrin-repeat proteins, and tandem repeat proteins. Ehrlichia sp. HF is a novel species in the genus Ehrlichia, as demonstrated through whole genome comparisons with six representative Ehrlichia species, subspecies, and strains, using average nucleotide identity, digital DNA-DNA hybridization, and core genome alignment sequence identity.

CONCLUSIONS

The genome of Ehrlichia sp. HF encodes all known virulence factors found in E. chaffeensis, substantiating it as a model Ehrlichia species to study fatal human ehrlichiosis. Comparisons between Ehrlichia sp. HF and E. chaffeensis will enable identification of in vivo virulence factors that are related to host specificity, disease severity, and host inflammatory responses. We propose to name Ehrlichia sp. HF as Ehrlichia japonica sp. nov. (type strain HF), to denote the geographic region where this bacterium was initially isolated.

Function and Regulation of Agrobacterium tumefaciens Cell Surface Structures that Promote Attachment

Agrobacterium tumefaciens attaches stably to plant host tissues and abiotic surfaces. During pathogenesis, physical attachment to the site of infection is a prerequisite to infection and horizontal gene transfer to the plant. Virulent and avirulent strains may also attach to plant tissue in more benign plant associations, and as with other soil microbes, to soil surfaces in the terrestrial environment. Although most A. tumefaciens virulence functions are encoded on the tumor-inducing plasmid, genes that direct general surface attachment are chromosomally encoded, and thus this process is not obligatorily tied to virulence, but is a more fundamental capacity. Several different cellular structures are known or suspected to contribute to the attachment process. The flagella influence surface attachment primarily via their propulsive activity, but control of their rotation during the transition to the attached state may be quite complex. A. tumefaciens produces several pili, including the Tad-type Ctp pili, and several plasmid-borne conjugal pili encoded by the Ti and At plasmids, as well as the so-called T-pilus, involved in interkingdom horizontal gene transfer. The Ctp pili promote reversible interactions with surfaces, whereas the conjugal and T-pili drive horizontal gene transfer (HGT) interactions with other cells and tissues. The T-pilus is likely to contribute to physical association with plant tissues during DNA transfer to plants. A. tumefaciens can synthesize a variety of polysaccharides including cellulose, curdlan (β-1,3 glucan), β-1,2 glucan (cyclic and linear), succinoglycan, and a localized polysaccharide(s) that is confined to a single cellular pole and is called the unipolar polysaccharide (UPP). Lipopolysaccharides are also in the outer leaflet of the outer membrane. Cellulose and curdlan production can influence attachment under certain conditions. The UPP is required for stable attachment under a range of conditions and on abiotic and biotic surfaces. Other factors that have been reported to play a role in attachment include the elusive protein called rhicadhesin. The process of surface attachment is under extensive regulatory control and can be modulated by environmental conditions, as well as by direct responses to surface contact. Complex transcriptional and post-transcriptional control circuitry underlies much of the production and deployment of these attachment functions.

CagW, a VirB6 homologue interacts with Cag-type IV secretion system substrate CagA in Helicobacter pylori

Protein translocating Cag type IV secretion system of Helicobacter pylori is a diverse multi-protein complex. Here, we have characterized one of its key subunit CagW to identify its interacting partners. Our results demonstrate for the first time that this VirB6 homologue interacts with the substrate of the secretion system CagA. CagW forms multimer and its absence affects cellular levels of pilus forming components, CagL, CagI and CagH. Our results support the notion that the protein is essential for the transport of CagA across the bacterial membrane barrier and would aid in improving our understanding of structural and functional aspects of the inner membrane part of Cag-T4SS channel complex for the passage of substrate CagA.

Biological Diversity and Evolution of Type IV Secretion Systems

The bacterial type IV secretion systems (T4SSs) are a highly functionally and structurally diverse superfamily of secretion systems found in many species of Gram-negative and -positive bacteria. Collectively, the T4SSs can translocate DNA and monomeric and multimeric protein substrates to a variety of bacterial and eukaryotic cell types. Detailed phylogenomics analyses have established that the T4SSs evolved from ancient conjugation machines whose original functions were to disseminate mobile DNA elements within and between bacterial species. How members of the T4SS superfamily evolved to recognize and translocate specific substrate repertoires to prokaryotic or eukaryotic target cells is a fascinating question from evolutionary, biological, and structural perspectives. In this chapter, we will summarize recent findings that have shaped our current view of the biological diversity of the T4SSs. We focus mainly on two subtypes, designated as the types IVA (T4ASS) and IVB (T4BSS) systems that respectively are represented by the paradigmatic Agrobacterium tumefaciens VirB/VirD4 and Legionella pneumophila Dot/Icm T4SSs. We present current information about the composition and architectures of these representative systems. We also describe how these and a few related T4ASS and T4BSS members evolved as specialized nanomachines through acquisition of novel domains or subunits, a process that ultimately generated extensive genetic and structural mosaicism among this secretion superfamily. Finally, we present new phylogenomics information establishing that the T4BSSs are much more broadly distributed than initially envisioned.

In Situ Imaging and Structure Determination of Bacterial Toxin Delivery Systems Using Electron Cryotomography

Determining the three-dimensional structure of biomacromolecules at high resolution in their native cellular environment is a major challenge for structural biology. Toward this end, electron cryotomography (ECT) allows large bio-macromolecular assemblies to be imaged directly in their hydrated physiological milieu to ~4 nm resolution. Combining ECT with other techniques like fluorescent imaging, immunogold labeling, and genetic manipulation has allowed the in situ investigation of complex biological processes at macromolecular resolution. Furthermore, the advent of cryogenic focused ion beam (FIB) milling has extended the domain of ECT to include regions even deep within thick eukaryotic cells. Anticipating two audiences (scientists who just want to understand the potential and general workflow involved and scientists who are learning how to do the work themselves), here we present both a broad overview of this kind of work and a step-by-step example protocol for ECT and subtomogram averaging using the Legionella pneumophila Dot/Icm type IV secretion system (T4SS) as a case study. While the general workflow is presented in step-by-step detail, we refer to online tutorials, user's manuals, and other training materials for the essential background understanding needed to perform each step.

Mechanisms of bacterial attachment to roots

The attachment of bacteria to roots constitutes the first physical step in many plant-microbe interactions. These interactions exert both positive and negative influences on agricultural systems depending on whether a growth-promoting, symbiotic or pathogenic relationship transpires. A common biphasic mechanism of root attachment exists across agriculturally important microbial species, including Rhizobium, Agrobacterium, Pseudomonas, Azospirillum and Salmonella. Attachment studies have revealed how plant-microbe interactions develop, and how to manipulate these relationships for agricultural benefit. Here, we review our current understanding of the molecular mechanisms governing plant-microbe root attachment and draw together a common biphasic model.

Mutation in ESBL Plasmid from Escherichia coli O104:H4 Leads Autoagglutination and Enhanced Plasmid Dissemination

Conjugative plasmids are one of the main driving force of wide-spreading of multidrug resistance (MDR) bacteria. They are self-transmittable via conjugation as carrying the required set of genes and cis-acting DNA locus for direct cell-to-cell transfer. IncI incompatibility plasmids are nowadays often associated with extended-spectrum beta-lactamases producing Enterobacteria in clinic and environment. pESBL-EA11 was isolated from Escherichia coli O104:H4 outbreak strain in Germany in 2011. During the previous study identifying transfer genes of pESBL-EA11, it was shown that transposon insertion at certain DNA region of the plasmid, referred to as Hft, resulted in great enhancement of transfer ability. This suggested that genetic modifications can enhance dissemination of MDR plasmids. Such ‘superspreader’ mutations have attracted little attention so far despite their high potential to worsen MDR spreading. Present study aimed to gain our understanding on regulatory elements that involved pESBL transfer. While previous studies of IncI plasmids indicated that immediate downstream gene of Hft, traA, is not essential for conjugative transfer, here we showed that overexpression of TraA in host cell elevated transfer rate of pESBL-EA11. Transposon insertion or certain nucleotide substitutions in Hft led strong TraA overexpression which resulted in activation of essential regulator TraB and likely overexpression of conjugative pili. Atmospheric Scanning Electron Microscopy observation suggested that IncI pili are distinct from other types of conjugative pili (such as long filamentous F-type pili) and rather expressed throughout the cell surface. High transfer efficiency in the mutant pESBL-EA11 was involved with hyperpiliation which facilitates cell-to-cell adhesion, including autoagglutination. The capability of plasmids to evolve to highly transmissible mutant is alarming, particularly it might also have adverse effect on host pathogenicity.

The Agrobacterium VirB/VirD4 T4SS: Mechanism and Architecture Defined Through In Vivo Mutagenesis and Chimeric Systems

The Agrobacterium tumefaciens VirB/VirD4 translocation machine is a member of a superfamily of translocators designated as type IV secretion systems (T4SSs) that function in many species of gram-negative and gram-positive bacteria. T4SSs evolved from ancestral conjugation systems for specialized purposes relating to bacterial colonization or infection. A. tumefaciens employs the VirB/VirD4 T4SS to deliver oncogenic DNA (T-DNA) and effector proteins to plant cells, causing the tumorous disease called crown gall. This T4SS elaborates both a cell-envelope-spanning channel and an extracellular pilus for establishing target cell contacts. Recent mechanistic and structural studies of the VirB/VirD4 T4SS and related conjugation systems in Escherichia coli have defined T4SS architectures, bases for substrate recruitment, the translocation route for DNA substrates, and steps in the pilus biogenesis pathway. In this review, we provide a brief history of A. tumefaciens VirB/VirD4 T4SS from its discovery in the 1980s to its current status as a paradigm for the T4SS superfamily. We discuss key advancements in defining VirB/VirD4 T4SS function and structure, and we highlight the power of in vivo mutational analyses and chimeric systems for identifying mechanistic themes and specialized adaptations of this fascinating nanomachine.

Agrobacterium delivers VirE2 protein into host cells via clathrin-mediated endocytosis

Agrobacterium tumefaciens can cause crown gall tumors on a wide range of host plants. As a natural genetic engineer, the bacterium can transfer both single-stranded DNA (ssDNA) [transferred DNA (T-DNA)] molecules and bacterial virulence proteins into various recipient cells. Among Agrobacterium-delivered proteins, VirE2 is an ssDNA binding protein that is involved in various steps of the transformation process. However, it is not clear how plant cells receive the T-DNA or protein molecules. Using a split-green fluorescent protein approach, we monitored the VirE2 delivery process inside plant cells in real time. We observed that A. tumefaciens delivered VirE2 from the bacterial lateral sides that were in close contact with plant membranes. VirE2 initially accumulated on plant cytoplasmic membranes at the entry points. VirE2-containing membranes were internalized through clathrin-mediated endocytosis to form endomembrane compartments. VirE2 colocalized with the early endosome marker SYP61 but not with the late endosome marker ARA6, suggesting that VirE2 escaped from early endosomes for subsequent trafficking inside the cells. Dual endocytic motifs at the carboxyl-terminal tail of VirE2 were involved in VirE2 internalization and could interact with the μ subunit of the plant clathrin-associated adaptor AP2 complex (AP2M). Both the VirE2 cargo motifs and AP2M were important for the transformation process. Because AP2-mediated endocytosis is well conserved, our data suggest that the A. tumefaciens pathogen hijacks conserved endocytic pathways to facilitate the delivery of virulence factors. This might be important for Agrobacterium to achieve both a wide host range and a high transformation efficiency.

Type IV Secretion in Agrobacterium tumefaciens and Development of Specific Inhibitors

The Agrobacterium tumefaciens VirB/D4 type IV secretion system (T4SS) comprises 12 membrane-bound proteins, and it assembles a surface-exposed T-pilus. It is considered to be the archetypical system that is generally used to orient the nomenclature of other T4SS. Whereas the sequence similarities between T4SSs from different organisms are often limited, the general mechanism of action appears to be conserved, and the evolutionary relationship to bacterial conjugation systems and to T4SSs from animal pathogens is well established. Agrobacterium is a natural genetic engineer that is extensively used for the generation of transgenic plants for research and for agro-biotechnological applications. It also served as an early model for the understanding of pathogen-host interactions and for the transfer of macromolecular virulence factors into host cells. The knowledge on the mechanism of its T4SS inspired the search for small molecules that inhibit the virulence of bacterial pathogens and of bacterial conjugation. Inhibitors of bacterial virulence and of conjugation have interesting potential as alternatives to antibiotics and as inhibitors of antimicrobial resistance gene transfer. Mechanistic work on the Agrobacterium T4SS will continue to inspire the search for inhibitor target sites and drug design.

Mechanisms of Conjugative Transfer and Type IV Secretion-Mediated Effector Transport in Gram-Positive Bacteria

Conjugative DNA transfer is the most important means to transfer antibiotic resistance genes and virulence determinants encoded by plasmids, integrative conjugative elements (ICE), and pathogenicity islands among bacteria. In gram-positive bacteria, there exist two types of conjugative systems, (i) type IV secretion system (T4SS)-dependent ones, like those encoded by the Enterococcus, Streptococcus, Staphylococcus, Bacillus, and Clostridia mobile genetic elements and (ii) T4SS-independent ones, as those found on Streptomyces plasmids. Interestingly, very recently, on the Streptococcus suis genome, the first gram-positive T4SS not only involved in conjugative DNA transfer but also in effector translocation to the host was detected. Although no T4SS core complex structure from gram-positive bacteria is available, several structures from T4SS protein key factors from Enterococcus and Clostridia plasmids have been solved. In this chapter, we summarize the current knowledge on the molecular mechanisms and structure-function relationships of the diverse conjugation machineries and emerging research needs focused on combatting infections and spread of multiple resistant gram-positive pathogens.

The Rickettsia type IV secretion system: unrealized complexity mired by gene family expansion

Many prokaryotes utilize type IV secretion systems (T4SSs) to translocate substrates (e.g. nucleoprotein, DNA, protein) across the cell envelope, and/or to elaborate surface structures (i.e. pili or adhesins). Among eight distinct T4SS classes, P-T4SSs are typified by the Agrobacterium tumefaciens vir T4SS, which is comprised of 12 scaffold components (VirB1-VirB11, VirD4). While most P-T4SSs include all 12 Vir proteins, some differ from the vir archetype by either containing additional scaffold components not analogous to Vir proteins or lacking one or more of the Vir proteins. In a special case, the Rickettsiales vir homolog (rvh) P-T4SS comprises unprecedented gene family expansion. rvh contains three families of gene duplications (rvhB9, rvhB8, rvhB4): RvhB9,8,4-I are conserved relative to equivalents in other P-T4SSs, while RvhB9,8,4-II have evolved atypical features that deviate substantially from other homologs. Furthermore, rvh contains five VirB6-like genes (rvhB6a-e), which are tandemly arrayed and contain large N- and C-terminal extensions. Our work herein focuses on the complexity underpinned by rvh gene family expansion. Furthermore, we describe an RvhB10 insertion, which occurs in a region that forms the T4SS pore. The significance of these curious properties to rvh structure and function is evaluated, shedding light on a highly complex T4SS.

Structural Biology of Bacterial Type IV Secretion Systems

Type IV secretion systems (T4SSs) are large multisubunit translocons, found in both gram-negative and gram-positive bacteria and in some archaea. These systems transport a diverse array of substrates from DNA and protein-DNA complexes to proteins, and play fundamental roles in both bacterial pathogenesis and bacterial adaptation to the cellular milieu in which bacteria live. This review describes the various biochemical and structural advances made toward understanding the biogenesis, architecture, and function of T4SSs.

Development of an efficient in vitro plant regeneration system amenable to Agrobacterium- mediated transformation of a recalcitrant grain legume blackgram (Vigna mungo L. Hepper)

An efficient, rapid and direct multiple shoot regeneration system amenable to Agrobacterium-mediated transformation from primary leaf with intact petiole of blackgram (Vigna mungo) is established for the first time. The effect of the explant type and its age, type and concentration of cytokinin and auxin either alone or in combination and genotype on multiple shoot regeneration efficiency and frequency was optimized. The primary leaf explants with petiole excised from 4-day-old seedlings directly developed multiple shoots (an average of 10 shoots/ explant) from the cut ends of the petiole in 95 % of the cultures on MSB (MS salts and B5 vitamins) medium containing 1.0 μM 6-benzylaminopurine. Elongated (2-3 cm) shoots were rooted on MSB medium with 2.5 μM indole-butyric acid and resulted plantlets were hardened and established in soil, where they resumed growth and reached maturity with normal seed set. The regenerated plants were morphologically similar to seed-raised plants and required 8 weeks time from initiation of culture to establish them in soil. The regeneration competent cells present at the cut ends of petiole are fully exposed and are, thus, easily accessible to Agrobacterium, making this plant regeneration protocol amenable for the production of transgenic plants. The protocol was further successfully used to develop fertile transgenic plants of blackgram using Agrobacterium tumefaciens strain EHA 105 carrying a binary vector pCAMBIA2301 that contains a neomycin phosphotransferase gene (nptII) and a β-glucuronidase (GUS) gene (uidA) interrupted with an intron. The presence and integration of transgenes in putative T0 plants were confirmed by polymerase chain reaction (PCR) and Southern blot hybridization, respectively. The transgenes were inherited in Mendelian fashion in T1 progeny and a transformation frequency of 1.3 % was obtained. This protocol can be effectively used for transferring new traits in blackgram and other legumes for their quantitative and qualitative improvements.

Maize (Zea mays L.)

Agrobacterium tumefaciens-mediated transformation is an effective method for introducing genes into maize. In this chapter, we describe a detailed protocol for genetic transformation of the maize genotype Hi II. Our starting plant material is immature embryos cocultivated with an Agrobacterium strain carrying a standard binary vector. In addition to step-by-step laboratory transformation procedures, we include extensive details in growing donor plants and caring for transgenic plants in the greenhouse.

Identification of a novel Afipia species isolated from an Indian flying fox

An old world fruit bat Pteropus giganteus, held in captivity and suffering from necrosis of its wing digits, failed to respond to antibiotic therapy and succumbed to the infection. Samples submitted to the National Centre for Foreign Animal Disease were tested for viral infection. Vero E6 cells exhibited minor but unique cytopathic effects on second blind passage, and full CPE by passage four. Utilizing an unbiased random amplification technique from cell culture supernatant, we identified a bacterium belonging to the Bradyrhizobiaceae. Purification of cell culture supernatant on TY media revealed a slow growing bacterial isolate. In this study using electron microscopy, 16S rRNA gene analysis and whole genome sequencing, we identify a novel bacterial species associated with the site of infection belonging to the genus Afipia. This genus of bacteria is very diverse, with only a limited number of species characterized. Afipia felis, previously described as the etiological agent to cause cat scratch disease, and Afipia septicemium, most recently shown to cause disease in humans, highlight the potential for members of this genus to form a branch of opportunistic pathogens within the Bradyrhizobiaceae. Increased utilization of next generation sequencing and genomics will aid in classifying additional members of this intriguing bacterial genera.

Structural biology of the Gram-negative bacterial conjugation systems

Conjugation, the process by which plasmid DNA is transferred from one bacterium to another, is mediated by type IV secretion systems (T4SSs). T4SSs are versatile systems that can transport not only DNA, but also toxins and effector proteins. Conjugative T4SSs comprise 12 proteins named VirB1-11 and VirD4 that assemble into a large membrane-spanning exporting machine. Before being transported, the DNA substrate is first processed on the cytoplasmic side by a complex called the relaxosome. The substrate is then targeted to the T4SS for export into a recipient cell. In this review, we describe the recent progress made in the structural biology of both the relaxosome and the T4SS.

Overexpression of the HspL Promotes Agrobacterium tumefaciens Virulence in Arabidopsis Under Heat Shock Conditions

Agrobacterium tumefaciens transfers a specific DNA fragment from the resident tumor-inducing (Ti) plasmid and effector virulence (Vir) proteins to plant cells during infection. A. tumefaciens VirB1-11 and VirD4 proteins assemble as the type IV secretion system (T4SS), which mediates transfer of the T-DNA and effector Vir protein into plant cells, thus resulting in crown gall disease in plants. Previous studies revealed that an α-crystallin-type, small heat-shock protein (HspL) is a more effective VirB8 chaperone than three other small heat-shock proteins (HspC, HspAT1, and HspAT2). Additionally, HspL contributes to efficient T4SS-mediated DNA transfer and tumorigenesis under room-temperature growth. In this study, we aimed to characterize the impact of HspL on Agrobacterium-mediated transformation efficiency under heat-shock treatment. During heat shock, transient transformation efficiency and VirB8 protein accumulation were lower in the hspL deletion mutant than in the wild type. Overexpression of HspL in A. tumefaciens enhanced the transient transformation efficiency in root explants of both susceptible and recalcitrant Arabidopsis ecotypes. In addition, the reduced transient transformation efficiency during heat stress was recovered by overexpression of HspL in A. tumefaciens. HspL may help maintain VirB8 homeostasis and elevate Agrobacterium-mediated transformation efficiency under both heat-shock and nonheat-shock growth.

Towards an integrated model of bacterial conjugation

Bacterial conjugation is one of the main mechanisms for horizontal gene transfer. It constitutes a key element in the dissemination of antibiotic resistance and virulence genes to human pathogenic bacteria. DNA transfer is mediated by a membrane-associated macromolecular machinery called Type IV secretion system (T4SS). T4SSs are involved not only in bacterial conjugation but also in the transport of virulence factors by pathogenic bacteria. Thus, the search for specific inhibitors of different T4SS components opens a novel approach to restrict plasmid dissemination. This review highlights recent biochemical and structural findings that shed new light on the molecular mechanisms of DNA and protein transport by T4SS. Based on these data, a model for pilus biogenesis and substrate transfer in conjugative systems is proposed. This model provides a renewed view of the mechanism that might help to envisage new strategies to curb the threating expansion of antibiotic resistance.

Historical account on gaining insights on the mechanism of crown gall tumorigenesis induced by Agrobacterium tumefaciens

The plant tumor disease known as crown gall was not called by that name until more recent times. Galls on plants were described by Malpighi (1679) who believed that these extraordinary growth are spontaneously produced. Agrobacterium was first isolated from tumors in 1897 by Fridiano Cavara in Napoli, Italy. After this bacterium was recognized to be the cause of crown gall disease, questions were raised on the mechanism by which it caused tumors on a variety of plants. Numerous very detailed studies led to the identification of Agrobacterium tumefaciens as the causal bacterium that cleverly transferred a genetic principle to plant host cells and integrated it into their chromosomes. Such studies have led to a variety of sophisticated mechanisms used by this organism to aid in its survival against competing microorganisms. Knowledge gained from these fundamental discoveries has opened many avenues for researchers to examine their primary organisms of study for similar mechanisms of pathogenesis in both plants and animals. These discoveries also advanced the genetic engineering of domesticated plants for improved food and fiber.

The Ctp type IVb pilus locus of Agrobacterium tumefaciens directs formation of the common pili and contributes to reversible surface attachment

Agrobacterium tumefaciens can adhere to plant tissues and abiotic surfaces and forms biofilms. Cell surface appendages called pili play an important role in adhesion and biofilm formation in diverse bacterial systems. The A. tumefaciens C58 genome sequence revealed the presence of the ctpABCDEFGHI genes (cluster of type IV pili; Atu0216 to Atu0224), homologous to tad-type pilus systems from several bacteria, including Aggregatibacter actinomycetemcomitans and Caulobacter crescentus. These systems fall into the type IVb pilus group, which can function in bacterial adhesion. Transmission electron microscopy of A. tumefaciens revealed the presence of filaments, significantly thinner than flagella and often bundled, associated with cell surfaces and shed into the external milieu. In-frame deletion mutations of all of the ctp genes, with the exception of ctpF, resulted in nonpiliated derivatives. Mutations in ctpA (a pilin homologue), ctpB, and ctpG decreased early attachment and biofilm formation. The adherence of the ctpA mutant could be restored by ectopic expression of the paralogous pilA gene. The ΔctpA ΔpilA double pilin mutant displayed a diminished biovolume and lower biofilm height than the wild type under flowing conditions. Surprisingly, however, the ctpCD, ctpE, ctpF, ctpH, and ctpI mutants formed normal biofilms and showed enhanced reversible attachment. In-frame deletion of the ctpA pilin gene in the ctpCD, ctpE, ctpF, ctpH, and ctpI mutants caused the same attachment-deficient phenotype as the ctpA single mutant. Collectively, these findings indicate that the ctp locus is involved in pilus assembly and that nonpiliated mutants, which retain the CtpA pilin, are proficient in attachment and adherence.

Non-additive costs and interactions alter the competitive dynamics of co-occurring ecologically distinct plasmids

Plasmids play an important role in shaping bacterial evolution and adaptation to heterogeneous environments. As modular genetic elements that are often conjugative, the selective pressures that act on plasmid-borne genes are distinct from those that act on the chromosome. Many bacteria are co-infected by multiple plasmids that impart niche-specific phenotypes. Thus, in addition to host-plasmid dynamics, interactions between co-infecting plasmids are likely to be important drivers of plasmid population dynamics, evolution and ecology. Agrobacterium tumefaciens is a facultative plant pathogen that commonly harbours two distinct megaplasmids. Virulence depends on the presence of the tumour-inducing (Ti) plasmid, with benefits that are primarily restricted to the disease environment. Here, we demonstrate that a second megaplasmid, the At plasmid, confers a competitive advantage in the rhizosphere. To assess the individual and interactive costs of these plasmids, we generated four isogenic derivatives: plasmidless, pAt only, pTi only and pAtpTi, and performed pairwise competitions under carbon-limiting conditions. These studies reveal a low cost to the virulence plasmid when outside of the disease environment, and a strikingly high cost to the At plasmid. In addition, the costs of pAt and pTi in the same host were significantly lower than predicted based on single plasmid costs, signifying the first demonstration of non-additivity between naturally occurring co-resident plasmids. Based on these empirically demonstrated costs and benefits, we developed a resource-consumer model to generate predictions about the frequencies of these genotypes in relevant environments, showing that non-additivity between co-residing plasmids allows for their stable coexistence across environments.

Agrobacterium: nature's genetic engineer

Agrobacterium was identified as the agent causing the plant tumor, crown gall over 100 years ago. Since then, studies have resulted in many surprising observations. Armin Braun demonstrated that Agrobacterium infected cells had unusual nutritional properties, and that the bacterium was necessary to start the infection but not for continued tumor development. He developed the concept of a tumor inducing principle (TIP), the factor that actually caused the disease. Thirty years later the TIP was shown to be a piece of a tumor inducing (Ti) plasmid excised by an endonuclease. In the next 20 years, most of the key features of the disease were described. The single-strand DNA (T-DNA) with the endonuclease attached is transferred through a type IV secretion system into the host cell where it is likely coated and protected from nucleases by a bacterial secreted protein to form the T-complex. A nuclear localization signal in the endonuclease guides the transferred strand (T-strand), into the nucleus where it is integrated randomly into the host chromosome. Other secreted proteins likely aid in uncoating the T-complex. The T-DNA encodes enzymes of auxin, cytokinin, and opine synthesis, the latter a food source for Agrobacterium. The genes associated with T-strand formation and transfer (vir) map to the Ti plasmid and are only expressed when the bacteria are in close association with a plant. Plant signals are recognized by a two-component regulatory system which activates vir genes. Chromosomal genes with pleiotropic functions also play important roles in plant transformation. The data now explain Braun's old observations and also explain why Agrobacterium is nature's genetic engineer. Any DNA inserted between the border sequences which define the T-DNA will be transferred and integrated into host cells. Thus, Agrobacterium has become the major vector in plant genetic engineering.

DNA substrate-induced activation of the Agrobacterium VirB/VirD4 type IV secretion system

The bitopic membrane protein VirB10 of the Agrobacterium VirB/VirD4 type IV secretion system (T4SS) undergoes a structural transition in response to sensing of ATP binding or hydrolysis by the channel ATPases VirD4 and VirB11. This transition, detectable as a change in protease susceptibility, is required for DNA substrate passage through the translocation channel. Here, we present evidence that DNA substrate engagement with VirD4 and VirB11 also is required for activation of VirB10. Several DNA substrates (oncogenic T-DNA and plasmids RSF1010 and pCloDF13) induced the VirB10 conformational change, each by mechanisms requiring relaxase processing at cognate oriT sequences. VirD2 relaxase deleted of its translocation signal or any of the characterized relaxases produced in the absence of cognate DNA substrates did not induce the structural transition. Translocated effector proteins, e.g., VirE2, VirE3, and VirF, also did not induce the transition. By mutational analyses, we supplied evidence that the N-terminal periplasmic loop of VirD4, in addition to its catalytic site, is essential for early-stage DNA substrate transfer and the VirB10 conformational change. Further studies of VirB11 mutants established that three T4SS-mediated processes, DNA transfer, protein transfer, and pilus production, can be uncoupled and that the latter two processes proceed independently of the VirB10 conformational change. Our findings support a general model whereby DNA ligand binding with VirD4 and VirB11 stimulates ATP binding/hydrolysis, which in turn activates VirB10 through a structural transition. This transition confers an open-channel configuration enabling passage of the DNA substrate to the cell surface.

The expanding bacterial type IV secretion lexicon

The bacterial type IV secretion systems (T4SSs) comprise a biologically diverse group of translocation systems functioning to deliver DNA or protein substrates from donor to target cells generally by a mechanism dependent on establishment of direct cell-to-cell contact. Members of one T4SS subfamily, the conjugation systems, mediate the widespread and rapid dissemination of antibiotic resistance and virulence traits among bacterial pathogens. Members of a second subfamily, the effector translocators, are used by often medically-important pathogens to deliver effector proteins to eukaryotic target cells during the course of infection. Here we summarize our current understanding of the structural and functional diversity of T4SSs and of the evolutionary processes shaping this diversity. We compare mechanistic and architectural features of T4SSs from Gram-negative and -positive species. Finally, we introduce the concept of the 'minimized' T4SSs; these are systems composed of a conserved set of 5-6 subunits that are distributed among many Gram-positive and some Gram-negative species.

Surface organelles assembled by secretion systems of Gram-negative bacteria: diversity in structure and function

Gram-negative bacteria express a wide variety of organelles on their cell surface. These surface structures may be the end products of secretion systems, such as the hair-like fibers assembled by the chaperone/usher (CU) and type IV pilus pathways, which generally function in adhesion to surfaces and bacterial-bacterial and bacterial-host interactions. Alternatively, the surface organelles may be integral components of the secretion machinery itself, such as the needle complex and pilus extensions formed by the type III and type IV secretion systems, which function in the delivery of bacterial effectors inside host cells. Bacterial surface structures perform functions critical for pathogenesis and have evolved to withstand forces exerted by the external environment and cope with defenses mounted by the host immune system. Given their essential roles in pathogenesis and exposed nature, bacterial surface structures also make attractive targets for therapeutic intervention. This review will describe the structure and function of surface organelles assembled by four different Gram-negative bacterial secretion systems: the CU pathway, the type IV pilus pathway, and the type III and type IV secretion systems.

Evolution of conjugation and type IV secretion systems

Genetic exchange by conjugation is responsible for the spread of resistance, virulence, and social traits among prokaryotes. Recent works unraveled the functioning of the underlying type IV secretion systems (T4SS) and its distribution and recruitment for other biological processes (exaptation), notably pathogenesis. We analyzed the phylogeny of key conjugation proteins to infer the evolutionary history of conjugation and T4SS. We show that single-stranded DNA (ssDNA) and double-stranded DNA (dsDNA) conjugation, while both based on a key AAA⁺ ATPase, diverged before the last common ancestor of bacteria. The two key ATPases of ssDNA conjugation are monophyletic, having diverged at an early stage from dsDNA translocases. Our data suggest that ssDNA conjugation arose first in diderm bacteria, possibly Proteobacteria, and then spread to other bacterial phyla, including bacterial monoderms and Archaea. Identifiable T4SS fall within the eight monophyletic groups, determined by both taxonomy and structure of the cell envelope. Transfer to monoderms might have occurred only once, but followed diverse adaptive paths. Remarkably, some Firmicutes developed a new conjugation system based on an atypical relaxase and an ATPase derived from a dsDNA translocase. The observed evolutionary rates and patterns of presence/absence of specific T4SS proteins show that conjugation systems are often and independently exapted for other functions. This work brings a natural basis for the classification of all kinds of conjugative systems, thus tackling a problem that is growing as fast as genomic databases. Our analysis provides the first global picture of the evolution of conjugation and shows how a self-transferrable complex multiprotein system has adapted to different taxa and often been recruited by the host. As conjugation systems became specific to certain clades and cell envelopes, they may have biased the rate and direction of gene transfer by conjugation within prokaryotes.

Structure of the VirB4 ATPase, alone and bound to the core complex of a type IV secretion system

Type IV secretion (T4S) systems mediate the transfer of proteins and DNA across the cell envelope of bacteria. These systems play important roles in bacterial pathogenesis and in horizontal transfer of antibiotic resistance. The VirB4 ATPase of the T4S system is essential for both the assembly of the system and substrate transfer. In this article, we present the crystal structure of the C-terminal domain of Thermoanaerobacter pseudethanolicus VirB4. This structure is strikingly similar to that of another T4S ATPase, VirD4, a protein that shares only 12% sequence identity with VirB4. The VirB4 domain purifies as a monomer, but the full-length protein is observed in a monomer-dimer equilibrium, even in the presence of nucleotides and DNAs. We also report the negative stain electron microscopy structure of the core complex of the T4S system of the Escherichia coli pKM101 plasmid, with VirB4 bound. In this structure, VirB4 is also monomeric and bound through its N-terminal domain to the core's VirB9 protein. Remarkably, VirB4 is observed bound to the side of the complex where it is ideally placed to play its known regulatory role in substrate transfer.

Alterations in Helicobacter pylori triggered by contact with gastric epithelial cells

Helicobacter pylori lives within the mucus layer of the human stomach, in close proximity to gastric epithelial cells. While a great deal is known about the effects of H. pylori on human cells and the specific bacterial products that mediate these effects, relatively little work has been done to investigate alterations in H. pylori that may be triggered by bacterial contact with human cells. In this review, we discuss the spectrum of changes in bacterial physiology and morphology that occur when H. pylori is in contact with gastric epithelial cells. Several studies have reported that cell contact causes alterations in H. pylori gene transcription. In addition, H. pylori contact with gastric epithelial cells promotes the formation of pilus-like structures at the bacteria–host cell interface. The formation of these structures requires multiple genes in the cag pathogenicity island, and these structures are proposed to have an important role in the type IV secretion system-dependent process through which CagA enters host cells. Finally, H. pylori contact with epithelial cells can promote bacterial replication and the formation of microcolonies, phenomena that are facilitated by the acquisition of iron and other nutrients from infected cells. In summary, the gastric epithelial cell surface represents an important niche for H. pylori, and upon entry into this niche, the bacteria alter their behavior in a manner that optimizes bacterial proliferation and persistent colonization of the host.

Processing and maturation of the pilin of the type IV secretion system encoded within the gonococcal genetic island

The type IV secretion system (T4SS) encoded within the gonococcal genetic island (GGI) of Neisseria gonorrhoeae has homology to the T4SS encoded on the F plasmid. The GGI encodes the putative pilin protein TraA and a serine protease TrbI, which is homologous to the TraF protein of the RP4 plasmid involved in circularization of pilin subunits of P-type pili. TraA was processed to a 68-amino acid long circular peptide by leader peptidase and TrbI. Processing occurred after co-translational membrane insertion and was independent of other proteins. Circularization occurred after removal of three C-terminal amino acids. Mutational analysis of TraA revealed limited flexibility at the cleavage and joining sites. Mutagenesis of TrbI showed that the conserved Lys-93 and Asp-155 are essential, whereas mutagenesis of Ser-52, the putative catalytic serine did not influence circularization. Further mutagenesis of other serine residues did not identify a catalytic serine, indicating that TrbI either contains redundant catalytic serine residues or does not function via a serine-lysine dyad mechanism. In vitro studies revealed that circularization occurs via a covalent intermediate between the C terminus of TraA and TrbI. The intermediate is processed to the circular form after cleavage of the N-terminal signal sequence. This is the first demonstration of a covalent intermediate in the circularization mechanism of conjugative pili.

Evidence for VirB4-mediated dislocation of membrane-integrated VirB2 pilin during biogenesis of the Agrobacterium VirB/VirD4 type IV secretion system

Agrobacterium VirB2 pilin is required for assembly of the VirB/VirD4 type IV secretion system (T4SS). The propilin is processed by signal sequence cleavage and covalent linkage of the N and C termini, and the cyclized pilin integrates into the inner membrane (IM) as a pool for assembly of the secretion channel and T pilus. Here, by use of the substituted cysteine accessibility method (SCAM), we defined the VirB2 IM topology and then identified distinct contributions of the T4SS ATPase subunits to the pilin structural organization. Labeling patterns of Cys-substituted pilins exposed to the membrane-impermeative, thiol-reactive reagent 3-(N-maleimidopropionyl)biocytin (MPB) supported a topology model in which two hydrophobic stretches comprise transmembrane domains, an intervening hydrophilic loop (residues 90 to 94) is cytoplasmic, and the hydrophilic N and C termini joined at residues 48 and 121 form a periplasmic loop. Interestingly, the VirB4 ATPase, but not a Walker A nucleoside triphosphate (NTP) binding motif mutant, induced (i) MPB labeling of Cys94, a residue that in the absence of the ATPase is located in the cytoplasmic loop, and (ii) release of pilin from the IM upon osmotic shock. These findings, coupled with evidence for VirB2-VirB4 complex formation by coimmunoprecipitation, support a model in which VirB4 functions as a dislocation motor to extract pilins from the IM during T4SS biogenesis. The VirB11 ATPase functioned together with VirB4 to induce a structural change in the pilin that was detectable by MPB labeling, suggestive of a role for VirB11 as a modulator of VirB4 dislocase activity.

Phylogenomics reveals a diverse Rickettsiales type IV secretion system

With an obligate intracellular lifestyle, Alphaproteobacteria of the order Rickettsiales have inextricably coevolved with their various eukaryotic hosts, resulting in small, reductive genomes and strict dependency on host resources. Unsurprisingly, large portions of Rickettsiales genomes encode proteins involved in transport and secretion. One particular transporter that has garnered recent attention from researchers is the type IV secretion system (T4SS). Homologous to the well-studied archetypal vir T4SS of Agrobacterium tumefaciens, the Rickettsiales vir homolog (rvh) T4SS is characterized primarily by duplication of several of its genes and scattered genomic distribution of all components in several conserved islets. Phylogeny estimation suggests a single event of ancestral acquirement of the rvh T4SS, likely from a nonalphaproteobacterial origin. Bioinformatics analysis of over 30 Rickettsiales genome sequences illustrates a conserved core rvh scaffold (lacking only a virB5 homolog), with lineage-specific diversification of several components (rvhB1, rvhB2, and rvhB9b), likely a result of modifications to cell envelope structure. This coevolution of the rvh T4SS and cell envelope morphology is probably driven by adaptations to various host cells, identifying the transporter as an important target for vaccine development. Despite the genetic intractability of Rickettsiales, recent advancements have been made in the characterization of several components of the rvh T4SS, as well as its putative regulators and substrates. While current data favor a role in effector translocation, functions in DNA uptake and release and/or conjugation cannot at present be ruled out, especially considering that a mechanism for plasmid transfer in Rickettsia spp. has yet to be proposed.

New insights into F-pilus structure, dynamics, and function

F-pili are thin, flexible filaments elaborated by F(+) cells of Escherichia coli. They belong to the class of Gram-negative pili that function in horizontal gene transfer. F-pili are initially required to establish contacts between DNA donor and recipient cells. Beyond that, F-pilus function, and that of other conjugative pili, has remained obscure and controversial. The idea that F-pili are dynamic structures was proposed 40 years ago. Initially, F-pili were thought to remain extended until another cell bound to the filament tip, whereupon the filament retracted to bring the contacted cell to the donor cell surface. Thereafter, secure surface-surface contacts would allow efficient DNA transfer. A later variant of this hypothesis was that F-pili are inherently dynamic, elongating and retracting even in the absence of exogenous signals. A very different hypothesis, also proposed first about 40 years ago, was that F-pili are conduits, presumably passive, for the transfer of DNA from donor to recipient. In this hypothesis, DNA transfer is not obligatorily coupled to F-pilus retraction. Here, we review recent data obtained by integrating long-established facts about the biology of F-pili with modern tools of fluorescence and electron microscopy. These data suggest that one function for F-pili is to search a large volume around donor cells in liquid culture for the presence of other cells. However, this may not be the only function. We show that F-pilin is also required at a second, largely undefined step occurring after cells have been brought into direct contact by F-pilus retraction.

[Establishment of high efficiency genetic transformation system of maize mediated by Agrobacterium tumefaciens]

In order to establish high-frequency regeneration and high-efficiency genetic transformation system in maize, the significance of the 11 factors influencing maize embryonic callus induction and 9 factors affecting embryonic callus differentiation was researched by orthogonal experiment. The results showed that genotype had highly significant impact on induction of embryonic callus. The concentration of 6-BA, AgNO3, 2,4-D, ABA, and medium are the significant factors. The Multi-comparison showed that ABA 2 mg/L has a significant influence. Among the callus differentiation factors, the genotype and 6-BA concentration showed a strong main effect, the concentrations of NAA, medium, KT and 2,4-D had significant impacts on callus differentiation. Southern blotting analysis demonstrated that the resistant callus rate under the selection pressure of 25 mg/L hygromycin was a reliable indicator for system optimization in resistance screening. The concentration of acetosyringone (AS) showed sensitive differences among genotypes. The highest transformation rate was found with the optimized combination of 24-25 degrees C for co-culture temperature, 0.7 ODx15 min for Agrobacterium tumefa-ciens concentration and incubation-time, and pH 5.5-6.2. By this optimized combination, the survival rate of resistant calli as an index for the stable transformation rates of inbred lines Huangzao 4 and Zong 31 by introducing GUS gene into maize inbred lines was as high as 48.6% and 46.2%, respectively.

Mechanisms and regulation of polar surface attachment in Agrobacterium tumefaciens

Agrobacterium tumefaciens is a plant pathogen that transfers a segment of its own DNA into host plants to cause Crown Gall disease. The infection process requires intimate contact between the infecting bacteria and the host tissue. A. tumefaciens attaches efficiently to plant tissues and to abiotic surfaces, and can establish complex biofilms at colonization sites. The dominant mode of attachment is via a single pole in contact with the surface. Several different appendages, adhesins and adhesives play roles during attachment, and foster the transition from free-swimming to sessile growth. This polar surface interaction reflects a more fundamental cellular asymmetry in A. tumefaciens that influences and is congruent with its attached lifestyle.

The structural biology of type IV secretion systems

Type IV secretion systems (T4SSs) are versatile secretion systems that are found in both Gram-negative and Gram-positive bacteria and secrete a wide range of substrates, from single proteins to protein-protein and protein-DNA complexes. They usually consist of 12 components that are organized into ATP-powered, double-membrane-spanning complexes. The structures of single soluble components or domains have been solved, but an understanding of how these structures come together has only recently begun to emerge. This Review focuses on the structural advances that have been made over the past 10 years and how the corresponding structural insights have helped to elucidate many of the details of the mechanism of type IV secretion.

Architectures and biogenesis of non-flagellar protein appendages in Gram-negative bacteria

Bacteria commonly expose non-flagellar proteinaceous appendages on their outer surfaces. These extracellular structures, called pili or fimbriae, are employed in attachment and invasion, biofilm formation, cell motility or protein and DNA transport across membranes. Over the past 15 years, the power of molecular and structural techniques has revolutionalized our understanding of the biogenesis, structure, function and mode of action of these bacterial organelles. Here, we review the five known classes of Gram-negative non-flagellar appendages from a biosynthetic and structural point of view.

Tissue specific response of Agrobacterium tumefaciens attachment to Sorghum bicolor (L) Moench

Agrobacterium mediated genetic transformation of plants have advantages over other methods, especially for making single copy transgenic plants with reduced chances of gene silencing and instability. However, monocotyledonous plant species could not utilize the full potential of this system because of possible limitations in Agrobacterium interaction with monocot plant cells. Agrobacterium attachment as a factor in genetic transformation was studied in the leaf, shoot apex, and leaf derived callus of sorghum (Sorghum bicolor (L) Moench). Pre-induction of Agrobacterium with acetosyringone was found necessary for Agrobacterium attachment to sorghum tissues. All the explants responded positively, with preferential Agrobacterium attachment and colonization around the tissues having actively dividing cells. Callus proved to be the best explant for Agrobacterium attachment as observed in scanning electron microscopy and transient GUS expression. Loss of Agrobacterium attachment was observed with an increase in the degree of tissue differentiation.

The effect of co-cultivation and selection parameters on Agrobacterium-mediated transformation of Chinese soybean varieties

In the present study, an efficient Agrobacterium-mediated gene transformation system was developed for soybean [Glycine max (L.) Merrill] based on the examinations of several factors affecting plant transformation efficiency. Increased transformation efficiencies were obtained when the soybean cotyledonary node were inoculated with the Agrobacterium inoculum added with 0.02% (v/v) surfactant (Silwet L-77). The applications of Silwet L-77 (0.02%) during infection and L-cysteine (600 mg l(-1)) during co-cultivation resulted in more significantly improved transformation efficiency than each of the two factors alone. The optimized temperature for infected explant co-cultivation was 22 degrees C. Regenerated transgenic shoots were selected and produced more efficiently with the modified selection scheme (initiation on shoot induction medium without hygromycin for 7 days, with 3 mg l(-1) hygromycin for 10 days, 5 mg l(-1) hygromycin for another 10 days, and elongation on shoot elongation medium with 8 mg l(-1) hygromycin). Using the optimized system, we obtained 145 morphologically normal and fertile independent transgenic plants in five important Chinese soybean varieties. The transformation efficacies ranged from 3.8 to 11.7%. Stable integration, expression and inheritance of the transgenes were confirmed by molecular and genetic analysis. T(1) plants were analyzed and transmission of transgenes to the T(1 )generation in a Mendelian fashion was verified. This optimized transformation system should be employed for efficient Agrobacterium-mediated soybean gene transformation.