SUMMARY Type IV secretion systems (T4SS) translocate DNA and protein substrates across prokaryotic cell envelopes generally by a mechanism requiring direct contact with a target cell. Three types of T4SS have been described: (i) conjugation systems, operationally defined as machines that translocate DNA substrates intercellularly by a contact-dependent process; (ii) effector translocator systems, functioning to deliver proteins or other macromolecules to eukaryotic target cells; and (iii) DNA release/uptake systems, which translocate DNA to or from the extracellular milieu. Studies of a few paradigmatic systems, notably the conjugation systems of plasmids F, R388, RP4, and pKM101 and the Agrobacterium tumefaciens VirB/VirD4 system, have supplied important insights into the structure, function, and mechanism of action of type IV secretion machines. Information on these systems is updated, with emphasis on recent exciting structural advances. An underappreciated feature of T4SS, most notably of the conjugation subfamily, is that they are widely distributed among many species of gram-negative and -positive bacteria, wall-less bacteria, and the Archaea. Conjugation-mediated lateral gene transfer has shaped the genomes of most if not all prokaryotes over evolutionary time and also contributed in the short term to the dissemination of antibiotic resistance and other virulence traits among medically important pathogens. How have these machines adapted to function across envelopes of distantly related microorganisms? A survey of T4SS functioning in phylogenetically diverse species highlights the biological complexity of these translocation systems and identifies common mechanistic themes as well as novel adaptations for specialized purposes relating to the modulation of the donor-target cell interaction.

Type IV secretion (T4S) systems are ancestrally related to bacterial conjugation machines. These systems assemble as a translocation channel, and often also as a surface filament or protein adhesin, at the envelopes of Gram-negative and Gram-positive bacteria. These organelles mediate the transfer of DNA and protein substrates to phylogenetically diverse prokaryotic and eukaryotic target cells. Many basic features of T4S are known, including structures of machine subunits, steps of machine assembly, substrates and substrate recognition mechanisms, and cellular consequences of substrate translocation. A recent advancement also has enabled definition of the translocation route for a DNA substrate through a T4S system of a Gram-negative bacterium. This review emphasizes the dynamics of assembly and function of model conjugation systems and the Agrobacterium tumefaciens VirB/D4 T4S system. We also summarize salient features of the increasingly studied effector translocator systems of mammalian pathogens.

Bacteria use type IV secretion systems for two fundamental objectives related to pathogenesisgenetic exchange and the delivery of effector molecules to eukaryotic target cells. Whereas gene acquisition is an important adaptive mechanism that enables pathogens to cope with a changing environment during invasion of the host, interactions between effector and host molecules can suppress defence mechanisms, facilitate intracellular growth and even induce the synthesis of nutrients that are beneficial to bacterial colonization. Rapid progress has been made towards defining the structures and functions of type IV secretion machines, identifying the effector molecules, and elucidating the mechanisms by which the translocated effectors subvert eukaryotic cellular processes during infection. The year 2003 marks the fiftieth anniversary of the first description of a TYPE IV SECRETION (T4S)SYSTEM: the CONJUGATION apparatus of the F plasmid 1 . This is a dynamic bacterial surface organelle, the activities of which are now known to include the contact-dependent delivery of DNA to bacterial recipients and the assembly and retraction of a conjugal PILUS 2 . In the past decade, reports describing systems that are ancestrally related to the F-transfer system and other conjugation machines have emerged. Instead of mediating DNA transfer between bacteria, these systems deliver DNA or protein substrates, known as effectors, to eukaryotic target cells during infection [3][4][5] . More recently, several new T4S systems have been described that are also ancestrally related to the conjugation machines, but these systems mediate the exchange of DNA with the extracellular milieu [6][7][8] . Collectively, this diversity of function in the face of a common ancestry makes the T4S machines attractive subjects for comparative studies that explore the dynamics of organelle assembly and action. Additionally, from a medical perspective, it is of enormous interest to develop a detailed understanding of how the inter-kingdom transfer of type IV effector molecules contributes to pathogenesis. This review will summarize the recent advances in our knowledge of T4S, NIH Public Access Author ManuscriptNat Rev Microbiol. Author manuscript; available in PMC 2013 December 27. NIH-PA Author ManuscriptNIH-PA Author Manuscript NIH-PA Author Manuscriptwith an emphasis on machine structure and function, and the activities of effectors after translocation into the eukaryotic host. The T4S familyThis fascinatingly versatile translocation family can be classified into three subfamilies, each of which contributes in unique ways to pathogenesis ( Fig. 1; Table 1). The largest subfamily, the conjugation systems, are found in most species of Gram-negative and Grampositive bacteria. These systems mediate DNA transfer both within and between phylogenetically diverse species, and some systems even deliver DNA to fungi, plants and human cells 2,9-13 . Conjugation is an important contributor to genome plasticity, and therefore bacterial fitness under changing en...

Bacteria use conjugation systems, a subfamily of the type IV secretion systems, to transfer DNA to recipient cells. Despite 50 years of research, the architecture and mechanism of action of the channel mediating DNA transfer across the bacterial cell envelope remains obscure. By use of a sensitive, quantifiable assay termed transfer DNA immunoprecipitation (TrIP), we identify contacts between a DNA substrate (T-DNA) and 6 of 12 components of the VirB/D4 conjugation system of the phytopathogen Agrobacterium tumefaciens. Our results define the translocation pathway for a DNA substrate through a bacterial conjugation machine, specifying the contributions of each subunit of the secretory apparatus to substrate passage.

Several bacterial pathogens utilize conjugation machines to export effector molecules during infection. Such systems are members of the type IV or 'adapted conjugation' secretion family. The prototypical type IV system is the Agrobacterium tumefaciens T-DNA transfer machine, which delivers oncogenic nucleoprotein particles to plant cells. Other pathogens, including Bordetella pertussis, Legionella pneumophila, Brucella spp. and Helicobacter pylori, use type IV machines to export effector proteins to the extracellular milieu or the mammalian cell cytosol.Gram-negative bacteria have adapted at least two cell-surface organelles for use in the delivery of macromolecules across kingdom boundaries by a cell-contact-dependent mechanism. The type III secretion systems are assembled from core components of the flagellar machine 1 . The type IV systems, the subject of this review, are built from core components of conjugation machines. This is a promiscuous secretion family both in terms of the translocated substrates -large nucleoprotein conjugation intermediates, an A/B toxin and monomeric proteins -and the phylogenetic diversity of cells targeted for substrate delivery -bacteria, fungi, plants and animals. In this review, we will summarize recent structure-function studies of the type IV systems, with an emphasis on the Agrobacterium tumefaciens T-DNA transfer machine.