IMG/PR: a database of plasmids from genomes and metagenomes with rich annotations and metadata.

Plasmids are mobile genetic elements found in many clades of Archaea and Bacteria. They drive horizontal gene transfer, impacting ecological and evolutionary processes within microbial communities, and hold substantial importance in human health and biotechnology. To support plasmid research and provide scientists with data of an unprecedented diversity of plasmid sequences, we introduce the IMG/PR database, a new resource encompassing 699 973 plasmid sequences derived from genomes, metagenomes and metatranscriptomes. IMG/PR is the first database to provide data of plasmid that were systematically identified from diverse microbiome samples. IMG/PR plasmids are associated with rich metadata that includes geographical and ecosystem information, host taxonomy, similarity to other plasmids, functional annotation, presence of genes involved in conjugation and antibiotic resistance. The database offers diverse methods for exploring its extensive plasmid collection, enabling users to navigate plasmids through metadata-centric queries, plasmid comparisons and BLAST searches. The web interface for IMG/PR is accessible at https://img.jgi.doe.gov/pr. Plasmid metadata and sequences can be downloaded from https://genome.jgi.doe.gov/portal/IMG_PR.

How to build segregation complexes in bacteria: Use bridges.

In this issue of Genes & Development, Graham and colleagues (pp. 1228-1238) examine how ParBs, which bind to prokaryotic centromere-like partition sites, spread into nearby nonspecific DNA and assemble into higher-order protein-DNA complexes. Spreading is accomplished by looping rather than one-dimensional filamentation, thereby compacting the DNA into an extensively bridged complex.

Nonspecific DNA binding by P1 ParA determines the distribution of plasmid partition and repressor activities.

The faithful segregation, or "partition," of many low-copy number bacterial plasmids is driven by plasmid-encoded ATPases that are represented by the P1 plasmid ParA protein. ParA binds to the bacterial nucleoid via an ATP-dependent nonspecific DNA (nsDNA)-binding activity, which is essential for partition. ParA also has a site-specific DNA-binding activity to the par operator (parOP), which requires either ATP or ADP, and which is essential for it to act as a transcriptional repressor but is dispensable for partition. Here we examine how DNA binding by ParA contributes to the relative distribution of its plasmid partition and repressor activities, using a ParA with an alanine substitution at Arg351, a residue previously predicted to participate in site-specific DNA binding. In vivo, the parAR351A allele is compromised for partition, but its repressor activity is dramatically improved so that it behaves as a "super-repressor." In vitro, ParAR351A binds and hydrolyzes ATP, and undergoes a specific conformational change required for nsDNA binding, but its nsDNA-binding activity is significantly damaged. This defect in turn significantly reduces the assembly and stability of partition complexes formed by the interaction of ParA with ParB, the centromere-binding protein, and DNA. In contrast, the R351A change shows only a mild defect in site-specific DNA binding. We conclude that the partition defect is due to altered nsDNA binding kinetics and affinity for the bacterial chromosome. Furthermore, the super-repressor phenotype is explained by an increased pool of non-nucleoid bound ParA that is competent to bind parOP and repress transcription.

ParA-mediated plasmid partition driven by protein pattern self-organization.

DNA segregation ensures the stable inheritance of genetic material prior to cell division. Many bacterial chromosomes and low-copy plasmids, such as the plasmids P1 and F, employ a three-component system to partition replicated genomes: a partition site on the DNA target, typically called parS, a partition site binding protein, typically called ParB, and a Walker-type ATPase, typically called ParA, which also binds non-specific DNA. In vivo, the ParA family of ATPases forms dynamic patterns over the nucleoid, but how ATP-driven patterning is involved in partition is unknown. We reconstituted and visualized ParA-mediated plasmid partition inside a DNA-carpeted flowcell, which acts as an artificial nucleoid. ParA and ParB transiently bridged plasmid to the DNA carpet. ParB-stimulated ATP hydrolysis by ParA resulted in ParA disassembly from the bridging complex and from the surrounding DNA carpet, which led to plasmid detachment. Our results support a diffusion-ratchet model, where ParB on the plasmid chases and redistributes the ParA gradient on the nucleoid, which in turn mobilizes the plasmid.

Dissection of the ATPase active site of P1 ParA reveals multiple active forms essential for plasmid partition.

The segregation, or partition, of bacterial plasmids is driven by the action of plasmid-encoded partition ATPases, which work to position plasmids inside the cell. The most common type of partition ATPase, generally called ParA, is represented by the P1 plasmid ParA protein. ParA interacts with P1 ParB (the site-specific DNA binding protein that recognizes the parS partition site), and interacts with the bacterial chromosome via an ATP-dependent nonspecific DNA binding activity. ParA also regulates expression of the par genes by acting as a transcriptional repressor. ParA requires ATP for multiple steps and in different ways during the partition process. Here, we analyze the properties of mutations in P1 ParA that are altered in a key lysine in the Walker A motif of the ATP binding site. Four different residues at this position (Lys, Glu, Gln, Arg) result in four different phenotypes in vivo. We focus particularly on the arginine substitution (K122R) because it results in a worse-than-null and dominant-negative phenotype called ParPD. We show that ParAK122R binds and hydrolyzes ATP, although the latter activity is reduced compared with wild-type. ParAK122R interacts with ParB, but the consequences of the interaction are damaged. The ability of ParB to stimulate the ATPase activity of ParA in vitro and its repressor activity in vivo is defective. The K122R mutation specifically damages the disassembly of ParA-ParB-DNA partition complexes, which we believe explains the ParPD phenotype in vivo.

ATP-regulated interactions between P1 ParA, ParB and non-specific DNA that are stabilized by the plasmid partition site, parS.

Localization of the P1 plasmid requires two proteins, ParA and ParB, which act on the plasmid partition site, parS. ParB is a site-specific DNA-binding protein and ParA is a Walker-type ATPase with non-specific DNA-binding activity. In vivo ParA binds the bacterial nucleoid and forms dynamic patterns that are governed by the ParB-parS partition complex on the plasmid. How these interactions drive plasmid movement and localization is not well understood. Here we have identified a large protein-DNA complex in vitro that requires ParA, ParB and ATP, and have characterized its assembly by sucrose gradient sedimentation and light scattering assays. ATP binding and hydrolysis mediated the assembly and disassembly of this complex, while ADP antagonized complex formation. The complex was not dependent on, but was stabilized by, parS. The properties indicate that ParA and ParB are binding and bridging multiple DNA molecules to create a large meshwork of protein-DNA molecules that involves both specific and non-specific DNA. We propose that this complex represents a dynamic adaptor complex between the plasmid and nucleoid, and further, that this interaction drives the redistribution of partition proteins and the plasmid over the nucleoid during partition.

Surfing biological surfaces: exploiting the nucleoid for partition and transport in bacteria.

The ParA family of ATPases is responsible for transporting bacterial chromosomes, plasmids and large protein machineries. ParAs pattern the nucleoid in vivo, but how patterning functions or is exploited in transport is of considerable debate. Here we discuss the process of self-organization into patterns on the bacterial nucleoid and explore how it relates to the molecular mechanism of ParA action. We review ParA-mediated DNA partition as a general mechanism of how ATP-driven protein gradients on biological surfaces can result in spatial organization on a mesoscale. We also discuss how the nucleoid acts as a formidable diffusion barrier for large bodies in the cell, and make the case that the ParA family evolved to overcome the barrier by exploiting the nucleoid as a matrix for movement.

Structural basis for ADP-mediated transcriptional regulation by P1 and P7 ParA.

The accurate segregation of DNA is essential for the faithful inheritance of genetic information. Segregation of the prototypical P1 plasmid par system requires two proteins, ParA and ParB, and a centromere. When bound to ATP, ParA mediates segregation by interacting with centromere-bound ParB, but when bound to ADP, ParA fulfils a different function: DNA-binding transcription autoregulation. The structure of ParA is unknown as is how distinct nucleotides arbitrate its different functions. To address these questions, we carried out structural and biochemical studies. Crystal structures show that ParA consists of an elongated N-terminal alpha-helix, which unexpectedly mediates dimerization, a winged-HTH and a Walker-box containing C-domain. Biochemical data confirm that apoParA forms dimers at physiological concentrations. Comparisons of four apoParA structures reveal a strikingly flexible dimer interface that allows ParA to adopt multiple conformations. The ParA-ADP structure shows that ADP-binding activates DNA binding using a bipartite mechanism. First, it locks in one specific dimer conformation, and second, it induces the folding of two DNA-binding basic motifs that we show are critical for operator binding.

Probing the N-terminus of ParB using cysteine-scanning mutagenesis and thiol modification.

Plasmid partition systems require site-specific DNA binding proteins to recognize the plasmid partition site, or centromere. When bound to the centromere, these proteins, typically called ParB, interact with the ParA ATPases, which in turn promote the proper positioning of plasmids prior to cell division. P1 ParB is a typical member of a major class of ParB-like proteins that are dimeric helix-turn-helix DNA binding proteins. The N-terminus of ParB contains the region that interacts with ParA and with itself, but it has been difficult to study because this region of the protein is flexible in solution. Here we describe the use of cysteine-scanning mutagenesis and thiol modification of the N-terminus of ParB to create tools to probe the interactions of ParB with itself, with ParA and with DNA. We introduce twelve single-cysteine substitutions across the N-terminus of ParB and show that most do not compromise the function of ParB and that none completely inactivate the protein in vivo. We test three of these ParB variants in vitro and show that they do not alter ParB function, measured by its ability to stimulate ParA ATPase activity and its site-specific DNA binding activity. We discuss that this approach will be generally applicable to the ParB-like proteins in this class of partition systems because of their natural low content of cysteines, and because our evidence suggests that many residues in the N-terminus are amenable to substitution by cysteine.

Chromosome dynamics: Rearranging the choreography of a multipartite bacterial genome.

A new study identifies a novel fusion between a linear and a circular bacterial chromosome, with unusual requirements for DNA recombination and replication. Understanding how cells accommodate this chromosome promises to inform analyses and elucidate mechanisms of chromosome dynamics in bacteria.

ATP control of dynamic P1 ParA-DNA interactions: a key role for the nucleoid in plasmid partition.

P1 ParA is a member of the Walker-type family of partition ATPases involved in the segregation of plasmids and bacterial chromosomes. ATPases of this class interact with DNA non-specifically in vitro and colocalize with the bacterial nucleoid to generate a variety of reported patterns in vivo. Here, we directly visualize ParA binding to DNA using total internal reflection fluorescence microscopy. This activity depends on, and is highly specific for ATP. DNA-binding activity is not coupled to ATP hydrolysis. Rather, ParA undergoes a slow multi-step conformational transition upon ATP binding, which licenses ParA to bind non-specific DNA. The kinetics provide a time-delay switch to allow slow cycling between the DNA binding and non-binding forms of ParA. We propose that this time delay, combined with stimulation of ParA's ATPase activity by ParB bound to the plasmid DNA, generates an uneven distribution of the nucleoid-associated ParA, and provides the motive force for plasmid segregation prior to cell division.

Structures of ParB bound to DNA reveal mechanism of partition complex formation.

The faithful inheritance of genetic information, which is essential for all organisms, requires accurate DNA partition (segregation) at cell division. In prokaryotes, partition is mediated by par systems, for which the P1 plasmid system of Escherichia coli is a prototype comprising a partition site and two proteins, ParA and ParB. To form the partition complex necessary for segregation, P1 ParB must recognize a complicated arrangement of A-box and B-box DNA motifs located on opposite ends of a sharply bent parS partition site of approximately 74 bp (refs 3-7). Here we describe structures of ParB bound to partition sites. ParB forms an asymmetric dimer with extended amino-terminal HTH (helix-turn-helix) domains that contact A-boxes. The two HTH domains emanate from a dimerized DNA-binding module composed of a six-stranded beta-sheet coiled-coil that binds B-boxes. Strikingly, these individual DNA-binding modules rotate freely about a flexible linker, enabling them to contact several arrangements of A- and B-boxes. Most notably, each DNA-binding element binds to and thus bridges adjacent DNA duplexes. These unique structural features of ParB explain how this protein can bind complex arrays of A- and B-box elements on adjacent DNA arms of the looped partition site.

Plasmid Localization and Partition in Enterobacteriaceae.

Plasmids are ubiquitous in the microbial world and have been identified in almost all species of bacteria that have been examined. Their localization inside the bacterial cell has been examined for about two decades; typically, they are not randomly distributed, and their positioning depends on copy number and their mode of segregation. Low-copy-number plasmids promote their own stable inheritance in their bacterial hosts by encoding active partition systems, which ensure that copies are positioned in both halves of a dividing cell. High-copy plasmids rely on passive diffusion of some copies, but many remain clustered together in the nucleoid-free regions of the cell. Here we review plasmid localization and partition (Par) systems, with particular emphasis on plasmids from Enterobacteriaceae and on recent results describing the in vivo localization properties and molecular mechanisms of each system. Partition systems also cause plasmid incompatibility such that distinct plasmids (with different replicons) with the same Par system cannot be stably maintained in the same cells. We discuss how partition-mediated incompatibility is a consequence of the partition mechanism.

ParB Partition Proteins: Complex Formation and Spreading at Bacterial and Plasmid Centromeres.

In bacteria, active partition systems contribute to the faithful segregation of both chromosomes and low-copy-number plasmids. Each system depends on a site-specific DNA binding protein to recognize and assemble a partition complex at a centromere-like site, commonly called parS. Many plasmid, and all chromosomal centromere-binding proteins are dimeric helix-turn-helix DNA binding proteins, which are commonly named ParB. Although the overall sequence conservation among ParBs is not high, the proteins share similar domain and functional organization, and they assemble into similar higher-order complexes. In vivo, ParBs "spread," that is, DNA binding extends away from the parS site into the surrounding non-specific DNA, a feature that reflects higher-order complex assembly. ParBs bridge and pair DNA at parS and non-specific DNA sites. ParB dimers interact with each other via flexible conformations of an N-terminal region. This review will focus on the properties of the HTH centromere-binding protein, in light of recent experimental evidence and models that are adding to our understanding of how these proteins assemble into large and dynamic partition complexes at and around their specific DNA sites.

Plasmid Partition Mechanisms.

The stable maintenance of low-copy-number plasmids in bacteria is actively driven by partition mechanisms that are responsible for the positioning of plasmids inside the cell. Partition systems are ubiquitous in the microbial world and are encoded by many bacterial chromosomes as well as plasmids. These systems, although different in sequence and mechanism, typically consist of two proteins and a DNA partition site, or prokaryotic centromere, on the plasmid or chromosome. One protein binds site-specifically to the centromere to form a partition complex, and the other protein uses the energy of nucleotide binding and hydrolysis to transport the plasmid, via interactions with this partition complex inside the cell. For plasmids, this minimal cassette is sufficient to direct proper segregation in bacterial cells. There has been significant progress in the last several years in our understanding of partition mechanisms. Two general areas that have developed are (i) the structural biology of partition proteins and their interactions with DNA and (ii) the action and dynamics of the partition ATPases that drive the process. In addition, systems that use tubulin-like GTPases to partition plasmids have recently been identified. In this chapter, we concentrate on these recent developments and the molecular details of plasmid partition mechanisms.

ParAB-mediated intermolecular association of plasmid P1 parS sites.

The P1 plasmid partition system depends on ParA-ParB proteins acting on centromere-like parS sites for a faithful plasmid segregation during the Escherichia coli cell cycle. In vivo we placed parS into host E. coli chromosome and on a Sop(+) F plasmid and found that the stability of a P1 plasmid deleted for parA-parB could be partially restored when parB was expressed in trans. In vitro, parS, conjugated to magnetic beads could capture free parS DNA fragment in presence of ParB. In vitro, ParA stimulated ParB-mediated association of intermolecular parS sites in an ATP-dependent manner. However, in the presence of ADP, ParA reduced ParB-mediated pairing to levels below that seen by ParB alone. ParB of P1 pairs the parS sites of plasmids in vivo and fragments in vitro. Our findings support a model whereby ParB complexes P1 plasmids, ParA-ATP stimulates this interaction and ParA-ADP inhibits ParB pairing activity in a parS-independent manner.

P1 partition complex assembly involves several modes of protein-DNA recognition.

Assembly of P1 plasmid partition complexes at the partition site, parS, is nucleated by a dimer of P1 ParB and Escherichia coli integration host factor (IHF), which promotes loading of more ParB dimers and the pairing of plasmids during the cell cycle. ParB binds several copies of two distinct recognition motifs, known as A- and B-boxes, which flank a bend in parS created by IHF binding. The recent crystal structure of ParB bound to a partial parS site revealed two relatively independent DNA-binding domains and raised the question of how a dimer of ParB recognizes its complicated arrangement of recognition motifs when it loads onto the full parS site in the presence of IHF. In this study, we addressed this question by examining ParB binding activities to parS mutants containing different combinations of the A- and B-box motifs in parS. Binding was measured to linear and supercoiled DNA in electrophoretic and filter binding assays, respectively. ParB showed preferences for certain motifs that are dependent on position and on plasmid topology. In the simplest arrangement, one motif on either side of the bend was sufficient to form a complex, although affinity differed depending on the motifs. Therefore, a ParB dimer can load onto parS in different ways, so that the initial ParB-IHF-parS complex consists of a mixture of different orientations of ParB. This arrangement supports a model in which parS motifs are available for interas well as intramolecular parS recognition.