The P1 plasmid in action: time-lapse photomicroscopy reveals some unexpected aspects of plasmid partition

Atypical low-copy number plasmid segregation systems, all in one?

Plasmid families harbor different maintenances functions, depending on their size and copy number. Low copy number plasmids rely on active partition systems, organizing a partition complex at specific centromere sites that is actively positioned using NTPase proteins. Some low copy number plasmids lack an active partition system, but carry atypical intracellular positioning systems using a single protein that binds to the centromere site but without an associated NTPase. These systems have been studied in the case of the Escherichia coli R388 and of the Staphylococcus aureus pSK1 plasmids. Here we review these two systems, which appear to be unrelated but share common features, such as their distribution on plasmids of medium size and copy number, certain activities of their centromere-binding proteins, StbA and Par, respectively, as well as their mode of action, which may involve dynamic interactions with the nucleoid-packed chromosome of their hosts.

Should Bacteriophages Be Classified as Parasites or Predators?

Bacteriophages are viruses infecting bacteria and propagating in bacterial cells. They were discovered over 100 years ago, and for decades they played crucial roles as models in genetics and molecular biology and as tools in genetic engineering and biotechnology. Now we also recognize their huge role in natural environment and their importance in human health and disease. Despite our understanding of bacteriophage mechanisms of development, these viruses are described as parasites or predators in the literature. From the biological point of view, there are fundamental differences between parasites and predators. Therefore, in this article, I asked whether bacteriophages should be classified as former or latter biological entities. Analysis of the literature and biological definitions led me to conclude that bacteriophages are parasites rather than predators and should be classified and described as such. If even more precise ecological classification is needed, bacteriophages can perhaps be included in the group of parasitoids. It might be the most appropriate formal classification of these viruses, especially if strictly virulent phages are considered, contrary to phages which lysogenize host cells and those which develop according to the permanent infection mode (or chronic cycle, like filamentous phages) revealing features of classical parasites.

Bacterial Vivisection: How Fluorescence-Based Imaging Techniques Shed a Light on the Inner Workings of Bacteria

The rise in fluorescence-based imaging techniques over the past 3 decades has improved the ability of researchers to scrutinize live cell biology at increased spatial and temporal resolution. In microbiology, these real-time vivisections structurally changed the view on the bacterial cell away from the "watery bag of enzymes" paradigm toward the perspective that these organisms are as complex as their eukaryotic counterparts. Capitalizing on the enormous potential of (time-lapse) fluorescence microscopy and the ever-extending pallet of corresponding probes, initial breakthroughs were made in unraveling the localization of proteins and monitoring real-time gene expression. However, later it became clear that the potential of this technique extends much further, paving the way for a focus-shift from observing single events within bacterial cells or populations to obtaining a more global picture at the intra- and intercellular level. In this review, we outline the current state of the art in fluorescence-based vivisection of bacteria and provide an overview of important case studies to exemplify how to use or combine different strategies to gain detailed information on the cell's physiology. The manuscript therefore consists of two separate (but interconnected) parts that can be read and consulted individually. The first part focuses on the fluorescent probe pallet and provides a perspective on modern methodologies for microscopy using these tools. The second section of the review takes the reader on a tour through the bacterial cell from cytoplasm to outer shell, describing strategies and methods to highlight architectural features and overall dynamics within cells.

Characterization of chromosomal and megaplasmid partitioning loci in Thermus thermophilus HB27

Background

In low-copy-number plasmids, the partitioning loci (par) act to ensure proper plasmid segregation and copy number maintenance in the daughter cells. In many bacterial species, par gene homologues are encoded on the chromosome, but their function is much less understood. In the two-replicon, polyploid genome of the hyperthermophilic bacterium Thermus thermophilus, both the chromosome and the megaplasmid encode par gene homologues (parABc and parABm, respectively). The mode of partitioning of the two replicons and the role of the two Par systems in the replication, segregation and maintenance of the genome copies are completely unknown in this organism.

Results

We generated a series of chromosomal and megaplasmid par mutants and sGFP reporter strains and analyzed them with respect to DNA segregation defects, genome copy number and replication origin localization. We show that the two ParB proteins specifically bind their cognate centromere-like sequences parS, and that both ParB-parS complexes localize at the cell poles. Deletion of the chromosomal parAB genes did not apparently affect the cell growth, the frequency of cells with aberrant nucleoids, or the chromosome and megaplasmid replication. In contrast, deletion of the megaplasmid parAB operon or of the parB gene was not possible, indicating essentiality of the megaplasmid-encoded Par system. A mutant expressing lower amounts of ParABm showed growth defects, a high frequency of cells with irregular nucleoids and a loss of a large portion of the megaplasmid. The truncated megaplasmid could not be partitioned appropriately, as interlinked megaplasmid molecules (catenenes) could be detected, and the ParBm-parSm complexes in this mutant lost their polar localization.

Conclusions

We show that in T. thermophilus the chromosomal par locus is not required for either the chromosomal or megaplasmid bulk DNA replication and segregation. In contrast, the megaplasmid Par system of T. thermophilus is needed for the proper replication and segregation of the megaplasmid, and is essential for its maintenance. The two Par sets in T. thermophilus appear to function in a replicon-specific manner. To our knowledge, this is the first analysis of Par systems in a polyploid bacterium.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1523-3) contains supplementary material, which is available to authorized users.

Defining the role of ATP hydrolysis in mitotic segregation of bacterial plasmids

Hydrolysis of ATP by partition ATPases, although considered a key step in the segregation mechanism that assures stable inheritance of plasmids, is intrinsically very weak. The cognate centromere-binding protein (CBP), together with DNA, stimulates the ATPase to hydrolyse ATP and to undertake the relocation that incites plasmid movement, apparently confirming the need for hydrolysis in partition. However, ATP-binding alone changes ATPase conformation and properties, making it difficult to rigorously distinguish the substrate and cofactor roles of ATP in vivo. We had shown that mutation of arginines R36 and R42 in the F plasmid CBP, SopB, reduces stimulation of SopA-catalyzed ATP hydrolysis without changing SopA-SopB affinity, suggesting the role of hydrolysis could be analyzed using SopA with normal conformational responses to ATP. Here, we report that strongly reducing SopB-mediated stimulation of ATP hydrolysis results in only slight destabilization of mini-F, although the instability, as well as an increase in mini-F clustering, is proportional to the ATPase deficit. Unexpectedly, the reduced stimulation also increased the frequency of SopA relocation over the nucleoid. The increase was due to drastic shortening of the period spent by SopA at nucleoid ends; average speed of migration per se was unchanged. Reduced ATP hydrolysis was also associated with pronounced deviations in positioning of mini-F, though time-averaged positions changed only modestly. Thus, by specifically targeting SopB-stimulated ATP hydrolysis our study reveals that even at levels of ATPase which reduce the efficiency of splitting clusters and the constancy of plasmid positioning, SopB still activates SopA mobility and plasmid positioning, and sustains near wild type levels of plasmid stability.

Genes enabling bacteria to survive and thrive in challenging environments are very often found on small, non-essential DNA molecules called plasmids. Many plasmids are naturally present in the cell in very few copies and so risk being lost from one of the daughter cells upon division. These plasmids elaborate a partition system, functionally similar to mitosis, which assures their faithful inheritance. Chromosomes also generally possess such systems. We know that partition systems involve two proteins, that one (B) stimulates the other (A) to hydrolyse ATP, and that upon binding to A protein ATP confers properties needed for partition. ATP's double action, as hydrolysis substrate and cofactor, complicates definition of its role in the mechanism. The novelty of our approach lies in use of B protein mutants that do not stimulate hydrolysis. Our results reveal that the major function of ATP hydrolysis is not to displace plasmid molecules to their positions in each cell half, as generally thought, but to split initial sibling plasmid pairs and prevent their reforming. This study is the first to dissect ATPase activity in vivo using normal A-protein ATPase, and so opens a new avenue to exploration of the mechanisms that ensure plasmid and chromosome inheritance.

Expression of a novel P22 ORFan gene reveals the phage carrier state in Salmonella typhimurium

We discovered a novel interaction between phage P22 and its host Salmonella Typhimurium LT2 that is characterized by a phage mediated and targeted derepression of the host dgo operon. Upon further investigation, this interaction was found to be instigated by an ORFan gene (designated pid for phage P22 encoded instigator of dgo expression) located on a previously unannotated moron locus in the late region of the P22 genome, and encoding an 86 amino acid protein of 9.3 kDa. Surprisingly, the Pid/dgo interaction was not observed during strict lytic or lysogenic proliferation of P22, and expression of pid was instead found to arise in cells that upon infection stably maintained an unintegrated phage chromosome that segregated asymmetrically upon subsequent cell divisions. Interestingly, among the emerging siblings, the feature of pid expression remained tightly linked to the cell inheriting this phage carrier state and became quenched in the other. As such, this study is the first to reveal molecular and genetic markers authenticating pseudolysogenic development, thereby exposing a novel mechanism, timing, and populational distribution in the realm of phage-host interactions.

Plasmid segregation: how to survive as an extra piece of DNA

Non-essential extra-chromosomal DNA elements such as plasmids are responsible for their own propagation in dividing host cells, and one means to ensure this is to carry a miniature active segregation system reminiscent of the mitotic spindle. Plasmids that are maintained at low numbers in prokaryotic cells have developed a range of such active partitioning systems, which are characterized by an impressive simplicity and efficiency and which are united by the use of dynamic, nucleotide-driven filaments to separate and position DNA molecules. A comparison of different plasmid segregation systems reveals (i) how unrelated filament-forming and DNA-binding proteins have been adopted and modified to create a range of simple DNA segregating complexes and (ii) how subtle changes in the few components of these DNA segregation machines has led to a remarkable diversity in the molecular mechanisms of closely related segregation systems. Here, our current understanding of plasmid segregation systems is reviewed and compared with other DNA segregation systems, and this is extended by a discussion of basic principles of plasmid segregation systems, evolutionary implications and the relationship between an autonomous DNA element and its host cell.

P1 plasmid segregation: accurate redistribution by dynamic plasmid pairing and separation

Low-copy-number plasmids, such as P1 and F, encode a type Ia partition system (P1par or Fsop) for active segregation of copies to daughter cells. Typical descriptions show a single central plasmid focus dividing and the products moving to the cell quarter regions, ensuring segregation. However, using improved optical and analytical tools and large cell populations, we show that P1 plasmid foci are very broadly distributed. Moreover, under most growth conditions, more than two foci are frequently present. Each focus contains either one or two plasmid copies. Replication and focus splitting occur at almost any position in the cell. The products then move rapidly apart for approximately 40% of the cell length. They then tend to maintain their relative positions. The segregating foci often pass close to or come to rest close to other foci in the cell. Foci frequently appear to fuse during these encounters. Such events occur several times in each cell and cell generation on average. We argue that foci pair with their neighbors and then actively separate again. The net result is an approximately even distribution of foci along the long cell axis on average. We show mathematically that trans-pairing and active separation could greatly increase the accuracy of segregation and would produce the distributions of foci that we observe. Plasmid pairing and separation may constitute a novel fine-tuning mechanism that takes the basic pattern created when plasmids separate after replication and converts it to a roughly even pattern that greatly improves the fidelity of plasmid segregation.

Bacterial partitioning proteins affect the subcellular location of broad-host-range plasmid RK2

It has been demonstrated that plasmids are not randomly distributed but are located symmetrically in mid-cell, or (1/4), (3/4) positions in bacterial cells. In this work we compared the localization of broad-host-range plasmid RK2 mini-replicons, which lack an active partitioning system, in Escherichia coli and Pseudomonas putida cells. In E. coli the location of the plasmid mini-replicon cluster was at the cell poles. In contrast, in Pseudomonas cells, as a result of the interaction of chromosomally encoded ParB protein with RK2 centromere-like sequences, these mini-derivatives were localized in the proximity of mid-cell, or (1/4), (3/4) positions. The expression of the Pseudomonas parAB genes in E. coli resulted in a positional change in the RK2 mini-derivative to the mid-cell or (1/4), (3/4) positions. Moreover, in a P. putida parAB mutant, both RK2 mini-derivatives and the entire RK2 plasmid exhibited disturbances of subcellular localization. These observations raise the possibility that in certain bacteria chromosomally encoded partitioning machinery could affect subcellular plasmid positioning.

Intracellular mobility of plasmid DNA is limited by the ParA family of partitioning systems

The highly conserved ParA family of partitioning systems is responsible for positioning DNA and protein complexes in bacteria. In Escherichia coli, plasmids that rely upon these systems are positioned at mid-cell and are repositioned at the quarter-cell positions after replication. How they remain fixed at these positions throughout the cell cycle is unknown. We use fluorescence recovery after photobleaching and time-lapse microscopy to measure plasmid mobility in living E. coli cells. We find that a minimalized version of plasmid RK2 that lacks its Par system is highly mobile, that the intact RK2 plasmid is relatively immobile, and that the addition of a Par system to the minimalized RK2 plasmid limits its mobility to that of the intact RK2. Mobility is thus the default state, and Par systems are required not only to position plasmids, but also to hold them at these positions. The intervention of Par systems is required continuously throughout the cell cycle to restrict plasmid movement that would, if unrestricted, subvert the segregation process. Our results reveal an important function for Par systems in plasmid DNA segregation that is likely to be conserved in bacteria.

A dynamic, mitotic-like mechanism for bacterial chromosome segregation

The mechanisms that mediate chromosome segregation in bacteria are poorly understood. Despite evidence of dynamic movement of chromosome regions, to date, mitotic-like mechanisms that act on the bacterial chromosome have not been demonstrated. Here we provide evidence that the Vibrio cholerae ParAI and ParBI proteins are components of an apparatus that pulls the origin region of the large V. cholerae chromosome to the cell pole and anchors it there. ParBI interacts with a conserved origin-proximal, centromere-like site (parSI) that, following chromosome replication, segregates asymmetrically from one pole to the other. While segregating, parSI stretches far away from neighboring chromosomal loci. ParAI forms a dynamic band that extends from the pole to the segregating ParBI/parSI complex. Movement of ParBI/parSI across the cell occurs in concert with ParAI retraction. Deletion of parAI disrupts proper origin localization and segregation dynamics, and parSI no longer separates from nearby regions. These data suggest that ParAI forms a dynamic structure that pulls the ParBI-bound chromosome to the pole in a process analogous to anaphase of eukaryotic mitosis.

The positioning of cytoplasmic protein clusters in bacteria

Cell division is a carefully orchestrated procedure. Bacterial cells have intricate mechanisms to ensure that genetic material is copied, proofread, and accurately partitioned into daughter cells. Partitioning now appears to also occur for some cytoplasmic proteins. Previously, using chromosomal fluorescent protein fusions, we demonstrated that a subset of Rhodobacter sphaeroides chemotaxis proteins colocalize to a discrete region within the bacterial cytoplasm. Using TlpT-yellow fluorescent protein as a marker for the position of the cytoplasmic protein clusters, we show most cells contain either one cluster localized at mid-cell or two clusters at the one-fourth and three-fourths positions of cell length. The number and positioning of these protein clusters depend on a previously unrecognized bacterial protein positioning factor, PpfA, which has homology to bacterial type I DNA partitioning factors. These data suggest that there is a mechanism involved in partitioning some cytoplasmic proteins upon cell division that is analogous to a mechanism seen for plasmid and chromosomal DNA.

Mechanisms for chromosome and plasmid segregation

The fundamental problems in duplicating and transmitting genetic information posed by the geometric and topological features of DNA, combined with its large size, are qualitatively similar for prokaryotic and eukaryotic chromosomes. The evolutionary solutions to these problems reveal common themes. However, depending on differences in their organization, ploidy, and copy number, chromosomes and plasmids display distinct segregation strategies as well. In bacteria, chromosome duplication, likely mediated by a stationary replication factory, is accompanied by rapid, directed migration of the daughter duplexes with assistance from DNA-compacting and perhaps translocating proteins. The segregation of unit-copy or low-copy bacterial plasmids is also regulated spatially and temporally by their respective partitioning systems. Eukaryotic chromosomes utilize variations of a basic pairing and unpairing mechanism for faithful segregation during mitosis and meiosis. Rather surprisingly, the yeast plasmid 2-micron circle also resorts to a similar scheme for equal partitioning during mitosis.

Gyrase inhibitors and thymine starvation disrupt the normal pattern of plasmid RK2 localization in Escherichia coli

Multicopy plasmids in Escherichia coli are not randomly distributed throughout the cell but exist as defined clusters that are localized at the mid-cell, or at the 1/4 and 3/4 cell length positions. To explore the factors that contribute to plasmid clustering and localization, E. coli cells carrying a plasmid RK2 derivative that can be tagged with a green fluorescent protein-LacI fusion protein were subjected to various conditions that interfere with plasmid superhelicity and/or DNA replication. The various treatments included thymine starvation and the addition of the gyrase inhibitors nalidixic acid and novobiocin. In each case, localization of plasmid clusters at the preferred positions was disrupted but the plasmids remained in clusters, suggesting that normal plasmid superhelicity and DNA synthesis in elongating cells are not required for the clustering of individual plasmid molecules. It was also observed that the inhibition of DNA replication by these treatments produced filaments in which the plasmid clusters were confined to one or two nucleoid bodies, which were located near the midline of the filament and were not evenly spaced throughout the filament, as is found in cells treated with cephalexin. Finally, the enhanced yellow fluorescent protein-RarA fusion protein was used to localize the replication complex in individual E. coli cells. Novobiocin and nalidixic acid treatment both resulted in rapid loss of RarA foci. Under these conditions the RK2 plasmid clusters were not disassembled, suggesting that a completely intact replication complex is not required for plasmid clustering.

The role of Par proteins in the active segregation of the P1 plasmid

The parS centromere-like site promotes active P1 plasmid segregation in the presence of P1 ParA and ParB proteins. At the modest growth rate used here, time-lapse and still photomicroscopy shows that the plasmid copies are clustered as a focus at the Escherichia coli cell centre. Just before cell division, the focus is actively divided and ejects bidirectionally into opposite halves of the dividing cell. In the absence of the wild-type parS binding protein ParB, a focus was formed, but generally did not go to the cell centre. The randomly placed focus did not divide and was inherited by one daughter cell only. In the absence of ParA, foci formed and frequently fixed to the cell centre. However, they failed to divide or eject and were left at the new cell pole of one cell at division. Thus, ParB appears to be required for recognition of the plasmid and its attachment to the cell centre, and ParA is required for focus division and energetic ejection from the cell centre. The ATPase active site mutation, parAK122E, blocked ejection. Mutant parAM314I ejected weakly, and the daughter foci took two generations to reach a new cell centre. This explains the novel alternation of segregation and missegregation in successive generations seen in time-lapse images of this mutant.

Probing plasmid partition with centromere-based incompatibility

Low-copy number plasmids of bacteria rely on specific centromeres for regular partition into daughter cells. When also present on a second plasmid, the centromere can render the two plasmids incompatible, disrupting partition and causing plasmid loss. We have investigated the basis of incompatibility exerted by the F plasmid centromere, sopC, to probe the mechanism of partition. Measurements of the effects of sopC at various gene dosages on destabilization of mini-F, on repression of the sopAB operon and on occupancy of mini-F DNA by the centromere-binding protein, SopB, revealed that among mechanisms previously proposed, no single one fully explained incompatibility. sopC on multicopy plasmids depleted SopB by titration and by contributing to repression. The resulting SopB deficit is proposed to delay partition complex formation and facilitate pairing between mini-F and the centromere vector, thereby increasing randomization of segregation. Unexpectedly, sopC on mini-P1 exerted strong incompatibility if the P1 parABS locus was absent. A mutation preventing the P1 replication initiation protein from pairing (handcuffing) reduced this strong incompatibility to the level expected for random segregation. The results indicate the importance of kinetic considerations and suggest that mini-F handcuffing promotes pairing of SopB-sopC complexes that can subsequently segregate as intact aggregates.

Segregation of the yeast plasmid: similarities and contrasts with bacterial plasmid partitioning

The high copy yeast plasmid 2 microm circle, like the well-studied low copy bacterial plasmids, utilizes two partitioning proteins and a cis-acting 'centromere'-like sequence for its stable propagation. Functionally, though, the protein and DNA constituents of the two partitioning systems are quite distinct. Key events in the yeast and bacterial segregation pathways are plasmid organization, localization, replication, 'counting' of replicated molecules and their distribution to daughter cells. We suggest that the two systems facilitate these common logistical steps by adapting to the physical, biochemical, and mechanical contexts in which the host chromosomes segregate.

Bacterial Mitotic Machineries

Here, we review recent progress that yields fundamental new insight into the molecular mechanisms behind plasmid and chromosome segregation in prokaryotic cells. In particular, we describe how prokaryotic actin homologs form mitotic machineries that segregate DNA before cell division. Thus, the ParM protein of plasmid R1 forms F actin-like filaments that separate and move plasmid DNA from mid-cell to the cell poles. Evidence from three different laboratories indicate that the morphogenetic MreB protein may be involved in segregation of the bacterial chromosome.

Segregation of the Escherichia coli chromosome terminus

We studied the segregation of the replication terminus of the Escherichia coli chromosome by time-lapse and still photomicroscopy. The replicated termini lie together at the cell centre. They rapidly segregate away from each other immediately before cell division. At fast growth rate, the copies move progressively and quickly toward the centres of the new-born cells. At slow growth rate, the termini usually remain near the inner cell pole and migrate to the cell centre in the middle of the cell cycle. A terminus domain of about 160kb, roughly centred on the dif recombination site, segregated as a unit at cell division. Sequences outside this domain segregated before division, giving two separate foci in predivision cells. Resolution of chromosome dimers via the terminus dif site requires the XerC recombinase and an activity of the FtsK protein that is thought to align the dif sequences at the cell centre. We found that anchoring of the termini at the cell centre and proper segregation at cell division occurred normally in the absence of recombination via the XerC recombinase. Anchoring and proper segregation were, however, frequently disrupted when the C-terminal domain of FtsK was truncated.

Clustering versus random segregation of plasmids lacking a partitioning function: a plasmid paradox?

Plasmids lacking a functional partition system are randomly distributed to the daughter cells; plasmid-free daughter cells are formed with a frequency of (1/2)2n per cell and cell generation where 2n is the (average) copy number at cell division. Hence, the unit of segregation is one plasmid copy. However, plasmids form clusters in the cells. A putative solution to this potential paradox is presented: one plasmid copy at a time is recruited from the plasmid clusters to the replication factories that are located in the cell centres. Hence, replication offers the means of declustering that is necessary in a growing host population. The daughter copies diffuse freely and each copy may with equal probability end up in either of the two cell halves. In this way, the random segregation of the plasmids is coupled to replication and occurs continuously during the cell cycle, and is not linked to cell division. The unit of segregation is the plasmid copy and not the plasmid clusters. In contrast, the two daughters of a Par+ plasmid are directed in opposite directions by the plasmid-encoded partition system, thereby assuring that each daughter cell receives the plasmid.