Transcription-coupled nucleoid architecture in bacteria

Rational Designed Hybrid Peptides Show up to a 6-Fold Increase in Antimicrobial Activity and Demonstrate Different Ultrastructural Changes as the Parental Peptides Measured by BioSAXS

Antimicrobial peptides (AMPs) are a promising class of compounds being developed against multi-drug resistant bacteria. Hybridization has been reported to increase antimicrobial activity. Here, two proline-rich peptides (consP1: VRKPPYLPRPRPRPL-CONH₂ and Bac5-v291: RWRRPIRRRPIRPPFWR-CONH₂) were combined with two arginine-isoleucine-rich peptides (optP1: KIILRIRWR-CONH₂ and optP7: KRRVRWIIW-CONH₂). Proline-rich antimicrobial peptides (PrAMPs) are known to inhibit the bacterial ribosome, shown also for Bac5-v291, whereas it is hypothesized a “dirty drug” model for the arginine-isoleucine-rich peptides. That hypothesis was underpinned by transmission electron microscopy and biological small-angle X-ray scattering (BioSAXS). The strength of BioSAXS is the power to detect ultrastructural changes in millions of cells in a short time (seconds) in a high-throughput manner. This information can be used to classify antimicrobial compounds into groups according to the ultrastructural changes they inflict on bacteria and how the bacteria react towards that assault. Based on previous studies, this correlates very well with different modes of action. Due to the novelty of this approach direct identification of the target of the antimicrobial compound is not yet fully established, more research is needed. More research is needed to address this limitation. The hybrid peptides showed a stronger antimicrobial activity compared to the proline-rich peptides, except when compared to Bac5-v291 against E. coli. The increase in activity compared to the arginine-isoleucine-rich peptides was up to 6-fold, however, it was not a general increase but was dependent on the combination of peptides and bacteria. BioSAXS experiments revealed that proline-rich peptides and arginine-isoleucine-rich peptides induce very different ultrastructural changes in E. coli, whereas a hybrid peptide (hyP7B5GK) shows changes, different to both parental peptides and the untreated control. These different ultrastructural changes indicated that the mode of action of the parental peptides might be different from each other as well as from the hybrid peptide hyP7B5GK. All peptides showed very low haemolytic activity, some of them showed a 100-fold or larger therapeutic window, demonstrating the potential for further drug development.

Single-Molecule/Cell Analyses Reveal Principles of Genome-Folding Mechanisms in the Three Domains of Life

Comparative structural/molecular biology by single-molecule analyses combined with single-cell dissection, mass spectroscopy, and biochemical reconstitution have been powerful tools for elucidating the mechanisms underlying genome DNA folding. All genomes in the three domains of life undergo stepwise folding from DNA to 30-40 nm fibers. Major protein players are histone (Eukarya and Archaea), Alba (Archaea), and HU (Bacteria) for fundamental structural units of the genome. In Euryarchaeota, a major archaeal phylum, either histone or HTa (the bacterial HU homolog) were found to wrap DNA. This finding divides archaea into two groups: those that use DNA-wrapping as the fundamental step in genome folding and those that do not. Archaeal transcription factor-like protein TrmBL2 has been suggested to be involved in genome folding and repression of horizontally acquired genes, similar to bacterial H-NS protein. Evolutionarily divergent SMC proteins contribute to the establishment of higher-order structures. Recent results are presented, including the use of Hi-C technology to reveal that archaeal SMC proteins are involved in higher-order genome folding, and the use of single-molecule tracking to reveal the detailed functions of bacterial and eukaryotic SMC proteins. Here, we highlight the similarities and differences in the DNA-folding mechanisms in the three domains of life.

RNA localization in prokaryotes: Where, when, how, and why

Only recently has it been recognized that the transcriptome of bacteria and archaea can be spatiotemporally regulated. All types of prokaryotic transcripts-rRNAs, tRNAs, mRNAs, and regulatory RNAs-may acquire specific localization and these patterns can be temporally regulated. In some cases bacterial RNAs reside in the vicinity of the transcription site, but in many others, transcripts show distinct localizations to the cytoplasm, the inner membrane, or the pole of rod-shaped species. This localization, which often overlaps with that of the encoded proteins, can be achieved either in a translation-dependent or translation-independent fashion. The latter implies that RNAs carry sequence-level features that determine their final localization with the aid of RNA-targeting factors. Localization of transcripts regulates their posttranscriptional fate by affecting their degradation and processing, translation efficiency, sRNA-mediated regulation, and/or propensity to undergo RNA modifications. By facilitating complex assembly and liquid-liquid phase separation, RNA localization is not only a consequence but also a driver of subcellular spatiotemporal complexity. We foresee that in the coming years the study of RNA localization in prokaryotes will produce important novel insights regarding the fundamental understanding of membrane-less subcellular organization and lead to practical outputs with biotechnological and therapeutic implications. This article is categorized under: RNA Export and Localization > RNA Localization Regulatory RNAs/RNAi/Riboswitches > Regulatory RNAs RNA Interactions with Proteins and Other Molecules > Protein-RNA Interactions: Functional Implications.

Different Proteins Mediate Step-Wise Chromosome Architectures in Thermoplasma acidophilum and Pyrobaculum calidifontis

Archaeal species encode a variety of distinct lineage-specific chromosomal proteins. We have previously shown that in Thermococcus kodakarensis, histone, Alba, and TrmBL2 play distinct roles in chromosome organization. Although our understanding of individual archaeal chromosomal proteins has been advancing, how archaeal chromosomes are folded into higher-order structures and how they are regulated are largely unknown. Here, we investigated the primary and higher-order structures of archaeal chromosomes from different archaeal lineages. Atomic force microscopy of chromosome spreads out of Thermoplasma acidophilum and Pyrobaculum calidifontis cells revealed 10-nm fibers and 30–40-nm globular structures, suggesting the occurrence of higher-order chromosomal folding. Our results also indicated that chromosome compaction occurs toward the stationary phase. Micrococcal nuclease digestion indicated that fundamental structural units of the chromosome exist in T. acidophilum and T. kodakarensis but not in P. calidifontis or Sulfolobus solfataricus. In vitro reconstitution showed that, in T. acidophilum, the bacterial HU protein homolog HTa formed a 6-nm fiber by wrapping DNA, and that Alba was responsible for the formation of the 10-nm fiber by binding along the DNA without wrapping. Remarkably, Alba could form different higher-order complexes with histone or HTa on DNA in vitro. Mass spectrometry detected HTa and Rad50 in the T. acidophilum chromosome but not in other species. A putative transcriptional regulator of the AsnC/Lrp family (Pcal_1183) was detected on the P. calidifontis chromosome, but not on that of other species studied. Putative membrane-associated proteins were detected in the chromosomes of the three archaeal species studied, including T. acidophilum, P. calidifontis, and T. kodakarensis. Collectively, our data show that Archaea use different combinations of proteins to achieve chromosomal architecture and functional regulation.

Architecture of the Escherichia coli nucleoid

How genomes are organized within cells and how the 3D architecture of a genome influences cellular functions are significant questions in biology. A bacterial genomic DNA resides inside cells in a highly condensed and functionally organized form called nucleoid (nucleus-like structure without a nuclear membrane). The Escherichia coli chromosome or nucleoid is composed of the genomic DNA, RNA, and protein. The nucleoid forms by condensation and functional arrangement of a single chromosomal DNA with the help of chromosomal architectural proteins and RNA molecules as well as DNA supercoiling. Although a high-resolution structure of a bacterial nucleoid is yet to come, five decades of research has established the following salient features of the E. coli nucleoid elaborated below: 1) The chromosomal DNA is on the average a negatively supercoiled molecule that is folded as plectonemic loops, which are confined into many independent topological domains due to supercoiling diffusion barriers; 2) The loops spatially organize into megabase size regions called macrodomains, which are defined by more frequent physical interactions among DNA sites within the same macrodomain than between different macrodomains; 3) The condensed and spatially organized DNA takes the form of a helical ellipsoid radially confined in the cell; and 4) The DNA in the chromosome appears to have a condition-dependent 3-D structure that is linked to gene expression so that the nucleoid architecture and gene transcription are tightly interdependent, influencing each other reciprocally. Current advents of high-resolution microscopy, single-molecule analysis and molecular structure determination of the components are expected to reveal the total structure and function of the bacterial nucleoid.

What Happens in the Staphylococcal Nucleoid under Oxidative Stress?

The evolutionary success of Staphylococcus aureus as an opportunistic human pathogen is largely attributed to its prominent abilities to cope with a variety of stresses and host bactericidal factors. Reactive oxygen species are important weapons in the host arsenal that inactivate phagocytosed pathogens, but S. aureus can survive in phagosomes and escape from phagocytic cells to establish infections. Molecular genetic analyses combined with atomic force microscopy have revealed that the MrgA protein (part of the Dps family of proteins) is induced specifically in response to oxidative stress and converts the nucleoid from the fibrous to the clogged state. This review collates a series of evidences on the staphylococcal nucleoid dynamics under oxidative stress, which is functionally and physically distinct from compacted Escherichia coli nucleoid under stationary phase. In addition, potential new roles of nucleoid clogging in the staphylococcal life cycle will be proposed.

Atomic Force Microscopy Imaging and Analysis of Prokaryotic Genome Organization

This protocol describes the application of atomic force microscopy for structural analysis of the prokaryotic and organellar nucleoids. It is based on a simple cell manipulation procedure that enables step-wise dissection of the nucleoid. The procedure includes (1) on-substrate-lysis of cells, and (2) enzyme treatment, followed by atomic force microscopy. This type of dissection analysis permits analysis of nucleoid structure ranging from the fundamental units assembled on DNA to higher order levels of organization. The combination with molecular-genetic and biochemical techniques further permits analysis of the functions of key nucleoid factors relevant to signal-induced structural re-organization or building up of basic structures, as seen for Dps in Escherichia coli, and TrmBL2 in Thermococcus kodakarensis. These systems are described here as examples of the successful application of AFM for this purpose. Moreover, we describe the procedures needed for quantitative analysis of the data.

Degradation of RNA during lysis of Escherichia coli cells in agarose plugs breaks the chromosome

The nucleoid of Escherichia coli comprises DNA, nucleoid associated proteins (NAPs) and RNA, whose role is unclear. We found that lysing bacterial cells embedded in agarose plugs in the presence of RNases caused massive fragmentation of the chromosomal DNA. This RNase-induced chromosomal fragmentation (RiCF) was completely dependent on the presence of RNase around lysing cells, while the maximal chromosomal breakage required fast cell lysis. Cell lysis in plugs without RNAse made the chromosomal DNA resistant to subsequent RNAse treatment. RiCF was not influenced by changes in the DNA supercoiling, but was influenced by growth temperature or age of the culture. RiCF was partially dependent on H-NS, histone-like nucleoid structuring- and global transcription regulator protein. The hupAB deletion of heat-unstable nucleoid protein (HU) caused increase in spontaneous fragmentation that was further increased when combined with deletions in two non-coding RNAs, nc1 and nc5. RiCF was completely dependent upon endonuclease I, a periplasmic deoxyribonuclease that is normally found inhibited by cellular RNA. Unlike RiCF, the spontaneous fragmentation in hupAB nc1 nc5 quadruple mutant was resistant to deletion of endonuclease I. RiCF-like phenomenon was observed without addition of RNase to agarose plugs if EDTA was significantly reduced during cell lysis. Addition of RNase under this condition was synergistic, breaking chromosomes into pieces too small to be retained by the pulsed field gels. RNase-independent fragmentation was qualitatively and quantitatively comparable to RiCF and was partially mediated by endonuclease I.

Cellular splicing factor UAP56 stimulates trimeric NP formation for assembly of functional influenza viral ribonucleoprotein complexes

The influenza virus RNA genome exists as a ribonucleoprotein (RNP) complex by interacting with NP, one of virus-encoded RNA binding proteins. It is proposed that trimeric NP is a functional form, but it is not clear how trimeric NP is formed and transferred to RNA. UAP56, a cellular splicing factor, functions as a molecular chaperone for NP and is required for the replication-coupled RNP formation of newly synthesized viral genome, but the details of NP transfer to viral RNA by UAP56 is unclear. Here we found that UAP56 is complexed with trimeric NP, but not monomeric NP. Gel filtration analysis and atomic force microscopy analysis indicated that the complex consists of two trimeric NP connected by UAP56. We also found that UAP56 stimulates trimeric NP formation from monomeric NP even at physiological salt concentrations. Thus, UAP56 facilitates the transfer of NP to viral RNA since trimeric NP has higher RNA binding activity than monomeric NP. Further, UAP56 represses the binding of excess amount of NP to RNA possibly by transferring trimeric NP. Collectively, we propose that UAP56 stimulates viral RNP formation through promotion of the assembly of trimeric NP and is important for the structural integrity of NP-RNA complex.

Use of small-angle X-ray scattering to resolve intracellular structure changes of Escherichia coli cells induced by antibiotic treatment

The application of small-angle X-ray scattering (SAXS) to whole Escherichia coli cells is challenging owing to the variety of internal constituents. To resolve their contributions, the outer shape was captured by ultra-small-angle X-ray scattering and combined with the internal structure resolved by SAXS. Building on these data, a model for the major structural components of E. coli was developed. It was possible to deduce information on the occupied volume, occurrence and average size of the most important intracellular constituents: ribosomes, DNA and proteins. E. coli was studied after treatment with three different antibiotic agents (chloramphenicol, tetracycline and rifampicin) and the impact on the intracellular constituents was monitored.

Small angle X-ray scattering as a high-throughput method to classify antimicrobial modes of action

Multi-drug resistant bacteria are currently undermining our health care system worldwide. While novel antimicrobial drugs, such as antimicrobial peptides, are urgently needed, identification of new modes of action is money and time consuming, and in addition current approaches are not available in a high throughput manner. Here we explore how small angle X-ray scattering (SAXS) as high throughput method can contribute to classify the mode of action for novel antimicrobials and therefore supports fast decision making in drug development. Using data bases for natural occurring antimicrobial peptides or predicting novel artificial peptides, many candidates can be discovered that will kill a selected target bacterium. However, in order to narrow down the selection it is important to know if these peptides follow all the same mode of action. In addition, the mode of action should be different from conventional antibiotics, in consequence peptide candidates can be developed further into drugs against multi-drug resistant bacteria. Here we used one short antimicrobial peptide with unknown mode of action and compared the ultrastructural changes of Escherichia coli cells after treatment with the peptide to cells treated with classic antibiotics. The key finding is that SAXS as a structure sensitive tool provides a rapid feedback on drug induced ultrastructural alterations in whole E. coli cells. We could demonstrate that ultrastructural changes depend on the used antibiotics and their specific mode of action. This is demonstrated using several well characterized antimicrobial compounds and the analysis of resulting SAXS curves by principal component analysis. To understand the result of the PCA analysis, the data is correlated with TEM images. In contrast to real space imaging techniques, SAXS allows to obtain nanoscale information averaged over approximately one million cells. The measurement takes only seconds, while conventional tests to identify a mode of action require days or weeks per single substance. The antimicrobial peptide showed a different mode of action as all tested antibiotics including polymyxin B and is therefore a good candidate for further drug development. We envision SAXS to become a useful tool within the high-throughput screening pipeline of modern drug discovery. This article is part of a Special Issue entitled: Antimicrobial peptides edited by Karl Lohner and Kai Hilpert.

Growth phase dependent changes in the structure and protein composition of nucleoid in Escherichia coli

The genomic DNA of bacteria is highly compacted in a single or a few bodies known as nucleoids. Here, we have isolated Escherichia coli nucleoid by sucrose density gradient centrifugation. The sedimentation rates, structures as well as protein/ DNA composition of isolated nucleoids were then compared under various growth phases. The nucleoid structures were found to undergo changes during the cell growth; i. e., the nucleoid structure in the stationary phase was more tightly compacted than that in the exponential phase. In addition to factor for inversion stimulation (Fis), histone-like nucleoid structuring protein (H-NS), heat-unstable nucleoid protein (HU) and integration host factor (IHF) here we have identified, three new candidates of E. coli nucleoid, namely DNA-binding protein from starved cells (Dps), host factor for phage Qβ (Hfq) and suppressor of td(-) phenotype A (StpA). Our results reveal that the major components of exponential phase nucleoid are Fis, HU, H-NS, StpA and Hfq, while Dps occupies more than half of the stationary phase nucleoid. It has been known for a while that Dps is the main nucleoid-associated protein at stationary phase. From these results and the prevailing information, we propose a model for growth phase dependent changes in the structure and protein composition of nucleoid in E. coli.

Atomic force microscopy analysis of the role of major DNA-binding proteins in organization of the nucleoid in Escherichia coli

Bacterial genomic DNA is packed within the nucleoid of the cell along with various proteins and RNAs. We previously showed that the nucleoid in log phase cells consist of fibrous structures with diameters ranging from 30 to 80 nm, and that these structures, upon RNase A treatment, are converted into homogeneous thinner fibers with diameter of 10 nm. In this study, we investigated the role of major DNA-binding proteins in nucleoid organization by analyzing the nucleoid of mutant Escherichia coli strains lacking HU, IHF, H–NS, StpA, Fis, or Hfq using atomic force microscopy. Deletion of particular DNA-binding protein genes altered the nucleoid structure in different ways, but did not release the naked DNA even after the treatment with RNase A. This suggests that major DNA-binding proteins are involved in the formation of higher order structure once 10-nm fiber structure is built up from naked DNA.

Early steps of double-strand break repair in Bacillus subtilis

All organisms rely on integrated networks to repair DNA double-strand breaks (DSBs) in order to preserve the integrity of the genetic information, to re-establish replication, and to ensure proper chromosomal segregation. Genetic, cytological, biochemical and structural approaches have been used to analyze how Bacillus subtilis senses DNA damage and responds to DSBs. RecN, which is among the first responders to DNA DSBs, promotes the ordered recruitment of repair proteins to the site of a lesion. Cells have evolved different mechanisms for efficient end processing to create a 3'-tailed duplex DNA, the substrate for RecA binding, in the repair of one- and two-ended DSBs. Strand continuity is re-established via homologous recombination (HR), utilizing an intact homologous DNA molecule as a template. In the absence of transient diploidy or of HR, however, two-ended DSBs can be directly re-ligated via error-prone non-homologous end-joining. Here we review recent findings that shed light on the early stages of DSB repair in Firmicutes.

Lamin B receptor recognizes specific modifications of histone H4 in heterochromatin formation

Inner nuclear membrane proteins provide a structural framework for chromatin, modulating transcription beneath the nuclear envelope. Lamin B receptor (LBR) is a classical inner nuclear membrane protein that associates with heterochromatin, and its mutations are known to cause Pelger-Huët anomaly in humans. However, the mechanisms by which LBR organizes heterochromatin remain to be elucidated. Here, we show that LBR represses transcription by binding to chromatin regions that are marked by specific histone modifications. The tudor domain (residues 1-62) of LBR primarily recognizes histone H4 lysine 20 dimethylation and is essential for chromatin compaction, whereas the whole nucleoplasmic region (residues 1-211) is required for transcriptional repression. We propose a model in which the nucleoplasmic domain of LBR tethers epigenetically marked chromatin to the nuclear envelope and transcriptional repressors are loaded onto the chromatin through their interaction with LBR.

A model for Escherichia coli chromosome packaging supports transcription factor-induced DNA domain formation

What physical mechanism leads to organization of a highly condensed and confined circular chromosome? Computational modeling shows that confinement-induced organization is able to overcome the chromosome's propensity to mix by the formation of topological domains. The experimentally observed high precision of separate subcellular positioning of loci (located on different chromosomal domains) in Escherichia coli naturally emerges as a result of entropic demixing of such chromosomal loops. We propose one possible mechanism for organizing these domains: regulatory control defined by the underlying E. coli gene regulatory network requires the colocalization of transcription factor genes and target genes. Investigating this assumption, we find the DNA chain to self-organize into several topologically distinguishable domains where the interplay between the entropic repulsion of chromosomal loops and their compression due to the confining geometry induces an effective nucleoid filament-type of structure. Thus, we propose that the physical structure of the chromosome is a direct result of regulatory interactions. To reproduce the observed precise ordering of the chromosome, we estimate that the domain sizes are distributed between 10 and 700 kb, in agreement with the size of topological domains identified in the context of DNA supercoiling.

Binding-activated localization microscopy of DNA structures

Many nucleic acid stains show a strong fluorescence enhancement upon binding to double-stranded DNA. Here we exploit this property to perform superresolution microscopy based on the localization of individual binding events. The dynamic labeling scheme and the optimization of fluorophore brightness yielded a resolution of ∼14 nm (fwhm) and a spatial sampling of 1/nm. We illustrate our approach with two different DNA-binding dyes and apply it to visualize the organization of the bacterial chromosome in fixed Escherichia coli cells. In general, the principle of binding-activated localization microscopy (BALM) can be extended to other dyes and targets such as protein structures.

Adaptation beyond the stress response: cell structure dynamics and population heterogeneity in Staphylococcus aureus

Staphylococcus aureus, a major opportunistic pathogen responsible for a broad spectrum of infections, naturally inhabits the human nasal cavity in about 30% of the population. The unique adaptive potential displayed by S. aureus has made it one of the major causes of nosocomial infections today, emphasized by the rapid emergence of multiple antibiotic-resistant strains over the past few decades. The uncanny ability to adapt to harsh environments is essential for staphylococcal persistence in infections or as a commensal, and a growing body of evidence has revealed critical roles in this process for cellular structural dynamics, and population heterogeneity. These two exciting areas of research are now being explored to identify new molecular mechanisms governing these adaptational strategies.

Mitochondrial DNA transcription regulation and nucleoid organization

Mitochondrial biogenesis is a complex process depending on both nuclear and mitochondrial DNA (mtDNA) transcription regulation to tightly coordinate mitochondrial levels and the cell's energy demand. The energy requirements for a cell to support its metabolic function can change in response to varying physiological conditions, such as, proliferation and differentiation. Therefore, mitochondrial transcription regulation is constantly being modulated in order to establish efficient mitochondrial oxidative metabolism and proper cellular function. The aim of this article is to review the function of major protein factors that are directly involved in the process of mtDNA transcription regulation, as well as, the importance of mitochondrial nucleoid structure and its influence on mtDNA segregation and transcription regulation. Here, we discuss the current knowledge on the molecular mode of action of transcription factors comprising the mitochondrial transcriptional machinery, as well as the action of nuclear receptors on regulatory regions of the mtDNA.

Histone and TK0471/TrmBL2 form a novel heterogeneous genome architecture in the hyperthermophilic archaeon Thermococcus kodakarensis

Being distinct from bacteria and eukaryotes, Archaea constitute a third domain of living things. The DNA replication, transcription, and translation machineries of Archaea are more similar to those of eukaryotes, whereas the genes involved in metabolic processes show more similarity to their bacterial counterparts. We report here that TK0471/TrmB-like 2 (TrmBL2), in addition to histone, is a novel type of abundant chromosomal protein in the model euryarchaeon Thermococcus kodakarensis . The chromosome of T. kodakarensis can be separated into regions enriched either with histone, in which the genetic material takes on a “beads-on-a-string” appearance, or with TK0471/TrmBL2, in which it assumes a thick fibrous structure. TK0471/TrmBL2 binds to both coding and intergenic regions and represses transcription when bound to the promoter region. These results show that the archaeal chromosome is organized into heterogeneous structures and that TK0471/TrmBL2 acts as a general chromosomal protein as well as a global transcriptional repressor.

Unveiling the role of Dps in the organization of mycobacterial nucleoid

In order to preserve genetic information in stress conditions, bacterial DNA is organized into higher order nucleoid structure. In this paper, with the help of Atomic Force Microscopy, we show the different structural changes in mycobacterial nucleoid at different points of growth in the presence of different concentrations of glucose in the medium. We also observe that in Mycobacterium smegmatis, two different Dps proteins (Dps1 and Dps2) promote two types of nucleoid organizations. At the late stationary phase, under low glucose availability, Dps1 binds to DNA to form a very stable toroid structure. On the other hand, under the same condition, Dps2-DNA complex forms an incompletely condensed toroid and finally forms a further stable coral reef structure in the presence of RNA. This coral reef structure is stable in high concentration of bivalent ion like Mg²⁺.

Spiral architecture of the nucleoid in Bdellovibrio bacteriovorus

We present a cryo-electron tomographic analysis of the three-dimensional architecture of a strain of the Gram-negative bacterium Bdellovibrio bacteriovorus in which endogenous MreB2 was replaced with monomeric teal fluorescent protein (mTFP)-labeled MreB2. In contrast to wild-type Bdellovibrio cells that predominantly displayed a compact nucleoid region, cells expressing mTFP-labeled MreB2 displayed a twisted spiral organization of the nucleoid. The more open structure of the MreB2-mTFP nucleoids enabled clear in situ visualization of ribosomes decorating the periphery of the nucleoid. Ribosomes also bordered the edges of more compact nucleoids from both wild-type cells and mutant cells. Surprisingly, MreB2-mTFP localized to the interface between the spiral nucleoid and the cytoplasm, suggesting an intimate connection between nucleoid architecture and MreB arrangement. Further, in contrast to wild-type cells, where a single tight chemoreceptor cluster localizes close to the single polar flagellum, MreB2-mTFP cells often displayed extended chemoreceptor arrays present at one or both poles and displayed multiple or inaccurately positioned flagella. Our findings provide direct structural evidence for spiral organization of the bacterial nucleoid and suggest a possible role for MreB in regulation of nucleoid architecture and localization of the chemotaxis apparatus.