Polar growth in the Alphaproteobacterial order Rhizobiales

The Sinorhizobium meliloti sensor histidine kinase CbrA contributes to free-living cell cycle regulation

Sinorhizobium meliloti is alternately capable of colonizing the soil as a free-living bacterium or establishing a chronic intracellular infection with its legume host for the purpose of nitrogen fixation. We previously identified the S. meliloti two-component sensor histidine kinase CbrA as playing an important role in regulating exopolysaccharide production, flagellar motility and symbiosis. Phylogenetic analysis of CbrA has highlighted its evolutionary relatedness to the Caulobacter crescentus sensor histidine kinases PleC and DivJ, which are involved in CtrA-dependent cell cycle regulation through the shared response regulator DivK. We therefore became interested in testing whether CbrA plays a role in regulating S. meliloti cell cycle processes. We find the loss of cbrA results in filamentous cell growth accompanied by cells that contain an aberrant genome complement, indicating CbrA plays a role in regulating cell division and possibly DNA segregation. S. meliloti DivK localizes to the old cell pole during distinct phases of the cell cycle in a phosphorylation-dependent manner. Loss of cbrA results in a significantly decreased rate of DivK polar localization when compared with the wild-type, suggesting CbrA helps regulate cell cycle processes by modulating DivK phosphorylation status as a kinase. Consistent with a presumptive decrease in DivK phosphorylation and activity, we also find the steady-state level of CtrA increased in cbrA mutants. Our data therefore demonstrate that CbrA contributes to free-living cell cycle regulation, which in light of its requirement for symbiosis, points to the potential importance of cell cycle regulation for establishing an effective host interaction.

Dynamic FtsA and FtsZ localization and outer membrane alterations during polar growth and cell division in Agrobacterium tumefaciens

Growth and cell division in rod-shaped bacteria have been primarily studied in species that grow predominantly by peptidoglycan (PG) synthesis along the length of the cell. Rhizobiales species, however, predominantly grow by PG synthesis at a single pole. Here we characterize the dynamic localization of several Agrobacterium tumefaciens components during the cell cycle. First, the lipophilic dye FM 4-64 predominantly stains the outer membranes of old poles versus growing poles. In cells about to divide, however, both poles are equally labeled with FM 4-64, but the constriction site is not. Second, the cell-division protein FtsA alternates from unipolar foci in the shortest cells to unipolar and midcell localization in cells of intermediate length, to strictly midcell localization in the longest cells undergoing septation. Third, the cell division protein FtsZ localizes in a cell-cycle pattern similar to, but more complex than, FtsA. Finally, because PG synthesis is spatially and temporally regulated during the cell cycle, we treated cells with sublethal concentrations of carbenicillin (Cb) to assess the role of penicillin-binding proteins in growth and cell division. Cb-treated cells formed midcell circumferential bulges, suggesting that interrupted PG synthesis destabilizes the septum. Midcell bulges contained bands or foci of FtsA-GFP and FtsZ-GFP and no FM 4-64 label, as in untreated cells. There were no abnormal morphologies at the growth poles in Cb-treated cells, suggesting unipolar growth uses Cb-insensitive PG synthesis enzymes.

Spatial control of the cell division site by the Min system in Escherichia coli

The Min system of Escherichia coli is involved in mediating placement of the cell division site at the midcell; this is accomplished through partitioning of the cell division inhibitor MinC to the cell poles to block aberrant polar division. The partitioning of MinC is achieved through its interaction with MinDE, which alternates its cellular distribution periodically between opposite cell poles throughout the cell cycle. This dynamic oscillation is the result of intricate molecular interactions occurring between the three Min proteins on the membrane in a spatiotemporal manner. In this minireview, we discuss recent developments in understanding the molecular mechanisms of the E. coli Min system from cellular, biochemical and biophysical perspectives. In addition, we propose a model that involves the balancing of different molecular interactions at different stages of the oscillation cycle.

Genome of the R-body producing marine alphaproteobacterium Labrenzia alexandrii type strain (DFL-11(T))

Labrenzia alexandrii Biebl et al. 2007 is a marine member of the family Rhodobacteraceae in the order Rhodobacterales, which has thus far only partially been characterized at the genome level. The bacterium is of interest because it lives in close association with the toxic dinoflagellate Alexandrium lusitanicum. Ultrastructural analysis reveals R-bodies within the bacterial cells, which are primarily known from obligate endosymbionts that trigger "killing traits" in ciliates (Paramecium spp.). Genomic traits of L. alexandrii DFL-11(T) are in accordance with these findings, as they include the reb genes putatively involved in R-body synthesis. Analysis of the two extrachromosomal elements suggests a role in heavy-metal resistance and exopolysaccharide formation, respectively. The 5,461,856 bp long genome with its 5,071 protein-coding and 73 RNA genes consists of one chromosome and two plasmids, and has been sequenced in the context of the Marine Microbial Initiative.

Coordination of division and development influences complex multicellular behavior in Agrobacterium tumefaciens

The α-Proteobacterium Agrobacterium tumefaciens has proteins homologous to known regulators that govern cell division and development in Caulobacter crescentus, many of which are also conserved among diverse α-Proteobacteria. In light of recent work demonstrating similarity between the division cycle of C. crescentus and that of A. tumefaciens, the functional conservation for this presumptive control pathway was examined. In C. crescentus the CtrA response regulator serves as the master regulator of cell cycle progression and cell division. CtrA activity is controlled by an integrated pair of multi-component phosphorelays: PleC/DivJ-DivK and CckA-ChpT-CtrA. Although several of the conserved orthologues appear to be essential in A. tumefaciens, deletions in pleC or divK were isolated and resulted in cell division defects, diminished swimming motility, and a decrease in biofilm formation. A. tumefaciens also has two additional pleC/divJhomologue sensor kinases called pdhS1 and pdhS2, absent in C. crescentus. Deletion of pdhS1 phenocopied the ΔpleC and ΔdivK mutants. Cells lacking pdhS2 morphologically resembled wild-type bacteria, but were decreased in swimming motility and elevated for biofilm formation, suggesting that pdhS2 may serve to regulate the motile to non-motile switch in A. tumefaciens. Genetic analysis suggests that the PleC/DivJ-DivK and CckA-ChpT-CtrA phosphorelays in A. tumefaciens are vertically-integrated, as in C. crescentus. A gain-of-function mutation in CckA (Y674D) was identified as a spontaneous suppressor of the ΔpleC motility phenotype. Thus, although the core architecture of the A. tumefaciens pathway resembles that of C. crescentus there are specific differences including additional regulators, divergent pathway architecture, and distinct target functions.

BtaE, an adhesin that belongs to the trimeric autotransporter family, is required for full virulence and defines a specific adhesive pole of Brucella suis

Brucella is responsible for brucellosis, one of the most common zoonoses worldwide that causes important economic losses in several countries. Increasing evidence indicates that adhesion of Brucella spp. to host cells is an important step to establish infection. We have previously shown that the BmaC unipolar monomeric autotransporter mediates the binding of Brucella suis to host cells through cell-associated fibronectin. Our genome analysis shows that the B. suis genome encodes several additional potential adhesins. In this work, we characterized a predicted trimeric autotransporter that we named BtaE. By expressing btaE in a nonadherent Escherichia coli strain and by phenotypic characterization of a B. suis ΔbtaE mutant, we showed that BtaE is involved in the binding of B. suis to hyaluronic acid. The B. suis ΔbtaE mutant exhibited a reduction in the adhesion to HeLa and A549 epithelial cells compared with the wild-type strain, and it was outcompeted by the wild-type strain in the binding to HeLa cells. The knockout btaE mutant showed an attenuated phenotype in the mouse model, indicating that BtaE is required for full virulence. BtaE was immunodetected on the bacterial surface at one cell pole. Using old and new pole markers, we observed that both the BmaC and BtaE adhesins are consistently associated with the new cell pole, suggesting that, in Brucella, the new pole is functionally differentiated for adhesion. This is consistent with the inherent polarization of this bacterium, and its role in the invasion process.

The physiology of bacterial cell division

Bacterial cell division is facilitated by the divisome, a dynamic multiprotein assembly localizing at mid-cell to synthesize the stress-bearing peptidoglycan and to constrict all cell envelope layers. Divisome assembly occurs in two steps and involves multiple interactions between more than 20 essential and accessory cell division proteins. Well before constriction and while the cell is still elongating, the tubulin-like FtsZ and early cell division proteins form a ring-like structure at mid-cell. Cell division starts once certain peptidoglycan enzymes and their activators have moved to the FtsZ-ring. Gram-negative bacteria like Escherichia coli simultaneously synthesize and cleave the septum peptidoglycan during division leading to a constriction. The outer membrane constricts together with the peptidoglycan layer with the help of the transenvelope spanning Tol-Pal system.

Regulation of apical growth and hyphal branching in Streptomyces

The filamentous bacteria Streptomyces grow by tip extension and through the initiation of new branches, and this apical growth is directed by a polarisome-like complex involving the essential polarity protein DivIVA. New branch sites must be marked de novo and, until recently, there was no understanding of how these new sites are selected. Equally, hyphal branching patterns are affected by environmental conditions, but there was no insight into how polar growth and hyphal branching might be regulated in response to external or internal cues. This review focuses on recent discoveries that reveal the principal mechanism of branch site selection in Streptomyces, and the first mechanism to be identified that regulates polarisome behaviour to modulate polar growth and hyphal branching.

In Situ probing of newly synthesized peptidoglycan in live bacteria with fluorescent D-amino acids

Tracking a bug's life: Peptidoglycan (PG) of diverse bacteria is labeled by exploiting the tolerance of cells for incorporating different non-natural D-amino acids. These nontoxic D-amino acids preferably label the sites of active PG synthesis, thereby enabling fine spatiotemporal tracking of cell-wall dynamics in phylogenetically and morphologically diverse bacteria. HCC = 7-hydroxycoumarin, NBD = 7-nitrobenzofurazan, TAMRA = carboxytetramethylrhodamine.

The Ser/Thr protein kinase AfsK regulates polar growth and hyphal branching in the filamentous bacteria Streptomyces

In cells that exhibit apical growth, mechanisms that regulate cell polarity are crucial for determination of cellular shape and for the adaptation of growth to intrinsic and extrinsic cues. Broadly conserved pathways control cell polarity in eukaryotes, but less is known about polarly growing prokaryotes. An evolutionarily ancient form of apical growth is found in the filamentous bacteria Streptomyces, and is directed by a polarisome-like complex involving the essential protein DivIVA. We report here that this bacterial polarization machinery is regulated by a eukaryotic-type Ser/Thr protein kinase, AfsK, which localizes to hyphal tips and phosphorylates DivIVA. During normal growth, AfsK regulates hyphal branching by modulating branch-site selection and some aspect of the underlying polarisome-splitting mechanism that controls branching of Streptomyces hyphae. Further, AfsK is activated by signals generated by the arrest of cell wall synthesis and directly communicates this to the polarisome by hyperphosphorylating DivIVA. Induction of high levels of DivIVA phosphorylation by using a constitutively active mutant AfsK causes disassembly of apical polarisomes, followed by establishment of multiple hyphal branches elsewhere in the cell, revealing a profound impact of this kinase on growth polarity. The function of AfsK is reminiscent of the phoshorylation of polarity proteins and polarisome components by Ser/Thr protein kinases in eukaryotes.

Quantitative image analysis and modeling indicate the Agrobacterium tumefaciens type IV secretion system is organized in a periodic pattern of foci

The Gram negative plant pathogen Agrobacterium tumefaciens is uniquely capable of genetically transforming eukaryotic host cells during the infection process. DNA and protein substrates are transferred into plant cells via a type IV secretion system (T4SS), which forms large cell-envelope spanning complexes at multiple sites around the bacterial circumference. To gain a detailed understanding of T4SS positioning, the spatial distribution of fluorescently labeled T4SS components was quantitatively assessed to distinguish between random and structured localization processes. Through deconvolution microscopy followed by Fourier analysis and modeling, T4SS foci were found to localize in a non-random periodic pattern. These results indicate that T4SS complexes are dependent on an underlying scaffold or assembly process to obtain an organized distribution suitable for effective delivery of substrates into host cells.

The histidine kinase PdhS controls cell cycle progression of the pathogenic alphaproteobacterium Brucella abortus

Bacterial differentiation is often associated with the asymmetric localization of regulatory proteins, such as histidine kinases. PdhS is an essential and polarly localized histidine kinase in the pathogenic alphaproteobacterium Brucella abortus. After cell division, PdhS is asymmetrically segregated between the two sibling cells, highlighting a differentiation event. However, the function(s) of PdhS in the B. abortus cell cycle remains unknown. We used an original approach, the pentapeptide scanning mutagenesis method, to generate a thermosensitive allele of pdhS. We report that a B. abortus strain carrying this pdhS allele displays growth arrest and an altered DivK-yellow fluorescent protein (YFP) polar localization at the restrictive temperature. Moreover, the production of a nonphosphorylatable PdhS protein or truncated PdhS proteins leads to dominant-negative effects by generating morphological defects consistent with the inhibition of cell division. In addition, we have used a domain mapping approach combined with yeast two-hybrid and fluorescence microscopy methods to better characterize the unusual PdhS sensory domain. We have identified a fragment of the PdhS sensory domain required for protein-protein interaction (amino acids [aa] 210 to 434), a fragment sufficient for polar localization (aa 1 to 434), and a fragment (aa 527 to 661) whose production in B. abortus correlates with the generation of cell shape alterations. The data support a model in which PdhS acts as an essential regulator of cell cycle progression in B. abortus and contribute to a better understanding of the differentiation program inherited by the two sibling cells.

The conserved polarity factor podJ1 impacts multiple cell envelope-associated functions in Sinorhizobium meliloti

Although diminutive in size, bacteria possess highly diverse and spatially confined cellular structures. Two related alphaproteobacteria, Sinorhizobium meliloti and Caulobacter crescentus, serve as models for investigating the genetic basis of morphological variations. S. meliloti, a symbiont of leguminous plants, synthesizes multiple flagella and no prosthecae, whereas C. crescentus, a freshwater bacterium, has a single polar flagellum and stalk. The podJ gene, originally identified in C. crescentus for its role in polar organelle development, is split into two adjacent open reading frames, podJ1 and podJ2, in S. meliloti. Deletion of podJ1 interferes with flagellar motility, exopolysaccharide production, cell envelope integrity, cell division and normal morphology, but not symbiosis. As in C. crescentus, the S. meliloti PodJ1 protein appears to act as a polarity beacon and localizes to the newer cell pole. Microarray analysis indicates that podJ1 affects the expression of at least 129 genes, the majority of which correspond to observed mutant phenotypes. Together, phenotypic characterization, microarray analysis and suppressor identification suggest that PodJ1 controls a core set of conserved elements, including flagellar and pili genes, the signalling proteins PleC and DivK, and the transcriptional activator TacA, while alternative downstream targets have evolved to suit the distinct lifestyles of individual species.

The scaffolding and signalling functions of a localization factor impact polar development

In the differentiating alphaproteobacterium Caulobacter crescentus, organelle synthesis at cell poles is critical to forming different progeny after cell division. Co-ordination of polar organelle synthesis, including pili and holdfast, and flagellum ejection, is mediated in part by the scaffolding protein PodJ. At the time of cell division, PodJ undergoes regulated processing to a short form that persists at the flagellar pole of swarmer cells. This study analyses how PodJ's role in structural and signalling protein localization impacts organelle synthesis. A PodJ mutant with an internal deletion exhibits reduced sensitivity to pili-tropic phage ΦCbK, resulting from reduced pilA gene expression, which can be linked to altered signalling protein localization. The phage sensitivity defect of a ΔpodJ mutant can be partially suppressed by ectopic pilA expression. Induction of PodJ processing, by manipulation of podJ itself or controlled perP expression, resulted in decreased pilus biogenesis and, when coupled with a podJ mutation that reduced pilA expression, led to complete loss of phage sensitivity. As a whole, the results show that PodJ's scaffolding role for structural and signalling proteins both contribute to flagellar pole organelle development.

Peptidoglycan plasticity in bacteria: stress-induced peptidoglycan editing by noncanonical D-amino acids

It has been generally assumed that the role of D-amino acids in bacterial physiology is rather limited. However, recent new evidence demonstrated that millimolar concentrations of noncanonical D-amino acids are synthesized and released to the environment by bacteria from diverse phyla. These D-amino acids help bacteria adapt to environmental challenges by modulating the structure and composition of the peptidoglycan (PG). This regulation, which appears to be well conserved among bacterial species, occurs principally through the incorporation of the D-amino acids into the terminus of the peptide moiety of muropeptides. These findings revived interest in studies investigating D-amino acids as an exciting and trendy topic in current microbiology, which considers them as fundamental players in different aspects of bacterial physiology. In this article, we provide an overview of the origins of research on the effects of D-amino acids in the biology of bacterial cell walls, including their recent implication as key factors for stress-associated PG remodeling.

A new slant to the Z ring and bacterial cell branch formation

Rod-shaped bacteria such as Escherichia coli accurately maintain their shape from generation to generation. The cytoskeletal proteins MreB and FtsZ, which respectively guide parallel growth of the sidewall and perpendicular growth of the division septum, are important to maintain a straight sidewall and uniformly rounded cell poles. FtsZ normally assembles into a ring at the cell midpoint, called the Z ring, which is oriented perpendicular to the cell's axis and is thus in perfect position to guide growth of a perpendicular septum. In this issue of Molecular Microbiology, Potluri et al. show that low molecular weight penicillin binding proteins, particularly PBP5, have a role in maintaining the perpendicular geometry of the Z ring and subsequent septum in E. coli. When these factors are absent or perturbed, division septa are readily deformed, which results in abnormal cell poles that often bifurcate over time to generate branches. The data suggest that cellular branching in E. coli is specifically induced by aberrant septation events caused by mis-oriented Z rings and not by deformation of a growing cell pole or emergence of new tips from the sidewall, which are likely mechanisms of branching in other bacterial families.