A dynamically localized histidine kinase controls the asymmetric distribution of polar pili proteins

Polar localization of the CckA histidine kinase and cell cycle periodicity of the essential master regulator CtrA in Caulobacter crescentus

The phosphorylated form of the response regulator CtrA represses DNA replication initiation and regulates the transcription of about 100 cell cycle-regulated genes in Caulobacter crescentus. CtrA activity fluctuates during the cell cycle, and its periodicity is a key element of the engine that drives cell cycle progression. The histidine kinase CckA controls the phosphorylation not only of CtrA but also of CpdR, whose unphosphorylated form promotes CtrA proteolysis. Thus, CckA has a central role in establishing the cell cycle periodicity of CtrA activity by controlling both its phosphorylation and stability. Evidence suggests that the polar localization of CckA during the cell cycle plays a role in CckA function. However, the exact pattern of CckA localization remains controversial. Here, we describe a thorough, quantitative analysis of the spatiotemporal distribution of a functional and chromosomally produced CckA-monomeric green fluorescent protein fusion that affects current models of cell cycle regulation. We also identify two cis-acting regions in CckA that are important for its proper localization and function. The disruption of a PAS-like motif in the sensor domain affects the stability of CckA accumulation at the poles. This is accompanied by a partial loss in CckA function. Shortening an extended linker between beta-sheets within the CckA catalysis-assisting ATP-binding domain has a more severe effect on CckA polar localization and function. This mutant strain exhibits a dramatic cell-to-cell variability in CpdR levels and CtrA cell cycle periodicity, suggesting that the cell cycle-coordinated polar localization of CckA may be important for the robustness of signal transduction and cell cycle progression.

Temporal controls of the asymmetric cell division cycle in Caulobacter crescentus

The asymmetric cell division cycle of Caulobacter crescentus is orchestrated by an elaborate gene-protein regulatory network, centered on three major control proteins, DnaA, GcrA and CtrA. The regulatory network is cast into a quantitative computational model to investigate in a systematic fashion how these three proteins control the relevant genetic, biochemical and physiological properties of proliferating bacteria. Different controls for both swarmer and stalked cell cycles are represented in the mathematical scheme. The model is validated against observed phenotypes of wild-type cells and relevant mutants, and it predicts the phenotypes of novel mutants and of known mutants under novel experimental conditions. Because the cell cycle control proteins of Caulobacter are conserved across many species of alpha-proteobacteria, the model we are proposing here may be applicable to other genera of importance to agriculture and medicine (e.g., Rhizobium, Brucella).

Organization and PprB-dependent control of the Pseudomonas aeruginosa tad Locus, involved in Flp pilus biology

Bacterial attachment to the substratum involves several cell surface organelles, including various types of pili. The Pseudomonas aeruginosa Tad machine assembles type IVb pili, which are required for adhesion to abiotic surfaces and to eukaryotic cells. Type IVb pili consist of a major subunit, the Flp pilin, processed by the FppA prepilin peptidase. In this study, we investigated the regulatory mechanism of the tad locus. We showed that the flp gene is expressed late in the stationary growth phase in aerobic conditions. We also showed that the tad locus was composed of five independent transcriptional units. We used transcriptional fusions to show that tad gene expression was positively controlled by the PprB response regulator. We subsequently showed that PprB bound to the promoter regions, directly controlling the expression of these genes. We then evaluated the contribution of two genes, tadF and rcpC, to type IVb pilus assembly. The deletion of these two genes had no effect on Flp production, pilus assembly, or Flp-mediated adhesion to abiotic surfaces in our conditions. However, our results suggest that the putative RcpC protein modifies the Flp pilin, thereby promoting Flp-dependent adhesion to eukaryotic cells.

Complex regulatory pathways coordinate cell-cycle progression and development in Caulobacter crescentus

Caulobacter crescentus has become the predominant bacterial model system to study the regulation of cell-cycle progression. Stage-specific processes such as chromosome replication and segregation, and cell division are coordinated with the development of four polar structures: the flagellum, pili, stalk, and holdfast. The production, activation, localization, and proteolysis of specific regulatory proteins at precise times during the cell cycle culminate in the ability of the cell to produce two physiologically distinct daughter cells. We examine the recent advances that have enhanced our understanding of the mechanisms of temporal and spatial regulation that occur during cell-cycle progression.

Two-component signaling systems and cell cycle control in Caulobacter crescentus

Recent work on the regulation ofprokaryotic growth and development by two-component systems (TCS) has revealed unsuspected levels of complexity. In the dimorphic freshwater bacterium Caulobacter crescentus, TCS provide stringent temporal and spatial control of cellular development and cell-cycle progression. While the environmental signals modulating TCS regulatory networks are largely unknown, the components of the network and their interactions with each other are increasingly well-defined. Here, we present an overview of TCS regulation of cell-cycle control in C. crescentus.

SpoT regulates DnaA stability and initiation of DNA replication in carbon-starved Caulobacter crescentus

Cell cycle progression and polar differentiation are temporally coordinated in Caulobacter crescentus. This oligotrophic bacterium divides asymmetrically to produce a motile swarmer cell that represses DNA replication and a sessile stalked cell that replicates its DNA. The initiation of DNA replication coincides with the proteolysis of the CtrA replication inhibitor and the accumulation of DnaA, the replication initiator, upon differentiation of the swarmer cell into a stalked cell. We analyzed the adaptive response of C. crescentus swarmer cells to carbon starvation and found that there was a block in both the swarmer-to-stalked cell polar differentiation program and the initiation of DNA replication. SpoT is a bifunctional synthase/hydrolase that controls the steady-state level of the stress-signaling nucleotide (p)ppGpp, and carbon starvation caused a SpoT-dependent increase in (p)ppGpp concentration. Carbon starvation activates DnaA proteolysis (B. Gorbatyuk and G. T. Marczynski, Mol. Microbiol. 55:1233-1245, 2005). We observed that SpoT is required for this phenomenon in swarmer cells, and in the absence of SpoT, carbon-starved swarmer cells inappropriately initiated DNA replication. Since SpoT controls (p)ppGpp abundance, we propose that this nucleotide relays carbon starvation signals to the cellular factors responsible for activating DnaA proteolysis, thereby inhibiting the initiation of DNA replication. SpoT, however, was not required for the carbon starvation block of the swarmer-to-stalked cell polar differentiation program. Thus, swarmer cells utilize at least two independent signaling pathways to relay carbon starvation signals: a SpoT-dependent pathway mediating the inhibition of DNA replication initiation, and a SpoT-independent pathway(s) that blocks morphological differentiation.

The dynamic interplay between a cell fate determinant and a lysozyme homolog drives the asymmetric division cycle of Caulobacter crescentus

Caulobacter crescentus divides asymmetrically into a swarmer cell and a stalked cell, a process that is governed by the imbalance in phosphorylated levels of the DivK cell fate determinant in the two cellular compartments. The asymmetric polar localization of the DivJ kinase results in its specific inheritance in the stalked daughter cell where it phosphorylates DivK. The mechanism for the polar positioning of DivJ is poorly understood. SpmX, an uncharacterized lysozyme homolog, is shown here to control DivJ localization and activation. In the absence of SpmX, DivJ is delocalized and dysfunctional, resulting in developmental defects caused by an insufficiency in phospho-DivK. While SpmX stimulates DivK phosphorylation in the stalked cell, unphosphorylated DivK in the swarmer cell activates an intricate transcriptional cascade that leads to the production of the spmX message. This event primes the swarmer cell for the impending transition into a stalked cell, a transition that is sparked by the abrupt accumulation and localization of SpmX to the future stalked cell pole. Localized SpmX then recruits and stimulates DivJ, and the resulting phospho-DivK implements the stalked cell fate. The dynamic interplay between SpmX and DivK is at the heart of the molecular circuitry that sustains the Caulobacter developmental cycle.

DivL performs critical cell cycle functions in Caulobacter crescentus independent of kinase activity

The Caulobacter cell cycle is regulated by a network of two-component signal transduction proteins. Phosphorylation and stability of the master transcriptional regulator CtrA are controlled by the CckA-ChpT phosphorelay, and CckA activity is modulated by another response regulator, DivK. In a screen to identify suppressors of the cold-sensitive divK341 mutant, we found point mutations in the essential gene divL. DivL is similar to histidine kinases but has a tyrosine instead of a histidine at the conserved phosphorylation site (Y550). Surprisingly, we found that the ATPase domain of DivL is not essential for Caulobacter viability. We show that DivL selectively affects CtrA phosphorylation but not CtrA proteolysis, indicating that DivL acts in a pathway independent of the CckA-ChpT phosphorelay. divL can be deleted in a strain overproducing the phosphomimetic protein CtrAD51E, but unlike DeltactrA cells expressing CtrAD51E, this strain is profoundly impaired in the control of chromosome replication and cell division. Thus, DivL performs a second function in addition to promoting CtrA phosphorylation. DivL is required for bipolar DivK localization and positively regulates DivK phosphorylation. Our results show that DivL controls two key cell cycle regulators, CtrA and DivK, and that phosphoryl transfer is not DivL's essential cellular activity.

Spatial complexity and control of a bacterial cell cycle

A major breakthrough in understanding the bacterial cell cycle is the discovery that bacteria exhibit a high degree of intracellular organization. Chromosomal loci and many protein complexes are positioned at particular subcellular sites. In this review, we examine recently discovered control mechanisms that make use of dynamically localized protein complexes to orchestrate the Caulobacter crescentus cell cycle. Protein localization, notably of signal transduction proteins, chromosome partition proteins, and proteases, serves to coordinate cell division with chromosome replication and cell differentiation. The developmental fate of daughter cells is decided before completion of cytokinesis, via the early establishment of cell polarity by the distribution of activated signaling proteins, bacterial cytoskeleton, and landmark proteins.

The tad locus: postcards from the widespread colonization island

The Tad (tight adherence) macromolecular transport system, which is present in many bacterial and archaeal species, represents an ancient and major new subtype of type II secretion. The tad genes are present on a genomic island named the widespread colonization island (WCI), and encode the machinery that is required for the assembly of adhesive Flp (fimbrial low-molecular-weight protein) pili. The tad genes are essential for biofilm formation, colonization and pathogenesis in the genera Aggregatibacter (Actinobacillus), Haemophilus, Pasteurella, Pseudomonas, Yersinia, Caulobacter and perhaps others. Here we review the structure, function and evolution of the Tad secretion system.

Putative type IV secretion genes in Bacillus anthracis

Although the physiology of Bacillus anthracis, the causative agent of anthrax, has been studied extensively, we still do not know how toxins are dispatched from the bacterial cell. Here, by means of distant homology and genome context analyses, we identify genes encoding putative type IV secretion system-related elements on the B. anthracis plasmids pXO1 and pXO2 and in the chromosome. We argue that this type IV secretion system-like system could be responsible for anthrax toxin secretion, although we also discuss the possibilities of its involvement in the processes of sporulation, germination or conjugation.

The symbiosis regulator CbrA modulates a complex regulatory network affecting the flagellar apparatus and cell envelope proteins

Sinorhizobium meliloti participates in a nitrogen-fixing symbiosis with legume plant host species of the genera Medicago, Melilotus, and Trigonella. We recently identified an S. meliloti two-component sensory histidine kinase, CbrA, which is absolutely required to establish a successful symbiosis with Medicago sativa (K. E. Gibson, G. R. Campbell, J. Lloret, and G. C. Walker, J. Bacteriol. 188:4508-4521, 2006). In addition to having a symbiotic defect, the cbrA::Tn5 mutant also has free-living phenotypes that suggest a cell envelope perturbation. Because the bases for these phenotypes are not well understood, we undertook an identification of CbrA-regulated genes. We performed a microarray analysis and compared the transcriptome of the cbrA::Tn5 mutant to that of the wild type. Our global analysis of gene expression identified 162 genes that are differentially expressed in the cbrA::Tn5 mutant, including those encoding proteins involved in motility and chemotaxis, metabolism, and cell envelope function. With regard to those genes with a known role in symbiosis, we observed increased expression of nine genes with overlapping functions in bacterial invasion of its host, which suggests that the mutant could be competent for invasion. Since these CbrA-repressed genes are vital to the invasion process, it appears that down-regulation of CbrA activity is important at this stage of nodule development. In contrast, our previous work showed that CbrA is required for bacteria to establish themselves within the host as nitrogen-fixing symbionts. Therefore, we propose a model in which CbrA functions as a developmental switch during symbiosis.

Holdfast formation in motile swarmer cells optimizes surface attachment during Caulobacter crescentus development

The adhesive holdfast is required for irreversible surface anchoring of Caulobacter crescentus cells. The holdfast is synthesized early during swarmer cell development and, together with pili and a functional flagellum, contributes to optimal attachment during cell differentiation. We present evidence that the timing of holdfast formation in swarmer cells is regulated posttranslationally and is dependent on the diguanylate cyclase PleD.

A phosphorelay system controls stalk biogenesis during cell cycle progression in Caulobacter crescentus

A fundamental question in developmental biology is how morphogenesis is coordinated with cell cycle progression. In Caulobacter crescentus, each cell cycle produces morphologically distinct daughter cells, a stalked cell and a flagellated swarmer cell. Construction of both the flagellum and stalk requires the alternative sigma factor RpoN (sigma(54)). Here we report that a sigma(54)-dependent activator, TacA, is required for cell cycle regulated stalk biogenesis by collaborating with RpoN to activate gene expression. We have also identified the first histidine phosphotransferase in C. crescentus, ShpA, and show that it too is required for stalk biogenesis. Using a systematic biochemical technique called phosphotransfer profiling we have identified a multicomponent phosphorelay which leads from the hybrid histidine kinase ShkA to ShpA and finally to TacA. This pathway functions in vivo to phosphorylate and hence, activate TacA. Finally, whole genome microarrays were used to identify candidate members of the TacA regulon, and we show that at least one target gene, staR, regulates stalk length. This is the first example of a general method for identifying the connectivity of a phosphorelay and can be applied to any organism with two-component signal transduction systems.

A Landmark Protein Essential for Establishing and Perpetuating the Polarity of a Bacterial Cell

Polarity is often an intrinsic property of the cell, yet little is known about its origin or its maintenance over generations. Here we identify a landmark protein, TipN, which acts as a spatial and temporal cue for setting up the correct polarity in the bacterium Caulobacter crescentus. TipN marks the new pole throughout most of the cell cycle, and its relocation to the nascent poles at the end of division provides a preexisting reference point for orienting the polarity axis in the progeny. Deletion of tipN causes pleiotropic polarity defects, including frequently reversed asymmetry in progeny size and mislocalization of proteins and organelles. Ectopic localization of TipN along the lateral side of the cell creates new axes of polarity leading to cell branching and formation of competent cell poles. Localization defects of the actin-like protein MreB in the DeltatipN mutant suggest that TipN is upstream of MreB in regulating cell polarity.

Bacterial Birth Scar Proteins Mark Future Flagellum Assembly Site

Many prokaryotic protein complexes underlie polar asymmetry. In Caulobacter crescentus, a flagellum is built exclusively at the pole that arose from the previous cell division. The basis for this pole specificity is unclear but could involve a cytokinetic birth scar that marks the newborn pole as the flagellum assembly site. We identified two developmental proteins, TipN and TipF, which localize to the division septum and the newborn pole after division. We show that septal localization of TipN/F depends on cytokinesis. Moreover, TipF, a c-di-GMP phosphodiesterase homolog, is a flagellum assembly factor that relies on TipN for proper positioning. In the absence of TipN, flagella are assembled at ectopic locations, and TipF is mislocalized to such sites. Thus TipN and TipF establish a link between bacterial cytokinesis and polar asymmetry, demonstrating that division does indeed leave a positional mark in its wake to direct the biogenesis of a polar organelle.

Cytokinesis signals truncation of the PodJ polarity factor by a cell cycle-regulated protease

We demonstrate that successive cleavage events involving regulated intramembrane proteolysis (Rip) occur as a function of time during the Caulobacter cell cycle. The proteolytic substrate PodJ(L) is a polar factor that recruits proteins required for polar organelle biogenesis to the correct cell pole at a defined time in the cell cycle. We have identified a periplasmic protease (PerP) that initiates the proteolytic sequence by truncating PodJ(L) to a form with altered activity (PodJ(S)). Expression of perP is regulated by a signal transduction system that activates cell type-specific transcription programs and conversion of PodJ(L) to PodJ(S) in response to the completion of cytokinesis. PodJ(S), sequestered to the progeny swarmer cell, is subsequently released from the polar membrane by the membrane metalloprotease MmpA for degradation during the swarmer-to-stalked cell transition. This sequence of proteolytic events contributes to the asymmetric localization of PodJ isoforms to the appropriate cell pole. Thus, temporal activation of the PerP protease and spatial restriction of the polar PodJ(L) substrate cooperatively control the cell cycle-dependent onset of Rip.

Dissection of functional domains of the polar localization factor PodJ in Caulobacter crescentus

The polar organelle development protein, PodJ, is important for proper establishment of polarity in Caulobacter crescentus. podJ null mutants are unable to form holdfast or pili, have reduced swarming motility, and have difficulty ejecting the flagellum during the swarmer to stalked cell transition. In this study, we create a series of truncation mutants to investigate functional domains of PodJ. We show that PodJ has a transmembrane domain between amino acids 600 and 670. We identify a periplasmic region important for pili production and a cytoplasmic region required for holdfast formation and swarming motility, and establish that PleC localization is not required for holdfast formation and motility in soft agar. Analysis of the mutants reveals that the last 54 amino acids of the protein negatively regulate processing of the full-length form of the protein, PodJ(L), to a shorter form, PodJ(S). Finally, we identify a cytoplasmic region of PodJ involved in targeting it to the flagellar pole, and a periplasmic region required for localization of PleC.

The role of polar localization in the function of an essential Caulobacter crescentus tyrosine kinase

DivL is an essential tyrosine kinase in Caulobacter crescentus that controls an early step in the cell division cycle. We show here that DivL dynamically localizes to the stalk-distal cell pole and less frequently to the stalked cell pole during the S-phase. The kinase is subsequently released from the cell poles late in division and remains dispersed in the newly divided progeny stalk and swarmer cells. Mutational analysis of DivL in a DivL-GFP fusion protein demonstrated that the extreme C-terminus and residues in the conserved four-helix bundle, which is the phosphorylation-dimerization domain, are important for localization. We speculate that the four-helix bundle of the core catalytic domain may serve as a recognition site for the "localization machinery". Unexpectedly, a DivL protein with mutations in the C-terminal localization sequence, and an intact catalytic domain, efficiently complemented a divL null mutation. Thus, subcellular localization of DivL is not essential to its function in cell division regulation. Regulation of cell division by DivL does, however, depend on its localization in the cell membrane.

MreB Actin-Mediated Segregation of a Specific Region of a Bacterial Chromosome

Faithful chromosome segregation is an essential component of cell division in all organisms. The eukaryotic mitotic machinery uses the cytoskeleton to move specific chromosomal regions. To investigate the potential role of the actin-like MreB protein in bacterial chromosome segregation, we first demonstrate that MreB is the direct target of the small molecule A22. We then demonstrate that A22 completely blocks the movement of newly replicated loci near the origin of replication but has no qualitative or quantitative effect on the segregation of other loci if added after origin segregation. MreB selectively interacts, directly or indirectly, with origin-proximal regions of the chromosome, arguing that the origin-proximal region segregates via an MreB-dependent mechanism not used by the rest of the chromosome.

Spatial complexity of mechanisms controlling a bacterial cell cycle

Cell cycle progression in Caulobacter is governed by a multilayered regulatory network linking chromosome replication with polar morphogenesis and cell division. Temporal and spatial regulation have emerged as the central themes, with the abundance, activity and subcellular location of key structural and regulatory proteins changing over the course of the cell cycle. An additional layer of complexity was recently uncovered, showing that each segment of the chromosome is located at a specific cellular position both during and after the completion of DNA replication, raising the possibility that this positioning contributes to temporal and spatial control of gene expression.

A membrane metalloprotease participates in the sequential degradation of a Caulobacter polarity determinant

Caulobacter crescentus assembles many of its cellular machines at distinct times and locations during the cell cycle. PodJ provides the spatial cues for the biogenesis of several polar organelles, including the pili, adhesive holdfast and chemotactic apparatus, by recruiting structural and regulatory proteins, such as CpaE and PleC, to a specific cell pole. PodJ is a protein with a single transmembrane domain that exists in two forms, full-length (PodJL) and truncated (PodJS), each appearing during a specific time period of the cell cycle to control different aspects of polar organelle development. PodJL is synthesized in the early predivisional cell and is later proteolytically converted to PodJS. During the swarmer-to-stalked transition, PodJS must be degraded to preserve asymmetry in the next cell cycle. We found that MmpA facilitates the degradation of PodJS. MmpA belongs to the site-2 protease (S2P) family of membrane-embedded zinc metalloproteases, which includes SpoIVFB and YluC of Bacillus subtilis and YaeL of Escherichia coli. MmpA appears to cleave within or near the transmembrane segment of PodJS, releasing it into the cytoplasm for complete proteolysis. While PodJS has a specific temporal and spatial address, MmpA is present throughout the cell cycle; furthermore, periplasmic fusion to mRFP1 suggested that MmpA is uniformly distributed around the cell. We also determined that mmpA and yaeL can complement each other in C. crescentus and E. coli, indicating functional conservation. Thus, the sequential degradation of PodJ appears to involve regulated intramembrane proteolysis (Rip) by MmpA.

Cytokinesis monitoring during development; rapid pole-to-pole shuttling of a signaling protein by localized kinase and phosphatase in Caulobacter

For successful generation of different cell types by asymmetric cell division, cell differentiation should be initiated only after completion of division. Here, we describe a control mechanism by which Caulobacter couples the initiation of a developmental program to the completion of cytokinesis. Genetic evidence indicates that localization of the signaling protein DivK at the flagellated pole prevents premature initiation of development. Photobleaching and FRET experiments show that polar localization of DivK is dynamic with rapid pole-to-pole shuttling of diffusible DivK generated by the localized activities of PleC phosphatase and DivJ kinase at opposite poles. This shuttling is interrupted upon completion of cytokinesis by the segregation of PleC and DivJ to different daughter cells, resulting in disruption of DivK localization at the flagellated pole and subsequent initiation of development in the flagellated progeny. Thus, dynamic polar localization of a diffusible protein provides a control mechanism that monitors cytokinesis to regulate development.

An actin-like gene can determine cell polarity in bacteria

Achieving proper polarity is essential for cellular function. In bacteria, cell polarity has been observed by using both morphological and molecular markers; however, no general regulators of bacterial cell polarity have been identified. Here we investigate the effect on cell polarity of two cytoskeletal elements previously implicated in cell shape determination. We find that the actin-like MreB protein mediates global cell polarity in Caulobacter crescentus, although the intermediate filament-like CreS protein influences cell shape without affecting cell polarity. MreB is organized in an axial spiral that is dynamically rearranged during the cell cycle, and MreB dynamics may be critical for the determination of cell polarity. By examining depletion and overexpression strains, we demonstrate that MreB is required both for the polar localization of the chromosomal origin sequence and the dynamic localization of regulatory proteins to the correct cell pole. We propose that the molecular polarity inherent in an actin-like filament is translated into a mechanism for directing global cell polarity.

Regulatory proteins with a sense of direction: cell cycle signalling network in Caulobacter

Localization of kinases and other signalling molecules at discrete cellular locations is often an essential component of signal transduction in eukaryotes. Caulobacter crescentus is a small, single-celled bacterium that presumably lacks intracellular organelles. Yet in Caulobacter, the subcellular distribution of several two-component signal transduction proteins involved in the control of polar morphogenesis and cell cycle progression changes from a fairly dispersed distribution to a tight accumulation at one or both poles in a spatial and temporal pattern that is reproduced during each cell cycle. This cell cycle-dependent choreography suggests that similarly to what happens in eukaryotes, protein localization provides a means of modulating signal transduction in bacteria. Recent studies have provided important insights into the biological role and the mechanisms for the differential localization of these bacterial signalling proteins during the Caulobacter cell cycle.

Spatial and temporal control of differentiation and cell cycle progression in Caulobacter crescentus

The dimorphic and intrinsically asymmetric bacterium Caulobacter crescentus has become an important model organism to study the bacterial cell cycle, cell polarity, and polar differentiation. A multifaceted regulatory network orchestrates the precise coordination between the development of polar organelles and the cell cycle. One master response regulator, CtrA, directly controls the initiation of chromosome replication as well as several aspects of polar morphogenesis and cell division. CtrA activity is temporally and spatially regulated by multiple partially redundant control mechanisms, such as transcription, phosphorylation, and targeted proteolysis. A multicomponent signal transduction network upstream CtrA, containing histidine kinases CckA, PleC, DivJ, and DivL and the essential response regulator DivK, contributes to the control of CtrA activity in response to cell cycle and developmental cues. An intriguing feature of this signaling network is the dynamic cell cycle-dependent polar localization of its components, which is believed to have a novel regulatory function.

Temporal and spatial regulation in prokaryotic cell cycle progression and development

Bacteria exhibit a high degree of intracellular organization, both in the timing of essential processes and in the placement of the chromosome, the division site, and individual structural and regulatory proteins. We examine the temporal and spatial regulation of the Caulobacter cell cycle, bacterial chromosome segregation and cytokinesis, and Bacillus subtilis sporulation. Mechanisms that control timing of cell cycle and developmental events include transcriptional cascades, regulated phosphorylation and proteolysis of signal transduction proteins, transient genetic asymmetry, and intercellular communication. Surprisingly, many signal transduction proteins are dynamically localized to specific subcellular addresses during the cell division cycle and sporulation, and proper localization is essential for their function. The Min proteins that govern division site selection in Escherichia coli may be the first example of a system that generates positional information de novo.

Cell cycle timing and developmental checkpoints in Caulobacter crescentus

Development in Caulobacter reflects a level of complexity once thought only to exist in eukaryotic cells. The cell cycle and development are not isolated from each other, but are interdependent processes. Checkpoints are in place to ensure that both cell cycle and developmental processes are completed accurately before the next stage is initiated. The timing of these processes is regulated by signal transduction networks that integrate signals from DNA replication, cell division and development. These signal transduction networks achieve precise timing of the cell cycle and development by regulating temporal gene expression, and protein activity by dynamic spatial localization within the cell and timed proteolysis.

A Bacterial Cell-Cycle Regulatory Network Operating in Time and Space

Transcriptional regulatory circuits provide only a fraction of the signaling pathways and regulatory mechanisms that control the bacterial cell cycle. The CtrA regulatory network, important in control of the Caulobacter cell cycle, illustrates the critical role of nontranscriptional pathways and temporally and spatially localized regulatory proteins. The system architecture of Caulobacter cell-cycle control involves top-down control of modular functions by a small number of master regulatory proteins with cross-module signaling coordinating the overall process. Modeling the cell cycle probably requires a top-down modeling approach and a hybrid control system modeling paradigm to treat its combined discrete and continuous characteristics.

The asymmetric spatial distribution of bacterial signal transduction proteins coordinates cell cycle events

The polar localization of signaling proteins that are essential for Caulobacter cell cycle control is temporally regulated. Here we provide evidence that phosphorylation of the essential response regulator, DivK, is required for both its function and its cell cycle-regulated localization. The asymmetric location of the DivJ and PleC histidine kinases and their antagonistic activities on the cellular concentration of phosphorylated DivK provide positional and temporal information for the ordered sequence of DivK localization during the cell cycle. DivJ activity on DivK affects its correct localization, which, in turn, is required for PleC function. Since DivJ and PleC regulate different cell cycle events, the interconnected function of these two histidine kinases through localization of a common response regulator provides a mechanism for coordinating cell cycle progression. Study of a DivK homolog in the morphologically symmetric bacterium Sinorhizobium meliloti suggests that this type of cell cycle mechanism is widespread in prokaryotes.

A lytic transglycosylase homologue, PleA, is required for the assembly of pili and the flagellum at the Caulobacter crescentus cell pole

Two distinct protein complexes, the flagellum and the pilus biogenesis machinery, are asymmetrically assembled at one pole of the Caulobacter predivisional cell. Cell division yields dissimilar daughter cells: a stalked cell and a swarmer cell that assembles several pili at the flagellated cell pole. Strains bearing mutations in the pleA gene are pililess and non-flagellated. The PleA protein contains a region that is similar to a peptidoglycan-hydrolytic active site, and a point mutation at this site in PleA results in the loss of flagellum and pili biogenesis. PleA was found to be required for the insertion of the outer membrane pilus secretion channel at the cell pole and for the accumulation of the PilA pilin subunit. PleA is also required for the assembly of substructures of the flagellar basal body hook complex that are located in or traverse the peptidoglycan layer. These results argue that PleA facilitates the assembly of envelope-spanning structures at the cell pole. In support of this, PleA was found to be present only during a short interval in the cell cycle that coincides with the assembly of the flagellum and the pilus secretion apparatus.

Generating and exploiting polarity in bacteria

Bacteria are often highly polarized, exhibiting specialized structures at or near the ends of the cell. Among such structures are actin-organizing centers, which mediate the movement of certain pathogenic bacteria within the cytoplasm of an animal host cell; organized arrays of membrane receptors, which govern chemosensory behavior in swimming bacteria; and asymmetrically positioned septa, which generate specialized progeny in differentiating bacteria. This polarization is orchestrated by complex and dynamic changes in the subcellular localization of signal transduction and cytoskeleton proteins as well as of specific regions of the chromosome. Recent work has provided information on how dynamic subcellular localization occurs and how it is exploited by the bacterial cell. The main task of a bacterial cell is to survive and duplicate itself. The bacterium must replicate its genetic material and divide at the correct site in the cell and at the correct time in the cell cycle with high precision. Each kind of bacterium also executes its own strategy to find nutrients in its habitat and to cope with conditions of stress from its environment. This involves moving toward food, adapting to environmental extremes, and, in many cases, entering and exploiting a eukaryotic host. These activities often involve processes that take place at or near the poles of the cell. Here we explore some of the schemes bacteria use to orchestrate dynamic changes at their poles and how these polar events execute cellular functions. In spite of their small size, bacteria have a remarkably complex internal organization and external architecture. Bacterial cells are inherently asymmetric, some more obviously so than others. The most easily recognized asymmetries involve surface structures, e.g., flagella, pili, and stalks that are preferentially assembled at one pole by many bacteria. "New" poles generated at the cell division plane differ from old poles from the previous round of cell division. Even in Escherichia coli, which is generally thought to be symmetrical, old poles are more static than new poles with respect to cell wall assembly (1), and they differ in the deposition of phospholipid domains (2). There are many instances of differential polar functions; among these is the preferential use of old poles when attaching to host cells as in the interaction of Bradyrhizobium with plant root hairs (3) or the polar pili-mediated attachment of the Pseudomonas aeruginosa pathogen to tracheal epithelia (4). An unusual polar organelle that mediates directed motility on solid surfaces is found in the nonpathogenic bacterium Myxococcus xanthus. The gliding motility of this bacterium is propelled by a nozzle-like structure that squirts a polysaccharide-containing slime from the pole of the cell (5). Interestingly, M. xanthus, which has nozzles at both poles, can reverse direction by closing one nozzle and opening the other in response to end-to-end interactions between cells.

The Caulobacter cell cycle: timing, spatial organization and checkpoints

In Caulobacter crescentus, a complex regulatory network integrates temporal and spatial information to control the ordered progression of the cell cycle, and to synchronize cell proliferation with development. Periodicity includes the timed synthesis, activation or destruction of key regulatory proteins, which activate a large number of genes at the appropriate time of the cell cycle. Checkpoints serve to couple cell division and polar development to the replication and segregation state of the chromosome. Asymmetrically positioned regulatory components are involved in the sequential positioning of polar organelles. New work sheds light on the spatial organization of cellular components involved in cell cycle progression and polar differentiation, and starts to define the molecular nature of checkpoints involved in cell cycle control and development.

Identification of a localization factor for the polar positioning of bacterial structural and regulatory proteins

Polar pili biogenesis in Caulobacter involves the asymmetric localization of the CpaE and CpaC components of the pili-specific secretion apparatus to one pole of the predivisional cell followed by the biosynthesis of the pili filaments in the daughter swarmer cell. The histidine kinase signaling protein, PleC, that controls the temporal accumulation of the PilA pilin subunit is asymmetrically localized to the pole at which pili are assembled. Here we identify a protein, PodJ, that provides the positional information for the polar localization of both PleC and CpaE. The PodJ protein was found to exist in two forms, a truncated 90-kDa and a full-length 110-kDa form, each controlling a different aspect of polar development and each localizing to the cell poles at a specific time in the cell cycle. When active PleC is delocalized in a DeltapodJ mutant, the accumulation of PilA, the downstream target of PleC signaling, is impaired, providing evidence that the polar localization of this histidine kinase stimulates the response signaled by a two-component system.