A regulatory checkpoint during flagellar biogenesis in Campylobacter jejuni initiates signal transduction to activate transcription of flagellar genes

PBP1A Directly Interacts with the Divisome Complex to Promote Septal Peptidoglycan Synthesis in Acinetobacter baumannii

The class A penicillin-binding proteins (aPBPs), PBP1A and PBP1B, are major peptidoglycan synthases that synthesize more than half of the peptidoglycan per generation in Escherichia coli. Whereas aPBPs have distinct roles in peptidoglycan biosynthesis during growth (i.e., elongation and division), they are semiredundant; disruption of either is rescued by the other to maintain envelope homeostasis and promote proper growth. Acinetobacter baumannii is a nosocomial pathogen that has a high propensity to overcome antimicrobial treatment. A. baumannii contains both PBP1A and PBP1B (encoded by mrcA and mrcB, respectively), but only mrcA deletion decreased fitness and contributed to colistin resistance through inactivation of lipooligosaccharide biosynthesis, indicating that PBP1B was not functionally redundant with the PBP1A activity. While previous studies suggested a distinct role for PBP1A in division, it was unknown whether its role in septal peptidoglycan biosynthesis was direct. Here, we show that A. baumannii PBP1A has a direct role in division through interactions with divisome components. PBP1A localizes to septal sites during growth, where it interacts with the transpeptidase PBP3, an essential division component that regulates daughter cell formation. PBP3 overexpression was sufficient to rescue the division defect in ΔmrcA A. baumannii; however, PBP1A overexpression was not sufficient to rescue the septal defect when PBP3 was inhibited, suggesting that their activity is not redundant. Overexpression of a major dd-carboxypeptidase, PBP5, also restored the canonical A. baumannii coccobacilli morphology in ΔmrcA cells. Together, these data support a direct role for PBP1A in A. baumannii division and highlights its role as a septal peptidoglycan synthase. IMPORTANCE Peptidoglycan biosynthesis is a validated target of β-lactam antibiotics, and it is critical that we understand essential processes in multidrug-resistant pathogens such as Acinetobacter baumannii. While model systems such as Escherichia coli have shown that PBP1A is associated with side wall peptidoglycan synthesis, we show herein that A. baumannii PBP1A directly interacts with the divisome component PBP3 to promote division, suggesting a unique role for the enzyme in this highly drug-resistant nosocomial pathogen. A. baumannii demonstrated unanticipated resistance and tolerance to envelope-targeting antibiotics, which may be driven by rewired peptidoglycan machinery and may underlie therapeutic failure during antibiotic treatment.

FlrA-independent production of flagellar proteins is required for proper flagellation in Shewanella putrefaciens

Flagella are multiprotein complexes whose assembly and positioning require complex spatiotemporal control. Flagellar assembly is thought to be controlled by several transcriptional tiers, which are mediated through various master regulators. Here, we revisited the regulation of flagellar genes in polarly flagellated gammaproteobacteria by the regulators FlrA, RpoN (σ⁵⁴ ) and FliA (σ²⁸ ) in Shewanella putrefaciens CN-32 at the transcript and protein level. We found that a number of regulatory and structural proteins were present in the absence of the main regulators, suggesting that initiation of flagella assembly and motor activation relies on the abundance control of only a few structural key components that are required for the formation of the MS- and C-ring and the flagellar type III secretion system. We identified FlrA-independent promoters driving expression of the regulators of flagellar number and positioning, FlhF and FlhG. Reduction of the gene expression levels from these promoters resulted in the emergence of hyperflagellation. This finding indicates that basal expression is required to adjust the flagellar counter in Shewanella. This is adding a deeper layer to the regulation of flagellar synthesis and assembly.

Insights into the Orchestration of Gene Transcription Regulators in Helicobacter pylori

Bacterial pathogens employ a general strategy to overcome host defenses by coordinating the virulence gene expression using dedicated regulatory systems that could raise intricate networks. During the last twenty years, many studies of Helicobacter pylori, a human pathogen responsible for various stomach diseases, have mainly focused on elucidating the mechanisms and functions of virulence factors. In parallel, numerous studies have focused on the molecular mechanisms that regulate gene transcription to attempt to understand the physiological changes of the bacterium during infection and adaptation to the environmental conditions it encounters. The number of regulatory proteins deduced from the genome sequence analyses responsible for the correct orchestration of gene transcription appears limited to 14 regulators and three sigma factors. Furthermore, evidence is accumulating for new and complex circuits regulating gene transcription and H. pylori virulence. Here, we focus on the molecular mechanisms used by H. pylori to control gene transcription as a function of the principal environmental changes.

Two-component regulatory systems in Helicobacter pylori and Campylobacter jejuni: Attractive targets for novel antibacterial drugs

Two-component regulatory systems (TCRS) are ubiquitous signal transduction mechanisms evolved by bacteria for sensing and adapting to the constant changes that occur in their environment. Typically consisting of two types of proteins, a membrane sensor kinase and an effector cytosolic response regulator, the TCRS modulate via transcriptional regulation a plethora of key physiological processes, thereby becoming essential for bacterial viability and/or pathogenicity and making them attractive targets for novel antibacterial drugs. Some members of the phylum Campylobacterota (formerly Epsilonproteobacteria), including Helicobacter pylori and Campylobacter jejuni, have been classified by WHO as “high priority pathogens” for research and development of new antimicrobials due to the rapid emergence and dissemination of resistance mechanisms against first-line antibiotics and the alarming increase of multidrug-resistant strains worldwide. Notably, these clinically relevant pathogens express a variety of TCRS and orphan response regulators, sometimes unique among its phylum, that control transcription, translation, energy metabolism and redox homeostasis, as well as the expression of relevant enzymes and virulence factors. In the present mini-review, we describe the signalling mechanisms and functional diversity of TCRS in H. pylori and C. jejuni, and provide an overview of the most recent findings in the use of these microbial molecules as potential novel therapeutic targets for the development of new antibiotics.

Insights into the Virulence of Campylobacter jejuni Associated with Two-Component Signal Transduction Systems and Single Regulators

Campylobacter jejuni is one of the major aetiologies of diarrhoea. Understanding the processes and virulence factors contributing to C. jejuni fitness is a cornerstone for developing mitigation strategies. Two-component signal transduction systems, known as two-component systems (TCSs), along with single regulators with no obvious cognate histidine kinase, help pathogens in interacting with their environments, but the available literature on C. jejuni is limited. A typical TCS possesses histidine kinase and response regulator proteins. The objective of this review was to provide insights into the virulence of C. jejuni associated with TCSs and single regulators. Despite limited research, TCSs are important contributors to the pathogenicity of C. jejuni by influencing motility (FlgSR), colonisation (DccRS), nutrient acquisition (PhosSR and BumSR), and stress response (RacRS). Of the single regulators, CbrR and CosR are involved in bile resistance and oxidative stress response, respectively. Cross-talks among TCSs complicate the full elucidation of their molecular mechanisms. Although progress has been made in characterising C. jejuni TCSs, shortfalls such as triggering signals, inability to induce mutations in some genes, or developing suitable in vivo models are still being encountered. Further research is expected to shed light on the unexplored sides of the C. jejuni TCSs, which may allow new drug discoveries and better control strategies.

Transcriptional organization and regulation of the Pseudomonas putida flagellar system

A single region of the Pseudomonas putida genome, designated the flagellar cluster, includes 59 genes potentially involved in the biogenesis and function of the flagellar system. Here, we combine bioinformatics and in vivo gene expression analyses to clarify the transcriptional organization and regulation of the flagellar genes in the cluster. We have identified 11 flagellar operons and characterized 22 primary and internal promoter regions. Our results indicate that synthesis of the flagellar apparatus and core chemotaxis machinery is regulated by a three-tier cascade in which fleQ is a Class I gene, standing at the top of the transcriptional hierarchy. FleQ- and σ⁵⁴ -dependent Class II genes encode most components of the flagellar structure, part of the chemotaxis machinery and multiple regulatory elements, including the flagellar σ factor FliA. FliA activation of Class III genes enables synthesis of the filament, one stator complex and completion of the chemotaxis apparatus. Accessory regulatory proteins and an intricate operon architecture add complexity to the regulation by providing feedback and feed-forward loops to the main circuit. Because of the high conservation of the gene arrangement and promoter motifs, we believe that the regulatory circuit presented here may also apply to other environmental pseudomonads.

Reduced Infection Efficiency of Phage NCTC 12673 on Non-Motile Campylobacter jejuni Strains Is Related to Oxidative Stress

Campylobacter jejuni is a Gram-negative foodborne pathogen that causes diarrheal disease and is associated with severe post-infectious sequelae. Bacteriophages (phages) are a possible means of reducing Campylobacter colonization in poultry to prevent downstream human infections. However, the factors influencing phage-host interactions must be better understood before this strategy can be predictably employed. Most studies have focused on Campylobacter phage binding to the host surface, with all phages classified as either capsule- or flagella-specific. Here we describe the characterization of a C. jejuni phage that requires functional flagellar glycosylation and motor genes for infection, without needing the flagella for adsorption to the cell surface. Through phage infectivity studies of targeted C. jejuni mutants, transcriptomic analysis of phage-resistant mutants, and genotypic and phenotypic analysis of a spontaneous phage variant capable of simultaneously overcoming flagellar gene dependence and sensitivity to oxidative stress, we have uncovered a link between oxidative stress, flagellar motility, and phage infectivity. Taken together, our results underscore the importance of understanding phage-host interactions beyond the cell surface and point to host oxidative stress state as an important and underappreciated consideration for future phage-host interaction studies.

Campylobacter jejuni BumSR directs a response to butyrate via sensor phosphatase activity to impact transcription and colonization

Campylobacter jejuni monitors intestinal metabolites produced by the host and microbiota to initiate intestinal colonization of avian and animal hosts for commensalism and infection of humans for diarrheal disease. We previously discovered that C. jejuni has the capacity to spatially discern different intestinal regions by sensing lactate and the short-chain fatty acids acetate and butyrate and then alter transcription of colonization factors appropriately for in vivo growth. In this study, we identified the C. jejuni butyrate-modulated regulon and discovered that the BumSR two-component signal transduction system (TCS) directs a response to butyrate by identifying mutants in a genetic screen defective for butyrate-modulated transcription. The BumSR TCS, which is important for infection of humans and optimal colonization of avian hosts, senses butyrate likely by indirect means to alter transcription of genes encoding important colonization determinants. Unlike many canonical TCSs, the predicted cytoplasmic sensor kinase BumS lacked in vitro autokinase activity, which would normally lead to phosphorylation of the cognate BumR response regulator. Instead, BumS has likely evolved mutations to naturally function as a phosphatase whose activity is influenced by exogenous butyrate to control the level of endogenous phosphorylation of BumR and its ability to alter transcription of target genes. To our knowledge, the BumSR TCS is the only bacterial signal transduction system identified so far that mediates responses to the microbiota-generated intestinal metabolite butyrate, an important factor for host intestinal health and homeostasis. Our findings suggest that butyrate sensing by this system is vital for C. jejuni colonization of multiple hosts.

Investigating the Role of FlhF Identifies Novel Interactions With Genes Involved in Flagellar Synthesis in Campylobacter jejuni

FlhF is a key protein required for complete flagellar synthesis, and its deletion results in the complete absence of a flagella and thus motility in Campylobacter jejuni. However, the specific mechanism still remains unknown. In this study, RNA-Seq, EMSAs, ChIP-qPCR and β-Galactosidase assays were performed to elucidate the novel interactions between FlhF and genes involved in flagellar synthesis. Results showed that FlhF has an overall influence on the transcription of flagellar genes with an flhF mutant displaying down-regulation of most flagellar related genes. FlhF can directly bind to the flgI promoter to regulate its expression, which has significant expression change in an flhF mutant. The possible binding site of FlhF to the flgI promoter was explored by continuously narrowing the flgI promoter region and performing further point mutations. Meanwhile, FlhF can directly bind to the promoters of rpoD, flgS, and fliA encoding early flagellin regulators, thereby directly or indirectly regulating the synthesis of class I, II, and III flagellar genes, respectively. Collectively, this study demonstrates that FlhF may directly regulate the transcription of flagellar genes by binding to their promoters as a transcriptional regulator, which will be helpful in understanding the mechanism of FlhF in flagellar biosynthetic and bacterial flagellation in general.

Propulsive nanomachines: the convergent evolution of archaella, flagella and cilia

Echoing the repeated convergent evolution of flight and vision in large eukaryotes, propulsive swimming motility has evolved independently in microbes in each of the three domains of life. Filamentous appendages – archaella in Archaea, flagella in Bacteria and cilia in Eukaryotes – wave, whip or rotate to propel microbes, overcoming diffusion and enabling colonization of new environments. The implementations of the three propulsive nanomachines are distinct, however: archaella and flagella rotate, while cilia beat or wave; flagella and cilia assemble at their tips, while archaella assemble at their base; archaella and cilia use ATP for motility, while flagella use ion-motive force. These underlying differences reflect the tinkering required to evolve a molecular machine, in which pre-existing machines in the appropriate contexts were iteratively co-opted for new functions and whose origins are reflected in their resultant mechanisms. Contemporary homologies suggest that archaella evolved from a non-rotary pilus, flagella from a non-rotary appendage or secretion system, and cilia from a passive sensory structure. Here, we review the structure, assembly, mechanism and homologies of the three distinct solutions as a foundation to better understand how propulsive nanomachines evolved three times independently and to highlight principles of molecular evolution.

A Polar Flagellar Transcriptional Program Mediated by Diverse Two-Component Signal Transduction Systems and Basal Flagellar Proteins Is Broadly Conserved in Polar Flagellates

Relative to peritrichous bacteria, polar flagellates possess regulatory systems that order flagellar gene transcription differently and produce flagella in specific numbers only at poles. How transcriptional and flagellar biogenesis regulatory systems are interlinked to promote the correct synthesis of polar flagella in diverse species has largely been unexplored. We found evidence for many Gram-negative polar flagellates encoding two-component signal transduction systems with activity linked to the formation of flagellar type III secretion systems to enable production of flagellar rod and hook proteins at a discrete, subsequent stage during flagellar assembly. This polar flagellar transcriptional program assists, in some manner, the FlhF/FlhG flagellar biogenesis regulatory system, which forms specific flagellation patterns in polar flagellates in maintaining flagellation and motility when activity of FlhF or FlhG might be altered. Our work provides insight into the multiple regulatory processes required for polar flagellation.

Bacterial flagella are rotating nanomachines required for motility. Flagellar gene expression and protein secretion are coordinated for efficient flagellar biogenesis. Polar flagellates, unlike peritrichous bacteria, commonly order flagellar rod and hook gene transcription as a separate step after production of the MS ring, C ring, and flagellar type III secretion system (fT3SS) core proteins that form a competent fT3SS. Conserved regulatory mechanisms in diverse polar flagellates to create this polar flagellar transcriptional program have not been thoroughly assimilated. Using in silico and genetic analyses and our previous findings in Campylobacter jejuni as a foundation, we observed a large subset of Gram-negative bacteria with the FlhF/FlhG regulatory system for polar flagellation to possess flagellum-associated two-component signal transduction systems (TCSs). We present data supporting a general theme in polar flagellates whereby MS ring, rotor, and fT3SS proteins contribute to a regulatory checkpoint during polar flagellar biogenesis. We demonstrate that Vibrio cholerae and Pseudomonas aeruginosa require the formation of this regulatory checkpoint for the TCSs to directly activate subsequent rod and hook gene transcription, which are hallmarks of the polar flagellar transcriptional program. By reprogramming transcription in V. cholerae to more closely follow the peritrichous flagellar transcriptional program, we discovered a link between the polar flagellar transcription program and the activity of FlhF/FlhG flagellar biogenesis regulators in which the transcriptional program allows polar flagellates to continue to produce flagella for motility when FlhF or FlhG activity may be altered. Our findings integrate flagellar transcriptional and biogenesis regulatory processes involved in polar flagellation in many species.

Protein Activity Sensing in Bacteria in Regulating Metabolism and Motility

Bacteria have evolved complex sensing and signaling systems to react to their changing environments, most of which are present in all domains of life. Canonical bacterial sensing and signaling modules, such as membrane-bound ligand-binding receptors and kinases, are very well described. However, there are distinct sensing mechanisms in bacteria that are less studied. For instance, the sensing of internal or external cues can also be mediated by changes in protein conformation, which can either be implicated in enzymatic reactions, transport channel formation or other important cellular functions. These activities can then feed into pathways of characterized kinases, which translocate the information to the DNA or other response units. This type of bacterial sensory activity has previously been termed protein activity sensing. In this review, we highlight the recent findings about this non-canonical sensory mechanism, as well as its involvement in metabolic functions and bacterial motility. Additionally, we explore some of the specific proteins and protein-protein interactions that mediate protein activity sensing and their downstream effects. The complex sensory activities covered in this review are important for bacterial navigation and gene regulation in their dynamic environment, be it host-associated, in microbial communities or free-living.

Diversification of Campylobacter jejuni Flagellar C-Ring Composition Impacts Its Structure and Function in Motility, Flagellar Assembly, and Cellular Processes

Bacterial flagella are reversible rotary motors that rotate external filaments for bacterial propulsion. Some flagellar motors have diversified by recruiting additional components that influence torque and rotation, but little is known about the possible diversification and evolution of core motor components. The mechanistic core of flagella is the cytoplasmic C ring, which functions as a rotor, directional switch, and assembly platform for the flagellar type III secretion system (fT3SS) ATPase. The C ring is composed of a ring of FliG proteins and a helical ring of surface presentation of antigen (SPOA) domains from the switch proteins FliM and one of two usually mutually exclusive paralogs, FliN or FliY. We investigated the composition, architecture, and function of the C ring of Campylobacter jejuni, which encodes FliG, FliM, and both FliY and FliN by a variety of interrogative approaches. We discovered a diversified C. jejuni C ring containing FliG, FliM, and both FliY, which functions as a classical FliN-like protein for flagellar assembly, and FliN, which has neofunctionalized into a structural role. Specific protein interactions drive the formation of a more complex heterooligomeric C. jejuni C-ring structure. We discovered that this complex C ring has additional cellular functions in polarly localizing FlhG for numerical regulation of flagellar biogenesis and spatial regulation of division. Furthermore, mutation of the C. jejuni C ring revealed a T3SS that was less dependent on its ATPase complex for assembly than were other systems. Our results highlight considerable evolved flagellar diversity that impacts motor output, biogenesis, and cellular processes in different species.IMPORTANCE The conserved core of bacterial flagellar motors reflects a shared evolutionary history that preserves the mechanisms essential for flagellar assembly, rotation, and directional switching. In this work, we describe an expanded and diversified set of core components in the Campylobacter jejuni flagellar C ring, the mechanistic core of the motor. Our work provides insight into how usually conserved core components may have diversified by gene duplication, enabling a division of labor of the ancestral protein between the two new proteins, acquisition of new roles in flagellar assembly and motility, and expansion of the function of the flagellum beyond motility, including spatial regulation of cell division and numerical control of flagellar biogenesis in C. jejuni Our results highlight that relatively small changes, such as gene duplications, can have substantial ramifications on the cellular roles of a molecular machine.

Campylobacter jejuni: collective components promoting a successful enteric lifestyle

Campylobacter jejuni is the leading cause of bacterial diarrhoeal disease in many areas of the world. The high incidence of sporadic cases of disease in humans is largely due to its prevalence as a zoonotic agent in animals, both in agriculture and in the wild. Compared with many other enteric bacterial pathogens, C. jejuni has strict growth and nutritional requirements and lacks many virulence and colonization determinants that are typically used by bacterial pathogens to infect hosts. Instead, C. jejuni has a different collection of factors and pathways not typically associated together in enteric pathogens to establish commensalism in many animal hosts and to promote diarrhoeal disease in the human population. In this Review, we discuss the cellular architecture and structure of C. jejuni, intraspecies genotypic variation, the multiple roles of the flagellum, specific nutritional and environmental growth requirements and how these factors contribute to in vivo growth in human and avian hosts, persistent colonization and pathogenesis of diarrhoeal disease.

A Chaperone for the Stator Units of a Bacterial Flagellum

The bacterial flagellum is a reversible rotating motor powered by ion transport through stator units, which also exert torque on the rotor component to turn the flagellum for motility. Species-specific adaptations to flagellar motors impact stator function to meet the demands of each species to sufficiently power flagellar rotation. We identified another evolutionary adaptation by discovering that FlgX of Campylobacter jejuni preserves the integrity of stator units by functioning as a chaperone to protect stator proteins from degradation by the FtsH protease complex due to the physiology of the bacterium. FlgX is required to maintain a level of stator units sufficient to power the naturally high-torque flagellar motor of C. jejuni for motility in intestinal mucosal layers to colonize hosts. Our work continues to identify an increasing number of adaptations to flagellar motors across bacterial species that provide the mechanics necessary for producing an effective rotating nanomachine for motility.

The stator units of the flagellum supply power to the flagellar motor via ion transport across the cytoplasmic membrane and generate torque on the rotor for rotation. Flagellar motors across bacterial species have evolved adaptations that impact and enhance stator function to meet the demands of each species, including producing stator units using different fuel types or various stator units for different motility modalities. Campylobacter jejuni produces one of the most complex and powerful flagellar motors by positioning 17 stator units at a greater radial distance than in most other bacteria to increase power and torque for high velocity of motility. We report another evolutionary adaptation impacting flagellar stators by identifying FlgX as a chaperone for C. jejuni stator units to ensure sufficient power and torque for flagellar rotation and motility. We discovered that FlgX maintains MotA and MotB stator protein integrity likely through a direct interaction with MotA that prevents their degradation. Suppressor analysis suggested that the physiology of C. jejuni drives the requirement for FlgX to protect stator units from proteolysis by the FtsH protease complex. C. jejuni ΔflgX was strongly attenuated for colonization of the natural avian host, but colonization capacity was greatly restored by a single mutation in MotA. These findings suggest that the likely sole function of FlgX is to preserve stator unit integrity for the motility required for host interactions. Our findings demonstrate another evolved adaptation in motile bacteria to ensure the equipment of the flagellar motor with sufficient power to generate torque for motility.

Colistin heteroresistance in Enterobacter cloacae is regulated by PhoPQ-dependent 4-amino-4-deoxy-l-arabinose addition to lipid A

The Enterobacter cloacae complex (ECC) consists of closely related bacteria commonly associated with the human microbiota. ECC are increasingly isolated from healthcare-associated infections, demonstrating that these Enterobacteriaceae are emerging nosocomial pathogens. ECC can rapidly acquire multidrug resistance to conventional antibiotics. Cationic antimicrobial peptides (CAMPs) have served as therapeutic alternatives because they target the highly conserved lipid A component of the Gram-negative outer membrane. Many Enterobacteriaceae fortify their outer membrane with cationic amine-containing moieties to prevent CAMP binding, which can lead to cell lysis. The PmrAB two-component system (TCS) directly activates 4-amino-4-deoxy-l-arabinose (l-Ara4N) biosynthesis to result in cationic amine moiety addition to lipid A in many Enterobacteriaceae such as E. coli and Salmonella. In contrast, PmrAB is dispensable for CAMP resistance in E. cloacae. Interestingly, some ECC clusters exhibit colistin heteroresistance, where a subpopulation of cells exhibit clinically significant resistance levels compared to the majority population. We demonstrate that E. cloacae lipid A is modified with l-Ara4N to induce CAMP heteroresistance and the regulatory mechanism is independent of the PmrAB_Ecl TCS. Instead, PhoP_Ecl binds to the arnB_Ecl promoter to induce l-Ara4N biosynthesis and PmrAB-independent addition to the lipid A disaccharolipid. Therefore, PhoPQ_Ecl contributes to regulation of CAMP heteroresistance in some ECC clusters.

WhiB6 regulation of ESX-1 gene expression is controlled by a negative feedback loop in Mycobacterium marinum

ESX (ESAT-6 system) export systems play diverse roles across mycobacterial species. Interestingly, genetic disruption of ESX systems in different species does not result in an accumulation of protein substrates in the mycobacterial cell. However, the mechanisms underlying this observation are elusive. We hypothesized that the levels of ESX substrates were regulated by a feedback-control mechanism, linking the levels of substrates to the secretory status of ESX systems. To test this hypothesis, we used a combination of genetic, transcriptomic, and proteomic approaches to define export-dependent mechanisms regulating the levels of ESX-1 substrates in Mycobacterium marinum WhiB6 is a transcription factor that regulates expression of genes encoding ESX-1 substrates. We found that, in the absence of the genes encoding conserved membrane components of the ESX-1 system, the expression of the whiB6 gene and genes encoding ESX-1 substrates were reduced. Accordingly, the levels of ESX-1 substrates were decreased, and WhiB6 was not detected in M. marinum strains lacking genes encoding ESX-1 components. We demonstrated that, in the absence of EccCb₁, a conserved ESX-1 component, substrate gene expression was restored by constitutive, but not native, expression of the whiB6 gene. Finally, we found that the loss of WhiB6 resulted in a virulent M. marinum strain with reduced ESX-1 secretion. Together, our findings demonstrate that the levels of ESX-1 substrates in M. marinum are fine-tuned by negative feedback control, linking the expression of the whiB6 gene to the presence, not the functionality, of the ESX-1 membrane complex.

Role of flagella and type four pili in the co-migration of Burkholderia terrae BS001 with fungal hyphae through soil

Burkholderia terrae BS001 has previously been found to be able to disperse along with growing fungal hyphae in soil, with the type-3 secretion system having a supportive role in this movement. In this study, we focus on the role of two motility- and adherence-associated appendages, i.e. type-4 pili (T4P) and flagella. Electron microcopy and motility testing revealed that strain BS001 produces polar flagella and can swim on semi-solid R2A agar. Flagellum- and T4P-negative mutants were then constructed to examine the ecological roles of the respective systems. Both in liquid media and on swimming agar, the mutant strains showed similar fitness to the wild-type strain in mixed culture. The flagellar mutant had completely lost its flagella, as well as its swimming capacity. It also lost its co-migration ability with two soil-exploring fungi, Lyophyllum sp. strain Karsten and Trichoderma asperellum 302, in soil microcosms. In contrast, the T4P mutant showed reduced surface twitching motility, whereas its co-migration ability in competition with the wild-type strain was slightly reduced. We conclude that the co-migration of strain BS001 with fungal hyphae through soil is dependent on the presence of functional flagella conferring swimming motility, with the T4P system having a minor effect.

DNA Supercoiling Regulates the Motility of Campylobacter jejuni and Is Altered by Growth in the Presence of Chicken Mucus

Campylobacter jejuni is the leading cause of bacterial gastroenteritis in humans, but relatively little is known about the global regulation of virulence factors during infection of chickens or humans. This study identified DNA supercoiling as playing a key role in regulating motility and flagellar protein production and found that this supercoiling-controlled regulon is induced by growth in chicken mucus. A direct correlation was observed between motility and resting DNA supercoiling levels in different strains of C. jejuni, and relaxation of DNA supercoiling resulted in decreased motility. Transcriptional analysis and Western immunoblotting revealed that a reduction in motility and DNA supercoiling affected the two-component regulatory system FlgRS and was associated with reduced FlgR expression, increased FlgS expression, and aberrant expression of flagellin subunits. Electron microscopy revealed that the flagellar structure remained intact. Growth in the presence of porcine mucin resulted in increased negative supercoiling, increased motility, increased FlgR expression, and reduced FlgS expression. Finally, this supercoiling-dependent regulon was shown to be induced by growth in chicken mucus, and the level of activation was dependent on the source of the mucus from within the chicken intestinal tract. In conclusion, this study reports for the first time the key role played by DNA supercoiling in regulating motility in C. jejuni and indicates that the induction of this supercoiling-induced regulon in response to mucus from different sources could play a critical role in regulating motility in vivo.

Although Campylobacter jejuni is the leading cause of bacterial gastroenteritis, very little is understood about how this pathogen controls the expression of genes involved in causing disease. This study for the first time identifies DNA supercoiling as a key regulator of motility in C. jejuni, which is essential for both pathogenesis and colonization. Altering the level of DNA supercoiling results in changes in motility levels, as well as changes in the expression of genes involved in flagellar gene regulation. Furthermore, spontaneous clones of the organism with different motility profiles have altered DNA supercoiling levels. Finally, mucus was identified as a key stimulator of changes in DNA supercoiling, and it was shown that mucus from different sites in the chicken intestine induced different levels of DNA supercoiling. In conclusion, this study implicates DNA supercoiling as a key regulator of motility in C. jejuni in vivo during colonization of the mucus layer.

Genetic and Transcriptional Analyses of the Flagellar Gene Cluster in Actinoplanes missouriensis

Actinoplanes missouriensis, a Gram-positive and soil-inhabiting bacterium, is a member of the rare actinomycetes. The filamentous cells produce sporangia, which contain hundreds of flagellated spores that can swim rapidly for a short period of time until they find niches for germination. These swimming cells are called zoospores, and the mechanism of this unique temporal flagellation has not been elucidated. Here, we report all of the flagellar genes in the bacterial genome and their expected function and contribution for flagellar morphogenesis. We identified a large flagellar gene cluster composed of 33 genes that encode the majority of proteins essential for assembling the functional flagella of Gram-positive bacteria. One noted exception to the cluster was the location of the fliQ gene, which was separated from the cluster. We examined the involvement of four genes in flagellar biosynthesis by gene disruption, fliQ, fliC, fliK, and lytA Furthermore, we performed a transcriptional analysis of the flagellar genes using RNA samples prepared from A. missouriensis grown on a sporangium-producing agar medium for 1, 3, 6, and 40 days. We demonstrated that the transcription of the flagellar genes was activated in conjunction with sporangium formation. Eleven transcriptional start points of the flagellar genes were determined using the rapid amplification of cDNA 5' ends (RACE) procedure, which revealed the highly conserved promoter sequence CTCA(N15-17)GCCGAA. This result suggests that a sigma factor is responsible for the transcription of all flagellar genes and that the flagellar structure assembles simultaneously.

IMPORTANCE

The biology of a zoospore is very interesting from the viewpoint of morphogenesis, survival strategy, and evolution. Here, we analyzed flagellar genes in A. missouriensis, which produces sporangia containing hundreds of flagellated spores each. Zoospores released from the sporangia swim for a short time before germination occurs. We identified a large flagellar gene cluster and an orphan flagellar gene (fliQ). These findings indicate that the zoospore flagellar components are typical of Gram-positive bacteria. However, the transcriptional analysis revealed that all flagellar genes are transcribed simultaneously during sporangium formation, a pattern differing from the orderly, regulated expression of flagellar genes in other bacteria, such as Salmonella and Escherichia coli These results suggest a novel regulatory mechanism for flagellar formation in A. missouriensis.

Regulation of Motility and Phenazine Pigment Production by FliA Is Cyclic-di-GMP Dependent in Pseudomonas aeruginosa PAO1

The transcription factor FliA, also called sigma 28, is a major regulator of bacterial flagellar biosynthesis genes. Growing evidence suggest that in addition to motility, FliA is involved in controlling numerous bacterial behaviors, even though the underlying regulatory mechanism remains unclear. By using a transcriptional fusion to gfp that responds to cyclic (c)-di-GMP, this study revealed a higher c-di-GMP concentration in the fliA deletion mutant of Pseudomonas aeruginosa than in its wild-type strain PAO1. A comparative analysis of transcriptome profiles of P. aeruginosa PAO1 and its fliA deletion mutant revealed an altered expression of several c-di-GMP-modulating enzyme-encoding genes in the fliA deletion mutant. Moreover, the downregulation of PA4367 (bifA), a Glu-Ala-Leu motif-containing phosphodiesterase, in the fliA deletion mutant was confirmed using the β-glucuronidase reporter gene assay. FliA also altered pyocyanin and pyorubin production by modulating the c-di-GMP concentration. Complementing the fliA mutant strain with bifA restored the motility defect and pigment overproduction of the fliA mutant. Our results indicate that in addition to regulating flagellar gene transcription, FliA can modulate the c-di-GMP concentration to regulate the swarming motility and phenazine pigment production in P. aeruginosa.

Diverse high-torque bacterial flagellar motors assemble wider stator rings using a conserved protein scaffold

Although it is known that diverse bacterial flagellar motors produce different torques, the mechanism underlying torque variation is unknown. To understand this difference better, we combined genetic analyses with electron cryo-tomography subtomogram averaging to determine in situ structures of flagellar motors that produce different torques, from Campylobacter and Vibrio species. For the first time, to our knowledge, our results unambiguously locate the torque-generating stator complexes and show that diverse high-torque motors use variants of an ancestrally related family of structures to scaffold incorporation of additional stator complexes at wider radii from the axial driveshaft than in the model enteric motor. We identify the protein components of these additional scaffold structures and elucidate their sequential assembly, demonstrating that they are required for stator-complex incorporation. These proteins are widespread, suggesting that different bacteria have tailored torques to specific environments by scaffolding alternative stator placement and number. Our results quantitatively account for different motor torques, complete the assignment of the locations of the major flagellar components, and provide crucial constraints for understanding mechanisms of torque generation and the evolution of multiprotein complexes.

FlhG employs diverse intrinsic domains and influences FlhF GTPase activity to numerically regulate polar flagellar biogenesis in Campylobacter jejuni

Flagellation in polar flagellates is one of the rare biosynthetic processes known to be numerically regulated in bacteria. Polar flagellates must spatially and numerically regulate flagellar biogenesis to create flagellation patterns for each species that are ideal for motility. FlhG ATPases numerically regulate polar flagellar biogenesis, yet FlhG orthologs are diverse in motif composition. We discovered that Campylobacter jejuni FlhG is at the center of a multipartite mechanism that likely influences a flagellar biosynthetic step to control flagellar number for amphitrichous flagellation, rather than suppressing activators of flagellar gene transcription as in Vibrio and Pseudomonas species. Unlike other FlhG orthologs, the FlhG ATPase domain was not required to regulate flagellar number in C. jejuni. Instead, two regions of C. jejuni FlhG that are absent or significantly altered in FlhG orthologs are involved in numerical regulation of flagellar biogenesis. Additionally, we found that C. jejuni FlhG influences FlhF GTPase activity, which may mechanistically contribute to flagellar number regulation. Our work suggests that FlhG ATPases divergently evolved in each polarly flagellated species to employ different intrinsic domains and extrinsic effectors to ultimately mediate a common output - precise numerical control of polar flagellar biogenesis required to create species-specific flagellation patterns optimal for motility.

Undiscovered regions on the molecular landscape of flagellar assembly

The bacterial flagellum is a motility structure and one of the most complicated motors in the biosphere. A flagellum consists of several dozens of building blocks in different stoichiometries and extends from the cytoplasm to the extracellular space. Flagellar biogenesis follows a strict spatio-temporal regime that is guided by a plethora of flagellar assembly factors and chaperones. The goal of this review is to summarize our current structural and mechanistic knowledge of this intricate process and to identify the undiscovered regions on the molecular landscape of flagellar assembly.

Functional Activation of the Flagellar Type III Secretion Export Apparatus

Flagella are assembled sequentially from the inside-out with morphogenetic checkpoints that enforce the temporal order of subunit addition. Here we show that flagellar basal bodies fail to proceed to hook assembly at high frequency in the absence of the monotopic protein SwrB of Bacillus subtilis. Genetic suppressor analysis indicates that SwrB activates the flagellar type III secretion export apparatus by the membrane protein FliP. Furthermore, mutants defective in the flagellar C-ring phenocopy the absence of SwrB for reduced hook frequency and C-ring defects may be bypassed either by SwrB overexpression or by a gain-of-function allele in the polymerization domain of FliG. We conclude that SwrB enhances the probability that the flagellar basal body adopts a conformation proficient for secretion to ensure that rod and hook subunits are not secreted in the absence of a suitable platform on which to polymerize.

A PAS domain-containing regulator controls flagella-flagella interactions in Campylobacter jejuni

The bipolar flagella of the foodborne bacterial pathogen Campylobacter jejuni confer motility, which is essential for virulence. The flagella of C. jejuni are post-translationally modified, but how this process is controlled is not well understood. In this work, we have identified a novel PAS-domain containing regulatory system, which modulates flagella-flagella interactions in C. jejuni. Inactivation of the cj1387c gene, encoding a YheO-like PAS6 domain linked to a helix-turn-helix domain, resulted in the generation of a tightly associated “cell-train” morphotype, where up to four cells were connected by their flagella. The morphotype was fully motile, resistant to vortexing, accompanied by increased autoagglutination, and was not observed in aflagellated cells. The Δcj1387c mutant displayed increased expression of the adjacent Cj1388 protein, which comprises of a single endoribonuclease L-PSP domain. Comparative genomics showed that cj1387c (yheO) orthologs in bacterial genomes are commonly linked to an adjacent cj1388 ortholog, with some bacteria, including C. jejuni, containing another cj1388-like gene (cj0327). Inactivation of the cj1388 and cj0327 genes resulted in decreased autoagglutination in Tween-20-supplemented media. The Δcj1388 and Δcj0327 mutants were also attenuated in a Galleria larvae-based infection model. Finally, substituting the sole cysteine in Cj1388 for serine prevented Cj1388 dimerization in non-reducing conditions, and resulted in decreased autoagglutination in the presence of Tween-20. We hypothesize that Cj1388 and Cj0327 modulate post-translational modification of the flagella through yet unidentified mechanisms, and propose naming Cj1387 the Campylobacter Flagella Interaction Regulator CfiR, and the Cj1388 and Cj0327 protein as CfiP and CfiQ, respectively.

Basal Body Structures Differentially Affect Transcription of RpoN- and FliA-Dependent Flagellar Genes in Helicobacter pylori

Flagellar biogenesis in Helicobacter pylori is regulated by a transcriptional hierarchy governed by three sigma factors, RpoD (σ(80)), RpoN (σ(54)), and FliA (σ(28)), that temporally coordinates gene expression with the assembly of the flagellum. Previous studies showed that loss of flagellar protein export apparatus components inhibits transcription of flagellar genes. The FlgS/FlgR two-component system activates transcription of RpoN-dependent genes though an unknown mechanism. To understand better the extent to which flagellar gene regulation is coupled to flagellar assembly, we disrupted flagellar biogenesis at various points and determined how these mutations affected transcription of RpoN-dependent (flaB and flgE) and FliA-dependent (flaA) genes. The MS ring (encoded by fliF) is one of the earliest flagellar structures assembled. Deletion of fliF resulted in the elimination of RpoN-dependent transcripts and an ∼4-fold decrease in flaA transcript levels. FliH is a cytoplasmic protein that functions with the C ring protein FliN to shuttle substrates to the export apparatus. Deletions of fliH and genes encoding C ring components (fliM and fliY) decreased transcript levels of flaB and flgE but had little or no effect on transcript levels of flaA. Transcript levels of flaB and flgE were elevated in mutants where genes encoding rod proteins (fliE and flgBC) were deleted, while transcript levels of flaA was reduced ∼2-fold in both mutants. We propose that FlgS responds to an assembly checkpoint associated with the export apparatus and that FliH and one or more C ring component assist FlgS in engaging this flagellar structure.

IMPORTANCE

The mechanisms used by bacteria to couple transcription of flagellar genes with assembly of the flagellum are poorly understood. The results from this study identified components of the H. pylori flagellar basal body that either positively or negatively affect expression of RpoN-dependent flagellar genes. Some of these basal body proteins may interact directly with regulatory proteins that control transcription of the H. pylori RpoN regulon, a hypothesis that can be tested by examining protein-protein interactions in vitro.

Helicobacter pylori FlhA Binds the Sensor Kinase and Flagellar Gene Regulatory Protein FlgS with High Affinity

Flagellar biogenesis is a complex process that involves multiple checkpoints to coordinate transcription of flagellar genes with the assembly of the flagellum. In Helicobacter pylori, transcription of the genes needed in the middle stage of flagellar biogenesis is governed by RpoN and the two-component system consisting of the histidine kinase FlgS and response regulator FlgR. In response to an unknown signal, FlgS autophosphorylates and transfers the phosphate to FlgR, initiating transcription from RpoN-dependent promoters. In the present study, export apparatus protein FlhA was examined as a potential signal protein. Deletion of its N-terminal cytoplasmic sequence dramatically decreased expression of two RpoN-dependent genes, flaB and flgE. Optical biosensing demonstrated a high-affinity interaction between FlgS and a peptide consisting of residues 1 to 25 of FlhA (FlhANT). The KD (equilibrium dissociation constant) was 21 nM and was characterized by fast-on (kon = 2.9 × 10(4) M(-1)s(-1)) and slow-off (koff = 6.2 × 10(-4) s(-1)) kinetics. FlgS did not bind peptides consisting of smaller fragments of the FlhANT sequence. Analysis of binding to purified fragments of FlgS demonstrated that the C-terminal portion of the protein containing the kinase domain binds FlhANT. FlhANT binding did not stimulate FlgS autophosphorylation in vitro, suggesting that FlhA facilitates interactions between FlgS and other structures required to stimulate autophosphorylation.

IMPORTANCE

The high-affinity binding of FlgS to FlhA characterized in this study points to an additional role for FlhA in flagellar assembly. Beyond its necessity for type III secretion, the N-terminal cytoplasmic sequence of FlhA is required for RpoN-dependent gene expression via interaction with the C-terminal kinase domain of FlgS.

Analysis of the activity and regulon of the two-component regulatory system composed by Cjj81176_1484 and Cjj81176_1483 of Campylobacter jejuni

Campylobacter jejuni is a leading cause of bacterial diarrheal disease and a frequent commensal of the intestinal tract in poultry and other animals. For optimal growth and colonization of hosts, C. jejuni employs two-component regulatory systems (TCSs) to monitor environmental conditions and promote proper expression of specific genes. We analyzed the potential of C. jejuni Cjj81176_1484 (Cjj1484) and Cjj81176_1483 (Cjj1483) to encode proteins of a cognate TCS that influences expression of genes possibly important for C. jejuni growth and colonization. Transcriptome analysis revealed that the regulons of the Cjj81176_1484 (Cjj1484) histidine kinase and the Cjj81176_1483 (Cjj1483) response regulator contain many common genes, suggesting that these proteins likely form a cognate TCS. We found that this TCS generally functions to repress expression of specific proteins with roles in metabolism, iron/heme acquisition, and respiration. Furthermore, the TCS repressed expression of Cjj81176_0438 and Cjj81176_0439, which had previously been found to encode a gluconate dehydrogenase complex required for commensal colonization of the chick intestinal tract. However, the TCS and other specific genes whose expression is repressed by the TCS were not required for colonization of chicks. We observed that the Cjj1483 response regulator binds target promoters in both unphosphorylated and phosphorylated forms and influences expression of some specific genes independently of the Cjj1484 histidine kinase. This work further expands the signaling mechanisms of C. jejuni and provides additional insights regarding the complex and multifactorial regulation of many genes involved in basic metabolism, respiration, and nutrient acquisition that the bacterium requires for optimal growth in different environments.

IMPORTANCE

Bacterial two-component regulatory systems (TCSs) link environmental cues to expression of specific genes that enable optimal bacterial growth or colonization of hosts. We found that the Campylobacter jejuni Cjj1484 histidine kinase and Cjj1483 response regulator function as a cognate TCS to largely repress expression of target genes encoding a gluconate dehydrogenase complex required for commensal colonization of the chick intestinal tract, as well as other genes encoding proteins for heme or iron acquisition, metabolism, and respiration. We also discovered different modes by which Cjj1483 may mediate repression with and without Cjj1484. This work provides insight into the signal transduction mechanisms of a leading cause of bacterial diarrheal disease and emphasizes the multifactorial and complex regulation of specific biological processes in C. jejuni.

The structure and regulation of flagella in Bacillus subtilis

Bacterial flagellar motility is among the most extensively studied physiological systems in biology, but most research has been restricted to using the highly similar Gram-negative species Escherichia coli and Salmonella enterica. Here, we review the recent advances in the study of flagellar structure and regulation of the distantly related and genetically tractable Gram-positive bacterium Bacillus subtilis. B. subtilis has a thicker layer of peptidoglycan and lacks the outer membrane of the Gram-negative bacteria; thus, not only phylogenetic separation but also differences in fundamental cell architecture contribute to deviations in flagellar structure and regulation. We speculate that a large number of flagella and the absence of a periplasm make B. subtilis a premier organism for the study of the earliest events in flagellar morphogenesis and the type III secretion system. Furthermore, B. subtilis has been instrumental in the study of heterogeneous gene transcription in subpopulations and of flagellar regulation at the translational and functional level.

Requirement of the flagellar protein export apparatus component FliO for optimal expression of flagellar genes in Helicobacter pylori

Flagellar biogenesis in Helicobacter pylori involves the coordinated expression of flagellar genes with assembly of the flagellum. The H. pylori flagellar genes are organized into three regulons based on the sigma factor needed for their transcription (RpoD [σ(80)], RpoN [σ(54)], or FliA [σ(28)]). Transcription of RpoN-dependent genes is activated by a two-component system consisting of the sensor kinase FlgS and the response regulator FlgR. While the cellular cues sensed by the FlgS/FlgR two-component system remain to be elucidated, previous studies revealed that disrupting certain components of the flagellar export apparatus inhibited transcription of the RpoN regulon. FliO is the least conserved of the membrane-bound components of the export apparatus and has not been annotated for any of the H. pylori genomes sequenced to date. A PSI-BLAST analysis identified a potential H. pylori FliO protein which membrane topology algorithms predict to possess a large N-terminal periplasmic domain that is absent from FliO of Escherichia coli and Salmonella, the paradigms for flagellar structure/function studies. FliO was necessary for flagellar biogenesis as well as wild-type levels of motility and transcription of RpoN-dependent and FliA-dependent flagellar genes in H. pylori strain B128. FliO also appears to be required for wild-type levels of the export apparatus protein FlhA in the membrane. Interestingly, the periplasmic and cytoplasmic domains were somewhat dispensable for flagellar gene regulation and assembly, suggesting that these domains have relatively minor roles in flagellar synthesis.

Themes and Variations: Regulation of RpoN-Dependent Flagellar Genes across Diverse Bacterial Species

Flagellar biogenesis in bacteria is a complex process in which the transcription of dozens of structural and regulatory genes is coordinated with the assembly of the flagellum. Although the overall process of flagellar biogenesis is conserved among bacteria, the mechanisms used to regulate flagellar gene expression vary greatly among different bacterial species. Many bacteria use the alternative sigma factor σ (54) (also known as RpoN) to transcribe specific sets of flagellar genes. These bacteria include members of the Epsilonproteobacteria (e.g., Helicobacter pylori and Campylobacter jejuni), Gammaproteobacteria (e.g., Vibrio and Pseudomonas species), and Alphaproteobacteria (e.g., Caulobacter crescentus). This review characterizes the flagellar transcriptional hierarchies in these bacteria and examines what is known about how flagellar gene regulation is linked with other processes including growth phase, quorum sensing, and host colonization.

Extracellular secretion of protease HtrA from Campylobacter jejuni is highly efficient and independent of its protease activity and flagellum

The serine protease HtrA of C. jejuni has been identified as a novel secreted virulence factor which opens cell-to-cell junctions by cleaving E-cadherin. Efficient C. jejuni transmigration across polarized human epithelial cells requires the intact flagellum and HtrA; however, the mechanism of HtrA secretion into the supernatant is unknown. Here we show that HtrA secretion is highly efficient and does not require its proteolytic activity because the protease-inactive S197A mutant is secreted like wild-type HtrA. In addition, the flagellar mutants ΔflaA/B, ΔfliI, ΔflgH, ΔflhA, ΔflhB, and ΔflgS were also able to secrete HtrA in high amounts, while they were strongly attenuated in secreting the well-known invasion antigen CiaB. We also tested several culture media and cell lines of different origin such as human, mouse, hamster, dog, and chicken for their ability to influence HtrA secretion. Interestingly, HtrA was effectively secreted in the presence of most but not all cell lines and media, albeit at different levels, but secretion was significantly higher when fetal calf serum (FCS) was added. These results demonstrate that HtrA secretion by Campylobacter proceeds independent of HtrA's protease activity, the flagellum and origin of cell lines, but can be strongly enhanced by molecular compound(s) present in FCS.