Spatiotemporal control of FlgZ activity impacts Pseudomonas aeruginosa flagellar motility

Spatial, temporal and numerical regulation of polar flagella assembly in Pseudomonas putida

The Gram-negative bacterium Pseudomonas putida bears a tuft of flagella at a single cell pole. New flagella must be assembled de novo every cell cycle to secure motility of both daughter cells. Here we show that the coordinated action of FimV, FlhF and FleN sets the location, timing and number of flagella assembled. The polar landmark proteins FimV and FlhF are independently targeted to the nascent new pole during or shortly after cell division, but FimV stabilizes FlhF association with the cell poles. FlhF determines the polar position of the flagella by targeting early flagellar components to the cell pole and preventing their nucleation at non-polar sites. FlhF also promotes efficient flagellar assembly and indirectly stimulates Class III flagellar promoter activation by promoting secretion of the anti-FliA anti-σ factor FlgM. The MinD-like ATPase FleN partitions between the cell poles and the cytoplasm. Cytoplasmic FleN regulates flagellar number by preventing excessive accumulation of FlhF at the cell poles that may otherwise lead to hyperflagellation, likely by antagonizing FleQ-dependent transcriptional activation. FimV is essential to FleN polar location. FimV and FleN temporally regulate the onset of flagellar assembly by preventing premature polar targeting of FlhF and the ensuing premature targeting of additional flagellar components. Our results shed new light on the mechanisms that ensure the timely assembly of the appropriate number of flagella at the correct polar location in polarly flagellated bacteria.

Molecular mechanism of flagellar motor rotation arrest in bacterial zoospores of Actinoplanes missouriensis before germination

Zoospores of the filamentous actinomycete Actinoplanes missouriensis swim vigorously using flagella and stop swimming to initiate germination in response to nutrient exposure. However, the molecular mechanisms underlying swimming cessation remain unknown. A protein (FtgA) of unknown function encoded by a chemotaxis gene cluster (che cluster-1) was found to be required for flagellar rotation arrest; the zoospores of ftgA-knockout mutants kept swimming awkwardly after germination. An ftgA-overexpressing strain exhibited a non-flagellated phenotype. Isolation of a suppressor strain from this strain and further in vivo experiments revealed that the extended N-terminal region of FliN, a component of the C-ring of the flagellar basal body, was involved in the function of FtgA; FliN-P101S canceled the flagellar rotation arrest by FtgA, as well as the negative effect of ftgA-overexpression on flagellation. Furthermore, bacterial two-hybrid assays suggested that FtgA interacted not only with the C-terminal core region of FliN but also with chemotaxis regulatory proteins CheA1 and CheW1-2, which are encoded by che cluster-1. We propose the following working model of motility regulation in A. missouriensis zoospores: the chemotaxis sensory complex initially captures FtgA to allow zoospores to swim and then releases FtgA to stop flagellar rotation (i.e., swimming) in response to external nutrient signals.

The global regulation of c-di-GMP and cAMP in bacteria

Nucleotide second messengers are highly versatile signaling molecules that regulate a variety of key biological processes in bacteria. The best-studied examples are cyclic AMP (cAMP) and bis-(3'-5')-cyclic dimeric guanosine monophosphate (c-di-GMP), which both act as global regulators. Global regulatory frameworks of c-di-GMP and cAMP in bacteria show several parallels but also significant variances. In this review, we illustrate the global regulatory models of the two nucleotide second messengers, compare the different regulatory frameworks between c-di-GMP and cAMP, and discuss the mechanisms and physiological significance of cross-regulation between c-di-GMP and cAMP. c-di-GMP responds to numerous signals dependent on a great number of metabolic enzymes, and it regulates various signal transduction pathways through its huge number of effectors with varying activities. In contrast, due to the limited quantity, the cAMP metabolic enzymes and its major effector are regulated at different levels by diverse signals. cAMP performs its global regulatory function primarily by controlling the transcription of a large number of genes via cAMP receptor protein (CRP) in most bacteria. This review can help us understand how bacteria use the two typical nucleotide second messengers to effectively coordinate and integrate various physiological processes, providing theoretical guidelines for future research.

Insights into the molecular basis of c-di-GMP signalling in Pseudomonas aeruginosa

The opportunistic human pathogen Pseudomonas aeruginosa can cause severe infections in immunocompromized people or cystic fibrosis (CF) patients. Because of its remarkable ability to invade the host and withstand the bacteriocidal effect of most conventional antibiotics, the infection caused by P. aeruginosa has become a major concern for human health. The switch from acute to chronic infection is governed by the second messenger bis-(3'-5')-cyclic dimeric guanosine mono-phosphate (c-di-GMP) in P. aeruginosa, and c-di-GMP is now recognized to regulate many important biological processes in pathogenesis. The c-di-GMP signalling mechanisms in P. aeruginosa have been studied extensively in the past decade, revealing complicated c-di-GMP metabolism and signalling network. In this review, the underlying mechanisms of this signalling network will be discussed, mainly focussing on how environmental cues regulate c-di-GMP signalling, protein-protein interaction mediated functional regulation, heterogeneity of c-di-GMP and cross talk between c-di-GMP signalling and other signalling systems. Understanding the molecular mechanism underlying the complex c-di-GMP signalling network would be beneficial for developing therapeutic approaches and antibacterial agents to combat the threat from P. aeruginosa.

The FilZ Protein Contains a Single PilZ Domain and Facilitates the Swarming Motility of Pseudoalteromonas sp. SM9913

Swarming regulation is complicated in flagellated bacteria, especially those possessing dual flagellar systems. It remains unclear whether and how the movement of the constitutive polar flagellum is regulated during swarming motility of these bacteria. Here, we report the downregulation of polar flagellar motility by the c-di-GMP effector FilZ in the marine sedimentary bacterium Pseudoalteromonas sp. SM9913. Strain SM9913 possesses two flagellar systems, and filZ is located in the lateral flagellar gene cluster. The function of FilZ is negatively controlled by intracellular c-di-GMP. Swarming in strain SM9913 consists of three periods. Deletion and overexpression of filZ revealed that, during the period when strain SM9913 expands quickly, FilZ facilitates swarming. In vitro pull-down and bacterial two-hybrid assays suggested that, in the absence of c-di-GMP, FilZ interacts with the CheW homolog A2230, which may be involved in the chemotactic signal transduction pathway to the polar flagellar motor protein FliM_p, to interfere with polar flagellar motility. When bound to c-di-GMP, FilZ loses its ability to interact with A2230. Bioinformatic investigation indicated that filZ-like genes are present in many bacteria with dual flagellar systems. Our findings demonstrate a novel mode of regulation of bacterial swarming motility.

Deep Profiling of the Proteome Dynamics of Pseudomonas aeruginosa Reference Strain PAO1 under Different Growth Conditions

As one of the most common bacterial pathogens causing nosocomial infections, Pseudomonas aeruginosa is highly adaptable to survive under various conditions. Here, we profiled the abundance dynamics of 3489 proteins across different growth stages in the P. aeruginosa reference strain PAO1 using data-independent acquisition-based quantitative proteomics. The proteins differentially expressed during the planktonic growth exhibit several distinct patterns of expression profiles and are relevant to various biological processes, highlighting the continuous adaptation of the PAO1 proteome during the transition from the acceleration phase to the stationary phase. By contrasting the protein expressions in a biofilm to planktonic cells, the known roles of T6SS, phenazine biosynthesis, quorum sensing, and c-di-GMP signaling in the biofilm formation process were confirmed. Additionally, we also discovered several new functional proteins that may play roles in the biofilm formation process. Lastly, we demonstrated the general concordance of protein expressions within operons across various growth states, which permits the study of coexpression protein units, and reversely, the study of regulatory components in the operon structure. Taken together, we present a high-quality and valuable resource on the proteomic dynamics of the P. aeruginosa reference strain PAO1, with the potential of advancing our understanding of the overall physiology of Pseudomonas bacteria.

GGDEF domain as spatial on-switch for a phosphodiesterase by interaction with landmark protein HubP

In bacteria, the monopolar localization of enzymes and protein complexes can result in a bimodal distribution of enzyme activity between the dividing cells and heterogeneity of cellular behaviors. In Shewanella putrefaciens, the multidomain hybrid diguanylate cyclase/phosphodiesterase PdeB, which degrades the secondary messenger c-di-GMP, is located at the flagellated cell pole. Here, we show that direct interaction between the inactive diguanylate cyclase (GGDEF) domain of PdeB and the FimV domain of the polar landmark protein HubP is crucial for full function of PdeB as a phosphodiesterase. Thus, the GGDEF domain serves as a spatially controlled on-switch that effectively restricts PdeBs activity to the flagellated cell pole. PdeB regulates abundance and activity of at least two crucial surface-interaction factors, the BpfA surface-adhesion protein and the MSHA type IV pilus. The heterogeneity in c-di-GMP concentrations, generated by differences in abundance and timing of polar appearance of PdeB, orchestrates the population behavior with respect to cell-surface interaction and environmental spreading.

Pseudomonas aeruginosa post-translational responses to elevated c-di-GMP levels

C-di-GMP signaling can directly influence bacterial behavior by affecting the functionality of c-di-GMP-binding proteins. In addition, c-di-GMP can exert a global effect on gene transcription or translation, e.g., via riboswitches or by binding to transcription factors. In this study, we investigated the effects of changes in intracellular c-di-GMP levels on gene expression and protein production in the opportunistic pathogen Pseudomonas aeruginosa. We induced c-di-GMP production via an ectopically introduced diguanylate cyclase and recorded the transcriptional, translational as well as proteomic profile of the cells. We demonstrate that rising levels of c-di-GMP under growth conditions otherwise characterized by low c-di-GMP levels caused a switch to a non-motile, auto-aggregative P. aeruginosa phenotype. This phenotypic switch became apparent before any c-di-GMP-dependent role on transcription, translation, or protein abundance was observed. Our results suggest that rising global c-di-GMP pools first affects the motility phenotype of P. aeruginosa by altering protein functionality and only then global gene transcription.

From Input to Output: The Lap/c-di-GMP Biofilm Regulatory Circuit

Biofilms are the dominant bacterial lifestyle. The regulation of the formation and dispersal of bacterial biofilms has been the subject of study in many organisms. Over the last two decades, the mechanisms of Pseudomonas fluorescens biofilm formation and regulation have emerged as among the best understood of any bacterial biofilm system. Biofilm formation by P. fluorescens occurs through the localization of an adhesin, LapA, to the outer membrane via a variant of the classical type I secretion system. The decision between biofilm formation and dispersal is mediated by LapD, a c-di-GMP receptor, and LapG, a periplasmic protease, which together control whether LapA is retained or released from the cell surface. LapA localization is also controlled by a complex network of c-di-GMP-metabolizing enzymes. This review describes the current understanding of LapA-mediated biofilm formation by P. fluorescens and discusses several emerging models for the regulation and function of this adhesin.

Taming the flagellar motor of pseudomonads with a nucleotide messenger

Pseudomonads rely on the flagellar motor to rotate a polar flagellum for swimming and swarming, and to sense surfaces for initiating the motile-to-sessile transition to adopt a surface-dwelling lifestyle. Deciphering the function and regulation of the flagellar motor is of paramount importance for understanding the behaviours of environmental and pathogenic pseudomonads. Recent studies disclosed the preeminent role played by the messenger c-di-GMP in controlling the real-time performance of the flagellar motor in pseudomonads. The studies revealed that c-di-GMP controls the dynamic exchange of flagellar stator units to regulate motor torque/speed and modulates the frequency of flagellar motor switching via the chemosensory signalling pathways. Apart from being a rotary motor, the flagellar motor is emerging as a mechanosensor that transduces surface-induced mechanical signals into an increase of cellular c-di-GMP concentration to initiate the cellular programs required for long-term colonization. Collectively, the studies generate long-awaited mechanistic insights into how c-di-GMP regulates bacterial motility and the motile-to-sessile transition. The new findings also raise the fundamental questions of how cellular c-di-GMP concentrations are dynamically coupled to flagellar output and the proton-motive force, and how c-di-GMP signalling is coordinated spatiotemporally to fine-tune flagellar response and the behaviour of pseudomonads in solutions and on surfaces.

c-di-GMP-related phenotypes are modulated by the interaction between a diguanylate cyclase and a polar hub protein

c-di-GMP is a major player in the switch between biofilm and motile lifestyles. Several bacteria exhibit a large number of c-di-GMP metabolizing proteins, thus a fine-tuning of this nucleotide levels may occur. It is hypothesized that some c-di-GMP metabolizing proteins would provide the global c-di-GMP levels inside the cell whereas others would maintain a localized pool, with the resulting c-di-GMP acting at the vicinity of its production. Although attractive, this hypothesis has yet to be demonstrated in Pseudomonas aeruginosa. We found that the diguanylate cyclase DgcP interacts with the cytosolic region of FimV, a polar peptidoglycan-binding protein involved in type IV pilus assembly. Moreover, DgcP is located at the cell poles in wild type cells but scattered in the cytoplasm of cells lacking FimV. Overexpression of dgcP leads to the classical phenotypes of high c-di-GMP levels (increased biofilm and impaired motilities) in the wild-type strain, but not in a ΔfimV background. Therefore, our findings suggest that DgcP activity is regulated by FimV. The polar localization of DgcP might contribute to a local c-di-GMP pool that can be sensed by other proteins at the cell pole, bringing to light a specialized function for a specific diguanylate cyclase.