Pseudomonas putida mutants in the exbBexbDtonB gene cluster are hypersensitive to environmental and chemical stressors

TonB systems are required for Aeromonas hydrophila motility by controlling the secretion of flagellin

The TonB system is required for the active transport of iron compounds across the outer membrane in Gram-negative bacteria. Our previous data indicated that three TonB systems act coordinately to contribute to the motility of Aeromonas hydrophila NJ-35. In this study, we found that flagellum biogenesis was defective in the ΔtonB123 mutant. Subcellular localization indicated that the flagellin subunits FlaA and FlaB were trapped in the cytoplasm of ΔtonB123 mutant with reduced molecular mass. Overexpression of FlaA or FlaB in the ΔtonB123 mutant was unable to restore the secretion of flagellin subunits. Further investigation demonstrated that flagellins in the ΔtonB123 mutant showed a weak affinity for the flagellin-specific chaperone FliS, which is necessary for the export of flagellins. Deglycosylation analysis indicated that flagellins in the cytoplasm of the ΔtonB123 mutant were almost nonglycosylated. Our data suggested that disruption of tonB123 impairs the formation of flagella by inhibiting flagellin glycosylation and decreasing the binding affinity of flagellin for the chaperone FliS. Taken together, our findings indicate a new role of the TonB system in flagellar biogenesis in A. hydrophila.

The TonBm-PocAB System Is Required for Maintenance of Membrane Integrity and Polar Position of Flagella in Pseudomonas putida

TonB-ExbB-ExbD-like energy transduction systems are widespread among Gram-negative bacteria. While most species have only one copy of tonB-exbBD genes, the Pseudomonas species possess more TonB-ExbBD homologues. One of them, the TonB3-PocA-PocB complex, was recently shown to be required for polar localization of FlhF and, thus, the flagella in Pseudomonas aeruginosa Here, we show that the orthologous TonB_m-PocA-PocB complex is important for polar localization of FlhF and flagella in Pseudomonas putida as well. Additionally, the system is necessary for maintaining membrane integrity, as the inactivation of the TonB_m-PocAB complex results in increased membrane permeability, lowered stress tolerance, and conditional cell lysis. Interestingly, the functionality of TonB_m-PocAB complex is more important for stationary than for exponentially growing bacteria. The whole-cell proteome analysis provided a likely explanation for this growth phase dependence, as extensive reprogramming was disclosed in an exponentially growing tonB_m deletion strain, while only a few proteomic changes, mostly downregulation of outer membrane proteins, were determined in the stationary-phase ΔtonB_m strain. We propose that this response in exponential phase, involving, inter alia, activation of AlgU and ColR regulons, can compensate for TonB_m-PocAB's deficiency, while stationary-phase cells are unable to alleviate the lack of TonB_m-PocAB. Our results suggest that mislocalization of flagella does not cause the membrane integrity problems; rather, the impaired membrane intactness of the TonB_m-PocAB-deficient strain could be the reason for the random placement of flagella.IMPORTANCE The ubiquitous Pseudomonas species are well adapted to survive in a wide variety of environments. Their success relies on their versatile metabolic, signaling, and transport ability but also on their high intrinsic tolerance to various stress factors. This is why the study of the stress-surviving mechanisms of Pseudomonas species is of utmost importance. The stress tolerance of Pseudomonads is mainly achieved through the high barrier property of their membranes. Here, we present evidence that the TonB-ExbBD-like TonB_m-PocAB system is involved in maintaining the membrane homeostasis of Pseudomonas putida, and its deficiency leads to lowered stress tolerance and conditional cell lysis.

Roles of three TonB systems in the iron utilization and virulence of the Aeromonas hydrophila Chinese epidemic strain NJ-35

The TonB system functions in iron transport and has been identified in certain Gram-negative bacteria. Recently, we reported three TonB systems in the Aeromonas hydrophila Chinese epidemic strain NJ-35, but the functions of these systems have not been thoroughly elucidated to date. In this study, we investigated the role of these TonB systems in A. hydrophila iron utilization and virulence. We found that tonB1 and tonB2 were preferentially transcribed in iron-chelated conditions, where gene expression levels were approximately 8- and 68-fold higher compared with iron-rich conditions, respectively; tonB3 was consistently transcribed at a low level under iron-repleted and iron-depleted conditions. Only the TonB2 system was required to utilize iron-binding proteins. The tonB123 mutant showed increased susceptibility to erythromycin and roxithromycin. In addition, all three tonB genes were involved in A. hydrophila virulence in zebrafish, and various phenotypes associated with environmental survival were changed with varying degrees in each tonB mutant. TonB2 plays a relatively major role in adhesion, motility, and biofilm formation, while TonB3 is more involved in the anti-phagocytosis of A. hydrophila. In each observed phenotype, no significant difference was found between the single- and double-deletion mutants, whereas the triple-deletion mutant exhibited the most serious defects, indicating that all three TonB systems of A. hydrophila coordinately complement one another. In conclusion, this study elucidates the importance of TonB in iron acquisition and virulence of A. hydrophila, which lays the foundation for future studies regarding the survival mechanisms of this bacterium in iron-restricted environments.

A role for TonB1 in biofilm formation and quorum sensing inPseudomonas aeruginosa

This study revealed that a Pseudomonas aeruginosa tonB1 mutant was unable to produce a mature biofilm and showed reduced swarming and twitching motilities compared with the parent strain. The tonB1 mutant was also found to produce significantly lower cell-free and cell-associated levels of the quorum sensing (QS) signal molecule 3-oxo-C12-AHL. Altered biofilm and motility phenotypes were restored to wildtype with the addition of exogenous N-acylhomoserine lactones. These functions were independent of the role of TonB1 in iron uptake. This is the first time that a link has been established between TonB1 activity and QS.

The ttgGHI solvent efflux pump operon of Pseudomonas putida DOT-T1E is located on a large self-transmissible plasmid

Pseudomonas putida DOT-T1E is a solvent-tolerant strain able to grow in the presence of > 1% (v/v) toluene in the culture medium. A set of multidrug efflux pumps have been found to play a major role in the tolerance of this bacterium to organic solvents (Rojas et al., J Bacteriol 183: 3967-3973). In the course of studies of the mechanisms underlying solvent tolerance in DOT-T1E, we isolated a spontaneous solvent-sensitive mutant derivative which had lost the genes encoding the TtgGHI efflux pump, the most important extrusion element in quantitative terms. Genomic comparisons between the mutant and its parental strain by microarray analysis revealed that in addition to the ttgVW-ttgGHI gene cluster, another group of genes, highly similar to those found in the Tn4653A and ISPpu12 transposable elements of the TOL plasmid pWW0 from P. putida mt-2, were also absent from this strain. Further analysis demonstrated that strain DOT-T1E harboured a large plasmid (named pGRT1) that was lost from the solvent-sensitive mutant. Mapping analysis revealed that the ttgVW-ttgGHI genes and the Tn4653A-like transposon are borne by the pGRT1 plasmid. Plasmid pGRT1 is highly stable and its frequency of loss is below 10(-8) per cell per generation under a variety of growth conditions, including nutritional and physical stresses. The pGRT1 plasmid is self-transmissible, and its acquisition by the toluene-sensitive P. putida KT2440 and Pseudomonas aeruginosa PAO1 increased the recipient's tolerance to toluene up to levels similar to those exhibited by P. putida DOT-T1E. We discuss the importance and potential benefits of this plasmid for the development of bacteria with enhanced solvent tolerance, and its potential impact for bioremediation and whole-cell biotransformations.

Analysis of environmental transcriptomes by DNA microarrays

In this work we investigated the correlations between global gene expression patterns and environmental parameters in natural ecosystems. We studied the preferential gene expression of the iron oxidizer bacterium Leptospirillum ferrooxidans to adapt its physiology to changes in the physicochemical parameters in its natural medium. Transcriptome analysis by DNA microarrays can proportionate an instant picture about the preferential gene expression between two different environmental samples. However, this type of analysis is very difficult and complex in natural ecosystems, mainly because of the broad biodiversity and multiple environmental parameters that may affect gene expression. The necessity of high-quality RNA preparations as well as complicated data analysis are also technological limitations. The low prokaryotic diversity of the extremely acidic and iron-rich waters of the Tinto River (Spain) ecosystem, where L. ferrooxidans is abundant, allows the opportunity to achieve global gene expression studies and to associate gene function with environmental parameters. We applied a total RNA amplification protocol validated previously for the amplification of the environmental transcriptome (meta-transcriptome). The meta-transcriptome of two sites from the Tinto River mainly differing in the salt and oxygen contents were amplified and analysed by a L. ferrooxidans DNA microarray. The results showed a clear preferential induction of genes involved in certain physicochemical parameters like: high salinity (ectAB, otsAB), low oxygen concentration (cydAB), iron uptake (fecA-exbBD-tonB), oxidative stress (carotenoid synthesis, oxyR, recG), potassium (kdpBAC) or phosphate concentrations (pstSCAB), etc. We conclude that specific gene expression patterns can be useful indicators for the physiological conditions in a defined ecosystem. Also, the upregulation of certain genes and operons reveals information about the environmental conditions (nutrient limitations, stresses, etc.).

A mutation in an exbD gene reduces tagetitoxin production by Pseudomonas syringae pv. tagetis

A mutant of Pseudomonas syringae pv. tagetis EB037 with limited ability to produce tagetitoxin was isolated after transposon mutagenesis and the mutation was characterized. The mutation occurred in a gene with a high degree of sequence identity to exbD. exbD is contiguous with tonB and exbB upstream and with a gene for a TonB-dependent receptor downstream. Using reverse transcription - polymerase chain reaction with RNA from the wild-type and exbD mutant strains, we demonstrated that the mutation in exbD did not have a polar affect on the expression of downstream genes. The exbD mutant was able to grow well in conditions where iron is not freely available. Siderophore production by the exbD mutant was similar to that of the wild-type strain. We conclude that the mutation in exbD disrupts tagetitoxin production without compromising iron metabolism. The results indicate that tagetitoxin export by P. syringae pv. tagetis involves an efflux pump that requires a functional TonB system that is not essential for normal iron metabolism.

The RpoT regulon of Pseudomonas putida DOT-T1E and its role in stress endurance against solvents

Pseudomonas putida encodes 20 extracytoplasmic sigma factors (ECFs). In this study, we show that one of these ECFs, known as ECF-Pp12 (PP3006), plays a role in tolerance of toluene and other organic solvents. Based on this finding, we have called the gene that encodes this new ECF rpoT. The rpoT gene forms an operon with the preceding gene and with the gene located downstream. The translated gene product of the open reading frame PP3005 is an inner membrane protein, whereas the PP3007 protein is periplasmic. A nonpolar DeltarpoT mutant was generated by homologous recombination, and survival of the mutant was tested under various stress conditions. The mutant strain was hypersensitive to toluene and other solvents but just as tolerant as the wild type of stress imposed by heat, antibiotics, NaCl, paraquat, sodium dodecyl sulfate, H(2)O(2), and benzoate. In the DeltarpoT mutant background, expression of around 50 transcriptional units was affected: 31 cistrons were upregulated, and 23 cistrons were downregulated. This indicates that about 1% of all P. putida genes are under the direct or indirect influence of RpoT. The rpoT gene controls the expression of a number of membrane proteins, including components of the respiratory chains, porins, transporters, and multidrug efflux pumps. Hypersensitivity of the P. putida RpoT-deficient mutant to organic solvents can be attributed to the fact that in the DeltarpoT strain, expression of the toluene efflux pump ttgGHI genes is severalfold lower than in the parental strain.

Growth phase-dependent expression of the Pseudomonas putida KT2440 transcriptional machinery analysed with a genome-wide DNA microarray

Bacterial transcriptional networks are built on a hierarchy of regulators, on top of which lie the components of the RNA polymerase (in particular the sigma factors) and the global control elements, which play a pivotal role. We have designed a genome-wide oligonucleotide-based DNA microarray for Pseudomonas putida KT2440. In combination with real-time reverse transcription polymerase chain reaction (RT-PCR), we have used it to analyse the expression pattern of the genes encoding the RNA polymerase subunits (the core enzyme and the 24 sigma factors), and various proteins involved in global regulation (Crc, Lrp, Fur, Anr, Fis, CsrA, IHF, HupA, HupB, HupN, BipA and several MvaT-like proteins), during the shift from exponential growth in rich medium into starvation and stress brought about by the entry into stationary phase. Expression of the genes encoding the RNA polymerase core and the vegetative sigma factor decreased in stationary phase, while that of sigma(S) increased. Data obtained for sigma(N), sigma(H), FliA and for the 19 extracytoplasmic function (ECF)-like sigma factors suggested that their mRNA levels change little upon entry into stationary phase. Expression of Crc, BipA, Fis, HupB, HupN and the MvaT-like protein PP3693 decreased in stationary phase, while that of HupA and the MvaT-like protein PP3765 increased significantly. Expression of IHF was indicative of post-transcriptional control. These results provide the first global study of the expression of the transcriptional machinery through the exponential stationary-phase shift in P. putida.

Role of iron and the TonB system in colonization of corn seeds and roots by Pseudomonas putida KT2440

Iron, which is abundant in corn (Zea mays L.) seeds, plays an important role in the initial establishment of Pseudomonas putida KT2440 populations on seeds. Sequestration of seed-borne iron by chelators decreases the capacity of KT2440 to initiate attachment to corn seeds. The importance of iron for this plant-bacteria interaction is further supported by the fact that mutations in the TonB system, which is key for iron uptake, result in reduced seed colonization. TonB is also a primary determinant of the fitness of P. putida in the rhizosphere, as a deletion mutant shows a clear competitive disadvantage during colonization of corn roots.