Siderophores and outer membrane proteins of antagonistic, plant-growth-stimulating, root-colonizing Pseudomonas spp

Proteomic response to phosphorus deficiency and aluminum stress of three aluminum-tolerant phosphobacteria isolated from acidic soils

Aluminum (Al)-tolerant phosphobacteria enhance plant growth in acidic soils by improving Al complexing and phosphorus (P) availability. However, the impact of Al stress and P deficiency on bacterial biochemistry and physiology remains unclear. We investigated the single and mutual effects of Al stress (10 mM) and P deficiency (0.05 mM) on the proteome of three aluminum-tolerant phosphobacteria: Enterobacter sp. 198, Enterobacter sp. RJAL6, and Klebsiella sp. RCJ4. Cultivated under varying conditions, P deficiency upregulated P metabolism proteins while Al exposure downregulated iron-sulfur and heme-containing proteins and upregulated iron acquisition proteins. This demonstrated that Al influence on iron homeostasis and bacterial central metabolism. This study offers crucial insights into bacterial behavior in acidic soils, benefiting the development of bioinoculants for crops facing Al toxicity and P deficiency. This investigation marks the first proteomic study on the interaction between high Al and P deficiency in acid soils-adapted bacteria.

Plant mineral nutrition and disease resistance: A significant linkage for sustainable crop protection

Complete and balanced nutrition has always been the first line of plant defense due to the direct involvement of mineral elements in plant protection. Mineral elements affect plant health directly by modulating the activity of redox enzymes or improving the plant vigor indirectly by altering root exudates, and changing microflora population dynamics, rhizosphere soil nutrient content, pH fluctuation, lignin deposition, and phytoalexin biosynthesis. Nitrogen (N) is one of the most important macronutrients having a significant impact on the host-pathogen axis. N negatively affects the plant's physical defense along with the production of antimicrobial compounds, but it significantly alleviates defense-related enzyme levels that can eventually assist in systemic resistance. Potassium (K) is an essential plant nutrient, when it is present in adequate concentration, it can certainly increase the plant's polyphenolic concentrations, which play a critical role in the defense mechanism. Although no distinguished role of phosphorus (P) is observed in plant disease resistance, a high P content may increase the plant's susceptibility toward the invader. Manganese (Mn) is one of the most important micronutrients, which have a vital effect on photosynthesis, lignin biosynthesis, and other plant metabolic functions. Zinc (Zn) is a part of enzymes that are involved in auxin synthesis, infectivity, phytotoxin, and mycotoxin production in pathogenic microorganisms. Similarly, many other nutrients also have variable effects on enhancing or decreasing the host susceptibility toward disease onset and progression, thereby making integrative plant nutrition an indispensable component of sustainable agriculture. However, there are still many factors influencing the triple interaction of host-pathogen-mineral elements, which are not yet unraveled. Thereby, the present review has summarized the recent progress regarding the use of macro- and micronutrients in sustainable agriculture and their role in plant disease resistance.

Graphene oxide influence in soil bacteria is dose dependent and changes at osmotic stress: growth variation, oxidative damage, antioxidant response, and plant growth promotion traits of a Rhizobium strain

Climate change events, such as drought, are increasing and soil bacteria can be severely affected. Moreover, the accumulation of emerging pollutants is expected to rapidly increase, and their impact on soil organisms, their interactions, and the services they provide is poorly known. The use of graphene oxide (GO) has been increasing due to its enormous potential for application in several areas and it is expected that concentration in soil will increase in the future, potentially causing disturbances in soil microorganisms not yet identified.Here we show the effects that GO nanosheets can cause on soil bacteria, in particular those that promote plant growth, in control and 10% polyethylene glycol (PEG) conditions. Low concentrations of GO nanosheets did not affect the growth of Rhizobium strain E20-8, but under osmotic stress (PEG) GO decreased bacterial growth even at lower concentrations. GO caused oxidative stress, with antioxidant mechanisms being induced to restrain damage, effectively at lower concentrations, but less effective at higher concentrations, and oxidative damage overcame. Under osmotic stress, alginate and glycine betaine osmoregulated the bacteria. Simultaneous exposure to PEG and GO induced oxidative damage. Plant growth promotion traits (indole acetic acid and siderophores production) were increased by osmotic stress and GO did not disturb these abilities. In the context of climate change, our findings might be relevant as they can form the premises for the implementation of crop production methodologies adapted to the new prevailing conditions, which include the presence of nanoparticles in the soil and more frequent and severe drought.

Evaluation of INSeq To Identify Genes Essential for Pseudomonas aeruginosa PGPR2 Corn Root Colonization

The reciprocal interaction between rhizosphere bacteria and their plant hosts results in a complex battery of genetic and physiological responses. In this study, we used insertion sequencing (INSeq) to reveal the genetic determinants responsible for the fitness of Pseudomonas aeruginosa PGPR2 during root colonization. We generated a random transposon mutant library of Pseudomonas aeruginosa PGPR2 comprising 39,500 unique insertions and identified genes required for growth in culture and on corn roots. A total of 108 genes were identified as contributing to the fitness of strain PGPR2 on roots. The importance in root colonization of four genes identified in the INSeq screen was verified by constructing deletion mutants in the genes and testing them for the ability to colonize corn roots singly or in competition with the wild type. All four mutants were affected in corn root colonization, displaying 5- to 100-fold reductions in populations in single inoculations, and all were outcompeted by the wild type by almost 100-fold after seven days on corn roots in mixed inoculations of the wild type and mutant. The genes identified in the screen had homology to genes involved in amino acid catabolism, stress adaptation, detoxification, signal transduction, and transport. INSeq technology proved a successful tool to identify fitness factors in Paeruginosa PGPR2 for root colonization.

Unearthing the genomes of plant-beneficial Pseudomonas model strains WCS358, WCS374 and WCS417

Background

Plant growth-promoting rhizobacteria (PGPR) can protect plants against pathogenic microbes through a diversity of mechanisms including competition for nutrients, production of antibiotics, and stimulation of the host immune system, a phenomenon called induced systemic resistance (ISR). In the past 30 years, the Pseudomonas spp. PGPR strains WCS358, WCS374 and WCS417 of the Willie Commelin Scholten (WCS) collection have been studied in detail in pioneering papers on the molecular basis of PGPR-mediated ISR and mechanisms of biological control of soil-borne pathogens via siderophore-mediated competition for iron.

Results

The genomes of the model WCS PGPR strains were sequenced and analyzed to unearth genetic cues related to biological questions that surfaced during the past 30 years of functional studies on these plant-beneficial microbes. Whole genome comparisons revealed important novel insights into iron acquisition strategies with consequences for both bacterial ecology and plant protection, specifics of bacterial determinants involved in plant-PGPR recognition, and diversity of protein secretion systems involved in microbe-microbe and microbe-plant communication. Furthermore, multi-locus sequence alignment and whole genome comparison revealed the taxonomic position of the WCS model strains within the Pseudomonas genus. Despite the enormous diversity of Pseudomonas spp. in soils, several plant-associated Pseudomonas spp. strains that have been isolated from different hosts at different geographic regions appear to be nearly isogenic to WCS358, WCS374, or WCS417. Interestingly, all these WCS look-a-likes have been selected because of their plant protective or plant growth-promoting properties.

Conclusions

The genome sequences of the model WCS strains revealed that they can be considered representatives of universally-present plant-beneficial Pseudomonas spp. With their well-characterized functions in the promotion of plant growth and health, the fully sequenced genomes of the WCS strains provide a genetic framework that allows for detailed analysis of the biological mechanisms of the plant-beneficial traits of these PGPR. Considering the increasing focus on the role of the root microbiome in plant health, functional genomics of the WCS strains will enhance our understanding of the diversity of functions of the root microbiome.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1632-z) contains supplementary material, which is available to authorized users.

Plant growth promoting activity of an auxin and siderophore producing isolate of Streptomyces under saline soil conditions

A biocontrol Streptomyces isolate (C) was tested for its plant growth promoting qualities under saline conditions. Exposure to elevated osmotic strengths up to 300 mM NaCl increased dry weight and cfu/ml significantly. The isolate C produced indolyl-3-acetic acid (IAA) into the medium in the amount of 2.4 μg/ml. The amount of auxin increased after adding salt and reached to 4.7 μg/ml in 300 mM NaCl. Biosynthesis of siderophore was detectable and increased in presence of NaCl. Streptomyces isolate C showed good solubilization of tricalcium phosphate in culture medium with 92 mg/l. Solubilization decreased in presence of NaCl. Soil treatment with isolate C increased the growth and development of wheat plant in normal and saline conditions. In this treatment there were significant increases in germination rate, percentage and uniformity, shoot length and dry weight compared to the control. Applying the bacterial inocula increased the concentration of N, P, Fe and Mn in wheat shoots grown in normal and saline soil, but had non-significant effect on other micro and macronutrients concentrations. Results of this study show that Streptomyces isolate C has potential to be utilized as biofertilizer in saline soils.

Physiological and Morphological Changes Induced by Nutrient Limitation of Pseudomonas fluorescens 378 in Continuous Culture

Pseudomonas fluorescens 378 was studied in continuous culture at a dilution rate of 0.05 or 0.15 h and under a limitation of carbon/energy, nitrogen, phosphorus, iron(III), or oxygen. Cultures were examined for nutritional consumption, production of biosurfactant AP-6 and lipase, and electron microscopy morphology. Morphological features were lysis and plasmolysis of the cells, vacuoles in the cells, granules in cell nuclei, and DNA coagulation during transmission electron microscopy preparation. Biosurfactant and lipase production were lost after 8 to 15 retention times, but under iron limitation and at low dilution rate they were maintained for more than 30 retention times. Consumption of nutrients varied between different cultures. Between 2.4 and 6.0 g of succinic acid per g (dry weight) was consumed; the highest value was obtained under phosphorus limitation. The uptake of nitrogen was mostly about 0.16 g/g (dry weight), and that of phosphorus varied between 13 and 58 mg/g (dry weight). Phosphorus-limited cells reduced their phosphorus consumption by at least 50% compared with other limitations. Cell morphology varied among different cultures. Up to 25% cell lysis occurred at the higher dilution rate. The frequencies of plasmolysis varied between 0 and 85%. Granules in nuclei were found in 65 to 100% of the cells. Vacuoles appeared mostly in low numbers, but at the lower dilution rate under phosphorus or iron limitation the frequencies increased to between 25 and 85%. At high dilution rate, the DNA coagulated in 30 to 70% of the cells. Multivariate data analysis demonstrated a general difference between the two tested dilution rates; i.e., both nutritional and morphological features differed more between the two tested dilution rates than between the different limitations. Cultures at the lower dilution rate changed more with time; this was especially pronounced for phosphorus or iron limitation. The data analysis also showed a correlation between plasmolysis or vacuoles in the cells and an increased carbon uptake under phosphorus limitation.

Agglutination, adherence, and root colonization by fluorescent pseudomonads

Two fractions of agglutination activity towards fluorescent pseudomonads were detected in root washes of potato, tomato, wheat, and bean. High-molecular-mass (>10 Da) components in crude root washes agglutinated only particular saprophytic, fluorescent Pseudomonas isolates. Ion-exchange treatment of the crude root washes resulted in preparations of lower-molecular-mass (10 to 10 Da) fractions which agglutinated almost all Pseudomonas isolates examined. Also, components able to suppress agglutination reactions of pseudomonads with the lower-molecular-mass root components were detected in crude root washes of all crops studied. Pseudomonas isolates were differentially agglutinated by both types of root components. The involvement of these two types of root components in short-term adherence and in colonization was studied in potato, tomato, and grass, using Pseudomonas isolates from these crops. Short-term adherence of isolates to roots was independent of their agglutination with either type of root components. With agglutination-negative mutants, the high-molecular-mass components seemed to be involved in adherence of Pseudomonas putida Corvallis to roots of all crops studied. Short-term adherence to roots of four Pseudomonas isolates could be influenced by addition of both crude and ion-exchange-treated root washes, depending on their agglutination phenotype with these root wash preparations. Potato root colonization by 10 different isolates from this crop, over a period of 7 days, was not correlated with their agglutination phenotype. Agg mutants of P. putida Corvallis were not impaired in root colonization. It is concluded that the root agglutinins studied can be involved in short-term adherence of pseudomonads to roots but do not play a decisive role in their root colonization.

Effect of Surface-Active Pseudomonas spp. on Leaf Wettability

Different strains of Pseudomonas putida and P. fluorescens isolated from the rhizosphere and phyllosphere were tested for surface activity in droplet cultures on polystyrene. Droplets of 6 of the 12 wild types tested spread over the surface during incubation, and these strains were considered surface active; strains not showing this reaction were considered non-surface active. Similar reactions were observed on pieces of wheat leaves. Supernatants from centrifuged broth cultures behaved like droplets of suspensions in broth; exposure to 100 degrees C destroyed the activity. Average contact angles of the supernatants of surface-active and non-surface-active strains on polystyrene were 24 degrees and 72 degrees , respectively. The minimal surface tension of supernatants of the surface-active strains was about 46 mN/m, whereas that of the non-surface-active strains was 64 mN/m (estimations from Zisman plots). After 6 days of incubation, wheat flag leaves sprayed with a dilute suspension of a surface-active strain of P. putida (WCS 358RR) showed a significant increase in leaf wettability, which was determined by contact angle measurements. Increasing the initial concentration of bacteria and the amount of nutrients in the inoculum sprayed on leaves reduced the contact angles from 138 degrees on leaves treated with antibiotics (control) to 43 degrees on leaves treated with surface-active bacteria. A closely related strain with no surface activity on polystyrene did not affect leaf wettability, although it was present in densities similar to those of the surface-active strain. Nutrients alone could occasionally also increase leaf wettability, apparently by stimulating naturally occurring surface-active bacteria. When estimating densities of Pseudomonas spp. underneath droplets with low contact angles, it appeared that populations on leaves treated with a surface-active strain could vary from about 10 to 10 CFU cm, suggesting that the surface effect may be prolonged after a decline of the population. The possible ecological implications are discussed.

Characterization of five novel Pseudomonas aeruginosa cell-surface signalling systems

Cell-surface signalling is a sophisticated regulatory mechanism used by Gram-negative bacteria to sense signals from outside the cell and transmit them into the cytoplasm. This regulatory system consists of an outer membrane-localized TonB-dependent receptor (TonB-dependent transducer), a cytoplasmic membrane-localized antisigma factor and an extracytoplasmic function (ECF) sigma factor. Pseudomonas aeruginosa contains 13 potential surface signalling systems of which only six have been studied in detail. In this work we have identified the regulons of five novel P. aeruginosa signalling systems. For that, the ECF sigmas PA0149, PA1912, PA2050, PA2093 and PA4896 have been overexpressed and their target gene candidates have been identified using DNA microarray, proteomic analysis, and/or lacZ reporter construct. All five ECF sigma factors control the production of one TonB-dependent transducer. Interestingly, two sigma factors, PA2050 and PA2093, regulate the synthesis of a second transducer. Furthermore, we show that although all these sigma factors seem to control putative (metal) transport systems, one of them also regulates the expression of P. aeruginosa pyocins. Finally, we also show that the PA1912-PA1911-PA1910 (designated FemI-FemR-FemA in this work) signalling system responds to the presence of the Mycobacterium siderophores mycobactin and carboxymycobactin and is involved in the utilization of these heterologous siderophores.

The heterologous siderophores ferrioxamine B and ferrichrome activate signaling pathways in Pseudomonas aeruginosa

Pseudomonas aeruginosa secretes two siderophores, pyoverdine and pyochelin, under iron-limiting conditions. These siderophores are recognized at the cell surface by specific outer membrane receptors, also known as TonB-dependent receptors. In addition, this bacterium is also able to incorporate many heterologous siderophores of bacterial or fungal origin, which is reflected by the presence of 32 additional genes encoding putative TonB-dependent receptors. In this work, we have used a proteomic approach to identify the inducing conditions for P. aeruginosa TonB-dependent receptors. In total, 11 of these receptors could be discerned under various conditions. Two of them are only produced in the presence of the hydroxamate siderophores ferrioxamine B and ferrichrome. Regulation of their synthesis is affected by both iron and the presence of a cognate siderophore. Analysis of the P. aeruginosa genome showed that both receptor genes are located next to a regulatory locus encoding an extracytoplasmic function sigma factor and a transmembrane sensor. The involvement of this putative regulatory locus in the specific induction of the ferrioxamine B and ferrichrome receptors has been demonstrated. These results show that P. aeruginosa has evolved multiple specific regulatory systems to allow the regulation of TonB-dependent receptors.

Novel lectin-like bacteriocins of biocontrol strain Pseudomonas fluorescens Pf-5

Bacteriocin LlpA, produced by Pseudomonas sp. strain BW11M1, is a peculiar antibacterial protein due to its homology to mannose-binding lectins mostly found in monocots (A. H. A. Parret, G. Schoofs, P. Proost, and R. De Mot, J. Bacteriol. 185:897-908, 2003). Biocontrol strain Pseudomonas fluorescens Pf-5 contains two llpA-like genes, named llpA1(Pf-5) and llpA2(Pf-5). Recombinant Escherichia coli cells expressing llpA1(Pf-5) or llpA2(Pf-5) acquired bacteriocin activity and secreted a 31-kDa protein cross-reacting with LlpA(BW11M1) antibodies. Antibacterial activity of the recombinant proteins was evidenced by gel overlay assays. Analysis of the antimicrobial spectrum indicated that LlpA1(Pf-5) and LlpA2(Pf-5) are able to inhibit P. fluorescens strains, as well as the related mushroom pathogen Pseudomonas tolaasii. LlpA-type bacteriocins are characterized by a domain structure consisting of tandem monocot mannose-binding lectin (MMBL) domains. Molecular phylogeny of these MMBL domains suggests that the individual MMBL domains within an LlpA protein have evolved separately toward a specific, as yet unknown, function or, alternatively, were acquired from different ancestral sources. Our observations are consistent with earlier observations, which hinted that MMBL-like bacteriocins represent a new family of antibacterial proteins, probably with a novel mode of action.

Characterization of antifungal metabolite produced by a new strain Pseudomonas aeruginosa PUPa3 that exhibits broad-spectrum antifungal activity and biofertilizing traits

AIM

To study the antifungal activity and plant beneficial traits of a broad-spectrum antagonistic fluorescent pseudomonad strain, PUPa3.

METHODS AND RESULTS

Strain PUPa3 was isolated from the rhizosphere soil of rice and identified as Pseudomonas aeruginosa on the basis of biochemical tests and by comparison of 16S rDNA sequences. This bacterium exhibits a broad-spectrum antifungal activity towards phytopathogenic fungi. The antifungal metabolite by PUPa3 was extracted, purified and characterized using nuclear magnetic resonance (NMR) and mass spectroscopy (MS). Production of indole-3-acetic acid (IAA), siderophores, phosphatase and protease in PUPa3 was determined. Strain PUPa3 did not produce hydrogen cyanide, cellulase and pectinase.

CONCLUSION

The antifungal metabolite produced by PUPa3 has been identified as phenazine-1-carboxamide (PCN) on the basis of NMR and MS data. Strain PUPa3 showed a broad-spectrum antifungal activity towards a range of phytopathogenic fungi. This bacterium also showed several plant growth-promoting traits but did not show the traits attributed to deleterious rhizobacteria.

SIGNIFICANCE AND IMPACT OF THE STUDY

Present study reports the production of PCN as well as IAA for the first time by a saprophytic P. aeruginosa strain PUPa3. Because of the production of siderophore, growth hormone, protease and phosphatase and its innate fungicidal potential, this strain can be used as biofertilizer and antagonist against a range of phytopathogenic fungi that infect rice, groundnut, tobacco, chili, mango, sugarcane, tea, cotton and banana.

Growth interactions during bacterial colonization of seedling rootlets

Rootlet elongation and bacterial growth on rootlets were determined after inoculation of cucumber and spinach seedlings with Pseudomonas strains differing in production of siderophores and HCN. Siderophore producers grew more profusely than nonproducers on both species and promoted rootlet elongation on cucumber. Coinoculation of siderophore producers and nonproducers resulted in restricted growth of the latter. The total populations of nonproducers of HCN in the presence of HCN producers were not decreased, but the tenacity of their association with the rootlet surface was altered.

The natural functions of secondary metabolites

Secondary metabolites, including antibiotics, are produced in nature and serve survival functions for the organisms producing them. The antibiotics are a heterogeneous group, the functions of some being related to and others being unrelated to their antimicrobial activities. Secondary metabolites serve: (i) as competitive weapons used against other bacteria, fungi, amoebae, plants, insects, and large animals; (ii) as metal transporting agents; (iii) as agents of symbiosis between microbes and plants, nematodes, insects, and higher animals; (iv) as sexual hormones; and (v) as differentiation effectors. Although antibiotics are not obligatory for sporulation, some secondary metabolites (including antibiotics) stimulate spore formation and inhibit or stimulate germination. Formation of secondary metabolites and spores are regulated by similar factors. This similarity could insure secondary metabolite production during sporulation. Thus the secondary metabolite can: (i) slow down germination of spores until a less competitive environment and more favorable conditions for growth exist; (ii) protect the dormant or initiated spore from consumption by amoebae; or (iii) cleanse the immediate environment of competing microorganisms during germination.

The rice inoculant strain Alcaligenes faecalis A15 is a nitrogen-fixing Pseudomonas stutzeri

The taxonomic position of the nitrogen-fixing rice isolate A15, previously classified as Alcaligenes faecalis, was reinvestigated. On the basis of its small subunit ribosomal RNA (16S rRNA) sequence this strain identifies as Pseudomonas stutzeri. Phenotyping and fatty acid profiling confirm this result. DNA:DNA hybridisations, using the optical renaturation rate method, between strain A15 and Pseudomonas stutzeri LMG 11199T revealed a mean DNA-binding of 77%. The identification was further corroborated by comparative sequence analysis of the oprF gene, which encodes the major outer membrane protein of rRNA homology group I pseudomonads. Furthermore we determined the nifH sequence of this strain and of two putative diazotrophic Pseudomonas spp. and made a comparative analysis with sequences of other diazotrophs. These Pseudomonas NifH sequences cluster with NifH sequences isolated from the rice rhizosphere by PCR and of proteobacteria from the beta and gamma subclasses.

Secondary metabolite- and endochitinase-dependent antagonism toward plant-pathogenic microfungi of pseudomonas fluorescens isolates from sugar beet rhizosphere

Forty-seven isolates representing all biovars of Pseudomonas fluorescens (biovars I to VI) were collected from the rhizosphere of field-grown sugar beet plants to select candidate strains for biological control of preemergence damping-off disease. The isolates were tested for in vitro antagonism toward the plant-pathogenic microfungi Pythium ultimum and Rhizoctonia solani in three different plate test media. Mechanisms of fungal inhibition were elucidated by tracing secondary-metabolite production and cell wall-degrading enzyme activity in the same media. Most biovars expressed a specific mechanism of antagonism, as represented by a unique antibiotic or enzyme production in the media. A lipopeptide antibiotic, viscosinamide, was produced independently of medium composition by P. fluorescens bv. I, whereas the antibiotic 2, 4-diacetylphloroglucinol was observed only in glucose-rich medium and only in P. fluorescens bv. II/IV. Both pathogens were inhibited by the two antibiotics. Finally, in low-glucose medium, a cell wall-degrading endochitinase activity in P. fluorescens bv. I, III, and VI was the apparent mechanism of antagonism toward R. solani. The viscosinamide-producing DR54 isolate (bv. I) was shown to be an effective candidate for biological control, as tested in a pot experiment with sugar beet seedlings infested with Pythium ultimum. The assignment of different patterns of fungal antagonism to the biovars of P. fluorescens is discussed in relation to an improved selection protocol for candidate strains to be used in biological control.

Initiation of biofilm formation in Pseudomonas fluorescens WCS365 proceeds via multiple, convergent signalling pathways: a genetic analysis

Populations of surface-attached microorganisms comprising either single or multiple species are commonly referred to as biofilms. Using a simple assay for the initiation of biofilm formation (e.g. attachment to an abiotic surface) by Pseudomonas fluorescens strain WCS365, we have shown that: (i) P. fluorescens can form biofilms on an abiotic surface when grown on a range of nutrients; (ii) protein synthesis is required for the early events of biofilm formation; (iii) one (or more) extracytoplasmic protein plays a role in interactions with an abiotic surface; (iv) the osmolarity of the medium affects the ability of the cell to form biofilms. We have isolated transposon mutants defective for the initiation of biofilm formation, which we term surface attachment defective (sad). Molecular analysis of the sad mutants revealed that the ClpP protein (a component of the cytoplasmic Clp protease) participates in biofilm formation in this organism. Our genetic analyses suggest that biofilm formation can proceed via multiple, convergent signalling pathways, which are regulated by various environmental signals. Finally, of the 24 sad mutants analysed in this study, only three had defects in genes of known function. This result suggests that our screen is uncovering novel aspects of bacterial physiology.

A two-component system plays an important role in the root-colonizing ability of Pseudomonas fluorescens strain WCS365

We describe the characterization of a novel Tn5lacZ colonization mutant of the efficiently colonizing Pseudomonas fluorescens strain WCS365, mutant strain PCL1210, which is at least 300- to 1,000-fold impaired in colonization of the potato root tip after co-inoculation of potato stem cuttings with a 1:1 mixture of mutant and parental cells. Similarly, the mutant is also impaired in colonization of tomato, wheat, and radish, indicating that the gene involved plays a role in the ability of P. fluorescens WCS365 to colonize a wide range of plant species. A 3.1-kb DNA fragment was found to be able to complement the observed mutation. The nucleotide sequence of the region around the Tn5lacZ insertion showed three open reading frames (ORFs). The transcriptional start site was determined. The operon is preceded by an integration host factor (IHF) binding site consensus sequence whereas no clear -10 and -35 sequences are present. The deduced amino acid sequences of the first two genes of the operon, designated as colR and colS, show strong similarity with known members of two-component regulatory systems. ColR has homology with the response regulators of the OmpR-PhoB subclass whereas ColS, the product of the gene in which the mutation resides, shows similarity to the sensor kinase members of these two-component systems. Hydrophobicity plots show that this hypothetical sensor kinase has two transmembrane domains, as is also known for other sensor kinases. The product of the third ORF, Orf222, shows no homology with known proteins. Only part of the orf222 gene is present in the colonization-complementing, 3.1-kb region, and it therefore does not play a role in complementation. No experimental evidence for a role of the ColR/ColS two-component system in the suspected colonization traits chemotaxis and transport of exudate compounds could be obtained. The function of this novel two-component system therefore remains to be elucidated. We conclude that colonization is an active process in which an environmental stimulus, through this two-component system, activates a so far unknown trait that is crucial for colonization.

Value of phenotyping methods as an initial screening of Pseudomonas aeruginosa in epidemiologic studies

When studying the epidemiology of Pseudomonas aeruginosa, determination of the similarity of isolates is crucial. In the present study the distinctive capacity of four phenotyping methods (antibiotic susceptibility patterns, serotyping, phage-typing and outer membrane protein [OMP] profile analysis) was determined and compared to pulsed-field gel electrophoresis (PFGE) of enzyme restricted chromosomal DNA. In all, 91 isolates of P. aeruginosa were cultured from ten patients. Antibiotic susceptibility patterns were concordant for all isolates. Serotyping yielded five, phage-typing eight, OMP profile analysis nine and PFGE seven distinct types of P. aeruginosa. Compared to PFGE, the distinctive capacities were 89% (81/91) for serotyping, 87% (79/91) for phage-typing, and 90% (82/91) for OMP profile analysis. When serotyping results were different, PFGE types also were different (exclusiveness 100%). However, isolates with the same serotype may have various PFGE patterns. In contrast, isolates with similar PFGE patterns could have different phage-types or OMP types. For the study of isolates of P. aeruginosa, serotyping provides a good initial selection to reduce the number of isolates that need to be genotyped.

Pseudomonas aeruginosa antigens as potential vaccines

Pseudomonas aeruginosa is one of the most important opportunistic bacterial pathogens in humans and animals. This organism is ubiquitous and has high intrinsic resistance to antibiotics due to the low permeability of the outer membrane and the presence of numerous multiple drug efflux pumps. Various cell-associated and secreted antigens of P. aeruginosa have been the subject of vaccine development. Among pseudomonas antigens, the mucoid substance, which is an extracellular slime consisting predominantly of alginate, was found to be heterogenous in terms of size and immunogenicity. High molecular mass alginate components (30-300 kDa) appear to contain conserved epitopes while lower molecular mass alginate components (10-30 kDa) possess conserved epitopes in addition to unique epitopes. Surface-exposed antigens including O-antigens (O-specific polysaccharide of LPS) or H-antigens (flagellar antigens) have been used for serotyping due to their highly immunogenic nature. Chemical structures of repeating units of O-specific polysaccharides have been elucidated and these data allowed the identification of 31 chemotypes of P. aeruginosa. Conserved epitopes among all serotypes of P. aeruginosa are located in the core oligosaccharide and the lipid A region of LPS and immunogens containing these epitopes induce cross-protective immunity in mice against different P. aeruginosa immunotypes. To examine the protective properties of OM proteins, a vaccine containing P. aeruginosa OM proteins of molecular masses ranging from 20 to 100 kDa has been used in pre-clinical and clinical trials. This vaccine was efficacious in animal models against P. aeruginosa challenge and induced high levels of specific antibodies in human volunteers. Plasma from human volunteers containing anti-P. aeruginosa antibodies provided passive protection and helped the recovery of 87% of patients with severe forms of P. aeruginosa infection. Vaccines prepared from P. aeruginosa ribosomes induced protective immunity in mice, but the efficacy of ribosomal vaccines in humans is not yet known. A number of recent studies indicated the potential of some P. aeruginosa antigens that deserve attention as new vaccine candidates. The outer core of LPS was implicated to be a ligand for binding of P. aeruginosa to airway and ocular epithelial cells of animals. However, heterogeneity exists in this outer core region among different serotypes. Epitopes in the inner core are highly conserved and it has been demonstrated to be surface-accessible, and not masked by O-specific polysaccharide. The use of an in vivo selection/expression technology (IVET) by a group of researchers identified a number of P. aeruginosa proteins that are expressed in vivo and essential for virulence. Two of these in vivo-expressed proteins are FptA (ferripyochelin receptor protein) and a homologue of an LPS biosynthetic enzyme. Our laboratory has identified a highly conserved protein, WbpM, and P. aeruginosa with a deficiency in this protein produces only rough LPS and became serum sensitive. Results from these studies have provided the foundation for a variety of vaccine formulations.

A novel cell surface polysaccharide in Pseudomonas putida WCS358, which shares characteristics with Escherichia coli K antigens, is not involved in root colonization

Previously we have shown that flagella and the O-specific polysaccharide of lipopolysaccharide play a role in colonization of the potato root by plant growth-promoting Pseudomonas strains WCS374 and WCS358. In this paper, we describe a novel cell surface-exposed structure in Pseudomonas putida WCS358 examined with a specific monoclonal antibody. This cell surface structure appeared to be a polysaccharide, which was accessible to the monoclonal antibody at the outer cell surface. Further study revealed that it does not contain 2-keto-3-deoxyoctonate, heptose, or lipid A, indicating that it is not a second type of lipopolysaccharide. Instead, the polysaccharide shared some characteristics with K antigen described for Escherichia coli. From a series of 49 different soil bacteria tested, only one other potato plant growth-promoting Pseudomonas strain reacted positively with the monoclonal antibody. Mutant cells lacking the novel antigen were efficiently isolated by an enrichment method involving magnetic antibodies. Mutant strains defective in the novel antigen contained normal lipopolysaccharide. One of these mutants was affected in neither its ability to adhere to sterile potato root pieces nor its ability to colonize potato roots. We conclude that the bacterial cell surface of P. putida WCS358 contains at least two different polysaccharide structures. These are the O-specific polysaccharide of lipopolysaccharide, which is relevant for potato root colonization, and the novel polysaccharide, which is not involved in adhesion to or colonization of the potato root.

Outer Membrane Protein Heterogeneity within Pseudomonas fluorescens and P. putida and Use of an OprF Antibody as a Probe for rRNA Homology Group I Pseudomonads

The electrophoretic patterns of outer membrane proteins of strains representing the biovars of Pseudomonas fluorescens and Pseudomonas putida were analyzed by gel electrophoresis. The outer membrane protein profiles were variable, and they were not useful for assigning strains to a specific biovar. However, three or four predominant outer membrane proteins migrating at 42 to 46 kDa, 33 to 38 kDa, and 20 to 22 kDa were conserved among the strains. They could be tentatively identified as OprE (44 kDa), OprF (38 kDa), OprH (21 kDa), and OprL (20.5 kDa), which are known proteins from Pseudomonas aeruginosa. A 37-kDa OprF-like protein was purified from P. fluorescens DF57 and used to raise a polyclonal antibody. In Western blot (immunoblot) analysis, this antibody reacted with OprF proteins from members of Pseudomonas rRNA homology group I but not with proteins from nonpseudomonads. The heterogeneity in M(infr) of OprF was greater among P. fluorescens strains than among P. putida strains. Immunofluorescence microscopy of intact cells demonstrated that the antibody recognized epitopes that were accessible only after unmasking by EDTA treatment. The antibody was used in a colony blotting assay to determine the percentage of rRNA homology group I pseudomonads among bacteria from the rhizosphere of barley. The bacteria were isolated on 10% tryptic soy agar, King's B agar, and the pseudomonad-specific medium Gould S1 agar. The estimate of OprF-containing CFU in rhizosphere soil obtained by colony blotting on 10% tryptic soy agar was about 2 and 14 times higher than the values obtained from King's agar and Gould S1 agar, respectively, indicating that not all fluorescent pseudomonads are scored on more specific media. The colonies reacting with the OprF antibody were verified as being rRNA homology group I pseudomonads by using the API 20NE system.

Multiple outer membrane receptors for uptake of ferric pseudobactins in Pseudomonas putida WCS358

Under iron limitation Pseudomonas putida WCS358 produces a fluorescent siderophore, pseudobactin 358, which, after complexing iron, is transported back into the cell via the specific outer membrane receptor PupA. In addition, this strain has the capacity to take up iron via a large variety of siderophores produced by other fluorescent pseudomonads. Putative receptor genes for such siderophores were identified in the chromosome of strain WCS358 by PCR using primers matching two domains conserved in four ferric pseudobactin receptors, including PupA. Eleven amplification products within the expected size range were obtained. Sequence analysis confirmed that the products were derived from genes encoding outer membrane receptors. Two complete receptor genes were isolated from a genomic library of P. putida WCS358. Both protein products are involved in the transport of a limited number of specific ferric pseudobactins. These results indicate that the ability of P. putida WCS358 to exploit many different heterologous pseudobactins is related to the presence of multiple outer membrane receptor proteins.

Localization of functional domains in the Escherichia coli coprogen receptor FhuE and the Pseudomonas putida ferric-pseudobactin 358 receptor PupA

Transport of ferric-siderophores across the outer membrane of gram-negative bacteria is mediated by specific outer membrane receptors. To localize the substrate-binding domain of the ferric-pseudobactin 358 receptor, PupA, of Pseudomonas putida WCS358, we constructed chimeric receptors in which different domains of PupA were replaced by the corresponding domains of the related ferric-pseudobactin receptors PupB and PupX, or the coprogen receptor FhuE of Escherichia coli. None of the chimeric proteins composed of pseudobactin receptor domains facilitated growth on any of the original substrates, or they showed only an extremely low efficiency. However, these receptors enabled cells of Pseudomonas BN8 to grow on media supplemented with uncharacterized siderophore preparations. These siderophore preparations were isolated from the culture supernatant of WCS358 cells carrying plasmids that contain genes of Pseudomonas B10 required for the biosynthesis of pseudobactin B10. Hybrid proteins that contained at least the amino-terminal 516 amino acids of mature FhuE were active as a receptor for coprogen and interacted with the E. coli TonB protein. A chimeric PupA-FhuE protein, containing the amino-terminal 94 amino acids of mature PupA, was also active as a coprogen receptor, but only in the presence of Pseudomonas TonB. It is concluded that the carboxy-terminal domain of ferric-pseudobactin receptors is important, but not sufficient, for ligand interaction, whereas binding of coprogen by the FhuE receptor is not dependent on this domain. Apparently, the ligand-binding sites of different receptors are located in different regions of the proteins. Furthermore, species-specific TonB binding by the PupA receptor is dependent on the amino-terminal domain of the receptor.

Construction of phoE-caa, a novel PCR- and immunologically detectable marker gene for Pseudomonas putida

In this paper we describe the construction and use in Pseudomonas putida WCS358 of phoE-caa, a novel hybrid marker gene, which allows monitoring both at the protein level by immunological methods and at the DNA level by PCR. The marker is based on the Escherichia coli outer membrane protein gene phoE and 75 bp of E. coli caa, which encode a nonbacteriocinic fragment of colicin A. This fragment contains an epitope which is recognized by monoclonal antibody (MAb) 1C11. As the epitope is contained in one of the cell surface-exposed loops of PhoE, whole cells of bacteria expressing the protein can be detected by using the MAb. The marker gene contains only E. coli sequences not coding for toxins and therefore can be considered environmentally safe. The hybrid PhoE-ColA protein was expressed in E. coli under conditions of phosphate starvation, and single cells could be detected by immunofluorescence microscopy with MAb 1C11. Using a wide-host-range vector the phoE-caa gene was introduced into P. putida WCS358. The gene appeared to be expressed under phosphate limitation in this species, and the gene product was present in the membrane fraction and reacted with MAb 1C11. The hybrid PhoE-ColA protein could be detected on whole cells of WCS358 mutant strains lacking (part of) the O-antigen of the lipopolysaccharide but not on wild-type WCS358 cells, unless these cells had previously been washed with 10 mM EDTA. In addition to immunodetection, the phoE-caa marker gene could be specifically detected by PCR with one primer directed to a part of the phoE sequence and a second primer that annealed to the caa insert.

Siderophore receptor PupA as a marker to monitor wild-type Pseudomonas putida WCS358 in natural environments

For application of genetically engineered fluorescent Pseudomonas spp., specific markers are required for monitoring of wild-type Pseudomonas strains and their genetically modified derivatives in natural environments. In this study, the specific siderophore receptor PupA of plant growth-promoting Pseudomonas putida WCS358 was used as a marker to monitor wild-type strain WCS358. After introduction into natural soil and rhizosphere environments, strain WCS358 could be recovered efficiently on a medium amended with 300 microM pseudobactin 358. Although low population densisties of indigenous pseudomonads (less than or equal to 10(3)/g of soil or root) were recovered on the pseudobactin 358-amended medium, subsequent agglutination assays with a WCS358-specific polyclonal antiserum enabled accurate monitoring of populations of wild-type strain WCS358 over a range of approximately 10(3) to 10(7) CFU/g of soil or root. Genetic analysis of the background population by PCR and Southern hybridization revealed that natural occurrence of the pupA gene was limited to a very small number of indigenous Pseudomonas spp. which are very closely related to P. putida WCS358. The PupA marker system enabled the study of differences in rhizosphere colonization among wild-type strain WCS358, rifampin-resistant derivative WCS358rr, and Tn5 mutant WCS358::xylE. Chromosomally mediated rifampin resistance did not affect the colonizing ability of P. putida WCS358. However, Tn5 mutant WCS358::xylE colonized the radish rhizosphere significantly less than did its parental strain.

Outer Membrane Proteins and Lipopolysaccharides in Pathovars of Xanthomonas campestris

Variations in the outer membrane proteins (OMPs) and lipopolysaccharides (LPSs) of 54 isolates belonging to 16 different pathovars of Xanthomonas campestris were characterized. OMP samples prepared by sarcosyl extraction of cell walls and LPS samples prepared by proteinase K treatment of sonicated cells were analyzed by sodium dodecyl sulfate-polyacrylamide gel electrophoresis in the presence of 4 M urea. In general, the OMP and LPS profiles within each pathovar were very similar but different from the profiles of other pathovars. Heterogeneity in OMP and LPS profiles was observed within X. campestris pv. campestris, X. campestris pv. translucens, and X. campestris pv. vesicatoria. LPSs were isolated from six X. campestris pathovars, which fell into two major groups on the basis of O antigenicity. The O antigens of X. campestris pv. begoniae, X. campestris pv. graminis, and X. campestris pv. translucens cross-reacted with each other; the other group consisted of X. campestris pv. campestris, X. campestris pv. pelargonii, and X. campestris pv. vesicatoria. A chemical analysis revealed a significant difference between the compositions of the neutral sugars of the LPSs of those two groups; the LPSs of the first group contained xylose and a 6-deoxy-3- O -methyl hexose, whereas the LPSs of the other group lacked both sugars.

Identification and characterization of a siderophore regulatory gene (pfrA) of Pseudomonas putida WCS358: homology to the alginate regulatory gene algQ of Pseudomonas aeruginosa

Genes encoding biosynthesis of pseudobactin 358 (a microbial iron transport agent) and its cognate outer membrane receptor protein, PupA, are transcribed only under iron limitation in plant growth-promoting Pseudomonas putida WCS358. Two cosmid clones were identified from a gene bank of WCS358 DNA which could independently and in an iron-dependent manner activate transcription from a WCS358 siderophore gene promoter in heterologous Pseudomonas strain A225. The functional region of one of the clones was localized by subcloning, transposon Tn3Gus mutagenesis, and DNA sequencing. Genomic transposon insertion mutants in the functional region lost the capacity to activate a siderophore gene promoter fusion transcriptionally; furthermore, these mutants no longer produced pseudobactin 358. The activating region consisted of a single gene designated pfrA (Pseudomonas ferric regulator). The pfrA gene codes for a single polypeptide, PfrA, of approximately 18 kDa, which has 58% identity to AlgQ (also known as AlgR2), a positive regulator involved in transcriptionally regulating alginate biosynthesis in Pseudomonas aeruginosa. Cross-complementation studies between the pfrA gene of P. putida and the algQ gene of P. aeruginosa revealed that pfrA can restore mucoidy (alginate production) in an algQ mutant and that algQ could poorly complement a pfrA genomic mutant. It is concluded that PfrA is involved in the positive regulation of siderophore biosynthetic genes in response to iron limitation; furthermore, pfrA and algQ appeared to be interchangeable between P. putida and P. aeruginosa.

Identification and characterization of the pupB gene encoding an inducible ferric-pseudobactin receptor of Pseudomonas putida WCS358

Pseudomonas putida WCS358 can transport iron complexed to a wide variety of pseudobactins produced by other Pseudomonas strains. The pupB gene encoding an outer membrane ferric-pseudobactin receptor was isolated from a genomic library of P. putida WCS358. The PupB receptor facilitated iron transport via two distinct heterologous siderophores, i.e. pseudobactin BN8 and pseudobactin BN7. The amino acid sequence deduced from the nucleotide sequence consisted of 804 amino acids (molecular weight 88,369) of which the N-terminal part was very similar to a prokaryotic leader peptide. The mature protein shared significant homology with the receptor for ferric-pseudobactin 358 (PupA) and contained three regions common to TonB-dependent receptor proteins of Escherichia coli. Interestingly, PupB expression was only observed in cells cultured in iron-deficient medium containing pseudobactin BN8 or pseudobactin BN7. This expression required a transcriptional unit, pupR, identified upstream of the structural pupB gene. Transposon Tn5 insertion mutants defective in PupB production still exhibited uptake of iron via pseudobactin BN8, although with reduced efficiency. Apparently, an additional transport system for this ferric-siderophore complex operates in this strain. In addition to pseudobactin BN8 also other heterologous siderophores were capable of inducing synthesis of specific high-molecular-weight outer membrane proteins in strain WCS358, which suggests the existence of multiple siderophore-inducible iron transport systems in this strain.

Identification and characterization of the exbB, exbD and tonB genes of Pseudomonas putida WCS358: their involvement in ferric-pseudobactin transport

Catechol-cephalosporins are siderophore-like antibiotics which are taken up by cells of Pseudomonas putida WCS358 via the ferric-siderophore transport pathway. Mutants of strain WCS358 were isolated that are resistant to high concentrations of these antibiotics. These mutants failed to grow under iron-limiting conditions, and could not utilize different ferric-siderophores. The mutants fall in three complementation groups. The nucleotide sequence determination identified three contiguous open reading frames, which were homologous to the exbB, exbD and tonB genes of Escherichia coli respectively. The deduced amino acid sequence of P. putida ExbB showed 58.6% homology with its E. coli homologue, but, unlike the E. coli protein, it has a N-terminal extension of 91 amino acids. The ExbD proteins are 64.8% homologous, whereas the TonB proteins only show 27.7% homology. The P. putida exbB gene could complement an E. coli exbB mutation, but the TonB proteins were not interchangeable between the species. It is concluded that P. putida WCS358 contains an energy-coupling system between the membranes, for active transport across the outer membrane, which is comprised of a TonB-like energy-transducing protein and two accessory proteins. This system is similar to, but not completely compatible with, the E. coli system.

Siderophore-mediated iron transport correlates with the presence of specific iron-regulated proteins in the outer membrane of Rhizobium meliloti

A universal chemical assay used to detect the production of siderophores in a range of Rhizobium strains showed that production is strain specific. Iron nutrition bioassays carried out on Rhizobium meliloti strains to determine cross-utilization of their siderophores showed that R. meliloti 2011, 220-5, and 220-3 could each use the siderophores produced by the other two but not the siderophore produced by R. meliloti DM4 (and vice versa). Mutants of R. meliloti 2011 and 220-5 defective in siderophore production were isolated by Tn5-mob mutagenesis. The Tn5-mob-containing EcoRI fragment of mutant R. meliloti 220-5-1 was cloned into pUC19. By using this fragment as a probe, the presence of a homologous region was observed in R. meliloti 2011 and 220-3 but not in R. meliloti DM4. A complementing cosmid from a gene bank of R. meliloti 2011 was identified by using the same probe. Introduction of this cosmid into R. meliloti 102F34, a strain not producing a siderophore, resulted in the ability of this strain to produce a siderophore and also in the ability to utilize the siderophores produced by R. meliloti 2011, 220-5, and 220-3 but not the siderophore produced by R. meliloti DM4. A comparative analysis of the outer membrane proteins prepared from iron-deficient cultures of R. meliloti 102F34 and 102F34 harboring the cosmid revealed the presence, in the latter, of a low-iron-induced outer membrane protein corresponding to a low-iron-induced protein in R. meliloti 2011, 220-5, and 220-3. This protein is not present in R. meliloti DM4. The results suggest that R. meliloti 2011, 220-5, and 220-3 produce siderophores that are identical or sufficiently similar in structure to be transported by the membrane transport system of each strain while also indicating that utilization of a particular siderophore is correlated with the presence of specific outer membrane proteins.

Traits of fluorescent Pseudomonas spp. involved in suppression of plant root pathogens

Certain members of the fluorescent pseudomonad group have been shown to be potential agents for the biocontrol of plant root diseases. The major problems with the commercialization of these beneficial strains are that few wild-type strains contain all the desired characteristics for this process and the performance of strains in different soil and climatic conditions is not reproducible. Consequently, prior to selection and/or improvement of suitable strains for biocontrol purposes, it is necessary to understand the important traits required for this purpose. The production of fluorescent siderophores (iron-binding compounds) and antibiotic compounds has been recognized as important for the inhibition of plant root pathogens. Efficient root colonization is also a prerequisite for successful biocontrol strains. This review discusses some of the characteristics of fluorescent pseudomonads that have been suggested to be important for biocontrol. The genetic organization and regulation of these processes is also examined. This information is necessary for attempts aimed at the improvement of strains based on deregulating pathways or introducing traits from one strain to another. The release of genetically engineered organisms into the environment is governed by regulations, and this aspect is summarized. The commercialization of fluorescent pseudomonads for the biological control of plant root diseases remains an exciting possibility. The understanding of the relevant characteristics will facilitate this process by enabling the direct selection and/or construction of strains which will perform under a variety of environmental conditions.

Purification of glucose-inducible outer membrane protein OprB of Pseudomonas putida and reconstitution of glucose-specific pores

A 43,000 molecular-weight, glucose-inducible, organic acid-repressible protein (OprB) was identified in the outer membrane of Pseudomonas putida. OprB was surface expressed in whole cells, had a high beta-sheet content, and was heat modifiable, as demonstrated by 125I-labeling, circular dichroism spectroscopy, and mobility on sodium dodecyl sulfate-polyacrylamide gel electrophoresis. OprB was extracted from outer membrane preparations by using 2% Lubrol PX with 10 mM EDTA and purified by DEAE-Sephacel ion exchange chromatography following ammonium sulfate precipitation. Reconstitution experiments with black lipid membranes showed that OprB formed small, cation-selective pores which bound glucose (KS = 110 mM) and other carbohydrates. However, the binding site of OprB appeared to be distinct from that of the maltodextrin-specific porin LamB from Escherichia coli.

Regulation of iron assimilation: nucleotide sequence analysis of an iron-regulated promoter from a fluorescent pseudomonad

An iron-regulated promoter was cloned on a 2.1 kb Bg/II fragment from Pseudomonas sp. strain M114 and fused to the lacZ reporter gene. Iron-regulated lacZ expression from the resulting construct (pSP1) in strain M114 was mediated via the Fur-like repressor which also regulates siderophore production in this strain. A 390 bp StuI-PstI internal fragment contained the necessary information for iron-regulated promoter expression. This fragment was sequenced and the initiation point for transcription was determined by primer extension analysis. The region directly upstream of the transcription start point contained no significant homology to known promoter consensus sequences. However the -16 to -25 bp region contained homology to four other iron-regulated pseudomonad promoters. Deletion of bases downstream from the transcriptional start did not affect the iron-regulated expression of the promoter. The -37 and -43 bp regions exhibited some homology to the 19 bp Escherichia coli Fur-binding consensus sequence. When expressed in E. coli (via a cloned transacting factor from strain M114) lacZ expression from pSP1 was found to be regulated by iron. A region of greater than 77 bases but less than 131 upstream from the transcriptional start was found to be necessary for promoter activity, further suggesting that a transcriptional activator may be required for expression.

The ferric-pseudobactin receptor PupA of Pseudomonas putida WCS358: homology to TonB-dependent Escherichia coli receptors and specificity of the protein

The initial step in the uptake of iron via ferric pseudobactin by the plant-growth-promoting Pseudomonas putida strain WCS358 is binding to a specific outer-membrane protein. The nucleotide sequence of the pupA structural gene, which codes for a ferric pseudobactin receptor, was determined. It contains a single open reading frame which potentially encodes a polypeptide of 819 amino acids, including a putative N-terminal signal sequence of 47 amino acids. Significant homology, concentrated in four boxes, was found with the TonB-dependent receptor proteins of Escherichia coli. The pupA mutant MH100 showed a residual efficiency of 30% in the uptake of 55Fe3+ complexed to pseudobactin 358, whereas the iron uptake of four other pseudobactins was not reduced at all. Cells of strain WCS374 supplemented with the pupA gene of strain WCS358 could transport ferric pseudobactin 358 but showed no affinity for three other pseudobactins. It is concluded that PupA is a specific receptor for ferric pseudobactin 358, and that strain WCS358 produces at least one other receptor for other pseudobactins.