Osmoadaptation in rhizobia: ectoine-induced salt tolerance

Roles and occurrences of microbiota in the osmoregulatory organs, gills and gut, in marine medaka upon hypotonic stress

Gills and gut are the two primary osmoregulatory organs in fish. Recently, studies have expanded beyond the osmoregulatory mechanisms of these organs to explore the microbiota communities inhabiting them. It is now known that microbial communities in both organs shift in response to osmotic stress. However, there are limited studies identifying the major contributors and co-occurrence among these microbiota in both organs under seawater and freshwater transfer conditions. The current data mining report performed a bioinformatics analysis on two previous published datasets from our group, aiming to provide insights into host-bacteria relationships under osmotic stress. We divided the samples into four groups: control seawater gills (LSW); control seawater gut (TSW); freshwater transfer gills (LFW); and freshwater transfer gut (TFW). Our results showed that LSW had higher diversities, richness, and evenness compared to TSW. However, both the LFW and LSW did not show any significant differences after the freshwater transfer experiment. We further applied co-occurrence network analysis and, for the first time, reported on the interactions of taxa shaping the community structure in these two organs. Moreover, we identified enriched ectoine biosynthesis in seawater samples, suggesting its potential role in seawater environments. Increased mRNA expression levels of Na+/K+-atpase, and cftr, were observed in gills after 6 h of ectoine treatment. These findings provide a foundation for future studies on host-bacteria interactions under osmotic stress.

Multiple Functions of Compatible Solute Ectoine and Strategies for Constructing Overproducers for Biobased Production

Ectoine and its derivative 5-hydroxyectoine are compatible solutes initially found in the hyperhalophilic bacterium Ectothiorhodospira halochloris, which inhabits the desert in Egypt. The habitat of ectoine producers implies the primary function of ectoine as a cytoprotectant against harsh conditions such as high salinity, drought, and high radiation. More extensive and in-depth studies have revealed the multiple functions of ectoine in its native producer bacterial cells and other types of cells and its biomolecular components (such as proteins and DNA) as a general protective agent. Its chemical properties as a bio-based amino acid derivative make it attractive for basic scientific research and related industries, such as the food/agricultural industry, cosmetic manufacturing, biologics, and therapeutic agent preparation. This article first discusses the functions and applications of ectoine and 5-hydroxyectoine. Subsequently, more emphasis was placed on advances in bio-based ectoine and/or 5-hydroxyectoine production. Strategies for developing more robust cell factories for highly efficient ectoine and/or 5-hydroxyectoine production are further discussed. We hope this review will provide a valuable reference for studies on the bio-based production of ectoine and 5-hydroxyectoine.

Streptomyces mahasarakhamensis sp. nov., an Endophytic Actinobacterium Isolated from Jasmine Rice and its Potential as plant Growth Promoter

Two endophytic actinobacteria, strains MK5T and MK7, were isolated from the surface-sterilized root of Jasmine rice (Oryza sativa KDML 105). These strains were aerobic actinobacteria with a well-developed substrate and aerial mycelia that formed spiral spore chains. The type strains that shared the high 16S rRNA gene sequence similarity with both strains were Streptomyces naganishii NBRC 12892T (99.4%), "Streptomyces griseicoloratus" TRM S81-3T (99.2%), and Streptomyces spiralis NBRC 14215T (98.9%). Strains MK5T and MK7 are the same species sharing a digital DNA-DNA hybridization (dDDH) value of 95.3% and a 16S rRNA gene sequence similarity of 100%. Chemotaxonomic data confirmed the affiliation of strains MK5T and MK7 to the genus Streptomyces. Strains MK5T and MK7 contained MK-9(H4) as a major menaquinone; the whole-cell sugar of both strains was galactose and glucose. The strain MK5T shared 93.4% average nucleotide identity (ANI)-Blast, 95.5% ANI-MUMmer, 93% average amino acid identity, and 61.3% dDDH with S. spiralis NBRC 14215T. The polyphasic approach confirmed that strain MK5T represents a novel species, and the name Streptomyces mahasarakhamensis sp. nov. is proposed. The type strain is MK5T (= TBRC 17754 = NRRL B-65683). Genome mining, using an in silico approach and searching biosynthesis gene clusters of strains MK5T and MK7, revealed that the genomes contained genes encoding proteins relating to plant growth promotion, bioactive compounds, and beneficial enzymes. Strains MK5T and MK7 could produce indole acetic acid and solubilize phosphate in vitro.

A host-specific diaminobutyrate aminotransferase contributes to symbiotic performance, homoserine metabolism, and competitiveness in the Rhizobium leguminosarum/Pisum sativum system

Rhizobium leguminosarum bv. viciae (Rlv) UPM791 effectively nodulates pea and lentil, but bacteroids contain a number of proteins differentially expressed depending on the host. One of these host-dependent proteins (C189) is similar to a diaminobutyrate-2-oxoglutarate aminotransferase (DABA-AT). DABA-AT activity was demonstrated with cell extracts and with purified protein, so C189 was renamed as Dat. The dat gene was strongly induced in the central, active area of pea nodules, but not in lentil. Mutants defective in dat were impaired in symbiotic performance with pea plants, exhibiting reduced shoot dry weight, smaller nodules, and a lower competitiveness for nodulation. In contrast, there were no significant differences between mutant and wild-type in symbiosis with lentil plants. A comparative metabolomic approach using cell-free extracts from bacteroids induced in pea and lentil showed significant differences among the strains in pea bacteroids whereas no significant differences were found in lentil. Targeted metabolomic analysis revealed that the dat mutation abolished the presence of 2,4-diaminobutyrate (DABA) in pea nodules, indicating that DABA-AT reaction is oriented toward the production of DABA from L-aspartate semialdehyde. This analysis also showed the presence of L-homoserine, a likely source of aspartate semialdehyde, in pea bacteroids but not in those induced in lentil. The dat mutant showed impaired growth when cells were grown with L-homoserine as nitrogen source. Inclusion of DABA or L-homoserine as N source suppressed pantothenate auxotropy in Rlv UPM791, suggesting DABA as source of the pantothenate precursor β-alanine. These data indicate that Rlv UPM791 Dat enzyme is part of an adaptation mechanism of this bacterium to a homoserine-rich environment such as pea nodule and rhizosphere.

Comparative genomics of the plant-growth promoting bacterium Sphingobium sp. strain AEW4 isolated from the rhizosphere of the beachgrass Ammophila breviligulata

Background

The genus Sphingobium within the class Alpha-proteobacteria contains a small number of plant-growth promoting rhizobacteria (PGPR), although it is mostly comprised of organisms that play an important role in biodegradation and bioremediation in sediments and sandy soils. A Sphingobium sp. isolate was obtained from the rhizosphere of the beachgrass Ammophila breviligulata with a variety of plant growth-promoting properties and designated as Sphingobium sp. strain AEW4.

Results

Analysis of the 16S rRNA gene as well as full genome nucleotide and amino acid identities revealed that this isolate is most similar to Sphingobium xenophagum and Sphingobium hydrophobicum. Comparative genomics analyses indicate that the genome of strain AEW4 contains unique features that explain its relationship with a plant host as a PGPR, including pathways involved in monosaccharide utilization, fermentation pathways, iron sequestration, and resistance to osmotic stress. Many of these unique features are not broadly distributed across the genus. In addition, pathways involved in the metabolism of salicylate and catechol, phenyl acetate degradation, and DNA repair were also identified in this organism but not in most closely related organisms.

Conclusion

The genome of Sphingobium sp. strain AEW4 contains a number of distinctive features that are crucial to explain its role as a plant-growth promoting rhizobacterium, and comparative genomics analyses support its classification as a relevant Sphingobium strain involved in plant growth promotion of beachgrass and other plants.

Supplementary Information

The online version contains supplementary material available at 10.1186/s12864-022-08738-8.

Developing Liquid Rhizobium Inoculants with Enhanced Long-Term Survival, Storage Stability, and Plant Growth Promotion Using Ectoine Additive

Liquid microbial inoculants have recently received great attention due to their vital roles for sustainable agricultural practices. However, long-term conservation under ambient temperature conditions and deleterious environmental factors might negatively impact microbial cell survival and limit their efficacy in the field. Thus, developing efficient liquid formulation providing prolonged survival of rhizobia in the final product and after an application is crucial. Therefore, this study investigates the effect of various additives on the long-term survival of rhizobia stored in liquid cultures at room temperature (25 °C) for 12 months. Various yeast sucrose media amended with polyvinylpyrrolidone (PVP) or gum arabic as colloidal agents in combination with ectoine (as a compatible solute) and/or glycerol were evaluated. A dramatic decline in viable cell count was obtained in formulas amended only with PVP from Log 8.5 to Log 5 in the first six months and then to Log 1.5 after 12 months. In contrast, rhizobia stored at PVP-based formulas amended with 10 mg L^‒1 ectoine exhibited almost constant survival level till the end of the storage period. The same trend was obtained using formulas based on gum arabic as a colloidal dispersing agent; however, less decline in cell count using a formula containing gum arabic alone as compared to using PVP. On the other hand, PVP based formulas exhibited higher viscosity compared with another formula. Increased viscosity till the 8th month of storage was achieved in the presence of ectoine indicating the increase of exopolymeric substances production. Electrophoretic protein pattern of rhizobial cells (stored for 12 months) exhibited several low molecular weight protein bands in cells stored in PVP based formula with ectoine as compared to the other treatments. Thus, the amendment of the liquid formulation of rhizobia bioinoculant with PVP plus ectoine not only improved cell survival but also enhanced the culture viscosity and consequently ameliorate the colonization and performance of rhizobial inoculants.

Evolutionary Timeline and Genomic Plasticity Underlying the Lifestyle Diversity in Rhizobiales

Bacteria form diverse interactions with eukaryotic hosts. This is well represented by the Rhizobiales, a clade of Alphaproteobacteria strategically important for their large diversity of lifestyles with implications for agricultural and medical research. To investigate their lifestyle evolution, we compiled a comprehensive data set of genomes and lifestyle information for over 1,000 Rhizobiales genomes. We show that the origins of major host-associated lineages in Rhizobiales broadly coincided with the emergences of their host plants/animals, suggesting bacterium-host interactions as a driving force in the evolution of Rhizobiales. We further found that, in addition to gene gains, preexisting traits and recurrent losses of specific genomic traits may have played underrecognized roles in the origin of host-associated lineages, providing clues to genetic engineering of microbial agricultural inoculants and prevention of the emergence of potential plant/animal pathogens.

Members of the order Rhizobiales include those capable of nitrogen fixation in nodules as well as pathogens of animals and plants. This lifestyle diversity has important implications for agricultural and medical research. Leveraging large-scale genomic data, we infer that Rhizobiales originated as a free-living ancestor ∼1,500 million years ago (Mya) and that the later emergence of host-associated lifestyles broadly coincided with the rise of their eukaryotic hosts. In particular, the first nodulating lineage arose from either Azorhizobium or Bradyrhizobium 150 to 80 Mya, a time range in general concurrent with the emergence of legumes. The rates of lifestyle transitions are highly variable; nodule association is more likely to be lost than gained, whereas animal association likely represents an evolutionary dead end. We searched for statistical correlations between gene presence and lifestyle and identified genes likely contributing to the transition and adaptation to the same lifestyle in divergent lineages. Among the genes potentially promoting successful transitions to major nodulation lineages, the nod and nif clusters for nodulation and nitrogen fixation, respectively, were repeatedly acquired during each transition; the fix, dct, and phb clusters involved in energy conservation under micro-oxic conditions were present in the nonnodulating ancestors; and the secretion systems were acquired in lineage-specific patterns. Our study data suggest that increased eukaryote diversity drives lifestyle diversification of bacteria and highlight both acquired and preexisting traits facilitating the origin of host association.

IMPORTANCE Bacteria form diverse interactions with eukaryotic hosts. This is well represented by the Rhizobiales, a clade of Alphaproteobacteria strategically important for their large diversity of lifestyles with implications for agricultural and medical research. To investigate their lifestyle evolution, we compiled a comprehensive data set of genomes and lifestyle information for over 1,000 Rhizobiales genomes. We show that the origins of major host-associated lineages in Rhizobiales broadly coincided with the emergences of their host plants/animals, suggesting bacterium-host interactions as a driving force in the evolution of Rhizobiales. We further found that, in addition to gene gains, preexisting traits and recurrent losses of specific genomic traits may have played underrecognized roles in the origin of host-associated lineages, providing clues to genetic engineering of microbial agricultural inoculants and prevention of the emergence of potential plant/animal pathogens.

Deciphering the Symbiotic Plant Microbiome: Translating the Most Recent Discoveries on Rhizobia for the Improvement of Agricultural Practices in Metal-Contaminated and High Saline Lands

Rhizosphere and plant-associated microorganisms have been intensely studied for their beneficial effects on plant growth and health. These mainly include nitrogen-fixing bacteria (NFB) and plant-growth promoting rhizobacteria (PGPR). This beneficial fraction is involved in major functions such as plant nutrition and plant resistance to biotic and abiotic stresses, which include water deficiency and heavy-metal contamination. Consequently, crop yield emerges as the net result of the interactions between the plant genome and its associated microbiome. Here, we provide a review covering recent studies on PGP rhizobia as effective inoculants for agricultural practices in harsh soil, and we propose models for inoculant combinations and genomic manipulation strategies to improve crop yield.

Metabolic and proteomic alteration in phytohormone-producing endophytic Bacillus amyloliquefaciens RWL-1 during methanol utilization

INTRODUCTION

Methanol utilization by bacteria is important for various industrial processes. Methylotrophic bacteria are taxonomically diverse and some species promote plant growth and induce stress tolerance. However, methylotrophic potential of bacterial endophytes is poorly understood.

OBJECTIVE

The current study aimed to evaluate the metabolomic and proteomic changes in endophytic Bacillus amyloliquefaciens RWL-1 caused by its methanol utilization and the resultant influence on its phytohormone production.

METHODS

B. amyloliquefaciens RWL-1 was grown in LB medium with different concentrations [0 (control), 0.5, 1, 1.5, 2, 2.5, 3, 3.5, and 4%) of methanol to examine its methylotrophic potential. SDS-PAGE analysis was carried out for bacterial protein confirmation. Moreover, the phytohormones (indole 3 acetic acid (IAA), gibberellins (GAs), abscisic acid (ABA)) produced by RWL-1 in methanol supplemented medium were quantified by GC-MS/SIM (6890N Network GC system, and 5973 Network Mass Selective Detector; Agilent Technologies, Santa Clara, CA, USA), while the antioxidants were estimated spectrophotometrically (T60 UV-VIS spectrophotometer, Leicester, UK). The amino acid quantification was carried out by amino acid analyzer (HITACHI L-8900, Japan). Furthermore, Nano-liquid chromatography (LC)-MS/MS analysis was performed with an Agilent system (Wilmington, DE, USA) for proteomic analysis while mascot algorithm (Matrix science, USA) was used to identify peptide sequences present in the protein sequence database.

RESULTS

RWL-1 showed significant growth in media supplemented with 2 and 3.5% methanol, when compared with other concentrations. Mass spectroscopy analysis revealed that RWL-1 utilizes methanol efficiently as a carbon source. In the presence of methanol, RWL-1 produced significantly higher levels of IAA but lower levels of ABA, when compared with the control. Further, enzymatic antioxidants and functional amino acids were significantly up-regulated, with predominant expression of glutamic acid and alanine. Nano-liquid chromatography, quadrupole time-of-flight analysis, and quantitative analysis of methanol-treated bacterial cells showed expression of eight different types of proteins, including detoxification proteins, unrecognized and unclassified enzymes with antioxidant properties, proteases, metabolism enzymes, ribosomal proteins, antioxidant proteins, chaperones, and heat shock proteins.

CONCLUSION

Results demonstrate that RWL-1 can significantly enhance its growth by utilizing methanol, and could produce phytohormones when growing in methanol-supplemented media, with increased expression of specific proteins and different biochemicals. These results will be useful in devising strategies for utilizing methylotrophic bacterial endophytes as alternative promoters of plant growth. Understanding RWL-1 ability to utilize methanol. The survival and phytohormones production by Bacillus amyloliquefaciens RWL-1 in methanol supplemented media whistle inducing metabolic and proteomic changes.

Production and characterization of ectoine using a moderately halophilic strain Halomonas salina BCRC17875

This study attempted to utilize Halomonas salina BCRC17875 to produce ectoine by optimizing the agitation speed and medium composition. In addition, the chemical structure of ectoine produced by H. salina BCRC17875 was determined. The results indicate that ectoine production reached 3.65 g/L at 38 h of cultivation when the agitation rate and NaCl concentration were fixed at 200 rpm and 2.0 M, respectively. It reached 9.20 g/L at 44 h of cultivation when the major medium components were yeast extract (56 g/L), glutamate (74.40 g/L), and ammonium sulfate (14 g/L). After the nitrogen concentration had been evaluated, evaluation of the nitrogen concentration revealed that the ectoine production reached 11.80 g/L at 44 h of cultivation when 56 g/L of yeast extract and 28 g/L of ammonium sulfate were used. Ectoine production reached 13.96 g/L at 44 h of cultivation when the carbon/nitrogen ratio was fixed at 3/1 using 84 g/L of yeast extract and 28 g/L of ammonium sulfate. Furthermore, the identification of ectoine were identified and characterized by fast atom bombardment mass spectrometry (FAB-MS) and ¹H NMR. The results demonstrated a fermentation strategy was successful in increasing ectoine production, and that the fermentation medium of ectoine had commercialization potential.

Metabolite profiling reveals abiotic stress tolerance in Tn5 mutant of Pseudomonas putida

Pseudomonas is an efficient plant growth–promoting rhizobacteria (PGPR); however, intolerance to drought and high temperature limit its application in agriculture as a bioinoculant. Transposon 5 (Tn5) mutagenesis was used to generate a stress tolerant mutant from a PGPR Pseudomonas putida NBRI1108 isolated from chickpea rhizosphere. A mutant NBRI1108T, selected after screening of nearly 10,000 transconjugants, exhibited significant tolerance towards high temperature and drought. Southern hybridization analysis of EcoRI and XhoI restricted genomic DNA of NBRI1108T confirmed that it had a single Tn5 insertion. The metabolic changes in the polar and non-polar extracts of NBRI1108 and NBRI1108T were examined using ¹H, ³¹P nuclear magnetic resonance (NMR) spectroscopy and gas chromatography-mass spectrometry (GC-MS). Thirty six chemically diverse metabolites consisting of amino acids, fatty acids and phospholipids were identified and quantified. Insertion of Tn5 influenced amino acid and phospholipid metabolism and resulted in significantly higher concentration of aspartic acid, glutamic acid, glycinebetaine, glycerophosphatidylcholine (GPC) and putrescine in NBRI1108T as compared to that in NBRI1108. The concentration of glutamic acid, glycinebetaine and GPC increased by 34%, 95% and 100%, respectively in the NBRI1108T as compared to that in NBRI1108. High concentration of glycerophosphatidylethanolamine (GPE) and undetected GPC in NBRI1108 indicates that biosynthesis of GPE may have taken place via the methylation pathway of phospholipid biosynthesis. However, high GPC and low GPE concentration in NBRI1108T suggest that methylation pathway and phosphatidylcholine synthase (PCS) pathway of phospholipid biosynthesis are being followed in the NBRI1108T. Application of multivariate principal component analysis (PCA) on the quantified metabolites revealed clear variations in NBRI1108 and NBRI1108T in polar and non-polar metabolites. Identification of abiotic stress tolerant metabolites from the NBRI1108T suggest that Tn5 mutagenesis enhanced tolerance towards high temperature and drought. Tolerance to drought was further confirmed in greenhouse experiments with maize as host plant, where NBRI1108T showed relatively high biomass under drought conditions.

Osmoprotection of Bacillus subtilis through import and proteolysis of proline-containing peptides

Bacillus subtilis can attain cellular protection against the detrimental effects of high osmolarity through osmotically induced de novo synthesis and uptake of the compatible solute l-proline. We have now found that B. subtilis can also exploit exogenously provided proline-containing peptides of various lengths and compositions as osmoprotectants. Osmoprotection by these types of peptides is generally dependent on their import via the peptide transport systems (Dpp, Opp, App, and DtpT) operating in B. subtilis and relies on their hydrolysis to liberate proline. The effectiveness with which proline-containing peptides confer osmoprotection varies considerably, and this can be correlated with the amount of the liberated and subsequently accumulated free proline by the osmotically stressed cell. Through gene disruption experiments, growth studies, and the quantification of the intracellular proline pool, we have identified the PapA (YqhT) and PapB (YkvY) peptidases as responsible for the hydrolysis of various types of Xaa-Pro dipeptides and Xaa-Pro-Xaa tripeptides. The PapA and PapB peptidases possess overlapping substrate specificities. In contrast, osmoprotection by peptides of various lengths and compositions with a proline residue positioned at their N terminus was not affected by defects in the PapA and PapB peptidases. Taken together, our data provide new insight into the physiology of the osmotic stress response of B. subtilis. They illustrate the flexibility of this ubiquitously distributed microorganism to effectively exploit environmental resources in its acclimatization to sustained high-osmolarity surroundings through the accumulation of compatible solutes.

Transcriptional analysis of major chaperone genes in salt-tolerant and salt-sensitive mesorhizobia

Salinity is an important abiotic stress that limits rhizobia-legume symbiosis, affecting plant growth, thus reducing crop productivity. Our aims were to evaluate the tolerance to salinity of native chickpea rhizobia as well as to investigate the expression of chaperone genes groEL, dnaKJ and clpB in both tolerant and sensitive isolates. One hundred and six native chickpea mesorhizobia were screened for salinity tolerance by measuring their growth with 1.5% and 3% NaCl. Most isolates were salt-sensitive, showing a growth below 20% compared to control. An association between salt tolerance and province of origin of the isolates was found. The transcriptional analysis by northern hybridization of chaperone genes was performed using tolerant and sensitive isolates belonging to different Mesorhizobium species. Upon salt shock, most isolates revealed a slight increase in the expression of the dnaK gene, whereas the groESL and clpB expression was unchanged or slightly repressed. No clear relationship was found between the chaperone genes induction and the level of salt tolerance of the isolates. This is the first report on transcriptional analysis of the major chaperones genes in chickpea mesorhizobia under salinity, which may contribute to a better understanding of the mechanisms that influence rhizobia salt tolerance.

Ectoines in cell stress protection: uses and biotechnological production

Microorganisms produce and accumulate compatible solutes aiming at protecting themselves from environmental stresses. Among them, the wide spread in nature ectoines are receiving increasing attention by the scientific community because of their multiple applications. In fact, increasing commercial demand has led to a multiplication of efforts in order to improve processes for their production. In this review, the importance of current and potential applications of ectoines as protecting agents for macromolecules, cells and tissues, together with their potential as therapeutic agents for certain diseases are analyzed and current theories for the understanding of the molecular basis of their biological activity are discussed. The genetic, biochemical and environmental determinants of ectoines biosynthesis by natural and engineered producers are described. The major limitations of current bioprocesses used for ectoines production are discussed, with emphasis on the different microorganisms, environments, molecular engineering and fermentation strategies used to optimize the production and recovery of ectoines. The combined application of both bioprocess and metabolic engineering strategies, allowing a deeper understanding of the main factors controlling the production process is also stated. Finally, this review aims to summarize and update the state of the art in ectoines uses and applications and industrial scale production using bacteria, emphasizing the importance of reactor design and operation strategies, together with the metabolic engineering aspects and the need for feedback between wet and in silico work to optimize bioproduction.

Transient Accumulation of Glycine Betaine and Dynamics of Endogenous Osmolytes in Salt-Stressed Cultures of Sinorhizobium meliloti

The fate of exogenously supplied glycine betaine and the dynamics of endogenous osmolytes were investigated throughout the growth cycle of salt-stressed cultures of strains of Sinorhizobium meliloti which differ in their ability to use glycine betaine as a growth substrate, but not as an osmoprotectant. We present (sup13)C nuclear magnetic resonance spectral and radiotracer evidence which demonstrates that glycine betaine is only transiently accumulated as a cytoplasmic osmolyte in young cultures of wild-type strains 102F34 and RCR2011. Specifically, these strains accumulate glycine betaine as a preferred osmolyte which virtually prevents the accumulation of endogenous osmolytes during the lag and early exponential phases of growth. Then, betaine levels in stressed cells decrease abruptly during the second half of the exponential phase. At this stage, the levels of glutamate and the dipeptide N-acetylglutaminylglutamine amide increase sharply so that the two endogenous solutes supplant glycine betaine in the ageing culture, in which it becomes a minor osmolyte because it is progressively catabolized. Ultimately, glycine betaine disappears when stressed cells reach the stationary phase. At this stage, wild-type strains of S. meliloti also accumulate the disaccharide trehalose as a third major endogenous osmolyte. By contrast, glycine betaine is always the dominant osmolyte and strongly suppresses the buildup of endogenous osmolytes at all stages of the growth cycle of a mutant strain, S. meliloti GMI766, which does not catabolize this exogenous osmoprotectant under any growth conditions.

RpoE2 of Sinorhizobium meliloti is necessary for trehalose synthesis and growth in hyperosmotic media

Adaptation to osmotic stress can be achieved by the accumulation of compatible solutes that aid in turgor maintenance and macromolecule stabilization. The genetic regulation of solute accumulation is poorly understood, and has been described well at the molecular level only in enterobacteria. In this study, we show the importance of the alternative sigma factor RpoE2 in Sinorhizobium meliloti osmoadaptation. Construction and characterization of an S. meliloti rpoE2 mutant revealed compromised growth in hyperosmotic media. This defect was due to the lack of trehalose, a minor carbohydrate osmolyte normally produced in the initial stages of growth and in stationary phase. We demonstrate here that all three trehalose synthesis pathways are RpoE2 dependent, but only the OtsA pathway is important for osmoinducible trehalose synthesis. Furthermore, we confirm that the absence of RpoE2-dependent induction of otsA is the cause of the osmotic phenotype of the rpoE2 mutant. In conclusion, we have highlighted that, despite its low level, trehalose is a crucial compatible solute in S. meliloti, and the OtsA pathway induced by RpoE2 is needed for its accumulation under hyperosmotic conditions.

Niche-specificity factors of a marine oil-degrading bacterium Alcanivorax borkumensis SK2

Alcanivorax borkumensis strain SK2 is a cosmopolitan hydrocarbonoclastic marine bacterium, with a specialized metabolism adapted to the degradation of petroleum oil hydrocarbons. Transposon mutagenesis was used for functional genome analysis of Alcanivorax SK2 to reveal the genetic basis of other environmentally relevant phenotypes, such as biofilm formation, adaptation to UV exposure, and to growth at either low temperature or high salinity. Forty-eight relevant transposon mutants deficient in any one of these environmentally responsive functions were isolated, and the corresponding genes interrupted by the mini-Tn5 element were sequenced using inverse PCR. Several cross connections between different phenotypes (e.g. biofilm and UV stress; biofilm and UV and osmoadaptation) on signal transduction level have been revealed, pointing at complex and tightly controlled cellular interactions involving oxygen as a primary messenger and cyclic-di-GMP as a secondary messenger required for Alcanivorax responses to environmental stresses. These results provide insights into bacterial function in a complex marine environment.

Osmotically induced synthesis of the compatible solute hydroxyectoine is mediated by an evolutionarily conserved ectoine hydroxylase

By using natural abundance (13)C NMR spectroscopy, we investigated the types of compatible solutes synthesized in a variety of Bacilli under high salinity growth conditions. Glutamate, proline, and ectoine were the dominant compatible solutes synthesized by the various Bacillus species. The majority of the inspected Bacilli produced the tetrahydropyrimidine ectoine in response to high salinity stress, and a subset of these also synthesized a hydroxylation derivative of ectoine, 5-hydroxyectoine. In Salibacillus salexigens, a representative of the ectoine- and 5-hydroxyectoine-producing species, ectoine production was linearly correlated with the salinity of the growth medium and dependent on an ectABC biosynthetic operon. The formation of 5-hydroxyectoine was primarily a stationary growth phase phenomenon. The enzyme responsible for ectoine hydroxylation (EctD) was purified from S. salexigens to apparent homogeneity. The EctD protein was shown in vitro to directly hydroxylate ectoine in a reaction dependent on iron(II), molecular oxygen, and 2-oxoglutarate. We identified the structural gene (ectD) for the ectoine hydroxylase in S. salexigens. Northern blot analysis showed that the transcript levels of the ectABC and ectD genes increased as a function of salinity. Many EctD-related proteins can be found in data base searches in various Bacteria. Each of these bacterial species also contains an ectABC ectoine biosynthetic gene cluster, suggesting that 5-hydroxyectoine biosynthesis strictly depends on the prior synthesis of ectoine. Our data base searches and the biochemical characterization of the EctD protein from S. salexigens suggest that the EctD-related ectoine hydroxylases are members of a new subfamily within the non-heme-containing, iron(II)- and 2-oxoglutarate-dependent dioxygenase superfamily (EC 1.14.11).

Osmoadaptative responses in the rhizobia nodulating Acacia isolated from south-eastern Moroccan Sahara

Four strains of rhizobia nodulating Acacia were isolated from the Moroccan desert soil by trapping with seedlings of Acacia gummifera and Acacia raddiana, and were studied for their ability to tolerate high salinity and dryness conditions. The strains MDSMC 2, MDSMC 18 and MDSMC 50 were halotolerant (they tolerated up to 1 M NaCl) and they accumulated glutamate and mannosucrose. The synthesis of the latter solute, which is the major endogenous osmolyte, is partially repressed in the presence of glycine betaine. The strain MDSMC 34 was less halotolerant (growth inhibited by a concentration greater than 0.5 M NaCl), and accumulated trehalose (as the main endogenous osmolyte) and glutamate. Rhizobia from the Moroccan desert soil were highly resistant to desiccation and their tolerance to dryness was stimulated by osmotic pretreatment. Thus, the accumulation of mannosucrose or trehalose by desert rhizobia represents both an osmoadaptative response and a part of a desiccation tolerance mechanism.

Interrelations between glycine betaine catabolism and methionine biosynthesis in Sinorhizobium meliloti strain 102F34

Methionine is produced by methylation of homocysteine. Sinorhizobium meliloti 102F34 possesses only one methionine synthase, which catalyzes the transfer of a methyl group from methyl tetrahydrofolate to homocysteine. This vitamin B(12)-dependent enzyme is encoded by the metH gene. Glycine betaine can also serve as an alternative methyl donor for homocysteine. This reaction is catalyzed by betaine-homocysteine methyl transferase (BHMT), an enzyme that has been characterized in humans and rats. An S. meliloti gene whose product is related to the human BHMT enzyme has been identified and named bmt. This enzyme is closely related to mammalian BHMTs but has no homology with previously described bacterial betaine methyl transferases. Glycine betaine inhibits the growth of an S. meliloti bmt mutant in low- and high-osmotic strength media, an effect that correlates with a decrease in the catabolism of glycine betaine. This inhibition was not observed with other betaines, like homobetaine, dimethylsulfoniopropionate, and trigonelline. The addition of methionine to the growth medium allowed a bmt mutant to recover growth despite the presence of glycine betaine. Methionine also stimulated glycine betaine catabolism in a bmt strain, suggesting the existence of another catabolic pathway. Inactivation of metH or bmt did not affect the nodulation efficiency of the mutants in the 102F34 strain background. Nevertheless, a metH strain was severely defective in competing with the wild-type strain in a coinoculation experiment.

Proline betaine uptake in Sinorhizobium meliloti: Characterization of Prb, an opp-like ABC transporter regulated by both proline betaine and salinity stress

Sinorhizobium meliloti uses proline betaine (PB) as an osmoprotectant when osmotically stressed and as an energy source in low-osmolarity environments. To fulfill this dual function, two separate PB transporters, BetS and Hut, that contribute to PB uptake at high and low osmolarity, respectively, have been previously identified. Here, we characterized a novel transport system that mediates the uptake of PB at both high and low osmolarities. Sequence analysis of Tn5-luxAB chromosomal insertions from several PB-inducible mutants has revealed the presence of a four-gene locus encoding the components of an ABC transporter, Prb, which belongs to the oligopeptide permease (Opp) family. Surprisingly, prb mutants were impaired in their ability to transport PB, and oligopeptides were not shown to be competitors for PB uptake. Further analysis of Prb specificity has shown its ability to take up other quaternary ammonium compounds such as choline and, to a lesser extent, glycine betaine. Interestingly, salt stress and PB were found to control prb expression in a positive and synergistic way and to increase Prb transport activity. At low osmolarity, Prb is largely implicated in PB uptake by stationary-phase cells, likely to provide PB as a source of carbon and nitrogen. Furthermore, at high osmolarity, the analysis of prb and betS single and double mutants demonstrated that Prb, together with BetS, is a key system for protection by PB.

Influence of high salinity on biofilm formation and benzoate assimilation by Pseudomonas aeruginosa

Pseudomonas species were used in bioremediation technologies. In situ conditions, such as marine salinity, could limit the degradation of hydrocarbons and aromatic compounds by the bacteria. Biofilm ability to tolerate environmental stress could be used to increase biorestoration. In this report, we used scanning confocal laser microscopy and microtiter dish assay to analyse the impact of hyperosmotic stress on biofilm formation by Pseudomonas aeruginosa. We used benzoate as the sole carbon source and the effect of the stress on its degradation was also studied. Hyperosmotic shock inhibited the biofilm development and decreased the degradation of benzoate. The osmoprotectant glycine betaine partially restored both the biofilm formation and benzoate degradation, suggesting that it could be used as a complement in bioremediation processes.

Characterization of salt tolerance in ectoine-transformed tobacco plants (Nicotiana tabaccum): photosynthesis, osmotic adjustment, and nitrogen partitioning

Ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyrimidinecarboxylic acid) biosynthetic genes (ect. ABC) from Halomonas elongata were introduced to tobacco plants using an Agrobacterium-mediated gene delivery system. The genes for ectoine biosynthesis were integrated in a stable manner into the tobacco genome and the corresponding transcripts were expressed. The concentration of ectoine under salt-stress conditions was higher in the roots than in leaves. A close relationship was found between stomatal conductance and the amount of transported nitrogen, suggesting that water transport through the xylem in the stem and transpiration may be involved in nitrogen transport to leaves. The data indicate that the turgor values of the ectoine transgenic lines increased with increasing salt concentration. The data revealed two ways in which ectoine enhanced salinity tolerance of tobacco plants. First, ectoine improved the maintenance of root function so that water is taken up consistently and supplied to shoots under saline conditions. Second, ectoine enhanced the nitrogen supply to leaves by increasing transpiration and by protecting Rubisco proteins from deleterious effects of salt, thereby improving the rate of photosynthesis.

Supplementation effects of hydroxyectoine on proline uptake of downshocked Brevibacterium sp. JCM 6894

Downshock treatment of the halotolerant Brevibacterium sp. JCM 6894 was a prerequisite for proline uptake which is a function for cell survival. Hydroxyectoine served as an effective stimulator for the proline uptake and cell survival of the downshocked cells of this strain. Duration of osmotic downshock, downshock strength, and the kinds of osmolyte affected the efficient rate of growth (ERG) and the uptake of proline. A shorter duration of osmotic downshock, that is Collapse

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MESH Headings

• Amino Acids, Diamino/pharmacology
• Brevibacterium/drug effects
• Brevibacterium/physiology
• Carbohydrates/pharmacology
• Carbon/metabolism
• Cell Proliferation/drug effects
• Culture Media
• Osmotic Pressure
• Proline/pharmacokinetics
• Proline/pharmacology

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Affiliation(s)

Chenxiang Wang Environmental Biochemistry Group, Research Center for Inland Seas, Kobe University, 5-1-1 Fukae, Kobe 658-0022, Japan
Daochen Zhu
Shinichi Nagata

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Effect of duration of osmotic downshock and coexisting glutamate on survival and uptake of ectoine in halotolerant Brevibacterium sp. JCM 6894

Halotolerant Brevibacterium sp. JCM 6894 that was subjected to an osmotic downshock (0.7 M NaCl to 0 M) was examined for its survival and uptake of ectoine in the presence of ectoine and/or carbon sources. In the presence of ectoine alone, the rates of ectoine uptake by the 1 h-downshocked cells were low and high in the absence and presence of 0.7 M NaCl, respectively, which were in parallel with the rates of cell growth. The presence of glutamate or amino acids together with ectoine exerted a stimulative effect on the survival of downshocked cells. The incubation time of the cells subjected to osmotic downshock strongly affected ectoine uptake as well as the cell growth of this strain, suggesting that the transporter of ectoine in the strain JCM 6894 was stimulated during the osmotic downshock for about 1 h. Different downshock strengths had marked effects on the rate of ectoine uptake when the downshocked cells were incubated in the presence of NaCl.

Ectoines as compatible solutes and carbon and energy sources for the halophilic bacterium Chromohalobacter salexigens

AIMS

To investigate the catabolism of ectoine and hydroxyectoine, which are the major compatible solutes synthesized by Chromohalobacter salexigens.

METHODS AND RESULTS

Growth curves performed in M63 minimal medium with low (0.75 mol l(-1) NaCl), optimal (1.5 mol l(-1) NaCl) or high (2.5 mol l(-1) NaCl) salinity revealed that betaine and ectoines were used as substrate for growth at optimal and high salt. Ectoine transport was maximal at optimal salinity, and showed 3- and 1.5-fold lower values at low and high salinity respectively. The salt-sensitive ectA mutant CHR62 showed an ectoine transport rate 6.8-fold higher than that of the wild type. Incubation of C. salexigens in a mixture of glucose and ectoine resulted in a biphasic growth pattern. However, CO(2) production due to ectoine catabolism was lower, but not completely abolished, in the presence of glucose. When used as the sole carbon source, glycine betaine effectively inhibited ectoine and hydroxyectoine synthesis at any salinity.

CONCLUSIONS

The catabolic pathways for ectoine and hydroxyectoine in C. salexigens operate at optimal and high (although less efficiently) salinity. Endogenous ectoine(s) may repress its own transport. Ectoine utilization was only partially repressed by glucose. Betaine, when used as carbon source, suppresses synthesis of ectoines even under high osmolarity conditions.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study is a previous step to the subsequent isolation and manipulation of the catabolic genes, so as to generate strains with enhanced production of ectoine and hydroxyectoine.

Efficient utilization of ectoine by halophilic Brevibacterium species and Escherichia coli subjected to osmotic downshock

Halophilic and non-halophilic bacteria subjected to osmotic downshock, from 0.7 M NaCl to deionized water, were examined for their survival, with the uptake and utilization of the cyclic amino acid ectoine, one of the representative compatible solutes, being taken into account. The uptake of ectoine added externally and survival of the cells were monitored as a function of incubation time in the presence and absence of NaCl. The halophilic Brevibacterium sp. JCM 6894 and B. epidermidis JCM 2593 actively accumulated ectoine regardless of the presence of NaCl, which led to cell survival. Brevibacterium casei JCM 2594 belonging to the same Brevibacterium species, however, revealed Na+-dependence of its uptake activity of ectoine. Non-halophilic Escherichia coli K-12 did not accumulate ectoine, and thereby this strain failed to survive irrespective of whether NaCl was present. The physiological meanings of the downshock procedure are discussed in connection with the uptake and the subsequent utilization of ectoine.

Ectoine-induced proteins in Sinorhizobium meliloti include an Ectoine ABC-type transporter involved in osmoprotection and ectoine catabolism

To understand the mechanisms of ectoine-induced osmoprotection in Sinorhizobium meliloti, a proteomic examination of S. meliloti cells grown in minimal medium supplemented with ectoine was undertaken. This revealed the induction of 10 proteins. The protein products of eight genes were identified by using matrix-assisted laser desorption ionization-time-of-flight mass spectrometry. Five of these genes, with four other genes whose products were not detected on two-dimensional gels, belong to the same gene cluster, which is localized on the pSymB megaplasmid. Four of the nine genes encode the characteristic components of an ATP-binding cassette transporter that was named ehu, for ectoine/hydroxyectoine uptake. This transporter was encoded by four genes (ehuA, ehuB, ehuC, and ehuD) that formed an operon with another gene cluster that contains five genes, named eutABCDE for ectoine utilization. On the basis of sequence homologies, eutABCDE encode enzymes with putative and hypothetical functions in ectoine catabolism. Analysis of the properties of ehuA and eutA mutants suggests that S. meliloti possesses at least one additional ectoine catabolic pathway as well as a lower-affinity transport system for ectoine and hydroxyectoine. The expression of ehuB, as determined by measurements of UidA activity, was shown to be induced by ectoine and hydroxyectoine but not by glycine betaine or by high osmolality.

Glucose 6-phosphate dehydrogenase is required for sucrose and trehalose to be efficient osmoprotectants in Sinorhizobium meliloti

Inactivation of the zwf gene in Sinorhizobium meliloti induces an osmosensitive phenotype and the loss of osmoprotection by trehalose and sucrose, but not by ectoine and glycine betaine. This phenotype is not linked to a defect in the biosynthesis of endogenous solutes. zwf expression is induced by high osmolarity, sucrose and trehalose, but is repressed by betaine. A zwf mutant is more sensitive than its parental strain to superoxide ions, suggesting that glucose 6-phosphate dehydrogenase involvement in the osmotic response most likely results from the production of reactive oxygen species during osmotic stress.

Role of ectoine in Vibrio cholerae osmoadaptation

Vibrio cholerae is both an intestinal pathogen and a microbe in the estuarine community. To persist in the estuarine environment, V. cholerae must adjust to changes in ionic composition and osmolarity. These changes in the aquatic environment have been correlated with cholera epidemics. In this work, we study the response of V. cholerae to increases in environmental osmolarity. Optimal growth of V. cholerae in minimal medium requires supplementation with 200 mM NaCl and KCl. However, when the NaCl concentration is increased beyond 200 mM, a proportionate delay in growth is observed. During this delay in growth, osmotic equilibrium is reached by cytoplasmic accumulation of small, uncharged solutes that are compatible with growth. We show that synthesis of the compatible solute ectoine and transport of the compatible solute glycine betaine impact the length of the osmoadaptive growth delay. We also demonstrate that high-osmolarity-adapted V. cholerae displays a growth advantage when competed against unadapted cells in high-osmolarity medium. In contrast, low-osmolarity-adapted V. cholerae displays no growth advantage when competed against high-osmolarity-adapted cells in low-osmolarity medium. These results may have implications for V. cholerae population dynamics when seawater and freshwater and their attendant microbes mix.

Construction and validation of a Sinorhizobium meliloti whole genome DNA microarray: genome-wide profiling of osmoadaptive gene expression

Based on the complete Sinorhizobium meliloti genome sequence we established DNA microarrays as a comprehensive tool for systematic genome-wide gene expression analysis in S. meliloti 1021. For these PCR fragment-based microarrays, called Sm6kPCR, a collection of probes for the 6207 predicted protein-coding genes consisting of 6046 gene-specific PCR fragments and 161 70 mer oligonucleotides was arrayed in high density on glass slides. To obtain these PCR fragments primer pairs were designed to amplify internal gene-specific DNA fragments of 80-350 bp. Additionally, these primers were characterized by a 5' extension that allowed for reamplification using standard primers after the first amplification employing the specific primers. In order to ascertain the quality of the Sm6kPCR microarrays and to validate gene expression studies in S. meliloti parallel hybridizations based on RNA samples obtained from cells cultured under identical conditions were performed. In addition, gene expression in S. meliloti in response to an osmotic upshift imposed by the addition of 0.38 M NaCl was monitored. 137 genes were identified showing significant changes in gene expression resulting from the osmotic upshift. From these genes 52 were induced and 85 genes were repressed. Among the genes displaying different RNA levels some functional groups could be identified that are particularly remarkable. Repression was observed for 8 genes related to motility and chemotaxis, 7 genes encoding amino acid biosynthesis enzymes and 15 genes involved in iron uptake whereas 14 genes involved in transport of small molecules and 4 genes related to polysaccharide biosynthesis were induced.

Osmotically regulated synthesis of the compatible solute ectoine in Bacillus pasteurii and related Bacillus spp

By using natural-abundance (13)C-nuclear magnetic resonance spectroscopy and high-performance liquid chromatography (HPLC) analysis we have investigated the types of compatible solutes that are synthesized de novo in a variety of Bacillus species under high-osmolality growth conditions. Five different patterns of compatible solute production were found among the 13 Bacillus species we studied. Bacillus subtilis, B. licheniformis, and B. megaterium produced proline; B. cereus, B. circulans, B. thuringiensis, Paenibacillus polymyxa, and Aneurinibacillus aneurinilyticus synthesized glutamate; B. alcalophilus, B. psychrophilus, and B. pasteurii synthesized ectoine; and Salibacillus (formerly Bacillus) salexigens produced both ectoine and hydroxyectoine, whereas Virgibacillus (formerly Bacillus) pantothenticus synthesized both ectoine and proline. Hence, the ability to produce the tetrahydropyrimidine ectoine under hyperosmotic growth conditions is widespread within the genus Bacillus and closely related taxa. To study ectoine biosynthesis within the group of Bacillus species in greater detail, we focused on B. pasteurii. We cloned and sequenced its ectoine biosynthetic genes (ectABC). The ectABC genes encode the diaminobutyric acid acetyltransferase (EctA), the diaminobutyric acid aminotransferase (EctB), and the ectoine synthase (EctC). Together these proteins constitute the ectoine biosynthetic pathway, and their heterologous expression in B. subtilis led to the production of ectoine. Northern blot analysis demonstrated that the ectABC genes are genetically organized as an operon whose expression is strongly enhanced when the osmolality of the growth medium is raised. Primer extension analysis allowed us to pinpoint the osmoregulated promoter of the B. pasteurii ectABC gene cluster. HPLC analysis of osmotically challenged B. pasteurii cells revealed that ectoine production within this bacterium is finely tuned and closely correlated with the osmolality of the growth medium. These observations together with the osmotic control of ectABC transcription suggest that the de novo synthesis of ectoine is an important facet in the cellular adaptation of B. pasteurii to high-osmolarity surroundings.

Uptake and utilization of ectoine by halotolerant Brevibacterium sp. JCM 6894 subjected to osmotic downshock

The concentration changes of the cyclic amino acid ectoine (1,4,5,6-tetrahydro-2-methyl-4-pyridine carboxylic acid) in Brevibacterium sp. JCM 6894 cells subjected to an osmotic downshock were investigated. When the cells grown in the presence of 2 M NaCl were suspended in deionized water, they immediately released about 60% of the ectoine synthesized intracellularly. During the subsequent incubation, we observed that both the extra- and intracellular concentrations of ectoine were reduced almost linearly with the incubation time. When ectoine was provided externally to the downshocked cells, a similar reduction in both intra- and extracellular ectoine concentrations was recognized. In addition, we observed an increase in ectoine accumulation at about 10 h of incubation, which indicates that ectoine was taken up by such downshocked cells in the absence of external Na+. Furthermore, the downshocked cells showed higher levels of survival, respiration, and growth in the presence of ectoine than in its absence. The ability to take ectoine up was induced in the cells grown in the presence of >0.25 M NaCl for >12 h. Thus, we conclude that even under the lower osmotic condition ectoine might be taken up and subsequently utilized by strain JCM 6894 subjected to the osmotic downshock, indicating that the uptake of ectoine by such cells occurred for the survival and growth of the cell itself rather than for cellular osmoregulation.

Synthesis, accumulation and hydrolysis of trehalose during growth of peanut rhizobia in hyperosmotic media

We examined and compared the activities of synthetic and hydrolytic enzymes involved in trehalose metabolism, in three peanut rhizobia strains grown in control, hypersaline, and non-ionic hyperosmotic media. Results indicated that the effects of hyperosmolarity on the synthesis and the degradation of the disaccharide were diverse. In the salt-tolerant slow-growing strain Bradyrhizobium sp. ATCC 10317, we observed increased synthesis and accumulation of trehalose under hyperosmolarity imposed by either NaCl or PEG-8000. In the other two peanut rhizobia strains, the disaccharide level did not change under hypersalinity. In the salt-sensitive slow-growing strain Bradyrhizobium sp. USDA 3187, intracellular trehalose diminished in late stationary phase-cells grown with PEG, this reduction was accompanied by both an increased activity of synthetic enzymes and a decreased activity of trehalase. In the salt-tolerant fast-growing strain Rhizobium sp. TAL 1000, we also observed a reduction of intracellular trehalose under PEG-mediated growth, this decrease was early and transiently accompanied by an enhancement of trehalase activity, afterwards, the activity of synthetic enzymes augmented.

Characterization of a Snorhizobium meliloti ATP-binding cassette histidine transporter also involved in betaine and proline uptake

The symbiotic soil bacterium Sinorhizobium meliloti uses the compatible solutes glycine betaine and proline betaine for both protection against osmotic stress and, at low osmolarities, as an energy source. A PCR strategy based on conserved domains in components of the glycine betaine uptake systems from Escherichia coli (ProU) and Bacillus subtilis (OpuA and OpuC) allowed us to identify a highly homologous ATP-binding cassette (ABC) binding protein-dependent transporter in S. meliloti. This system was encoded by three genes (hutXWV) of an operon which also contained a fourth gene (hutH2) encoding a putative histidase, which is an enzyme involved in the first step of histidine catabolism. Site-directed mutagenesis of the gene encoding the periplasmic binding protein (hutX) and of the gene encoding the cytoplasmic ATPase (hutV) was done to study the substrate specificity of this transporter and its contribution in betaine uptake. These mutants showed a 50% reduction in high-affinity uptake of histidine, proline, and proline betaine and about a 30% reduction in low-affinity glycine betaine transport. When histidine was used as a nitrogen source, a 30% inhibition of growth was observed in hut mutants (hutX and hutH2). Expression analysis of the hut operon determined using a hutX-lacZ fusion revealed induction by histidine, but not by salt stress, suggesting this uptake system has a catabolic role rather than being involved in osmoprotection. To our knowledge, Hut is the first characterized histidine ABC transporter also involved in proline and betaine uptake.

Osmoprotection by pipecolic acid in Sinorhizobium meliloti: specific effects of D and L isomers

DL-Pipecolic acid (DL-PIP) promotes growth restoration of Sinorhizobium meliloti cells facing inhibitory hyperosmolarity. Surprisingly, D and L isomers of this imino acid supplied separately were not effective. The uptake of L-PIP was significantly favored in the presence of the D isomer and by a hyperosmotic stress. Chromatographic analysis of the intracellular solutes showed that stressed cells did not accumulate radiolabeled L-PIP. Rather, it participates in the synthesis of the main endogenous osmolytes (glutamate and the dipeptide N-acetylglutaminylglutamine amide) during the lag phase, thus providing a means for the stressed cells to recover the osmotic balance. (13)C nuclear magnetic resonance analysis was used to determine the fate of D-PIP taken into the cells. In the absence of L-PIP, the imported D isomer was readily degraded. Supplied together with its L isomer, D-PIP was accumulated temporarily and thus might contribute together with the endogenous osmolytes to enhance the internal osmotic strength. Furthermore, it started to disappear from the cytosol when the L isomer was no longer available in the culture medium (during the late exponential phase of growth). Together, these results show an uncommon mechanism of protection of osmotically stressed cells of S. meliloti. It was proved, for the first time, that the presence of the two isomers of the same molecule is necessary for it to manifest an osmoprotective activity. Indeed, D-PIP seems to play a major role in cellular osmoadaptation through both its own accumulation and improvement of the utilization of the L isomer as an immediate precursor of endogenous osmolytes.

Is the accumulation of osmoprotectant the unique mechanism involved in bacterial osmoprotection?

Sucrose, trehalose, maltose, cellobiose, gentiobiose, turanose and palatinose are very unusual osmoprotectants for Sinorhizobium meliloti, because these compounds, unlike other bacterial osmoprotectants, do not accumulate as cytosolic osmolytes in salt-stressed S. meliloti cells. Rather, these compounds were catabolized during early exponential growth, and contributed to enhance the cytosolic levels of the two endogenously-synthesized osmolytes: glutamate and the dipeptide N-acetylglutaminylglutamine amide (NAGGN). Furthermore, all of the disaccharides that acted as powerful osmoprotectants shared the same uptake routes in S. meliloti. Here, we show that these disaccharides, in fact, belong to a new family of non-accumulated sinorhizobial osmoprotectants and that two mechanisms of osmoprotection coexist in S. meliloti.

Ectoine, the compatible solute of Halomonas elongata, confers hyperosmotic tolerance in cultured tobacco cells

1,4,5,6-Tetrahydro-2-methyl-4-pyrimidinecarboxylic acid (ectoine) functions as a compatible osmolyte in the moderate halophile Halomonas elongata OUT30018. Ectoine is biosynthesized by three successive enzyme reactions from aspartic beta-semialdehyde. The genes encoding the enzymes involved in the biosynthesis, ectA, ectB, and ectC, encoding L-2,4-diaminobutyric acid acetyltransferase, L-2, 4-diaminobutyric acid transaminase, and L-ectoine synthase, respectively, have been previously cloned. To investigate the function of ectoine as a compatible solute in plant cells, the three genes were individually placed under the control of the cauliflower mosaic virus 35S promoter and introduced together into cultured tobacco (Nicotiana tabacum L.) cv Bright Yellow 2 (BY2) cells. The transgenic BY2 cells accumulated a small quantity of ectoine (14-79 nmol g(-1) fresh weight) and showed increased tolerance to hyperosmotic shock (900 mOsm). Furthermore, the transgenic BY2 cells exhibited a normal growth pattern even under hyperosmotic conditions (up to 530 mOsm), in which the growth of the untransformed BY2 (wild type) cells was obviously delayed. These results suggest that genetically engineered synthesis of ectoine results in the increased hyperosmotic tolerance of cultured tobacco BY2 cells despite the low level of accumulation of the solute.

Glycine betaine, carnitine, and choline enhance salinity tolerance and prevent the accumulation of sodium to a level inhibiting growth of Tetragenococcus halophila

Natural-abundance (13)C-nuclear magnetic resonance was used to probe the intracellular organic solute content of the moderately halophilic bacterium Tetragenococcus halophila. When grown in complex growth media supplemented or not with NaCl, T. halophila accumulates glycine betaine and carnitine. Unlike other moderate halophiles, T. halophila was not able to produce potent osmoprotectants (such as ectoines and glycine betaine) through de novo synthesis when cultured in defined medium under hyperosmotic constraint. Addition of 2 mM carnitine, glycine betaine, or choline to defined medium improved growth parameters, not only at high salinity (up to 2.5 M NaCl) but also in media lacking NaCl. These compounds were taken up when available in the surrounding medium. The transport activity occurred at low and high salinities and seems to be constitutive. Glycine betaine and carnitine were accumulated by T. halophila in an unmodified form, while exogenously provided choline led to an intracellular accumulation of glycine betaine. This is the first evidence of the existence of a choline-glycine betaine pathway in a lactic acid bacterium. An assay showed that the compatible solutes strikingly repressed the accumulation of glutamate and slightly increased the intracellular potassium level only at high salinity. Interestingly, osmoprotectant-treated cells were able to maintain the intracellular sodium concentration at a relatively constant level (200 to 300 nmol/mg [dry weight]), independent of the NaCl concentration of the medium. In contrast, in the absence of osmoprotectant, the intracellular sodium content increased sharply from 200 to 2,060 nmol/mg (dry weight) when the salinity of the medium was raised from 1 to 2 M. Indeed, the imported compatible solutes play an actual role in regulating the intracellular Na(+) content and confer a much higher salt tolerance to T. halophila.