Hydroxyectoine is superior to trehalose for anhydrobiotic engineering of Pseudomonas putida KT2440

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.

Seed-Encapsulation of Desiccation-Tolerant Microorganisms for the Protection of Maize from Drought: Phenotyping Effects of a New Dry Bioformulation

Droughts and high temperatures deeply affect crop production. The use of desiccation-tolerant (or xerotolerant) microorganisms able to protect plants from droughts represents a promising alternative. These xerotolerant microorganisms have previously been used to modulate plant responses and improve their tolerance to drought. In addition, these microorganisms could be stored and used in dry formats, which would improve their viability and resilience at a much lower cost than current market alternatives. In the present study we analyze the possibility of using strains of xerotolerant Actinobacteria in encapsulated format on seeds. Under this formulation, we carried out greenhouse with farming soil with maize plants. Under greenhouse conditions, the plants showed greater resistance to drought, as well as increased growth and production yield, but not as well in field trials. This alternative could represent a useful tool to improve water efficiency in crops for drought-affected areas or affected by water scarcity.

Influence of rehydration on transcriptome during resuscitation of desiccated Pseudomonas putida KT2440

Purpose

Pseudomonas putida KT2440 is a desiccation-sensitive bacterium that loses culturability after 15 days of air desiccation. We have previously shown that P. putida KT2440 can develop a viable but nonculturable (VBNC) state after being exposed to desiccation stress and eventually recover when desiccated cells are rehydrated for at least 24 h.

Methods

To determine which genes of transport, oxidation-reduction, and transcription processes could be involved in the return of P. putida KT2440 to the culturable state, a transcriptome analysis was carried out comparing the gene expression of non-desiccated samples with samples subjected to desiccation followed by 20 min of rehydration or desiccation followed by 24 h of rehydration.

Results

Desiccation stress triggered a VBNC state of P. putida. The major response was detected after 24 h of rehydration with 148 upregulated and 42 downregulated genes. During the VBNC state, P. putida activated transmembrane transport processes like that of siderophores through a TonB-dependent transporter and putative polyhydric alcohol transport systems. Prolonged rehydration with distilled water resuscitated P. putida KT2440 cells activating the catabolism of phenylalanine/tyrosine to provide energy and carbon for ubiquinone biosynthesis while maintaining a reduced protein synthesis. On the other hand, the interruption of the TonB-dependent receptor gene (PP_1446) increased desiccation survival of the mutant strain.

Conclusion

The activation of the iron transport system (TonB-dependent siderophore receptor) and alcohol transport can be helping the VBNC state of P. putida. Activation of catabolism of phenylalanine/tyrosine and reduced protein synthesis was needed for resuscitation from the VBNC state.

Degradation of the microbial stress protectants and chemical chaperones ectoine and hydroxyectoine by a bacterial hydrolase-deacetylase complex

When faced with increased osmolarity in the environment, many bacterial cells accumulate the compatible solute ectoine and its derivative 5-hydroxyectoine. Both compounds are not only potent osmostress protectants, but also serve as effective chemical chaperones stabilizing protein functionality. Ectoines are energy-rich nitrogen and carbon sources that have an ecological impact that shapes microbial communities. Although the biochemistry of ectoine and 5-hydroxyectoine biosynthesis is well understood, our understanding of their catabolism is only rudimentary. Here, we combined biochemical and structural approaches to unravel the core of ectoine and 5-hydroxy-ectoine catabolisms. We show that a conserved enzyme bimodule consisting of the EutD ectoine/5-hydroxyectoine hydrolase and the EutE deacetylase degrades both ectoines. We determined the high-resolution crystal structures of both enzymes, derived from the salt-tolerant bacteria Ruegeria pomeroyi and Halomonas elongata These structures, either in their apo-forms or in forms capturing substrates or intermediates, provided detailed insights into the catalytic cores of the EutD and EutE enzymes. The combined biochemical and structural results indicate that the EutD homodimer opens the pyrimidine ring of ectoine through an unusual covalent intermediate, N-α-2 acetyl-l-2,4-diaminobutyrate (α-ADABA). We found that α-ADABA is then deacetylated by the zinc-dependent EutE monomer into diaminobutyric acid (DABA), which is further catabolized to l-aspartate. We observed that the EutD-EutE bimodule synthesizes exclusively the α-, but not the γ-isomers of ADABA or hydroxy-ADABA. Of note, α-ADABA is known to induce the MocR/GabR-type repressor EnuR, which controls the expression of many ectoine catabolic genes clusters. We conclude that hydroxy-α-ADABA might serve a similar function.

Dealing with water stress and microbial preservation

The relevance of preserving microorganisms has been well accepted for several decades. Interest is now shifting towards investigating adequate preservation methods to improve microbial survival rates and to preserve new taxa of previously considered unculturable microorganisms. In addition, a growing interest in preserving fragile microbial consortia or communities with biotechnological interest motivates the improvement of preservation methods. In the present study, we reviewed the effect of water availability in microbial diversity shift. We describe the effect of drought on microorganisms at the molecular level and their molecular responses to this life-threatening challenge focusing on the production of xeroprotectants. We also review the interspecies interactions of those drought-tolerant microorganisms with other sensitive organisms including neighbouring prokaryotes and eukaryotes such as plants, and the potential role of these microorganisms at determining the ecological composition of stressed environments. We emphasize the importance of applying the knowledge derived from the molecular mechanisms used by desiccation-tolerant microorganisms for the improvement of the preservation techniques. An overview of the current and newer techniques for preserving microorganisms and microbial communities is provided. The biotechnological interest in preserving pure cultures, microbial consortia and communities is also discussed.

Desiccation-induced cell damage in bacteria and the relevance for inoculant production

Plant growth-promoting bacteria show great potential for use in agriculture although efficient application remains challenging to achieve. Cells often lose viability during inoculant production and application, jeopardizing the efficacy of the inoculant. Since desiccation has been documented to be the primary stress factor affecting the decrease in survival, obtaining xerotolerance in plant growth-promoting bacteria is appealing. The molecular damage that occurs by drying bacteria has been broadly investigated, although a complete view is still lacking due to the complex nature of the process. Mechanic, structural, and metabolic changes that occur as a result of water depletion may potentially afflict lethal damage to membranes, DNA, and proteins. Bacteria respond to these harsh conditions by increasing production of exopolysaccharides, changing composition of the membrane, improving the stability of proteins, reducing oxidative stress, and repairing DNA damage. This review provides insight into the complex nature of desiccation stress in bacteria in order to facilitate strategic choices to improve survival and shelf life of newly developed inoculants. KEY POINTS: Desiccation-induced damage affects most major macromolecules in bacteria. Most bacteria are not xerotolerant despite multiple endogenous adaption mechanisms. Sensitivity to drying severely hampers inoculant quality.

Narrative of a versatile and adept species Pseudomonas putida

Pseudomonas putidais a fast-growing bacterium found mostly in temperate soil and water habitats. The metabolic versatility ofP. putidamakes this organism attractive for biotechnological applications such as biodegradation of environmental pollutants and synthesis of added-value chemicals (biocatalysis). This organism has been extensively studied in respect to various stress responses, mechanisms of genetic plasticity and transcriptional regulation of catabolic genes.P. putidais able to colonize the surface of living organisms, but is generally considered to be of low virulence. A number ofP. putidastrains are able to promote plant growth. The aim of this review is to give historical overview of the discovery of the speciesP. putidaand isolation and characterization ofP. putidastrains displaying potential for biotechnological applications. This review also discusses some major findings inP. putidaresearch encompassing regulation of catabolic operons, stress-tolerance mechanisms and mechanisms affecting evolvability of bacteria under conditions of environmental stress.

Desiccation-induced viable but nonculturable state in Pseudomonas putida KT2440, a survival strategy

The potential of Pseudomonas putida KT2440 to act as a plant-growth promoter or as a bioremediator of toxic compounds can be affected by desiccation. In the present work, the bacterial survival ratio (BSR) in response to air desiccation was evaluated for P. putida KT2440 in the presence of different protectors. The BSR in the presence of nonreducing disaccharides, such as trehalose, was high after 15 days of desiccation stress (occurring at 30°C and 50% relative humidity), whereas in the absence of a protector the bacterial counts diminished to nondetectable numbers (ca 2.8 log CFU/mL). The LIVE/DEAD staining method showed that bacteria protected with trehalose maintained increased numbers of green cells after desiccation while cells without protection were all observed to be red. This indicated that nonprotected bacteria had compromised membrane integrity. However, when nonprotected bacteria subjected to 18 days of desiccation stress were rehydrated for a short time with maize root exudates or for 48 h with water (prolonged rehydration), the bacterial counts were as high as that observed for those not subjected to desiccation stress, suggesting that the cells entered the viable but nonculturable (VBNC) state under desiccation and that they returned to a culturable state after those means of rehydration. Interestingly an increase in the green color intensity of cells that returned to a culturable state was observed using LIVE/DEAD staining method, indicating an improvement in their membrane integrity. Cellular activity in the VBNC state was determined. A GFP-tagged P. putida strain expressing GFP constitutively was subjected to desiccation. After 12 days of desiccation, the GFP-tagged strain lost culturability, but it exhibited active GFP expression, which in turn made the cells green. Furthermore, the expression of 16S rRNA, rpoN (housekeeping), mutL, mutS (encoding proteins from the mismatch repair complex), and oprH (encoding an outer membrane protein) were examined by RT-PCR. All evaluated genes were expressed by both types of cells, culturable and nonculturable, indicating active molecular processes during the VBNC state.

Anti-Melanogenic Effects of Hydroxyectoine via MITF Inhibition by JNK, p38, and AKT Pathways in B16F10 Melanoma Cells

Melanin plays a role in determining human skin color of a person, and a large amount of melanin makes the skin color look darkened. The proper amount of melanin formation protects our skin from UV radiation, but excessive melanin production causes hyperpigmentation and leads to freckles, melasma, and lentigo. In this study, we investigated the inhibitory effect of hydroxyectoine on melanogenesis and its mechanism in B16F10 cells. Melanin content and cellular tyrosinase activity were determined. The expression of microphthalmia-associated transcription factor (MITF), and the activities of tyrosinase and other melanogenesis-related enzymes, such as tyrosinase-related protein 1 (TRP-1) and tyrosinase-related protein 2, were also examined. Hydroxyectoine treatment significantly inhibited melanin production and intracellular tyrosinase activity in a dose-dependent manner. Western blot analysis showed that hydroxyectoine also reduced the expressions of tyrosinase and TRP-1. In addition, hydroxyectoine significantly reduced the expression of MITF, a major regulator of melanin production, and inhibited the phosphorylation of p38, c-Jun N-terminal kinase, and activated the protein kinase B. The results demonstrated that hydroxyectoine inhibits the expression of MITF through the inhibition or activation of melanin-related signaling pathways and downregulates melanogenesis by inhibiting melanogenic enzyme expression and tyrosinase activity. Hydroxyectoine has potential value in functional cosmetics applications, such as whitening.

Illuminating the catalytic core of ectoine synthase through structural and biochemical analysis

Ectoine synthase (EctC) is the signature enzyme for the production of ectoine, a compatible solute and chemical chaperone widely synthesized by bacteria as a cellular defense against the detrimental effects of osmotic stress. EctC catalyzes the last step in ectoine synthesis through cyclo-condensation of the EctA-formed substrate N-gamma-acetyl-L-2,4-diaminobutyric acid via a water elimination reaction. We have biochemically and structurally characterized the EctC enzyme from the thermo-tolerant bacterium Paenibacillus lautus (Pl). EctC is a member of the cupin superfamily and forms dimers, both in solution and in crystals. We obtained high-resolution crystal structures of the (Pl)EctC protein in forms that contain (i) the catalytically important iron, (ii) iron and the substrate N-gamma-acetyl-L-2,4-diaminobutyric acid, and (iii) iron and the enzyme reaction product ectoine. These crystal structures lay the framework for a proposal for the EctC-mediated water-elimination reaction mechanism. Residues involved in coordinating the metal, the substrate, or the product within the active site of ectoine synthase are highly conserved among a large group of EctC-type proteins. Collectively, the biochemical, mutational, and structural data reported here yielded detailed insight into the structure-function relationship of the (Pl)EctC enzyme and are relevant for a deeper understanding of the ectoine synthase family as a whole.

Recent examples of α-ketoglutarate-dependent mononuclear non-haem iron enzymes in natural product biosyntheses

Covering: up to 2018 α-Ketoglutarate (αKG, also known as 2-oxoglutarate)-dependent mononuclear non-haem iron (αKG-NHFe) enzymes catalyze a wide range of biochemical reactions, including hydroxylation, ring fragmentation, C-C bond cleavage, epimerization, desaturation, endoperoxidation and heterocycle formation. These enzymes utilize iron(ii) as the metallo-cofactor and αKG as the co-substrate. Herein, we summarize several novel αKG-NHFe enzymes involved in natural product biosyntheses discovered in recent years, including halogenation reactions, amino acid modifications and tailoring reactions in the biosynthesis of terpenes, lipids, fatty acids and phosphonates. We also conducted a survey of the currently available structures of αKG-NHFe enzymes, in which αKG binds to the metallo-centre bidentately through either a proximal- or distal-type binding mode. Future structure-function and structure-reactivity relationship investigations will provide crucial information regarding how activities in this large class of enzymes have been fine-tuned in nature.

Transcriptomic and Ectoine Analysis of Halotolerant Nocardiopsis gilva YIM 90087T Under Salt Stress

The genus Nocardiopsis is an unique actinobacterial group that widely distributed in hypersaline environments. In this study, we investigated the growth conditions, transcriptome analysis, production and accumulation of ectoine by Nocardiopsis gilva YIM 90087^T under salt stress. The colony color of N. gilva YIM 90087^T changed from yellow to white under salt stress conditions. Accumulation of ectoine and hydroxyectoine in cells was an efficient way to regulate osmotic pressure. The ectoine synthesis was studied by transferring the related genes (ectA, ectB, and ectC) to Escherichia coli. Transcriptomic analysis showed that the pathways of ABC transporters (ko02010) and glycine, serine, and threonine metabolism (ko00260) played a vital role under salt stress environment. The ectABC from N. gilva YIM 90087^T was activated under the salt stress. Addition of exogenous ectoine and hydroxyectoine were helpful to protect N. gilva YIM 90087^T from salt stress.

Role of the Extremolytes Ectoine and Hydroxyectoine as Stress Protectants and Nutrients: Genetics, Phylogenomics, Biochemistry, and Structural Analysis

Fluctuations in environmental osmolarity are ubiquitous stress factors in many natural habitats of microorganisms, as they inevitably trigger osmotically instigated fluxes of water across the semi-permeable cytoplasmic membrane. Under hyperosmotic conditions, many microorganisms fend off the detrimental effects of water efflux and the ensuing dehydration of the cytoplasm and drop in turgor through the accumulation of a restricted class of organic osmolytes, the compatible solutes. Ectoine and its derivative 5-hydroxyectoine are prominent members of these compounds and are synthesized widely by members of the Bacteria and a few Archaea and Eukarya in response to high salinity/osmolarity and/or growth temperature extremes. Ectoines have excellent function-preserving properties, attributes that have led to their description as chemical chaperones and fostered the development of an industrial-scale biotechnological production process for their exploitation in biotechnology, skin care, and medicine. We review, here, the current knowledge on the biochemistry of the ectoine/hydroxyectoine biosynthetic enzymes and the available crystal structures of some of them, explore the genetics of the underlying biosynthetic genes and their transcriptional regulation, and present an extensive phylogenomic analysis of the ectoine/hydroxyectoine biosynthetic genes. In addition, we address the biochemistry, phylogenomics, and genetic regulation for the alternative use of ectoines as nutrients.

Maintenance and assessment of cell viability in formulation of non-sporulating bacterial inoculants

The application of beneficial, plant-associated microorganisms is a sustainable approach to improving crop performance in agriculture. However, microbial inoculants are often susceptible to prolonged periods of storage and deleterious environmental factors, which negatively impact their viability and ultimately limit efficacy in the field. This particularly concerns non-sporulating bacteria. To overcome this challenge, the availability of protective formulations is crucial. Numerous parameters influence the viability of microbial cells, with drying procedures generally being among the most critical ones. Thus, technological advances to attenuate the desiccation stress imposed on living cells are key to successful formulation development. In this review, we discuss the core aspects important to consider when aiming at high cell viability of non-sporulating bacteria to be applied as microbial inoculants in agriculture. We elaborate the suitability of commonly applied drying methods (freeze-drying, vacuum-drying, spray-drying, fluidized bed-drying, air-drying) and potential measures to prevent cell damage from desiccation (externally applied protectants, stress pre-conditioning, triggering of exopolysaccharide secretion, 'helper' strains). Furthermore, we point out methods for assessing bacterial viability, such as colony counting, spectrophotometry, microcalorimetry, flow cytometry and viability qPCR. Choosing appropriate technologies for maintenance of cell viability and evaluation thereof will render formulation development more efficient. This in turn will aid in utilizing the vast potential of promising, plant beneficial bacteria as sustainable alternatives to standard agrochemicals.

Tinkering with Osmotically Controlled Transcription Allows Enhanced Production and Excretion of Ectoine and Hydroxyectoine from a Microbial Cell Factory

Ectoine and hydroxyectoine are widely synthesized by members of the Bacteria and a few members of the Archaea as potent osmostress protectants. We have studied the salient features of the osmostress-responsive promoter directing the transcription of the ectoine/hydroxyectoine biosynthetic gene cluster from the plant-root-associated bacterium Pseudomonas stutzeri by transferring it into Escherichia coli, an enterobacterium that does not produce ectoines naturally. Using ect-lacZ reporter fusions, we found that the heterologous ect promoter reacted with exquisite sensitivity in its transcriptional profile to graded increases in sustained high salinity, responded to a true osmotic signal, and required the buildup of an osmotically effective gradient across the cytoplasmic membrane for its induction. The involvement of the -10, -35, and spacer regions of the sigma-70-type ect promoter in setting promoter strength and response to osmotic stress was assessed through site-directed mutagenesis. Moderate changes in the ect promoter sequence that increase its resemblance to housekeeping sigma-70-type promoters of E. coli afforded substantially enhanced expression, both in the absence and in the presence of osmotic stress. Building on this set of ect promoter mutants, we engineered an E. coli chassis strain for the heterologous production of ectoines. This synthetic cell factory lacks the genes for the osmostress-responsive synthesis of trehalose and the compatible solute importers ProP and ProU, and it continuously excretes ectoines into the growth medium. By combining appropriate host strains and different plasmid variants, excretion of ectoine, hydroxyectoine, or a mixture of both compounds was achieved under mild osmotic stress conditions.IMPORTANCE Ectoines are compatible solutes, organic osmolytes that are used by microorganisms to fend off the negative consequences of high environmental osmolarity on cellular physiology. An understanding of the salient features of osmostress-responsive promoters directing the expression of the ectoine/hydroxyectoine biosynthetic gene clusters is lacking. We exploited the ect promoter from an ectoine/hydroxyectoine-producing soil bacterium for such a study by transferring it into a surrogate bacterial host. Despite the fact that E. coli does not synthesize ectoines naturally, the ect promoter retained its exquisitely sensitive osmotic control, indicating that osmoregulation of ect transcription is an inherent feature of the promoter and its flanking sequences. These sequences were narrowed to a 116-bp DNA fragment. Ectoines have interesting commercial applications. Building on data from a site-directed mutagenesis study of the ect promoter, we designed a synthetic cell factory that secretes ectoine, hydroxyectoine, or a mixture of both compounds into the growth medium.

Transcriptional regulation of ectoine catabolism in response to multiple metabolic and environmental cues

Ectoine and hydroxyectoine are effective microbial osmostress protectants, but can also serve as versatile nutrients for bacteria. We have studied the genetic regulation of ectoine and hydroxyectoine import and catabolism in the marine Roseobacter species Ruegeria pomeroyi and identified three transcriptional regulators involved in these processes: the GabR/MocR-type repressor EnuR, the feast and famine-type regulator AsnC and the two-component system NtrYX. The corresponding genes are widely associated with ectoine and hydroxyectoine uptake and catabolic gene clusters (enuR, asnC), and with microorganisms predicted to consume ectoines (ntrYX). EnuR contains a covalently bound pyridoxal-5'-phosphate as a co-factor and the chemistry underlying the functioning of MocR/GabR-type regulators typically requires a system-specific low molecular mass effector molecule. Through ligand binding studies with purified EnuR, we identified N-(alpha)-L-acetyl-2,4-diaminobutyric acid and L-2,4-diaminobutyric acid as inducers for EnuR that are generated through ectoine catabolism. AsnC/Lrp-type proteins can wrap DNA into nucleosome-like structures, and we found that the asnC gene was essential for use of ectoines as nutrients. Furthermore, we discovered through transposon mutagenesis that the NtrYX two-component system is required for their catabolism. Database searches suggest that our findings have important ramifications for an understanding of the molecular biology of most microbial consumers of ectoines.

Engineering Rugged Field Assays to Detect Hazardous Chemicals Using Spore-Based Bacterial Biosensors

Bacterial whole cell-based biosensors have been genetically engineered to achieve selective and reliable detection of a wide range of hazardous chemicals. Although whole-cell biosensors demonstrate many advantages for field-based detection of target analytes, there are still some challenges that need to be addressed. Most notably, their often modest shelf life and need for special handling and storage make them challenging to use in situations where access to reagents, instrumentation, and expertise are limited. These problems can be circumvented by developing biosensors in Bacillus spores, which can be engineered to address all of these concerns. In its sporulated state, a whole cell-based biosensor has a remarkably long life span and is exceptionally resistant to environmental insult. When these spores are germinated for use in analytical techniques, they show no loss in performance, even after long periods of storage under harsh conditions. In this chapter, we will discuss the development and use of whole cell-based sensors, their adaptation to spore-based biosensors, their current applications, and future directions in the field.

A New Physiological Role for the DNA Molecule as a Protector against Drying Stress in Desiccation-Tolerant Microorganisms

The DNA molecule is associated with the role of encoding information required to produce RNA which is translated into proteins needed by the cell. This encoding involves information transmission to offspring or to other organisms by horizontal transfer. However, despite the abundance of this molecule in both the cell and the environment, its physiological role seems to be restricted mainly to that of a coding and inheritance molecule. In this paper, we report a new physiological role for the DNA molecule as involved in protection against desiccation, in addition to its well-established main information transfer and other recently reported functions such as bio-film formation in eDNA form. Desiccation-tolerant microorganisms such as Microbacterium sp. 3J1 significantly upregulate genes involved in DNA synthesis to produce DNA as part of their defensive mechanisms to protect protein structures and functions from drying according to RNA-seq analysis. We have observed the intracellular overproduction of DNA in two desiccation-tolerant microorganisms, Microbacterium sp. 3J1 and Arthrobacter siccitolerans 4J27, in response to desiccation signals. In addition, this conclusion can be made from our observations that synthetic DNA protects two proteins from drying and when part of a xeroprotectant preparation, DNA from various organisms including desiccation-sensitive species, does the same. Removal of DNA by nuclease treatment results in absence of this additive protective effect. We validated this role in biochemical and biophysical assays in proteins and occurs in trans even with short, single chains of synthetically produced DNA.

Plant Drought Tolerance Enhancement by Trehalose Production of Desiccation-Tolerant Microorganisms

A collection of desiccation-tolerant xeroprotectant-producing microorganisms was screened for their ability to protect plants against drought, and their role as plant growth-promoting rhizobacteria was investigated in two different crops (tomato and pepper). The most commonly described biochemical mechanisms for plant protection against drought by microorganisms including the production of phytohormones, antioxidants and xeroprotectants were analyzed. In particular, the degree of plant protection against drought provided by these microorganisms was characterized. After studying the findings and comparing them with results of the closest taxonomic relatives at the species and strain levels, we propose that trehalose produced by these microorganisms is correlated with their ability to protect plants against drought. This proposal is based on the increased protection of plants against drought by the desiccation-sensitive microorganism Pseudomonas putida KT2440, which expresses the otsAB genes for trehalose biosynthesis in trans.

Feeding on compatible solutes: A substrate-induced pathway for uptake and catabolism of ectoines and its genetic control by EnuR

Ectoine and 5-hydroxyectoine are widely synthesized microbial osmostress protectants. They are also versatile nutrients but their catabolism and the genetic regulation of the corresponding genes are incompletely understood. Using the marine bacterium Ruegeria pomeroyi DSS-3, we investigated the utilization of ectoines and propose a seven steps comprising catabolic route that entails an initial conversion of 5-hydroxyectoine to ectoine, the opening of the ectoine ring, and the subsequent degradation of this intermediate to l-aspartate. The catabolic genes are co-transcribed with three genes encoding a 5-hydroxyectoine/ectoine-specific TRAP transporter. A chromosomal deletion of this entire gene cluster abolishes the utilization of ectoines as carbon and nitrogen sources. The presence of ectoines in the growth medium triggers enhanced expression of the importer and catabolic operon, a process dependent on a substrate-inducible promoter that precedes this gene cluster. EnuR, a member of the MocR/GabR-type transcriptional regulators, controls the activity of this promoter and functions as a repressor. EnuR contains a covalently bound pyridoxal-5'-phosphate, and we suggest that this co-factor is critical for the substrate-mediated induction of the 5-hydroxyectoine/ectoine import and catabolic genes. Bioinformatics showed that ectoine consumers are restricted to the Proteobacteria and that EnuR is likely a central regulator for most ectoine/5-hydroxyectoine catabolic genes.

EctD-mediated biotransformation of the chemical chaperone ectoine into hydroxyectoine and its mechanosensitive channel-independent excretion

Background

Ectoine and its derivative 5-hydroxyectoine are cytoprotectants widely synthesized by microorganisms as a defense against the detrimental effects of high osmolarity on cellular physiology and growth. Both ectoines possess the ability to preserve the functionality of proteins, macromolecular complexes, and even entire cells, attributes that led to their description as chemical chaperones. As a consequence, there is growing interest in using ectoines for biotechnological purposes, in skin care, and in medical applications. 5-Hydroxyectoine is synthesized from ectoine through a region- and stereo-specific hydroxylation reaction mediated by the EctD enzyme, a member of the non-heme-containing iron(II) and 2-oxoglutarate-dependent dioxygenases. This chemical modification endows the newly formed 5-hydroxyectoine with either superior or different stress- protecting and stabilizing properties. Microorganisms producing 5-hydroxyectoine typically contain a mixture of both ectoines. We aimed to establish a recombinant microbial cell factory where 5-hydroxyectoine is (i) produced in highly purified form, and (ii) secreted into the growth medium.

Results

We used an Escherichia coli strain (FF4169) defective in the synthesis of the osmostress protectant trehalose as the chassis for our recombinant cell factory. We expressed in this strain a plasmid-encoded ectD gene from Pseudomonas stutzeri A1501 under the control of the anhydrotetracycline-inducible tet promoter. We chose the ectoine hydroxylase from P. stutzeri A1501 for our cell factory after a careful comparison of the in vivo performance of seven different EctD proteins. In the final set-up of the cell factory, ectoine was provided to salt-stressed cultures of strain FF4169 (pMP41; ectD⁺). Ectoine was imported into the cells via the osmotically inducible ProP and ProU transport systems, intracellularly converted to 5-hydroxyectoine, which was then almost quantitatively secreted into the growth medium. Experiments with an E. coli mutant lacking all currently known mechanosensitive channels (MscL, MscS, MscK, MscM) revealed that the release of 5-hydroxyectoine under osmotic steady-state conditions occurred independently of these microbial safety valves. In shake-flask experiments, 2.13 g l⁻¹ ectoine (15 mM) was completely converted into 5-hydroxyectoine within 24 h.

Conclusions

We describe here a recombinant E. coli cell factory for the production and secretion of the chemical chaperone 5-hydroxyectoine free from contaminating ectoine.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0525-4) contains supplementary material, which is available to authorized users.

Microbial inoculation of seed for improved crop performance: issues and opportunities

There is increasing interest in the use of beneficial microorganisms as alternatives to chemical pesticides and synthetic fertilisers in agricultural production. Application of beneficial microorganisms to seeds is an efficient mechanism for placement of microbial inocula into soil where they will be well positioned to colonise seedling roots and protect against soil-borne diseases and pests. However, despite the long history of inoculation of legume seeds with Rhizobia spp. and clear laboratory demonstration of the ability of a wide range of other beneficial microorganisms to improve crop performance, there are still very few commercially available microbial seed inoculants. Seed inoculation techniques used for research purposes are often not feasible at a commercial scale and there are significant technical challenges in maintaining viable microbial inocula on seed throughout commercial seed treatment processes and storage. Further research is needed before the benefits of a wide range of environmentally sensitive potential seed inoculants can be captured for use in agriculture, ecosystem restoration and bioremediation. There is no single solution to the challenge of improving the ability of seed inoculants to establish and function consistently in the field. Development of novel formulations that maintain the viability of both inoculant and seed during storage will result from multidisciplinary research in microbial and seed physiology and adjuvant chemistry.

Strangers in the archaeal world: osmostress-responsive biosynthesis of ectoine and hydroxyectoine by the marine thaumarchaeon Nitrosopumilus maritimus

Ectoine and hydroxyectoine are compatible solutes widely synthesized by members of the Bacteria to cope with high osmolarity surroundings. Inspection of 557 archaeal genomes revealed that only 12 strains affiliated with the Nitrosopumilus, Methanothrix or Methanobacterium genera harbour ectoine/hydroxyectoine gene clusters. Phylogenetic considerations suggest that these Archaea have acquired these genes through horizontal gene transfer events. Using the Thaumarchaeon 'Candidatus Nitrosopumilus maritimus' as an example, we demonstrate that the transcription of its ectABCD genes is osmotically induced and functional since it leads to the production of both ectoine and hydroxyectoine. The ectoine synthase and the ectoine hydroxylase were biochemically characterized, and their properties resemble those of their counterparts from Bacteria. Transcriptional analysis of osmotically stressed 'Ca. N. maritimus' cells demonstrated that they possess an ectoine/hydroxyectoine gene cluster (hyp-ectABCD-mscS) different from those recognized previously since it contains a gene for an MscS-type mechanosensitive channel. Complementation experiments with an Escherichia coli mutant lacking all known mechanosensitive channel proteins demonstrated that the (Nm)MscS protein is functional. Hence, 'Ca. N. maritimus' cells cope with high salinity not only through enhanced synthesis of osmostress-protective ectoines but they already prepare themselves simultaneously for an eventually occurring osmotic down-shock by enhancing the production of a safety-valve (NmMscS).

Enhancement of solvent tolerance in Pseudomonas sp. BCNU 106 with trehalose

As the solvent hyper-resistant Pseudomonas sp. BCNU 106 experiences limited growth with solvents, a strategy is therefore needed to allow better growth to broaden its performance in biotechnological applications. Pseudomonas sp. BCNU 106 was cultivated in a medium supplemented with 0·05 mol l(-1) trehalose, and the cell survival was observed during subsequent growth with 1% (v/v) toluene. Exogenously added trehalose was transported into the cells and conferred protection against toluene stress. BCNU 106 grown in the presence of exogenous trehalose showed higher solvent tolerance, it can thus have more potential for biotransformation and biodegradation.

SIGNIFICANCE AND IMPACT OF THE STUDY

This study shows that exogenously supplemented trehalose confers protection against toluene stress and enhances the bacterial cell growth in the presence of toluene. This is of importance to the mass cultivation of solvent-tolerant bacteria, where some of the growth-related limitations of solvent-tolerant bacteria can be overcome, and their performance in biotechnological applications for biotransformation and biodegradation broadened.

Overproduction, crystallization and X-ray diffraction data analysis of ectoine synthase from the cold-adapted marine bacterium Sphingopyxis alaskensis

Ectoine biosynthetic genes (ectABC) are widely distributed in bacteria. Microorganisms that carry them make copious amounts of ectoine as a cell protectant in response to high-osmolarity challenges. Ectoine synthase (EctC; EC 4.2.1.108) is the key enzyme for the production of this compatible solute and mediates the last step of ectoine biosynthesis. It catalyzes the ring closure of the cyclic ectoine molecule. A codon-optimized version of ectC from Sphingopyxis alaskensis (Sa) was used for overproduction of SaEctC protein carrying a Strep-tag II peptide at its carboxy-terminus. The recombinant SaEctC-Strep-tag II protein was purified to near-homogeneity from Escherichia coli cell extracts by affinity chromatography. Size-exclusion chromatography revealed that it is a dimer in solution. The SaEctC-Strep-tag II protein was crystallized using the sitting-drop vapour-diffusion method and crystals that diffracted to 1.0 Å resolution were obtained.

Genome Sequence of Leucobacter sp. 4J7B1, a Plant-Osmoprotectant Soil Microorganism

We report the first genome sequence for Leucobacter sp. 4J7B1, a newly described desiccation-tolerant strain. The complete genome sequence of Leucobacter sp. 4J7B1 has been sequenced and is estimated to be around 3.5 Mb in size, with an average GC content of 62.18%. We predict 2,953 protein-coding sequences.

Abiotic stress protection by ecologically abundant dimethylsulfoniopropionate and its natural and synthetic derivatives: insights from Bacillus subtilis

Dimethylsulfoniopropionate (DMSP) is an abundant osmolyte and anti-stress compound produced primarily in marine ecosystems. After its release into the environment, microorganisms can exploit DMSP as a source of sulfur and carbon, or accumulate it as an osmoprotectant. However, import systems for this ecophysiologically important compatible solute, and its stress-protective properties for microorganisms that do not produce it are insufficiently understood. Here we address these questions using a well-characterized set of Bacillus subtilis mutants to chemically profile the influence of DMSP import on stress resistance, the osmostress-adaptive proline pool and on osmotically controlled gene expression. We included in this study the naturally occurring selenium analogue of DMSP, dimethylseleniopropionate (DMSeP), as well as a set of synthetic DMSP derivatives. We found that DMSP is not a nutrient for B. subtilis, but it serves as an excellent stress protectant against challenges conferred by sustained high salinity or lasting extremes in both low and high growth temperatures. DMSeP and synthetic DMSP derivatives retain part of these stress protective attributes, but DMSP is clearly the more effective stress protectant. We identified the promiscuous and widely distributed ABC transporter OpuC as a high-affinity uptake system not only for DMSP, but also for its natural and synthetic derivatives.

Crystal structure of the ectoine hydroxylase, a snapshot of the active site

Ectoine and its derivative 5-hydroxyectoine are compatible solutes that are widely synthesized by bacteria to cope physiologically with osmotic stress. They also serve as chemical chaperones and maintain the functionality of macromolecules. 5-Hydroxyectoine is produced from ectoine through a stereo-specific hydroxylation, an enzymatic reaction catalyzed by the ectoine hydroxylase (EctD). The EctD protein is a member of the non-heme-containing iron(II) and 2-oxoglutarate-dependent dioxygenase superfamily and is evolutionarily well conserved. We studied the ectoine hydroxylase from the cold-adapted marine ultra-microbacterium Sphingopyxis alaskensis (Sa) and found that the purified SaEctD protein is a homodimer in solution. We determined the SaEctD crystal structure in its apo-form, complexed with the iron catalyst, and in a form that contained iron, the co-substrate 2-oxoglutarate, and the reaction product of EctD, 5-hydroxyectoine. The iron and 2-oxoglutarate ligands are bound within the EctD active site in a fashion similar to that found in other members of the dioxygenase superfamily. 5-Hydroxyectoine, however, is coordinated by EctD in manner different from that found in high affinity solute receptor proteins operating in conjunction with microbial import systems for ectoines. Our crystallographic analysis provides a detailed view into the active site of the ectoine hydroxylase and exposes an intricate network of interactions between the enzyme and its ligands that collectively ensure the hydroxylation of the ectoine substrate in a position- and stereo-specific manner.

Plant-derived compatible solutes proline betaine and betonicine confer enhanced osmotic and temperature stress tolerance to Bacillus subtilis

L-Proline is a widely used compatible solute and is employed by Bacillus subtilis, through both synthesis and uptake, as an osmostress protectant. Here, we assessed the stress-protective potential of the plant-derived L-proline derivatives N-methyl-L-proline, L-proline betaine (stachydrine), trans-4-L-hydroxproline and trans-4-hydroxy-L-proline betaine (betonicine) for cells challenged by high salinity or extremes in growth temperature. l-Proline betaine and betonicine conferred salt stress protection, but trans-4-L-hydroxyproline and N-methyl-L-proline was unable to do so. Except for L-proline, none of these compounds served as a nutrient for B. subtilis. L-Proline betaine was a considerably better osmostress protectant than betonicine, and its import strongly reduced the l-proline pool produced by B. subtilis under osmotic stress conditions, whereas a supply of betonicine affected the L-proline pool only modestly. Both compounds downregulated the transcription of the osmotically inducible opuA operon, albeit to different extents. Mutant studies revealed that L-proline betaine was taken up via the ATP-binding cassette transporters OpuA and OpuC, and the betaine-choline-carnitine-transporter-type carrier OpuD; betonicine was imported only through OpuA and OpuC. L-Proline betaine and betonicine also served as temperature stress protectants. A striking difference between these chemically closely related compounds was observed: L-proline betaine was an excellent cold stress protectant, but did not provide heat stress protection, whereas the reverse was true for betonicine. Both compounds were primarily imported in temperature-challenged cells via the high-capacity OpuA transporter. We developed an in silico model for the OpuAC-betonicine complex based on the crystal structure of the OpuAC solute receptor complexed with L-proline betaine.

Genome Sequence of Arthrobacter siccitolerans 4J27, a Xeroprotectant-Producing Desiccation-Tolerant Microorganism

We report the first genome sequence for Arthrobacter siccitolerans 4J27, a newly described desiccation-tolerant species. The complete genome of A. siccitolerans 4J27 has been sequenced and is estimated to be around 5.3 Mb in size, with an average GC content of 65.13%. We predict 4,480 protein-coding sequences (CDSs).

Glass-forming property of hydroxyectoine is the cause of its superior function as a desiccation protectant

We were able to demonstrate that hydroxyectoine, in contrast to ectoine, is a good glass-forming compound. Fourier transform infrared and spin label electron spin resonance studies of dry ectoine and hydroxyectoine have shown that the superior glass-forming properties of hydroxyectoine result from stronger intermolecular H-bonds with the OH group of hydroxyectoine. Spin probe experiments have also shown that better molecular immobilization in dry hydroxyectoine provides better redox stability of the molecules embedded in this dry matrix. With a glass transition temperature of 87°C (vs. 47°C for ectoine) hydroxyectoine displays remarkable desiccation protection properties, on a par with sucrose and trehalose. This explains its accumulation in response to increased salinity and elevated temperature by halophiles such as Halomonas elongata and its successful application in ``anhydrobiotic engineering'' of both enzymes and whole cells.

Arthrobacter siccitolerans sp. nov., a highly desiccation-tolerant, xeroprotectant-producing strain isolated from dry soil

A novel desiccation-tolerant, xeroprotectant-producing bacterium, designated strain 4J27(T), was isolated from a Nerium oleander rhizosphere subjected to seasonal drought in Granada, Spain. Phylogenetic analysis based on 16S rRNA gene sequencing placed the isolate within the genus Arthrobacter, its closest relative being Arthrobacter phenanthrenivorans Shep3 DSM 18606(T), with which it showed 99.23 % 16S rRNA gene sequence similarity. DNA-DNA hybridization measurements showed less than 25 % relatedness between strain 4J27(T) and Arthrobacter phenanthrenivorans DSM 18606(T). The DNA base composition of strain 4J27(T) was 65.3 mol%. The main fatty acids were anteiso C15 : 0, anteiso C17 : 0, C16 : 0 and iso C16 : 0 and the major menaquinone was MK-9 (H2). The peptidoglycan type was A3α with an l-Lys-l-Ser-l-Thr-l-Ala interpeptide bridge. The bacterium tested positive for catalase activity and negative for oxidase activity. Phylogenetic, chemotaxonomic and phenotypic analyses indicated that the desiccation-tolerant strain 4J27(T) represents a novel species within the genus Arthrobacter, for which the name Arthrobacter siccitolerans is proposed. The type strain is 4J27(T) ( = CECT 8257(T) = LMG 27359(T)).

Molecular dynamics simulations and structure-guided mutagenesis provide insight into the architecture of the catalytic core of the ectoine hydroxylase

Many bacteria amass compatible solutes to fend-off the detrimental effects of high osmolarity on cellular physiology and water content. These solutes also function as stabilizers of macromolecules, a property for which they are referred to as chemical chaperones. The tetrahydropyrimidine ectoine is such a compatible solute and is widely synthesized by members of the Bacteria. Many ectoine producers also synthesize the stress protectant 5-hydroxyectoine from the precursor ectoine, a process that is catalyzed by the ectoine hydroxylase (EctD). The EctD enzyme is a member of the non-heme-containing iron(II) and 2-oxoglutarate-dependent dioxygenase superfamily. A crystal structure of the EctD protein from the moderate halophile Virgibacillus salexigens has previously been reported and revealed the coordination of the iron catalyst, but it lacked the substrate ectoine and the co-substrate 2-oxoglutarate. Here we used this crystal structure as a template to assess the likely positioning of the ectoine and 2-oxoglutarate ligands within the active site by structural comparison, molecular dynamics simulations, and site-directed mutagenesis. Collectively, these approaches suggest the positioning of the iron, ectoine, and 2-oxoglutarate ligands in close proximity to each other and with a spatial orientation that will allow the region-selective and stereo-specific hydroxylation of (4S)-ectoine to (4S,5S)-5-hydroxyectoine. Our study thus provides a view into the catalytic core of the ectoine hydroxylase and suggests an intricate network of interactions between the three ligands and evolutionarily highly conserved residues in members of the EctD protein family.

Serratia entomophila bet gene induction and the impact of glycine betaine accumulation on desiccation tolerance

AIMS

The genes involved in choline transport and oxidation to glycine betaine in the biopesticidal bacterium Serratia entomophila were characterized, and the potential of osmoprotectants, coupled with increased NaCl concentrations, to improve the desiccation tolerance of this species was investigated.

METHODS AND RESULTS

Serratia entomophila carries sequences similar to the Escherichia coli betTIBA genes encoding a choline transporter and dehydrogenase, a betaine aldehyde dehydrogenase and a regulatory protein. Disruption of betA abolished the ability of Ser. entomophila to utilize choline as a carbon source. Quantitative reverse-transcriptase PCR analysis revealed that betA transcription was reduced compared to that of the upstream genes in the operon, and that NaCl and choline induced bet gene expression. Glycine betaine and choline increased the NaCl tolerance of Ser. entomophila, and osmotically preconditioned cultures survived better than control cultures following desiccation and immediately after application to agricultural soil.

CONCLUSIONS

Addition of glycine betaine and NaCl to growth medium can greatly enhance the desiccation survival of Ser. entomophila, and its initial survival in soil.

SIGNIFICANCE AND IMPACT OF THE STUDY

Serratia entomophila is sensitive to desiccation and does not persist under low soil moisture conditions. Techniques described here for enhancing the desiccation survival of Ser. entomophila can be used to improve formulations of this bacterium, and allow its application under a wider range of environmental conditions.

Role of trehalose in heat and desiccation tolerance in the soil bacterium Rhizobium etli

BACKGROUND

The compatible solute trehalose is involved in the osmostress response of Rhizobium etli, the microsymbiont of Phaseolus vulgaris. In this work, we reconstructed trehalose metabolism in R. etli, and investigated its role in cellular adaptation and survival to heat and desiccation stress under free living conditions.

RESULTS

Besides trehalose as major compatible solute, R. etli CE3 also accumulated glutamate and, if present in the medium, mannitol. Putative genes for trehalose synthesis (otsAB/treS/treZY), uptake (aglEFGK/thuEFGK) and degradation (thuAB/treC) were scattered among the chromosome and plasmids p42a, p42c, p42e, and p42f, and in some instances found redundant. Two copies of the otsA gene, encoding trehalose-6-P-synthase, were located in the chromosome (otsAch) and plasmid p42a (otsAa), and the latter seemed to be acquired by horizontal transfer. High temperature alone did not influence growth of R. etli, but a combination of high temperature and osmotic stress was more deleterious for growth than osmotic stress alone. Although high temperature induced some trehalose synthesis by R. etli, trehalose biosynthesis was mainly triggered by osmotic stress. However, an otsAch mutant, unable to synthesize trehalose in minimal medium, showed impaired growth at high temperature, suggesting that trehalose plays a role in thermoprotection of R. etli. Desiccation tolerance by R. etli wild type cells was dependent of high trehalose production by osmotic pre-conditioned cells. Cells of the mutant strain otsAch showed ca. 3-fold lower survival levels than the wild type strain after drying, and a null viability after 4 days storage.

CONCLUSIONS

Our findings suggest a beneficial effect of osmotic stress in R. etli tolerance to desiccation, and an important role of trehalose on the response of R. etli to high temperature and desiccation stress.

Xeroprotectants for the stabilization of biomaterials

With the advancement of science and technology, it is crucial to have effective preservation methods for the stable long-term storage of biological material (biomaterials). As an alternative to cryopreservation, various techniques have been developed, which are based on the survival mechanism of anhydrobiotic organisms. In this sense, it has been found that the synthesis of xeroprotectants can effectively stabilize biomaterials in a dry state. The most widely studied xeroprotectant is trehalose, which has excellent properties for the stabilization of certain proteins, bacteria, and biological membranes. There have also been attempts to apply trehalose to the stabilization of eukaryotic cells but without conclusive results. Consequently, a xeroprotectant or method that is useful for the stable drying of a particular biomaterial might not necessarily be suitable for another one. This article provides an overview of recent advances in the use of new techniques to stabilize biomaterials and compare xeroprotectants with other more standard methods.

Role of trehalose in salinity and temperature tolerance in the model halophilic bacterium Chromohalobacter salexigens

The disaccharide trehalose is considered as a universal stress molecule, protecting cells and biomolecules from injuries imposed by high osmolarity, heat, oxidation, desiccation and freezing. Chromohalobacter salexigens is a halophilic and extremely halotolerant γ-proteobacterium of the family Halomonadaceae. In this work, we have investigated the role of trehalose as a protectant against salinity, temperature and desiccation in C. salexigens. A mutant deficient in the trehalose-6-phosphate synthase gene (otsA::Ω) was not affected in its salt or heat tolerance, but double mutants ectoine- and trehalose-deficient, or hydroxyectoine-reduced and trehalose-deficient, displayed an osmo- and thermosensitive phenotype, respectively. This suggests a role of trehalose as a secondary solute involved in osmo- (at least at low salinity) and thermoprotection of C. salexigens. Interestingly, trehalose synthesis was osmoregulated at the transcriptional level, and thermoregulated at the post-transcriptional level, suggesting that C. salexigens cells need to be pre-conditioned by osmotic stress, in order to be able to quickly synthesize trehalose in response to heat stress. C. salexigens was more sensitive to desiccation than E. coli and desiccation tolerance was slightly improved when cells were grown at high temperature. Under these conditions, single mutants affected in the synthesis of trehalose or hydroxyectoine were more sensitive to desiccation than the wild-type strain. However, given the low survival rates of the wild type, the involvement of trehalose and hydroxyectoine in C. salexigens response to desiccation could not be firmly established.

Biotechnological uses of desiccation-tolerant microorganisms for the rhizoremediation of soils subjected to seasonal drought

Plant growth-promoting rhizobacteria (PGPR) increase the viability and health of host plants when they colonize roots and engage in associative symbiosis (Bashan et al. 2004). In return, PGPR viability is increased by host plant roots by the provision of nutrients and a more protective environment (Richardson et al. in Plant Soil 321:305-339, 2009). The PGPR have great potential in agriculture since the combination of certain microorganisms and plants can increase crop production and increase protection against frost, salinity, drought and other environmental stresses such as the presence of xenobiotic pollutants. But there is a great challenge in combining plants and microorganisms without compromising the viability of either microorganisms or seeds. In this paper, we review how anhydrobiotic engineering can be used for the formulation of biotechnological tools that guarantee the supply of both plants and microorganisms in the dry state. We also describe the application of this technology for the selection of desiccation-tolerant PGPR for polycyclic aromatic hydrocarbons bioremediation, in soils subjected to seasonal drought, by the rhizoremediation process.

Unexpected property of ectoine synthase and its application for synthesis of the engineered compatible solute ADPC

A new cyclic amino acid was detected in a deletion mutant of the moderately halophilic bacterium Halomonas elongata deficient in ectoine synthesis. Using mass spectroscopy (MS) and nuclear magnetic resonance (NMR) techniques, the substance was identified as 5-amino-3,4-dihydro-2H-pyrrole-2-carboxylate (ADPC). We were able to demonstrate that ADPC is the product of a side reaction of lone ectoine synthase (EC 4.2.1.108), which forms ADPC by cyclic condensation of glutamine. This reaction was shown to be reversible. Subsequently, a number of ectoine derivatives, in particular 4,5-dihydro-2-methylimidazole-4-carboxylate (DHMICA) and homoectoine, were also shown to be cleaved by ectoine synthase, which is classified as a hydro-lyase. This study thus reports for the first time that ectoine synthase accepts more than one substrate and is a reversible enzyme able to catalyze both the intramolecular condensation into and the hydrolytic cleavage of cyclic amino acid derivatives. As ADPC supports growth of bacteria under salt stress conditions and stabilizes enzymes against freeze-thaw denaturation, it displays typical properties of compatible solutes. As ADPC has not yet been described as a natural compound, it is presented here as the first man-made compatible solute created through genetic engineering.

Rapid method for isolation of desiccation-tolerant strains and xeroprotectants

A novel biotechnological process has been developed for the isolation of desiccation-tolerant microorganisms and their xeroprotectants, i.e., compatible solutes involved in long-term stability of biomolecules in the dry state. Following exposure of soil samples to chloroform, we isolated a collection of desiccation-tolerant microorganisms. This collection was screened for the production of xeroprotectants by a variation of the bacterial milking (osmotic downshock) procedure and by a novel air-drying/rehydration ("dry milking") incubation method. The resultant solutes were shown to protect both proteins and living cells against desiccation damage, thereby validating them as xeroprotectants. Nuclear magnetic resonance (NMR) analytical studies were performed to identify the xeroprotectants; synthetic mixtures of these compounds were shown to perform similarly to natural isolates in drying experiments with proteins and cells. This new approach has biotechnological and environmental implications for the identification of new xeroprotectants of commercial and therapeutic value.

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.