Membrane topology of the acyl-lipid desaturase from Bacillus subtilis

A multi-species direct-fed microbial supplement alters the milk lipidome of dairy cows

The study evaluated the effects of supplementing a multi-species direct-fed microbial (DFM) on the milk lipidome of lactating dairy cows. Twenty-four multiparous Holstein cows (41 ± 7 d in milk) were used in a randomized complete block design with experimental duration of 91 d. Cows were blocked based on energy-corrected milk yield from a 14-d pretreatment period, and were assigned randomly within each block to the following treatments: (1) control (CON): corn silage-based total mixed ration without DFM; or (2) BOV+: basal diet top-dressed with a DFM containing a mixture of Lactobacillus animalis (LA-51), Propionibacterium freudenreichii (PF-24), Bacillus subtilis (CH201), and Bacillus licheniformis (CH200) at 11.8 × 109 cfu/d. Milk samples were taken from morning and evening milkings on 2 consecutive days of each week of the pretreatment and treatment periods. Separate composites of pretreatment period and treatment period samples were prepared for individual cows and used for lipidome analysis. Lipidome analysis of the milk samples was performed using an ultra-high-performance liquid chromatograph linked to a quadrupole time-of-flight mass spectrometer in both positive and negative ionizations. The relative concentrations of 14 lipid species, including long-chain polyunsaturated fatty acids (LC-PUFA) such as FA 20:8 and FA 28:7 and triacylglycerides (TG) such as TG 40:3 and TG 54:2, were increased [false discovery rate (FDR) ≤0.05], whereas 13 lipid species, including saturated FA 24:0 and TG 40:0 were decreased (FDR ≤0.05) by supplemental BOV+. The relative concentration of de novo FA in milk was greater, whereas that of preformed FA was lower in dairy cows supplemented with BOV+. Results from this study demonstrate the potential of a DFM containing L. animalis, P. freudenreichii, Bacillus subtilis, and B. licheniformis to alter the milk lipidome in lactating dairy cows toward increased relative concentration of LC-PUFA, which might offer a healthier profile of FA to consumers with its associated health benefits.

Desaturases: Structural and mechanistic insights into the biosynthesis of unsaturated fatty acids

This review highlights the key role of fatty acid desaturases in the synthesis of naturally occurring, more common and not unsaturated fatty acids. The three major classes of fatty acid desaturases, such as acyl-lipid, acyl-acyl carrier protein and acyl-coenzyme A, are described in detail, with particular attention to the cellular localisation, the structure, the substrate and product specificity and the expression and regulation of desaturase genes. The review also gives an insight into the biocatalytic reaction of fatty acid desaturation by covering the general and more class-specific mechanistic studies around the synthesis of unsaturated fatty acids Finally, we conclude the review by looking at the numerous novel applications for desaturases in order to meet the very high demand for polyunsaturated fatty acids, taking into account the opportunity for the development of new, more efficient, easily reproducible, sustainable bioengineering advances in the field.

Membrane fatty acid desaturase: biosynthesis, mechanism, and architecture

Fatty acid desaturase catalyzes the desaturation reactions by inserting double bonds into the fatty acyl chain, producing unsaturated fatty acids, which play a vital part in the synthesis of polyunsaturated fatty acids. Though soluble fatty acid desaturases have been described extensively in advanced organisms, there are very limited studies of membrane fatty acid desaturases due to their difficulties in producing a sufficient amount of recombinant desaturases. However, the advancement of technology has shown substantial progress towards the development of elucidating crystal structures of membrane fatty acid desaturase, thus, allowing modification of structure to be manipulated. Understanding the structure, mechanism, and biosynthesis of fatty acid desaturase lay a foundation for the potential production of various strategies associated with alteration and modifications of polyunsaturated fatty acids. This manuscript presents the current state of knowledge and understanding about the structure, mechanisms, and biosynthesis of fatty acid desaturase. In addition, the role of unsaturated fatty acid desaturases in health and diseases is also encompassed. This will be useful in understanding the molecular basis and structural protein of fatty acid desaturase that are significant for the advancement of therapeutic strategies associated with the improvement of health status. KEY POINTS: • Current state of knowledge and understanding about the biosynthesis, mechanisms, and structure of fatty acid desaturase. • The role of unsaturated fatty acid desaturase. • The molecular basis and structural protein elucidated the crystal structure of fatty acid desaturase.

Transcription factors FabR and FadR regulate cold adaptability and spoilage potential of Shewanella baltica

Shewanella baltica is the specific spoilage microorganism of Pseudosciaena crocea during low-temperature storage. Exploring the correlation between cold adaptability and spoilage potential may provide a new perspective for prolonging shelf life of aquatic products. In the present study, we investigated the synthesis pathway of unsaturated fatty acid (UFA) responsible for regulating cold adaptability in Shewanella baltica and its effect on spoilage potential. FabR and FadR, as key regulators of membrane unsaturated fatty acids synthesis pathway, were identified in S. baltica. FabR was significantly down-regulated at 4 °C compared to at 30 °C, yet FadR displayed the opposite results. By overexpressing fabR and fadR genes at 4 °C, we found that FabR and FadR had negative and positive effects on UFA content and membrane fluidity as well as spoilage potential, respectively. These data indicated that FabR and FadR functioned collectively to increase the membrane fluidity for better cold adaptability at low temperature, resulting in the maintenance of spoilage potential of S. baltica.

Adaptation of the Marine Bacterium Shewanella baltica to Low Temperature Stress

Marine bacteria display significant versatility in adaptation to variations in the environment and stress conditions, including temperature shifts. Shewanella baltica plays a major role in denitrification and bioremediation in the marine environment, but is also identified to be responsible for spoilage of ice-stored seafood. We aimed to characterize transcriptional response of S. baltica to cold stress in order to achieve a better insight into mechanisms governing its adaptation. We exposed bacterial cells to 8 °C for 90 and 180 min, and assessed changes in the bacterial transcriptome with RNA sequencing validated with the RT-qPCR method. We found that S. baltica general response to cold stress is associated with massive downregulation of gene expression, which covered about 70% of differentially expressed genes. Enrichment analysis revealed upregulation of only few pathways, including aminoacyl-tRNA biosynthesis, sulfur metabolism and the flagellar assembly process. Downregulation was observed for fatty acid degradation, amino acid metabolism and a bacterial secretion system. We found that the entire type II secretion system was transcriptionally shut down at low temperatures. We also observed transcriptional reprogramming through the induction of RpoE and repression of RpoD sigma factors to mediate the cold stress response. Our study revealed how diverse and complex the cold stress response in S. baltica is.

Pseudomonas putida Δ9-fatty acid desaturase: Gene cloning, expression, and function in the cationic surfactants stress

Pseudomonas putida counteract the fluidizing effect of cationic surfactants decreasing the content of membrane unsaturated fatty acid (UFA). A Δ9-fatty acid desaturase gene (desA) from P. putida was isolated, cloned, and successfully expressed in Escherichia coli, a Δ9 desaturase deficient organism. desA consists of 1185 bp and codes for 394 amino acids. The deduced amino acid sequence reveals three histidine clusters and a hydropathy profile, typical of membrane-bound desaturases. Validating desA expression in E. coli cells, the amount of palmitoleic acid increased from 2.05 to 7.36%, with the concomitant increase in membrane fluidity (fluorescence polarization value decrease from 0.13 ± 0.03 to 0.09 ± 0.02). Also, when DesA activity was assayed in vivo, the percentage of UFA obtained from exogenous palmitic acid [1-¹⁴ C] increased 10-fold. In contrast, when cells expressing desA were exposed 15 min at sublethal concentration of cationic surfactants, the amount of UFA was 82% lower than that detected in cells non-exposed. Thus, the decrease in UFA content to counteract the fluidizing effect of cationic surfactants can be correlated with reduction of DesA activity.

Structural determinant of functionality in acyl lipid desaturases

Little is known about the structure-function relationship of membrane-bound lipid desaturases. Using a domain-swapping strategy, we found that the N terminus (comprising the two first transmembrane segments) region of Bacillus cereus DesA desaturase improves Bacillus subtilis Des activity. In addition, the replacement of the first two transmembrane domains from Bacillus licheniformis inactive open reading frame (ORF) BL02692 with the corresponding domain from DesA was sufficient to resurrect this enzyme. Unexpectedly, we were able to restore the activity of ORF BL02692 with a single substitution (Cys40Tyr) of a cysteine localized in the first transmembrane domain close to the lipid-water interface. Substitution of eight residues (Gly90, Trp104, Lys172, His228, Pro257, Leu275, Tyr282, and Leu284) by site-directed mutagenesis produced inactive variants of DesA. Homology modeling of DesA revealed that His228 is part of the metal binding center, together with the canonical His boxes. Trp104 shapes the hydrophobic tunnel, whereas Gly90 and Lys172 are probably involved in substrate binding/recognition. Pro257, Leu275, Tyr282, and Leu284 might be relevant for the structural arrangement of the active site or interaction with electron donors. This study reveals the role of the N-terminal region of Δ5 phospholipid desaturases and the individual residues necessary for the activity of this class of enzymes.

Biotechnological potential of insect fatty acid-modifying enzymes

There are more than one million described insect species. This species richness is reflected in the diversity of insect metabolic processes. In particular, biosynthesis of secondary metabolites, such as defensive compounds and chemical signals, encompasses an extraordinarily wide range of chemicals that are generally unparalleled among natural products from other organisms. Insect genomes, transcriptomes and proteomes thus offer a valuable resource for discovery of novel enzymes with potential for biotechnological applications. Here, we focus on fatty acid (FA) metabolism-related enzymes, notably the fatty acyl desaturases and fatty acyl reductases involved in the biosynthesis of FA-derived pheromones. Research on insect pheromone-biosynthetic enzymes, which exhibit diverse enzymatic properties, has the potential to broaden the understanding of enzyme specificity determinants and contribute to engineering of enzymes with desired properties for biotechnological production of FA derivatives. Additionally, the application of such pheromone-biosynthetic enzymes represents an environmentally friendly and economic alternative to the chemical synthesis of pheromones that are used in insect pest management strategies.

Lipid bilayer stress in obesity-linked inflammatory and metabolic disorders

The maintenance of the characteristic lipid compositions and physicochemical properties of biological membranes is essential for their proper function. Mechanisms allowing to sense and restore membrane homeostasis have been identified in prokaryotes for a long time and more recently in eukaryotes. A membrane remodeling can result from aberrant metabolism as seen in obesity. In this review, we describe how such lipid bilayer stress can account for the modulation of membrane proteins involved in the pathogenesis of obesity-linked inflammatory and metabolic disorders. We address the case of the Toll-like receptor 4 that is implicated in the obesity-related low grade inflammation and insulin resistance. The lipid raft-mediated TLR4 activation is promoted by an enrichment of the plasma membrane with saturated lipids or cholesterol increasing the lipid phase order. We discuss of the plasma membrane Na, K-ATPase that illustrates a new concept according to which direct interactions between specific residues and particular lipids determine both stability and activity of the pump in parallel with indirect effects of the lipid bilayer. The closely related sarco(endo)-plasmic Ca-ATPase embedded in the more fluid ER membrane seems to be more sensitive to a lipid bilayer stress as demonstrated by its inactivation in cholesterol-loaded macrophages or its inhibition mediated by an increased PtdCho/PtdEtn ratio in obese mice hepatocytes. Finally, we describe the model recently proposed for the activation of the conserved IRE-1 protein through alterations in the ER membrane lipid packing and thickness. Such IRE-1 activation could occur in response to abnormal lipid synthesis and membrane remodeling as observed in hepatocytes exposed to excess nutrients. Since the IRE-1/XBP1 branch also stimulates the lipid synthesis, this pathway could create a vicious cycle "lipogenesis-ER lipid bilayer stress-lipogenesis" amplifying hepatic ER pathology and the obesity-linked systemic metabolic defects.

Classification and substrate head-group specificity of membrane fatty acid desaturases

Membrane fatty acid desaturases are a diverse superfamily of enzymes that catalyze the introduction of double bonds into fatty acids. They are essential in a range of metabolic processes, such as the production of omega-3 fatty acids. However, our structure-function understanding of this superfamily is still developing and their range of activities and substrate specificities are broad, and often overlapping, which has made their systematic characterization challenging. A central issue with characterizing these proteins has been the lack of a structural model, which has been overcome with the recent publication of the crystal structures of two mammalian fatty acid desaturases. In this work, we have used sequence similarity networks to investigate the similarity among over 5000 related membrane fatty acid desaturase sequences, leading to a detailed classification of the superfamily, families and subfamilies with regard to their function and substrate head-group specificity. This work will facilitate rapid prediction of the function and specificity of new and existing sequences, as well as forming a basis for future efforts to manipulate the substrate specificity of these proteins for biotechnology applications.

Molecular Cloning and Functional Expression of a Δ9- Fatty Acid Desaturase from an Antarctic Pseudomonas sp. A3

Fatty acid desaturase enzymes play an essential role in the synthesis of unsaturated fatty acids. Pseudomonas sp. A3 was found to produce a large amount of palmitoleic and oleic acids after incubation at low temperatures. Using polymerase Chain Reaction (PCR), a novel Δ9- fatty acid desaturase gene was isolated, cloned, and successfully expressed in Escherichia coli. The gene was designated as PA3FAD9 and has an open reading frame of 1,185 bp which codes for 394 amino acids with a predicted molecular weight of 45 kDa. The activity of the gene product was confirmed via GCMS, which showed a functional putative Δ9-fatty acid desaturase capable of increasing the total amount of cellular unsaturated fatty acids of the E. coli cells expressing the gene. The results demonstrate that the cellular palmitoleic acids have increased two-fold upon expression at 15°C using only 0.1 mM IPTG. Therefore, PA3FAD9 from Pseudomonas sp.A3 codes for a Δ9-fatty acid desaturase-like protein which was actively expressed in E. coli.

Molecular and biochemical characterization of the OLE-1 high-oleic castor seed (Ricinus communis L.) mutant

MAIN CONCLUSION

The natural OLE-1 high-oleic castor mutant has been characterized, demonstrating that point mutations in the FAH12 gene are responsible for the high-oleic phenotype. The contribution of each mutation was evaluated by heterologous expression in yeast, and lipid studies in developing OLE-1 seeds provided new evidence of unusual fatty acids channeling into TAGs. Ricinus communis L. is a plant of the Euphorbiaceae family well known for producing seeds whose oil has a very high ricinoleic (12-hydroxyoctadecenoic) acid content. Castor oil is considered the only commercially renewable source of hydroxylated fatty acids, which have many applications as chemical reactants. Accordingly, there has been great interest in the field of plant lipid biotechnology to define how ricinoleic acid is synthesized, which could also provide information that might serve to increase the content of other unusual fatty acids in oil crops. Accordingly, we set out to study the biochemistry of castor oil synthesis by characterizing a natural castor bean mutant deficient in ricinoleic acid synthesis (OLE-1). This mutant accumulates high levels of oleic acid and displays remarkable alterations in its seed lipid composition. To identify enzymes that are critical for this phenotype in castor oil, we cloned and sequenced the oleate desaturase (FAD2) and hydroxylase (FAH12) genes from wild-type and OLE-1 castor bean plants and analyzed their expression in different tissues. Heterologous expression in yeast confirmed that three modifications to the OLE-1 FAH12 protein were responsible for its weaker hydroxylase activity. In addition, we studied the expression of the genes involved in this biosynthetic pathway at different developmental stages, as well as that of other genes involved in lipid biosynthesis, both in wild-type and mutant seeds.

Unexpected functional diversity in the fatty acid desaturases of the flour beetle Tribolium castaneum and identification of key residues determining activity

Desaturases catalyse modifications to fatty acids which are essential to homeostasis and for pheromone and defensive chemical production. All desaturases of the flour beetle Tribolium castaneum were investigated via query of the sequenced genome which yielded 15 putative acyl-Coenzyme A genes. Eleven desaturase mRNA were obtained in full length and functionally expressed in yeast. Phylogenetic analysis separated the desaturases into 4 distinct clades; one clade contained conserved beetle Δ9 desaturases, second clade was Tribolium-specific having diverse activities including Δ5, Δ9 and Δ12 desaturation and the other 2 clades had mixed insect representatives. Three members of this clade contained unusual inserted sequences of ∼20 residues in the C-terminal region and were related to desaturases that all contained similar inserts. Deletion of the entirety of the insert in the flour beetle Δ12 desaturase abolished its activity but this was partially restored by the reintroduction of two histidine residues, suggesting the histidine(s) are required for activity but the full length insert is not. Five new desaturase activities were discovered: Δ9 desaturation of C12:0-C16:0 substrates; two unprecedented Δ5 enzymes acting on C18:0 and C16:0; Δ9 activity exclusively on C16:0 and a further stearate Δ9 desaturase. qPCR analysis ruled out a role in sex pheromone synthesis for the Δ5 and Δ9/C16:0 desaturases. The flour beetle genome has underpinned an examination of all transcribed desaturases in the organism and revealed a diversity of novel and unusual activities, an improved understanding of the evolutionary relationships among insect desaturases and sequence determinants of activity.

Δ12-Fatty acid desaturase from Candida parapsilosis is a multifunctional desaturase producing a range of polyunsaturated and hydroxylated fatty acids

Numerous Δ12-, Δ15- and multifunctional membrane fatty acid desaturases (FADs) have been identified in fungi, revealing great variability in the enzymatic specificities of FADs involved in biosynthesis of polyunsaturated fatty acids (PUFAs). Here, we report gene isolation and characterization of novel Δ12/Δ15- and Δ15-FADs named CpFad2 and CpFad3, respectively, from the opportunistic pathogenic yeast Candida parapsilosis. Overexpression of CpFad3 in Saccharomyces cerevisiae strains supplemented with linoleic acid (Δ9,Δ12-18:2) and hexadecadienoic acid (Δ9,Δ12-16:2) leads to accumulation of Δ15-PUFAs, i.e., α-linolenic acid (Δ9,Δ12,Δ15-18:3) and hexadecatrienoic acid with an unusual terminal double bond (Δ9,Δ12,Δ15-16:3). CpFad2 produces a range of Δ12- and Δ15-PUFAs. The major products of CpFad2 are linoleic and hexadecadienoic acid (Δ9,Δ12-16:2), accompanied by α-linolenic acid and hexadecatrienoic acid (Δ9,Δ12,Δ15-16:3). Using GC/MS analysis of trimethylsilyl derivatives, we identified ricinoleic acid (12-hydroxy-9-octadecenoic acid) as an additional product of CpFad2. These results demonstrate that CpFAD2 is a multifunctional FAD and indicate that detailed analysis of fatty acid derivatives might uncover a range of enzymatic selectivities in other Δ12-FADs from budding yeasts (Ascomycota: Saccharomycotina).

ω3 fatty acid desaturases from microorganisms: structure, function, evolution, and biotechnological use

The biosynthesis of very-long-chain polyunsaturated fatty acids involves an alternating process of fatty acid desaturation and elongation catalyzed by complex series of enzymes. ω3 desaturase plays an important role in converting ω6 fatty acids into ω3 fatty acids. Genes for this desaturase have been identified and characterized in a wide range of microorganisms, including cyanobacteria, yeasts, molds, and microalgae. Like all fatty acid desaturases, ω3 desaturase is structurally characterized by the presence of three highly conserved histidine-rich motifs; however, unlike some desaturases, it lacks a cytochrome b5-like domain. Understanding the structure, function, and evolution of ω3 desaturases, particularly their substrate specificities in the biosynthesis of very-long-chain polyunsaturated fatty acids, lays the foundation for potential production of various ω3 fatty acids in transgenic microorganisms.

The role of desaturases in the biosynthesis of marking pheromones in bumblebee males

Bumblebee males (Hymenoptera) produce species-specific labial gland secretions called marking pheromones (MPs). MPs generally consist of terpenoids and fatty-acid-derived aliphatic compounds with various chain lengths predominantly containing one or no double bonds. The unsaturated fatty-acid-derived MP components were hypothesized to be produced by fatty acid desaturases (FADs) that exhibit diverse substrate specificities. To address this hypothesis, we isolated and functionally characterized FADs from three bumblebee species: Bombus lucorum, Bombus terrestris, and Bombus lapidarius. By employing RNA sequencing of the male labial glands and fat bodies of B. lucorum and B. terrestris, we identified five paralogous FAD-like sequences but only two FAD lineages were abundant and differentially expressed in the labial glands. We found that abundant FAD lineages were also expressed in the labial gland and fat body of Bombus lapidarius. Functional characterization of FADs in a yeast expression system confirmed that Δ4-FADs exhibited a unique Δ4-desaturase activity exclusively on 14-carbon fatty acyls and Δ9-FADs displayed Δ9-desaturase activity on 14- to 18-carbon fatty acyls. These results indicate that Δ9-FADs are involved in the biosynthesis of major unsaturated components of MPs in B. lucorum and B. lapidarius despite the diverse MP composition of these bumblebee species. The contribution of lipases, acyltransferases, esterases, and fatty acid reductases to production of the species-specific MP composition is also discussed in light of the transcriptomic data obtained in this study.

Identification and characterization of new Δ-17 fatty acid desaturases

ω-3 fatty acid desaturase is a key enzyme for the biosynthesis of ω-3 polyunsaturated fatty acids via the oxidative desaturase/elongase pathways. Here we report the identification of three ω-3 desaturases from oomycetes, Pythium aphanidermatum, Phytophthora sojae, and Phytophthora ramorum. These new ω-3 desaturases share 55 % identity at the amino acid level with the known Δ-17 desaturase of Saprolegnia diclina, and about 31 % identity with the bifunctional Δ-12/Δ-15 desaturase of Fusarium monoliforme. The three enzymes were expressed in either wild-type or codon optimized form in an engineered arachidonic acid producing strain of Yarrowia lipolytica to study their activity and substrate specificity. All three were able to convert the ω-6 arachidonic acid to the ω-3 eicosapentanoic acid, with a substrate conversion efficiency of 54-65 %. These enzymes have a broad ω-6 fatty acid substrate spectrum, including both C18 and C20 ω-6 fatty acids although they prefer the C20 substrates, and have strong Δ-17 desaturase activity but weaker Δ-15 desaturase activity. Thus, they belong to the Δ-17 desaturase class. Unlike the previously identified bifunctional Δ-12/Δ-15 desaturase from F. monoliforme, they lack Δ-12 desaturase activity. The newly identified Δ-17 desaturases could use fatty acids in both acyl-CoA and phospholipid fraction as substrates. The identification of these Δ-17 desaturases provides a set of powerful new tools for genetic engineering of microbes and plants to produce ω-3 fatty acids, such as eicosapentanoic acid and docosahexanoic acid, at high levels.

Identification of a conserved protein involved in anaerobic unsaturated fatty acid synthesis in Neiserria gonorrhoeae: implications for facultative and obligate anaerobes that lack FabA

Transcriptome analysis of the facultative anaerobe, Neisseria gonorrhoeae, revealed that many genes of unknown function were induced under anaerobic conditions. Mutation of one such gene, NGO1024, encoding a protein belonging to the 2-nitropropane dioxygenase-like superfamily of proteins, was found to result in an inability of gonococci to grow anaerobically. Anaerobic growth of an NG1024 mutant was restored upon supplementation with unsaturated fatty acids (UFA), but not with the saturated fatty acid palmitate. Gonococcal fatty acid profiles confirmed that NGO1024 was involved in UFA synthesis anaerobically, but not aerobically, demonstrating that gonococci contain two distinct pathways for the production of UFAs, with a yet unidentified aerobic mechanism, and an anaerobic mechanism involving NGO1024. Expression of genes involved in classical anaerobic UFA synthesis, fabA, fabM and fabB, was toxic in gonococci and unable to complement a NGO1024 mutation, suggesting that the chemistry involved in gonococcal anaerobic UFA synthesis is distinct from that of the classical pathway. NGO1024 homologues, which we suggest naming UfaA, form a distinct lineage within the 2-nitropropane dioxygenase-like superfamily, and are found in many facultative and obligate anaerobes that produce UFAs but lack fabA, suggesting that UfaA is part of a widespread pathway involved in UFA synthesis.

Role of ferredoxin and flavodoxins in Bacillus subtilis fatty acid desaturation

The Bacillus subtilis acyl lipid desaturase (Δ5-Des) is an iron-dependent integral membrane protein able to selectively introduce double bonds into long-chain fatty acids. In the last decade since its discovery, the molecular mechanism of Δ5-Des expression has been studied extensively. However, the mechanism of desaturation, which must rely on unknown bacterial proteins for electron transfer, has not yet been explored. The B. subtilis genome encodes three proteins that can act as potential electron donors of Δ5-Des, ferredoxin (Fer) and two flavodoxins (Flds) (YkuN and YkuP), which are encoded by the ykuNOP operon. Here we report that the disruption of either the fer gene or the ykuNOP operon decreases the desaturation of palmitic acid by ∼30%. Nevertheless, a fer ykuNOP mutant abolished the desaturation reaction almost completely. Our results establish Fer and the two Flds as redox partners for Δ5-Des and suggest that the Fer and Fld proteins could function physiologically in the biosynthesis of unsaturated fatty acids in B. subtilis. Although Flds have extensively been described as partners in a number of redox processes, this is the first report describing their role as electron donors in the fatty acid desaturation reaction.

Role of heme and heme-proteins in trypanosomatid essential metabolic pathways

Around the world, trypanosomatids are known for being etiological agents of several highly disabling and often fatal diseases like Chagas disease (Trypanosoma cruzi), leishmaniasis (Leishmania spp.), and African trypanosomiasis (Trypanosoma brucei). Throughout their life cycle, they must cope with diverse environmental conditions, and the mechanisms involved in these processes are crucial for their survival. In this review, we describe the role of heme in several essential metabolic pathways of these protozoans. Notwithstanding trypanosomatids lack of the complete heme biosynthetic pathway, we focus our discussion in the metabolic role played for important heme-proteins, like cytochromes. Although several genes for different types of cytochromes, involved in mitochondrial respiration, polyunsaturated fatty acid metabolism, and sterol biosynthesis, are annotated at the Tritryp Genome Project, the encoded proteins have not yet been deeply studied. We pointed our attention into relevant aspects of these protein functions that are amenable to be considered for rational design of trypanocidal agents.

Effects of different carbon sources on the growth, fatty acids production, and expression of three desaturase genes of Mortierella alpina ATCC 16266

On the molecular and biochemical levels, the effects of different carbon sources on biomass production, fatty acid biosynthesis, and gene expression of three desaturases were investigated in Mortierella alpina ATCC 16266, at a stationary phase, which is an important filamentous fungus capable of producing various polyunsaturated fatty acids (PUFAs). The maximum mycelial biomass was achieved using sucrose as carbon source. However, the highest productivity of total lipids was shown to be no biomass associated. In addition, glucose was the preferred carbon source for the expression of three desaturase genes compared to others, but the change at the corresponding fatty acid product's level of these desaturase genes was not in accordance with the change measured at the mRNA level among those carbon sources that we utilized. Significant discrepancies between the mRNA expression and the product abundance may indicate post-transcriptional regulatory mechanisms of these desaturases.

Membrane thickness cue for cold sensing in a bacterium

Thermosensors are ubiquitous integral membrane proteins found in all kinds of life. They are involved in many physiological roles, including membrane remodeling, chemotaxis, touch, and pain [1-3], but, the mechanism by which their transmembrane (TM) domains transmit temperature signals is largely unknown. The histidine kinase DesK from Bacillus subtilis is the paradigmatic example of a membrane-bound thermosensor suited to remodel membrane fluidity when the temperature drops below approximately 30°C [1, 4] providing, thus, a tractable system for investigating the mechanism of TM-mediated input-output control of thermal adaptation. Here we show that the multimembrane-spanning domain from DesK can be simplified into a chimerical single-membrane-spanning minimal sensor (MS) that fully retains, in vivo and in vitro, the sensing properties of the parental system. The MS N terminus contains three hydrophilic amino acids near the lipid-water interface creating an instability hot spot. Mutational analysis of this boundary-sensitive beacon revealed that membrane thickness controls the signaling state of the sensor by dictating the hydration level of the metastable hydrophilic spot. Guided by these results we biochemically demonstrated that the MS signal transmission activity is sensitive to bilayer thickness. Membrane thickness could be a general cue for sensing temperature in many organisms.

Differences in cold adaptation of Bacillus subtilis under anaerobic and aerobic conditions

Bacillus subtilis, which grows under aerobic conditions, employs fatty acid desaturase (Des) to fluidize its membrane when subjected to temperature downshift. Des requires molecular oxygen for its activity, and its expression is regulated by DesK-DesR, a two-component system. Transcription of des is induced by the temperature downshift and is decreased when membrane fluidity is restored. B. subtilis is also capable of anaerobic growth by nitrate or nitrite respiration. We studied the mechanism of cold adaptation in B. subtilis under anaerobic conditions that were predicted to inhibit Des activity. We found that in anaerobiosis, in contrast to aerobic growth, the induction of des expression after temperature downshift (from 37 degrees C to 25 degrees C) was not downregulated. However, the transfer from anaerobic to aerobic conditions rapidly restored the downregulation. Under both aerobic and anaerobic conditions, the induction of des expression was substantially reduced by the addition of external fluidizing oleic acid and was fully dependent on the DesK-DesR two-component regulatory system. Fatty acid analysis proved that there was no desaturation after des induction under anaerobic conditions despite the presence of high levels of the des protein product, which was shown by immunoblot analysis. The cold adaptation of B. subtilis in anaerobiosis is therefore mediated exclusively by the increased anteiso/iso ratio of branched-chain fatty acids and not by the temporarily increased level of unsaturated fatty acids that is typical under aerobic conditions. The degrees of membrane fluidization, as measured by diphenylhexatriene fluorescence anisotropy, were found to be similar under both aerobic and anaerobic conditions.

Evidence that the yeast desaturase Ole1p exists as a dimer in vivo

Desaturase enzymes are composed of two classes, the structurally well characterized soluble class found predominantly in the plastids of higher plants and the more widely distributed but poorly structurally defined integral membrane class. Despite their distinct evolutionary origins, the two classes both require an iron cofactor and molecular oxygen for activity and are inhibited by azide and cyanide, suggesting strong mechanistic similarities. The fact that the soluble desaturase is active as a homodimer prompted us test the hypothesis that an archetypal integral membrane desaturase from Saccharomyces cerevisiae, the Delta(9)-acyl-Co-A desaturase Ole1p, also exhibits a dimeric organization. Ole1p was chosen because it is one of the best characterized integral membrane desaturase and because it retains activity when fused with epitope tags. FLAG-Ole1p was detected by Western blotting of immunoprecipitates in which anti-Myc antibodies were used for capture from yeast extracts co-expressing Ole1p-Myc and Ole1p-FLAG. Interaction was confirmed by two independent bimolecular complementation assays (i.e. the split ubiquitin system and the split luciferase system). Co-expression of active and inactive Ole1p subunits resulted in an approximately 75% suppression of the accumulation of palmitoleic acid, demonstrating that the physiologically active form of Ole1p in vivo is the dimer in which both protomers must be functional.

Crystallization and preliminary X-ray crystallographic studies of DesR, a thermosensing response regulator in a two-component signalling system from Streptococcus pneumoniae

The response regulator DesR, which activates the transcription of the des gene by binding to a regulatory region, is essential for controlling the fluidity of membrane phospholipids. DesR from Streptococcus pneumoniae was overexpressed in Escherichia coli. The protein was purified and crystallized for structural analysis. Diffraction data were collected to 1.7 A resolution using synchrotron radiation and the crystals belonged to the orthorhombic space group P2(1)2(1)2(1), with unit-cell parameters a = 46.91, b = 71.38, c = 117.73 A. Assuming the presence of a dimer in the asymmetric unit, this corresponds to a V(M) of 2.21 A(3) Da(-1).

Signalling pathways controlling fatty acid desaturation

Microorganisms, plants and animals regulate the synthesis of unsaturated fatty acids (UFAs) during changing environmental conditions as well as in response to nutrients. Unsaturation of fatty acid chains has important structural roles in cell membranes: a proper ratio of saturated to UFAs contributes to membrane fluidity. Alterations in this ratio have been implicated in various disease states including cardiovascular diseases, immune disorders, cancer and obesity. They are also the major components of triglycerides and intermediates in the synthesis of biologically active molecules such as eicosanoids, which mediates fever, inflammation and neurotransmission. UFAs homeostasis in many organisms is achieved by feedback regulation of fatty acid desaturases gene transcription. Here, we review recently discovered components and mechanisms of the regulatory machinery governing the transcription of fatty acid desaturases in bacteria, yeast and animals.

Transmembrane topology of the Acr3 family arsenite transporter from Bacillus subtilis

The transmembrane topology of the Acr3 family arsenite transporter Acr3 from Bacillus subtilis was analysed experimentally using translational fusions with alkaline phosphatase and green fluorescent protein and in silico by topology modelling. Initial topology prediction resulted in two models with 9 and 10 TM helices respectively. 32 fusion constructs were made between truncated forms of acr3 and the reporter genes at 17 different sites throughout the acr3 sequence to discriminate between these models. Nine strong reporter protein signals provided information about the majority of the locations of the cytoplasmic and extracellular loops of Acr3 and showed that both the N- and the C-termini are located in the cytoplasm. Two ambiguous data points indicated the possibility of an alternative 8 helix topology. This possibility was investigated using another 10 fusion variants, but no experimental support for the 8 TM topology was obtained. We therefore conclude that Acr3 has 10 transmembrane helices. Overall, the loops which connect the membrane spanning segments are short, with cytoplasmic loops being somewhat longer than the extracellular loops. The study provides the first ever experimentally derived structural information on a protein of the Acr3 family which constitutes one of the largest classes of arsenite transporters.

Truncation mutants highlight a critical role for the N- and C-termini of the Spirulina Delta(6) desaturase in determining regioselectivity

The results of our previous study on heterologous expression in Escherichia coli of the gene desD, which encodes Spirulina Delta(6) desaturase, showed that co-expression with an immediate electron donor-either cytochrome b ( 5 ) or ferredoxin-was required for the production of GLA (gamma-linolenic acid), the product of the reaction catalyzed by Delta(6) desaturase. Since a system for stable transformation of Spirulina is not available, studies concerning Spirulina-enzyme characterization have been carried out in heterologous hosts. In this present study, the focus is on the role of the enzyme's N- and C-termini, which are possibly located in the cytoplasmic phase. Truncated enzymes were expressed in E. coli by employing the pTrcHisA expression system. The truncation of the N- and C-terminus by 10 (N10 and C10) and 30 (N30 and C30) amino acids, respectively, altered the enzyme's regioselective mode from one that measures from a preexisting double bond to that measuring from the methyl end of the substrate.

A small membrane-peripheral region close to the active center determines regioselectivity of membrane-bound fatty acid desaturases from Aspergillus nidulans

Fatty acid desaturases catalyze the introduction of double bonds at specific positions of an acyl chain and are categorized according to their substrate specificity and regioselectivity. The current understanding of membrane-bound desaturases is based on mutant studies, biochemical topology analysis, and the comparison of related enzymes with divergent functionality. Because structural information is lacking, the principles of membrane-bound desaturase specificity are still not understood despite of substantial research efforts. Here we compare two membrane-bound fatty acid desaturases from Aspergillus nidulans: a strictly monofunctional oleoyl-Delta12 desaturase and a processive bifunctional oleoyl-Delta12/linoleoyl-omega3 desaturase. The high similarities in the primary sequences of the enzymes provide an ideal starting point for the systematic analysis of factors determining substrate specificity and bifunctionality. Based on the most current topology models, both desaturases were divided into nine domains, and the domains of the monofunctional Delta12 desaturase were systematically exchanged for their respective corresponding matches of the bifunctional sister enzyme. Catalytic capacities of hybrid enzymes were tested by heterologous expression in yeast, followed by biochemical characterization of the resulting fatty acid patterns. The individual exchange of two domains of a length of 18 or 49 amino acids each resulted in bifunctional Delta12/omega3 activity of the previously monofunctional parental enzyme. Sufficient determinants of fatty acid desaturase substrate specificity and bifunctionality could, thus, be narrowed down to a membrane-peripheral region close to the catalytic site defined by conserved histidine-rich motifs in the topology model.

Primary Structure, Regioselectivity, and Evolution of the Membrane-bound Fatty Acid Desaturases of Claviceps purpurea

Two cDNAs with sequence similarity to fatty acid desaturase genes were isolated from the phytopathogenic fungus, Claviceps purpurea. The predicted amino acid sequences of the corresponding genes, named CpDes12 and CpDesX, share 87% identity. Phylogenetic analysis indicates that CpDes12 and CpDesX arose by gene duplication of an ancestral Delta(12)-desaturase gene after the divergence of Nectriaceae and Clavicipitaceae. Functional expression of CpDes12 and CpDesX in yeast (Saccharomyces cerevisiae) indicated that CpDes12 is primarily a "Delta(12)"-desaturase, whereas CpDesX is a novel desaturase catalyzing "Delta(12)," "Delta(15)," and "omega(3)" types of desaturation with omega(3) activity predominating. CpDesX sequentially desaturates both 16:1-9c and 18:1-9c to give 16:3-9c,12c,15c and 18:3-9c,12c,15c, respectively. In addition, it could also act as an omega(3)-desaturase converting omega(6)-polyunsaturates 18:3-6c,9c,12c, 20:3-8c,11c,14c, and 20:4-5c,8c,11c,14c to their omega(3) counterparts 18:4-6c,9c,12c,15c, 20:4-8c,11c,14c,17c, and 20:5-5c,8c,11c,14c,17c, respectively. By using reciprocal site-directed mutagenesis, we demonstrated that two residues (isoleucine at 152 and alanine at 206) are critical in defining the catalytic specificity of these enzymes and the C-terminal amino acid sequence (residues 302-477) was also found to be important. These data provide insights into the nature of regioselectivity in membrane-bound fatty acid desaturases and the relevant structural determinants. The authors suggest that the regios-electivity of such enzymes may be best understood by considering the relative importance of more than one regioselective preference. In this view, CpDesX is designated as anu + 3(omega(3)) desaturase, which primarily references an existing double bond (nu + 3 regioselectivity) and secondarily shows preference for omega(3) desaturation.

Biosynthesis of polyunsaturated fatty acids in lower eukaryotes

Polyunsaturated fatty acids have important structural roles in cell membranes. They are also intermediates in the synthesis of biologically active molecules such as eicosanoids, which mediate fever, inflammation, blood pressure and neurotransmission. Arachidonic and docosahexaenoic acids are essential components of brain tissues and, through their involvement in the development of neural and retinal functions, important dietary nutrients for neonatal babies. Lower eukaryotes are particularly rich in C20-22 polyunsaturated fatty acids. Fungi and marine microalgae are currently used to produce nutraceutic oils. Other protists and algae are being studied because of the variability in their enzymes involved in polyunsaturated fatty acid biosynthesis. Such enzymes could be used as source for the production of transgenic organisms able to synthesize designed oils for human diet or, in the case of parasitic protozoa, they might be identified as putative chemotherapeutic targets. Polyunsaturated fatty acids can be synthesized by two different pathways: an anaerobic one, by using polyketide synthase related enzymes, and an aerobic one, which involves the action of elongases and oxygen dependent desaturases. Desaturases can be classified into three main types, depending on which of the consecutive steps of polyunsaturated fatty acid synthesis they are involved with. The enzymes may be specialized to act on: saturated substrates (type I); mono- and di-unsaturated fatty acids by introducing additional double bonds at the methyl-end site of the existing double bonds (type II); or the carboxy half ('front-end') of polyunsaturated ones (type III). Type III desaturases require the alternating action of elongases. A description of the enzymes that have been isolated and functionally characterized is provided, in order to highlight the different pathways found in lower eukaryotes.

Mutational and functional analysis of the beta-carotene ketolase involved in the production of canthaxanthin and astaxanthin

Biosynthesis of the commercial carotenoids canthaxanthin and astaxanthin requires beta-carotene ketolase. The functional importance of the conserved amino acid residues of this enzyme from Paracoccus sp. strain N81106 (formerly classified as Agrobacterium aurantiacum) was analyzed by alanine-scanning mutagenesis. Mutations in the three highly conserved histidine motifs involved in iron coordination abolished its ability to catalyze the formation of ketocarotenoids. This supports the hypothesis that the CrtW ketolase belongs to the family of iron-dependent integral membrane proteins. Most of the mutations generated at other highly conserved residues resulted in partial activity. All partially active mutants showed a higher amount of adonixanthin accumulation than did the wild type when expressed in Escherichia coli cells harboring the zeaxanthin biosynthetic gene cluster. Some of the partially active mutants also produced a significant amount of echinenone when expressed in cells producing beta-carotene. In fact, expression of a mutant carrying D117A resulted in the accumulation of echinenone as the predominant carotenoid. These observations indicate that partial inactivation of the CrtW ketolase can often lead to the production of monoketolated intermediates. In order to improve the conversion rate of astaxanthin catalyzed by the CrtW ketolase, a color screening system was developed. Three randomly generated mutants, carrying L175M, M99V, and M99I, were identified to have improved activity. These mutants are potentially useful in pathway engineering for the production of astaxanthin.

Two aerobic pathways for the formation of unsaturated fatty acids in Pseudomonas aeruginosa

The double bond in anaerobic unsaturated fatty acid (UFA) biosynthesis is introduced by the FabA dehydratase/isomerase of the bacterial type II fatty acid biosynthetic pathway. A DeltafabA mutant of Pseudomonas aeruginosa grew aerobically, but required a UFA supplement for anaerobic growth. Wild-type cells produced 18:1Delta11 as the principal UFA, whereas the DeltafabA strain produced only 16:1Delta9. The double bond in the 16:1Delta9 was introduced after phospholipid formation and was localized in the sn-2 position. Two predicted membrane proteins, DesA and DesB, possessed the conserved histidine clusters characteristic of fatty acid desaturases. The DeltafabADeltadesA double mutant required exogenous fatty acids for growth but the DeltafabAdesB double mutant did not. Exogenous stearate was converted to 18:1Delta9 and supported the growth of DeltafabADeltadesA double mutant. A DeltafabADeltadesAdesB triple mutant was unable to desaturate exogenous stearate and was an UFA auxotroph. We detected a 2.5-fold increase in desA expression in DeltafabA mutants, whereas desB expression was derepressed by the deletion of the gene encoding a transcriptional repressor DesT. These data add two aerobic desaturases to the enzymes used for fatty acid metabolism in proteobacteria: DesA, a 2-position phospholipid Delta9-desaturase that supplements the anaerobic FabA pathway, and DesB, an inducible acyl-CoA Delta9-desaturase whose expression is repressed by DesT.

The desaturase from Bacillus subtilis, a promising tool for the selective olefination of phospholipids

The Delta5-desaturase from Bacillus subtilis has been cloned in Escherichia coli BL21 cells and its enzyme activity has been investigated as a function of temperature and oxygenation by analyzing methyl ester adducts from the total lipid extract in GC-MS measurements. The present data bring out that the activity of recombinant Delta5-desaturase, at 20-22 degrees C and 20% oxygen, is surprisingly high yielding 22% of C16:1,Delta5 (5-cis-palmitoleic acid) and 13% C18:2, Delta5 Delta11 (efedrenic acid). Lower amounts of other mono- and doubly-Delta5-unsaturated fatty acids were also detected. These findings demonstrate that Delta5-desaturase can accept a multiplicity of substrates and is endowed with an unprecedented activity among other acyl-lipid desaturases thus representing a unique tool for the production of rare Delta5 unsaturated fatty acid derivatives.

Membrane topology of mouse stearoyl-CoA desaturase 1

Stearoyl-CoA desaturase (SCD) is an integral membrane protein anchored in the endoplasmic reticulum. It catalyzes the biosynthesis of monounsaturated fatty acids that are required for the synthesis of triglycerides, cholesteryl esters, and phospholipids. Four mouse isoforms of SCD (SCD1-4) and two human isoforms have been characterized. In the current study, we characterize the topology of the mouse SCD1 isoform. Hydropathy analysis of the 355-amino acid mouse SCD1 protein predicts that the protein contains four transmembrane domains (TMDs) and three loops connecting the membrane-spanning domains. To define the topology of the protein, recombinant SCD1 constructs containing epitope tags were transiently expressed in HeLa cells and analyzed by indirect immunofluorescence and cysteine derivatization. Our data provide evidence that the N and C termini of SCD1 are oriented toward the cytosol with four transmembrane domains separated by two very short hydrophilic loops in the ER lumen and one large hydrophilic loop in the cytosol. In addition, based on the previous observation that SCD is a thiol enzyme, we sought to investigate whether the cysteine residues were essential for enzyme activity through mutagenesis studies, and our data suggest that the cysteines in SCD are not catalytically essential.

Biosynthesis of iso-fatty acids in myxobacteria

The fatty acid (FA) profiles of the myxobacteria Stigmatella aurantiaca and Myxococcus xanthus were investigated by acidic methanolysis of total cell extracts and GC or GC-MS analysis. The main components were 13-methyltetradecanoic acid (iso-15:0) and (Z)-hexadec-11-enoic acid (16:1, omega-5 cis). The biosynthesis of iso-FAs was investigated in several feeding experiments. Feeding of isovaleric acid (IVA) to a mutant impaired in the degradation of leucine to isovaleryl-CoA (IV-CoA)(bkd mutant) of M. xanthus only increased the amount of iso-odd FAs, whereas feeding of isobutyric acid (IBA) gave increased amounts only of iso-even FAs. In contrast, a bkd mutant of S. aurantiaca gave increased amounts of iso-odd and iso-even fatty acids in both experiments. We assumed that in S. aurantiacaalpha-oxidation takes place. [D(7)]-15-Methylhexadecanoic acid was synthesised and fed to S. aurantiaca as well as [D(10)]leucine and [D(8)]valine to elucidate this pathway in more detail. The iso-fatty acid was degraded by alpha- and beta-oxidation steps. [D(10)]Leucine was strongly incorporated into iso-odd and iso-even fatty acids, whereas the incorporation rates for [D(8)]valine into both types of fatty acids were low. Thus alpha-oxidation plays an important role in the biosynthesis of iso-fatty acids in S. aurantiaca. The incorporation rates observed after feeding of [D(10)]leucine and [D(8)]valine are the highest for iso-17:0 compared to the other acids. This indicates the central role of iso-17:0 in the biosynthesis of iso-FAs. The shorter homologues seem to be formed mainly by alpha-oxidation and beta-oxidation of this acid. After feeding of traces of unsaturated counterparts of this labelled FA occurred in the extracts indicating that desaturases are active in the biosynthesis of unsaturated fatty acids in S. aurantiaca.

The Bacillus subtilis desaturase: a model to understand phospholipid modification and temperature sensing

Most fatty acid desaturases are members of a large superfamily of integral membrane, O2-dependent, iron-containing enzymes that insert double bonds into previously synthesized fatty acyl chains. The cold shock-induced, membrane-bound desaturase from Bacillus subtilis (Delta5-Des) uses existing phospholipids as substrates to introduce a cis-double bond at the fifth position of the fatty acyl chain. While essentially no three-dimensional structural information is available for these difficult-to-purify enzymes, experimental analysis of the topology of Delta5-Des has provided a model that might be extended to most acyl-lipid desaturases. In addition, studies of the cold-induced expression of Delta5-Des led to the identification of a two-component system composed of a membrane-associated kinase, DesK, and a transcriptional regulator, DesR, which stringently controls the transcription of the des gene, coding for the desaturase. A model for sensing and transduction of low-temperature signals has emerged from our results, which we discuss in the context of transcriptional regulation of membrane lipid fluidity homeostasis.

The fatty acid profile of vegetative Azotobacter vinelandii ATCC 12837: growth phase-dependence

Fatty acids of Azotobacter vinelandii ATCC 12837 were determined at various times during aerobic vegetative growth at 30 degrees C to provide baseline data for studying the effects of chemical agents on the organism's survival and fatty acid biosynthesis. Palmitate (16:0) was the highest at 36.7+/-4.3 mol% (mean+/-SD) after the first 5 h in fresh culture, decreasing slightly to 33.4+/-2.6 mol% at 49 h. The other fatty acids were therefore each normalized as a ratio of 16:0. At 5 h, as a ratio of 16:0, myristate (14:0) was 0.14+/-0.06, palmitoleate (16:1cDelta9-10) 0.13+/-0.06, oleate (18:1cDelta9-10) 0.21+/-0.12, cis-vaccenate (18:1cDelta11-12) 0.30+/-0.17 and stearate (18:0) 0.68+/-0.02. As the growth phase advanced to 49 h, 14:0 and 16:1cDelta9-10 increased, 18:1cDelta9-10 decreased and cis-vaccenate reciprocally increased, whereas 18:0 decreased. These suggest that the saturated fatty acid biosynthesis pathway yielded 16:0 and 18:0 in the 5-h lag period. By desaturation, 18:0 formed the unsaturated fatty acid (UFA) 18:1cDelta9-10. As the culture aged, the anaerobic UFA biosynthesis pathway formed 16:1cDelta9-10, which was elongated to 18:1cDelta11-12. These fatty acid alterations represent a homeoviscous adaptation, modulating the microbe's membrane lipid viscosity for optimal cellular function.

Membrane topology analysis of cyclic glucan synthase, a virulence determinant of Brucella abortus

Brucella abortus cyclic glucan synthase (Cgs) is a 316-kDa (2,831-amino-acid) integral inner membrane protein that is responsible for the synthesis of cyclic beta-1,2-glucan by a novel mechanism in which the enzyme itself acts as a protein intermediate. B. abortus Cgs uses UDP-glucose as a sugar donor and has the three enzymatic activities necessary for synthesis of the cyclic polysaccharide (i.e., initiation, elongation, and cyclization). Cyclic glucan is required in B. abortus for effective host interaction and complete expression of virulence. To gain further insight into the structure and mechanism of action of B. abortus Cgs, we studied the membrane topology of the protein using a combination of in silico predictions, a genetic approach involving the construction of fusions between the cgs gene and the genes encoding alkaline phosphatase (phoA) and beta-galactosidase (lacZ), and site-directed chemical labeling of lysine residues. We found that B. abortus Cgs is a polytopic membrane protein with the amino and carboxyl termini located in the cytoplasm and with six transmembrane segments, transmembrane segments I (residues 419 to 441), II (residues 452 to 474), III (residues 819 to 841), IV (residues 847 to 869), V (residues 939 to 961), and VI (residues 968 to 990). The six transmembrane segments determine four large cytoplasmic domains and three very small periplasmic regions.