Characterization of the gene coding for GDP-mannose dehydrogenase (algD) from Azotobacter vinelandii

The Azotobacter vinelandii AlgU regulon during vegetative growth and encysting conditions: A proteomic approach

In the Pseduomonadacea family, the extracytoplasmic function sigma factor AlgU is crucial to withstand adverse conditions. Azotobacter vinelandii, a closed relative of Pseudomonas aeruginosa, has been a model for cellular differentiation in Gram-negative bacteria since it forms desiccation-resistant cysts. Previous work demonstrated the essential role of AlgU to withstand oxidative stress and on A. vinelandii differentiation, particularly for the positive control of alginate production. In this study, the AlgU regulon was dissected by a proteomic approach under vegetative growing conditions and upon encystment induction. Our results revealed several molecular targets that explained the requirement of this sigma factor during oxidative stress and extended its role in alginate production. Furthermore, we demonstrate that AlgU was necessary to produce alkyl resorcinols, a type of aromatic lipids that conform the cell membrane of the differentiated cell. AlgU was also found to positively regulate stress resistance proteins such as OsmC, LEA-1, or proteins involved in trehalose synthesis. A position-specific scoring-matrix (PSSM) was generated based on the consensus sequence recognized by AlgU in P. aeruginosa, which allowed the identification of direct AlgU targets in the A. vinelandii genome. This work further expands our knowledge about the function of the ECF sigma factor AlgU in A. vinelandii and contributes to explains its key regulatory role under adverse conditions.

Computational modeling of the molecular basis for the calcium-dependence of the mannuronan C-5 epimerase AvAlgE6 from Azotobacter vinelandii

The mannuronan C-5 epimerases catalyze epimerization of β-d-mannuronic acid to α-l-guluronic acid in alginate polymers. The seven extracellular Azotobacter vinelandii epimerases (AvAlgE1-7) are calcium-dependent, and calcium is essential for the structural integrity of their carbohydrate binding R-modules. Ca²⁺ is also found in the crystal structures of the A-modules, where it is suggested to play a structural role. In this study, the structure of the catalytic A-module of the A. vinelandii mannuronan C-5 epimerase AvAlgE6 is used to investigate the role of this Ca²⁺. Molecular dynamics (MD) simulations with and without calcium reveal the possible importance of the bound Ca²⁺ in the hydrophobic packing of β-sheets. In addition, a putative calcium binding site is found in the active site, indicating a potential direct role of this calcium in the catalysis. According to the literature, two of the residues coordinating calcium in this site are essential for the activity. MD simulations of the interaction with bound substrate indicate that the presence of a calcium ion in this binding site increases the binding strength. Further, explicit calculations of the substrate dissociation pathways with umbrella sampling simulations show and energetically higher dissociation barrier when calcium is present. The present study eludes to a putative catalytic role of calcium in the charge neutralizing first step of the enzymatic reaction. In addition to the importance for understanding these enzymes' molecular mechanisms, this could have implications for engineering strategies of the epimerases in industrial alginate processing.

Genetic Regulation of Alginate Production in Azotobacter vinelandii a Bacterium of Biotechnological Interest: A Mini-Review

Alginates are a family of polymers composed of guluronate and mannuronate monomers joined by β (1–4) links. The different types of alginates have variations in their monomer content and molecular weight, which determine the rheological properties and their applications. In industry, alginates are commonly used as additives capable of viscosifying, stabilizing, emulsifying, and gelling aqueous solutions. Recently, additional specialized biomedical uses have been reported for this polymer. Currently, the production of alginates is based on the harvesting of seaweeds; however, the composition and structure of the extracts are highly variable. The production of alginates for specialized applications requires a precise composition of monomers and molecular weight, which could be achieved using bacterial production systems such as those based on Azotobacter vinelandii, a free-living, non-pathogenic bacterium. In this mini-review, we analyze the latest advances in the regulation of alginate synthesis in this model.

Insights into the roles of charged residues in substrate binding and mode of action of mannuronan C-5 epimerase AlgE4

Mannuronan C-5 epimerases catalyse the epimerization of monomer residues in the polysaccharide alginate, changing the physical properties of the biopolymer. The enzymes are utilized to tailor alginate to numerous biological functions by alginate-producing organisms. The underlying molecular mechanisms that control the processive movement of the epimerase along the substrate chain is still elusive. To study this, we have used an interdisciplinary approach combining molecular dynamics simulations with experimental methods from mutant studies of AlgE4, where initial epimerase activity and product formation were addressed with NMR spectroscopy, and characteristics of enzyme-substrate interactions were obtained with isothermal titration calorimetry and optical tweezers. Positive charges lining the substrate-binding groove of AlgE4 appear to control the initial binding of poly-mannuronate, and binding also seems to be mediated by both electrostatic and hydrophobic interactions. After the catalytic reaction, negatively charged enzyme residues might facilitate dissociation of alginate from the positive residues, working like electrostatic switches, allowing the substrate to translocate in the binding groove. Molecular simulations show translocation increments of two monosaccharide units before the next productive binding event resulting in MG-block formation, with the epimerase moving with its N-terminus towards the reducing end of the alginate chain. Our results indicate that the charge pair R343-D345 might be directly involved in conformational changes of a loop that can be important for binding and dissociation. The computational and experimental approaches used in this study complement each other, allowing for a better understanding of individual residues' roles in binding and movement along the alginate chains.

Functional characterization of three Azotobacter chroococcum alginate-modifying enzymes related to the Azotobacter vinelandii AlgE mannuronan C-5-epimerase family

Bacterial alginate initially consists of 1–4-linked β-D-mannuronic acid residues (M) which can be later epimerized to α-L-guluronic acid (G). The family of AlgE mannuronan C-5-epimerases from Azotobacter vinelandii has been extensively studied, and three genes putatively encoding AlgE-type epimerases have recently been identified in the genome of Azotobacter chroococcum. The three A. chroococcum genes, here designated AcalgE1, AcalgE2 and AcalgE3, were recombinantly expressed in Escherichia coli and the gene products were partially purified. The catalytic activities of the enzymes were stimulated by the addition of calcium ions in vitro. AcAlgE1 displayed epimerase activity and was able to introduce long G-blocks in the alginate substrate, preferentially by attacking M residues next to pre-existing G residues. AcAlgE2 and AcAlgE3 were found to display lyase activities with a substrate preference toward M-alginate. AcAlgE2 solely accepted M residues in the positions − 1 and + 2 relative to the cleavage site, while AcAlgE3 could accept either M or G residues in these two positions. Both AcAlgE2 and AcAlgE3 were bifunctional and could also catalyze epimerization of M to G. Together, we demonstrate that A. chroococcum encodes three different AlgE-like alginate-modifying enzymes and the biotechnological and biological impact of these findings are discussed.

RpoS controls the expression and the transport of the AlgE1-7 epimerases in Azotobacter vinelandii

Azotobacter vinelandii produces differentiated cells, called cysts, surrounded by two alginate layers, which are necessary for their desiccation resistance. This alginate contains variable proportions of guluronate residues, resulting from the activity of seven extracytoplasmic epimerases, AlgE1-7. These enzymes are exported by a system secretion encoded by the eexDEF operon; mutants lacking the AlgE1-7 epimerases, the EexDEF or the RpoS sigma factor produce alginate, but are unable to form desiccation resistant cysts. Herein, we found that RpoS was required for full transcription of the algE1-7 and eexDEF genes. We found that the AlgE1-7 protein levels were diminished in the rpoS mutant strain. In addition, the alginate produced in the absence of RpoS was more viscous in the presence of proteases, a phenotype similar to that of the eexD mutant. Primer extension analysis located two promoters for the eexDEF operon, one of them was RpoS-dependent. Thus, during encysting conditions, RpoS coordinates the expression of both the AlgE1-7 epimerases and the EexDEF protein complex responsible for their transport.

[Development of a liquid fermentation system and encystment for a nitrogen-fixing bacterium strain having biofertilizer potential]

The indiscriminate use of chemical fertilizers has contributed to the deterioration of the biological, physical and chemical properties of the soil, resulting in the loss of its productive capacity. For this reason, the use of biofertilizers has emerged as a technological alternative. The objective of this research was to develop a suitable liquid fermentation system and encystment for the multiplication of Azotobacter chroococcum AC1 strain, a bacterium employed in a biofertilizer formulation produced at present by CARPOICA, Colombia. Sequential statistical designs were used to determine the conditions in the fermentation system. The interaction between agitation, aeration and pH was evaluated on the viable biomass (CFU/ml) of AC1. In addition, the encystment ability of the strain was evaluated using two encystment agents and the potential plant growth-promoting rhizobacteria (PGPR) activity was assessed by different techniques, such as nitrogen fixation by ARA, phosphate solubilization by the phospho-molybdenum-blue reaction and indolic compound production by colorimetric reaction using the Salkowski reagent. Results showed significant effects (p<0.05) on the viable biomass in the three conditions (pH, aeration and agitation) tested individually, in one dual interaction and one tripartite interaction, were demonstrated to have a positive effect on the response variable aeration and agitation. The addition of the two encystment agents evaluated, AE01 and AE02, demonstrated the ability of AC1 to form cysts under stress conditions. Likewise, fermentation and encystment conditions did not affect the biological activities tested.

The signaling protein MucG negatively affects the production and the molecular mass of alginate in Azotobacter vinelandii

Azotobacter vinelandii is a soil bacterium that produces the polysaccharide alginate. In this work, we identified a miniTn5 mutant, named GG9, which showed increased alginate production of higher molecular mass, and increased expression of the alginate biosynthetic genes algD and alg8 when compared to its parental strain. The miniTn5 was inserted within ORF Avin07920 encoding a hypothetical protein. Avin07910, located immediately downstream and predicted to form an operon with Avin07920, encodes an inner membrane multi-domain signaling protein here named mucG. Insertional inactivation of mucG resulted in a phenotype of increased alginate production of higher molecular mass similar to that of mutant GG9. The MucG protein contains a periplasmic and putative HAMP and PAS domains, which are linked to GGDEF and EAL domains. The last two domains are potentially involved in the synthesis and degradation, respectively, of bis-(3'-5')-cyclic dimeric GMP (c-di-GMP), a secondary messenger that has been reported to be essential for alginate production. Therefore, we hypothesized that the negative effect of MucG on the production of this polymer could be explained by the putative phosphodiesterase activity of the EAL domain. Indeed, we found that alanine replacement mutagenesis of the MucG EAL motif or deletion of the entire EAL domain resulted in increased alginate production of higher molecular mass similar to the GG9 and mucG mutants. To our knowledge, this is the first reported protein that simultaneous affects the production of alginate and its molecular mass.

Alginate-modifying enzymes: biological roles and biotechnological uses

Alginate denotes a group of industrially important 1-4-linked biopolymers composed of the C-5-epimers β-D-mannuronic acid (M) and α-L-guluronic acid (G). The polysaccharide is manufactured from brown algae where it constitutes the main structural cell wall polymer. The physical properties of a given alginate molecule, e.g., gel-strength, water-binding capacity, viscosity and biocompatibility, are determined by polymer length, the relative amount and distribution of G residues and the acetyl content, all of which are controlled by alginate modifying enzymes. Alginate has also been isolated from some bacteria belonging to the genera Pseudomonas and Azotobacter, and bacterially synthesized alginate may be O-acetylated at O-2 and/or O-3. Initially, alginate is synthesized as polymannuronic acid, and some M residues are subsequently epimerized to G residues. In bacteria a mannuronan C-5-epimerase (AlgG) and an alginate acetylase (AlgX) are integral parts of the protein complex necessary for alginate polymerization and export. All alginate-producing bacteria use periplasmic alginate lyases to remove alginate molecules aberrantly released to the periplasm. Alginate lyases are also produced by organisms that utilize alginate as carbon source. Most alginate-producing organisms encode more than one mannuronan C-5 epimerase, each introducing its specific pattern of G residues. Acetylation protects against further epimerization and from most alginate lyases. An enzyme from Pseudomonas syringae with alginate deacetylase activity has been reported. Functional and structural studies reveal that alginate lyases and epimerases have related enzyme mechanisms and catalytic sites. Alginate lyases are now utilized as tools for alginate characterization. Secreted epimerases have been shown to function well in vitro, and have been engineered further in order to obtain enzymes that can provide alginates with new and desired properties for use in medical and pharmaceutical applications.

Enzymatic modifications of exopolysaccharides enhance bacterial persistence

Biofilms are surface-attached communities of bacterial cells embedded in a self-produced matrix that are found ubiquitously in nature. The biofilm matrix is composed of various extracellular polymeric substances, which confer advantages to the encapsulated bacteria by protecting them from eradication. The matrix composition varies between species and is dependent on the environmental niche that the bacteria inhabit. Exopolysaccharides (EPS) play a variety of important roles in biofilm formation in numerous bacterial species. The ability of bacteria to thrive in a broad range of environmental settings is reflected in part by the structural diversity of the EPS produced both within individual bacterial strains as well as by different species. This variability is achieved through polymerization of distinct sugar moieties into homo- or hetero-polymers, as well as post-polymerization modification of the polysaccharide. Specific enzymes that are unique to the production of each polymer can transfer or remove non-carbohydrate moieties, or in other cases, epimerize the sugar units. These modifications alter the physicochemical properties of the polymer, which in turn can affect bacterial pathogenicity, virulence, and environmental adaptability. Herein, we review the diversity of modifications that the EPS alginate, the Pel polysaccharide, Vibrio polysaccharide, cepacian, glycosaminoglycans, and poly-N-acetyl-glucosamine undergo during biosynthesis. These are EPS produced by human pathogenic bacteria for which studies have begun to unravel the effect modifications have on their physicochemical and biological properties. The biological advantages these polymer modifications confer to the bacteria that produce them will be discussed. The expanding list of identified modifications will allow future efforts to focus on linking these modifications to specific biosynthetic genes and biofilm phenotypes.

Detection of Pseudomonas aeruginosa by a triplex polymerase chain reaction assay based on lasI/R and gyrB genes

Pseudomonas aeruginosa is a nosocomial pathogen, which, due to its inherent and acquired resistance to a wide range of antibiotics, causes high mortality rates. Therefore, rapid detection of the bacterium with high specificity and sensitivity plays a critical role in the control of the pathogenic bacterium. The aim of this study was to evaluate the accuracy and specificity of a prompt detection of the bacterium based on a triplex polymerase chain reaction that amplifies the lasI, lasR and gyrB genes. For this purpose, 30 clinical isolates of P. aeruginosa and 30 wound biopsy samples were retrieved from clinical diagnostic laboratories. After the extraction of the chromosomal DNA, the desired genes were amplified using uniplex and triplex PCR with appropriate primers. The specificity of the primers was evaluated by a comparison of the PCR results for P. aeruginosa clinical samples and non-Pseudomonas species control samples. The sensitivity of the primers was determined using a serial dilution of the genomic DNA template (100 ng to 100 fg) and by a comparison of the PCR and bacterial culture results. The results showed that the triplex PCR assay was positive for all of the samples (100%), while the PCR identifications were negative for non-Pseudomonas species. Additionally, at 10(-4) and 10(-5) diluted genomic DNA from P. aeruginosa (10 pg and 1 pg), the triplex PCR test was positive for the Las and gyrB genes in all of the samples, respectively. Based on these results, the designed primers can be used for the rapid, specific and sensitive diagnosis of P. aeruginosa in a triplex PCR assay.

A small heat-shock protein (Hsp20) regulated by RpoS is essential for cyst desiccation resistance in Azotobacter vinelandii

In Azotobacter vinelandii, a cyst-forming bacterium, the alternative sigma factor RpoS is essential to the formation of cysts resistant to desiccation and to synthesis of the cyst-specific lipids, alkylresorcinols. In this study, we carried out a proteome analysis of vegetative cells and cysts of A. vinelandii strain AEIV and its rpoS mutant derivative AErpoS. This analysis allowed us to identify a small heat-shock protein, Hsp20, as one of the most abundant proteins of cysts regulated by RpoS. Inactivation of hsp20 did not affect the synthesis of alkylresorcinols or the formation of cysts with WT morphology; however, the cysts formed by the hsp20 mutant strain were unable to resist desiccation. We also demonstrated that expression of hsp20 from an RpoS-independent promoter in the AErpoS mutant strain is not enough to restore the phenotype of resistance to desiccation. These results indicate that Hsp20 is essential for the resistance to desiccation of A. vinelandii cysts, probably by preventing the aggregation of proteins caused by the lack of water. To our knowledge, this is the first report of a small heat-shock protein that is essential for desiccation resistance in bacteria.

Alginate synthesis in Azotobacter vinelandii is increased by reducing the intracellular production of ubiquinone

Azotobacter vinelandii, a soil nitrogen fixing bacterium, produces alginate a polysaccharide with industrial and medical relevant applications. In this work, we characterized a miniTn5 mutant, named GG101, that showed a 14-fold increase in the specific production of alginate when grown diazotrophically on solid minimal medium comparing to the parental E strain (also named AEIV). Quantitative real-time reverse transcription PCR analysis indicated that this increased alginate production was due to higher expression levels of several biosynthetic alg genes such as algD. Sequencing of the locus interrupted in GG101 indicated that the miniTn5 was inserted in the positive strand, and 10 bp upstream the start codon of the gene ubiA, encoding the enzyme for the second step in the biosynthesis of ubiquinone (Q8). Both the transcription of ubiA and the content of Q8 are decreased in the mutant GG101 when compared to the wild-type strain E. Genetic complementation of mutant GG101 with a wild-type copy of the ubiCA genes restored the content of Q8 and reduced the production of alginate to levels similar to those of the parental E strain. Furthermore, respirometric analysis showed a reproducible decrease of about 8 % in the respiratory capacity of mutant GG101, at exponential phase of growth in liquid minimal medium. Collectively, our data show that a decreased content in Q8 results in higher levels of alginate in A. vinelandii.

Production of glucuronic acid-based polysaccharides by microbial fermentation for biomedical applications

This review provides an overview of the properties, different biosynthetic machineries, and biotechnological production processes of four microbially derived glucuronic acid-based polysaccharides that are of interest for diverse biomedical purposes. In particular, the utilization of hyaluronic acid and heparin sulfate in high-value medical applications is already well established, whereas chondroitin sulfate and alginate show high potential within this ever-growing field. Furthermore, new strategies exploiting genetically engineered microorganisms generated through improving naturally existing pathways or de novo designed ones are described. These new developments result in increased fermentation titers, and thereby, pave the way towards feasible, or at least improved, process economy. Moreover, these strategies also allow for the future possibility of producing tailor-made biopolymers with specified characteristics, even novel molecules.

Roles of RpoS and PsrA in cyst formation and alkylresorcinol synthesis in Azotobacter vinelandii

Azotobacter vinelandii is a soil bacterium that undergoes differentiation to form cysts that are resistant to desiccation. Upon induction of cyst formation, the bacterium synthesizes alkylresorcinols that are present in cysts but not in vegetative cells. Alternative sigma factors play important roles in differentiation. In A. vinelandii, AlgU (sigma E) is involved in controlling the loss of flagella upon induction of encystment. We investigated the involvement of the sigma factor RpoS in cyst formation in A. vinelandii. We analysed the transcriptional regulation of the rpoS gene by PsrA, the main regulator of rpoS in Pseudomonas species, which are closely related to A. vinelandii. Inactivation of rpoS resulted in the inability to form cysts resistant to desiccation and to produce cyst-specific alkylresorcinols, whereas inactivation of psrA reduced by 50 % both production of alkylresorcinols and formation of cysts resistant to desiccation. Electrophoretic mobility shift assays revealed specific binding of PsrA to the rpoS promoter region and that inactivation of psrA reduced rpoS transcription by 60 %. These results indicate that RpoS and PsrA are involved in regulation of encystment and alkylresorcinol synthesis in A. vinelandii.

Two-stage fermentation process for alginate production by Azotobacter vinelandii mutant altered in poly-beta-hydroxybutyrate (PHB) synthesis

AIMS

A two-stage fermentation strategy, based on batch cultures conducted first under non-oxygen-limited conditions, and later under oxygen-limited conditions, was used to improve alginate production by Azotobacter vinelandii (AT6), a strain impaired in poly-beta-hydroxybutyrate (PHB) production.

METHODS AND RESULTS

The use of sucrose as carbon source, as well as a high oxygen concentration (10%), allowed to obtain a maximum biomass concentration of 7.5 g l(-1) in the first stage of cultivation. In the second stage, the cultures were limited by oxygen (oxygen close to 0%) and fed with a sucrose solution at high concentration. Under those conditions, the growth rate decreased considerably and the cells used the carbon source mainly for alginate biosynthesis, obtaining a maximum concentration of 9.5 g l(-1), after 50 h of cultivation.

CONCLUSION

Alginate concentration obtained from the AT6 strain was two times higher than that obtained using the wild-type strain (ATCC 9046) and was the highest reported in the literature. However, the mean molecular mass of the alginate produced in the second stage of the process by the mutant AT6 was lower (400 kDa) than the polymer molecular mass obtained from the cultures developed with the parental strain (950 kDa).

SIGNIFICANCE AND IMPACT OF THE STUDY

The use of a mutant of A. vinelandii impaired in the PHB production in combination with a two-stage fermentation process could be a feasible strategy for the production of alginate at industrial level.

Characterization of three new Azotobacter vinelandii alginate lyases, one of which is involved in cyst germination

Alginates are polysaccharides composed of 1-4-linked beta-D-mannuronic acid and alpha-L-guluronic acid. The polymer can be degraded by alginate lyases, which cleave the polysaccharide using a beta-elimination reaction. Two such lyases have previously been identified in the soil bacterium Azotobacter vinelandii, as follows: the periplasmic AlgL and the secreted bifunctional mannuronan C-5 epimerase and alginate lyase AlgE7. In this work, we describe the properties of three new lyases from this bacterium, AlyA1, AlyA2, and AlyA3, all of which belong to the PL7 family of polysaccharide lyases. One of the enzymes, AlyA3, also contains a C-terminal module similar to those of proteins secreted by a type I secretion system, and its activity is stimulated by Ca(2+). All three enzymes preferably cleave the bond between guluronic acid and mannuronic acid, resulting in a guluronic acid residue at the new reducing end, but AlyA3 also degrades the other three possible bonds in alginate. Strains containing interrupted versions of alyA1, alyA3, and algE7 were constructed, and their phenotypes were analyzed. Genetically pure alyA2 mutants were not obtained, suggesting that this gene product may be important for the bacterium during vegetative growth. After centrifugation, cultures from the algE7 mutants form a large pellet containing alginate, indicating that AlgE7 is involved in the release of alginate from the cells. Upon encountering adverse growth conditions, A. vinelandii will form a resting stage called cyst. Alginate is a necessary part of the protective cyst coat, and we show here that strains lacking alyA3 germinate poorly compared to wild-type cells.

Genome sequence of Azotobacter vinelandii, an obligate aerobe specialized to support diverse anaerobic metabolic processes

Azotobacter vinelandii is a soil bacterium related to the Pseudomonas genus that fixes nitrogen under aerobic conditions while simultaneously protecting nitrogenase from oxygen damage. In response to carbon availability, this organism undergoes a simple differentiation process to form cysts that are resistant to drought and other physical and chemical agents. Here we report the complete genome sequence of A. vinelandii DJ, which has a single circular genome of 5,365,318 bp. In order to reconcile an obligate aerobic lifestyle with exquisitely oxygen-sensitive processes, A. vinelandii is specialized in terms of its complement of respiratory proteins. It is able to produce alginate, a polymer that further protects the organism from excess exogenous oxygen, and it has multiple duplications of alginate modification genes, which may alter alginate composition in response to oxygen availability. The genome analysis identified the chromosomal locations of the genes coding for the three known oxygen-sensitive nitrogenases, as well as genes coding for other oxygen-sensitive enzymes, such as carbon monoxide dehydrogenase and formate dehydrogenase. These findings offer new prospects for the wider application of A. vinelandii as a host for the production and characterization of oxygen-sensitive proteins.

Isolation and characterization of Azotobacter vinelandii mutants impaired in alkylresorcinol synthesis: alkylresorcinols are not essential for cyst desiccation resistance

During encystment of Azotobacter vinelandii, a family of alkylresorcinols (ARs) and alkylpyrones (APs) are synthesized. In the mature cyst, these lipids replace the membrane phospholipids and are also components of the layers covering the cyst. In this study, A. vinelandii strains unable to synthesize ARs were isolated after mini-Tn5 mutagenesis. Cloning and nucleotide sequencing of the affected loci revealed the presence of the transposons within the arsA gene of the previously reported arsABCD gene cluster, which encodes a type I fatty acid synthase. A mutant strain (SW-A) carrying an arsA mutation allowing transcription of arsBCD was constructed and shown to be unable to produce ARs, indicating that the ArsA protein is essential for the synthesis of these phenolic lipids. Transcription of arsA was induced 200-fold in cells undergoing encystment, but only 14-fold in aged cultures of A. vinelandii, in accordance with AR synthesis and cyst formation percentages under the two conditions. Although it was previously reported that the inactivation of arsB abolishes AR synthesis and results in a failure in encystment, the arsA mutants were able to form cysts resistant to desiccation. These data indicate that ARs play a structural role in the exine layer of the cysts, but they are not essential for either cyst formation or for desiccation resistance.

The Na+-translocating NADH : ubiquinone oxidoreductase of Azotobacter vinelandii negatively regulates alginate synthesis

Azotobacter vinelandii is a nitrogen-fixing soil bacterium that produces the exopolysaccharide alginate. In this report we describe the isolation and characterization of A. vinelandii strain GG4, which carries an nqrE : : Tn5 mutation resulting in alginate overproduction. The nqrE gene encodes a subunit of the Na+-translocating NADH : ubiquinone oxidoreductase (Na+-NQR). As expected, Na+-NQR activity was abolished in mutant GG4. When this strain was complemented with the nqrEF genes this activity was restored and alginate production was reduced to wild-type levels. Na+-NQR may be the main sodium pump of A. vinelandii under the conditions tested (∼2 mM Na+) since no Na+/H+-antiporter activity was detected. Collectively our results indicate that in A. vinelandii the lack of Na+-NQR activity caused the absence of a transmembrane Na+ gradient and an increase in alginate production.

Encystment of Azotobacter nigricans grown diazotrophically on kerosene as sole carbon source

Encystment of Azotobacter nigricans was induced by its diazotrophic cultivation on kerosene. Its growth and nitrogenase activity were affected by kerosene in comparison to cultures grown on sucrose. Electron microscopy of vegetative cells showed that when nitrogenase activity was higher and the poly-beta-hydroxybutyrate granules were not present to a significant extent, peripheral bodies were abundant. After 8 days of culture on kerosene, the presence of cysts with intracellular bunches of poly-beta-hydroxybutyrate granules was observed. Germination of cysts bears germinating multicelled yet unbroken capsule cysts with up to three cells inside. This is the first report of encystment induction of Azotobacter species grown on kerosene.

flhDC, but not fleQ, regulates flagella biogenesis in Azotobacter vinelandii, and is under AlgU and CydR negative control

Azotobacter vinelandii is a nitrogen-fixing soil bacterium that undergoes differentiation to form cysts resistant to desiccation. Upon encystment, this bacterium becomes non-motile. As in enteric bacteria, motility in A. vinelandii occurs through the use of peritrichous flagella. Pseudomonas aeruginosa, a phylogenetically close relative of A. vinelandii, possesses a single polar flagellum. The FlhDC proteins are the master regulators of flagella and motility in enterobacteria, whereas FleQ is the master regulator in P. aeruginosa, and it is under AlgU (sigmaE) negative control. At present, nothing is known about the organization and expression of flagella genes in A. vinelandii. Here, we identified the flagella gene cluster of this bacterium. Homologues of the master regulatory genes flhDC and fleQ are present in A. vinelandii. Inactivation of flhDC, but not fleQ, impaired flagella biogenesis and motility. We present evidence indicating that a negative effect of the AlgU sigma factor on flhDC expression causes loss of motility in A. vinelandii, and that CydR (a homologue of Fnr) is under AlgU control and has a negative effect on flhDC expression. Taken together, these results suggest the existence of a cascade consisting of AlgU and CydR that negatively controls expression of flhDC; the results also suggest that the block in flagella synthesis under encystment conditions centres on flhDC repression by the AlgU–CydR cascade.

Structural and mutational characterization of the catalytic A-module of the mannuronan C-5-epimerase AlgE4 from Azotobacter vinelandii

Alginate is a family of linear copolymers of (1-->4)-linked beta-d-mannuronic acid and its C-5 epimer alpha-l-guluronic acid. The polymer is first produced as polymannuronic acid and the guluronic acid residues are then introduced at the polymer level by mannuronan C-5-epimerases. The structure of the catalytic A-module of the Azotobacter vinelandii mannuronan C-5-epimerase AlgE4 has been determined by x-ray crystallography at 2.1-A resolution. AlgE4A folds into a right-handed parallel beta-helix structure originally found in pectate lyase C and subsequently in several polysaccharide lyases and hydrolases. The beta-helix is composed of four parallel beta-sheets, comprising 12 complete turns, and has an amphipathic alpha-helix near the N terminus. The catalytic site is positioned in a positively charged cleft formed by loops extending from the surface encompassing Asp(152), an amino acid previously shown to be important for the reaction. Site-directed mutagenesis further implicates Tyr(149), His(154), and Asp(178) as being essential for activity. Tyr(149) probably acts as the proton acceptor, whereas His(154) is the proton donor in the epimerization reaction.

Genetics of bacterial alginate: alginate genes distribution, organization and biosynthesis in bacteria

Bacterial alginate genes are chromosomal and fairly widespread among rRNA homology group I Pseudomonads and Azotobacter. In both genera, the genetic pathway of alginate biosynthesis is mostly similar and the identified genes are identically organized into biosynthetic, regulatory and genetic switching clusters. In spite of these similarities,still there are transcriptional and functional variations between P. aeruginosa and A. vinelandii. In P. aeruginosa all biosynthetic genes except algC transcribe in polycistronic manner under the control of algD promoter while in A. vinelandii, these are organized into many transcriptional units. Of these, algA and algC are transcribed each from two different and algD from three different promoters. Unlike P. aeruginosa, the promoters of these transcriptional units except one of algC and algD are algT-independent. Both bacterial species carry homologous algG gene for Ca(2+)-independent epimerization. But besides algG, A. vinelandii also has algE1-7 genes which encode C-5-epimerases involved in the complex steps of Ca(2+)-dependent epimerization. A hierarchy of alginate genes expression under sigma(22)(algT) control exists in P. aeruginosa where algT is required for transcription of the response regulators algB and algR, which in turn are necessary for expression of algD and its downstream biosynthetic genes. Although algTmucABCD genes cluster play similar regulatory roles in both P. aeruginosa and A. vinelandii but unlike, transcription of A. vinelandii, algR is independent of sigma(22). These differences could be due to the fact that in A. vinelandii alginate plays a role as an integrated part in desiccation-resistant cyst which is not found in P. aeruginosa.

Identification of salt stress inducible genes that control cell envelope related functions in Azospirillum brasilense Sp7

Plant growth promoting rhizobacteria such as Azospirillum brasilense are agronomically important as they are frequently used for crop inoculation. But adverse factors such as increasing soil salinity limit their survival, multiplication and phytostimulatory effect. In order to understand the role of the genes involved in the adaptation of A. brasilense Sp7 to salt stress, a mutant library (6,800 mutants) was constructed after random integration of a mini-Transposon Tn5 derivative containing a promoterless gusA and oriV. The library was screened for salt stress inducible Gus activity on minimal malate agar medium containing NaCl and 5-bromo-4-chloro-3-indolyl-beta-D: -glucuronide. Salt stress responsiveness of the promoters was estimated by quantifying GusA activity in the presence and absence of NaCl stress using p-nitrophenyl-beta-D: -glucuronide as a substrate. In 11 mutants showing high levels of gusA expression in the presence of salt-stress, the partial nucleotide sequence of the DNA region flanking the site of Tn5 insertion was determined and analysed using the NCBI-BLAST programs. Similarity searches revealed that 10 out of the 11 genes sequenced showed notable similarity with genes involved in functions related to modulation in the composition of exopolysaccharides, capsular polysaccharides, lipopolysaccharides, peptidoglycan and lipid bilayer of the cell envelope. Induction of cell envelope related genes in response to salt stress and salt sensitive phenotype of several mutants in A. brasilense indicate a prominent role of cell envelope in salt-stress adaptation.

Molecular and bioengineering strategies to improve alginate and polydydroxyalkanoate production by Azotobacter vinelandii

Several aspects of alginate and PHB synthesis in Azotobacter vinelandii at a molecular level have been elucidated in articles published during the last ten years. It is now clear that alginate and PHB synthesis are under a very complex genetic control. Genetic modification of A. vinelandii has produced a number of very interesting mutants which have particular traits for alginate production. One of these mutants has been shown to produce the alginate with the highest mean molecular mass so far reported. Recent work has also shed light on the factors determining molecular mass distribution; the most important of these being identified as; dissolved oxygen tension and specific growth rate. The use of specific mutants has been very useful for the correct analysis and interpretation of the factors affecting polymerization. Recent scale-up/down work on alginate production has shown that oxygen limitation is crucial for producing alginate of high molecular mass, a condition which is optimized in shake flasks and which can now be reproduced in stirred fermenters. It is clear that the phenotypes of mutants grown on plates are not necessarily reproducible when the strains are tested in lab or bench scale fermenters. In the case of PHB, A. vinelandii has shown itself able to produce relatively large amounts of this polymer of high molecular weight on cheap substrates, even allowing for simple extraction processes. The development of fermentation strategies has also shown promising results in terms of improving productivity. The understanding of the regulatory mechanisms involved in the control of PHB synthesis, and of its metabolic relationships, has increased considerably, making way for new potential strategies for the further improvement of PHB production. Overall, the use of a multidisciplinary approach, integrating molecular and bioengineering aspects is a necessity for optimizing alginate and PHB production in A. vinelandii.

Identification and characterization of an Azotobacter vinelandii type I secretion system responsible for export of the AlgE-type mannuronan C-5-epimerases

Alginate is a linear copolymer of beta-d-mannuronic acid and its C-5-epimer, alpha-l-guluronic acid. During biosynthesis, the polymer is first made as mannuronan, and various fractions of the monomers are then epimerized to guluronic acid by mannuronan C-5-epimerases. The Azotobacter vinelandii genome encodes a family of seven extracellular such epimerases (AlgE1 to AlgE7) which display motifs characteristic for proteins secreted via a type I pathway. Putative ATPase-binding cassette regions from the genome draft sequence of the A. vinelandii OP strain and experimentally verified type I transporters from other species were compared. This analysis led to the identification of one putative A. vinelandii type I system (eexDEF). The corresponding genes were individually disrupted in A. vinelandii strain E, and Western blot analysis using polyclonal antibodies against all AlgE epimerases showed that these proteins were present in wild-type culture supernatants but absent from the eex mutant supernatants. Consistent with this, the wild-type strain and the eex mutants produced alginate with about 20% guluronic acid and almost pure mannuronan (< or =2% guluronic acid), respectively. The A. vinelandii wild type is able to enter a particular desiccation-tolerant resting stage designated cyst. At this stage, the cells are surrounded by a rigid coat in which alginate is a major constituent. Such a coat was formed by wild-type cells in a particular growth medium but was missing in the eex mutants. These mutants were also found to be unable to survive desiccation. The reason for this is probably that continuous stretches of guluronic acid residues are needed for alginate gel formation to take place.

Bacterial alginates: from biosynthesis to applications

Alginate is a polysaccharide belonging to the family of linear (unbranched), non-repeating copolymers, consisting of variable amounts of beta-D-mannuronic acid and its C5-epimer alpha- L-guluronic acid linked via beta-1,4-glycosidic bonds. Like DNA, alginate is a negatively charged polymer, imparting material properties ranging from viscous solutions to gel-like structures in the presence of divalent cations. Bacterial alginates are synthesized by only two bacterial genera, Pseudomonas and Azotobacter, and have been extensively studied over the last 40 years. While primarily synthesized in form of polymannuronic acid, alginate undergoes chemical modifications comprising acetylation and epimerization, which occurs during periplasmic transfer and before final export through the outer membrane. Alginate with its unique material properties and characteristics has been increasingly considered as biomaterial for medical applications. The genetic modification of alginate producing microorganisms could enable biotechnological production of new alginates with unique, tailor-made properties, suitable for medical and industrial applications.

Hypercyst mutants in Rhodospirillum centenum identify regulatory loci involved in cyst cell differentiation

Rhodospirillum centenum is a purple photosynthetic bacterium that forms resting cyst cells when starved for nutrients. In this study, we demonstrate that chalcone synthase gene (chsA) expression is developmentally regulated, with expression of chsA increasing up to 86-fold upon induction of the cyst developmental cycle. Screening for mini-Tn5-induced mutants that exhibit elevated chsA::lacZ expression has led to the isolation of a set of R. centenum mutants that display increased chsA gene expression concomitant with constitutive induction of the cyst developmental cycle. These "hypercyst" mutants have lost the ability to regulate cyst cell formation in response to nutrient availability. Sequence analysis indicates that the mini-Tn5-disrupted genes code for a variety of factors, including metabolic enzymes and a large set of potential regulatory factors, including four gene products with homology to histidine sensor kinases and three with homology to response regulators. Several of the disrupted genes also have sequence similarity to che-like signal transduction components.

Construction and analyses of hybrid Azotobacter vinelandii mannuronan C-5 epimerases with new epimerization pattern characteristics

The secreted mannuronan C-5 epimerases from Azotobacter vinelandii form a family of seven homologous modular type enzymes, which appear to have evolved through duplications and point mutations in the individual modules. The catalytic A modules of these enzymes are responsible for generating the characteristic sequence distribution patterns of G residues in the industrially important polymer alginate by epimerizing M (beta-D-mannuronic acid) moieties to G (alpha-L-guluronic acid). Forty-six different hybrid enzymes were constructed by exchanging parts of the sequences encoding the A modules of AlgE2 (generates consecutive stretches of G residues) and AlgE4 (generates alternating structures). These hybrid enzymes introduce a variety of new monomer-sequence patterns into their substrates, and some regions important for the subsite specificity or processivity of the enzymes were identified. By using time-resolved NMR spectroscopy, it became clear that the rates for introducing alternating structures and consecutive stretches of G residues are different for each enzyme, and that it is the ratio between these rates that determines the overall epimerization pattern. These findings open up new possibilities in biotechnology and in studies of the many biological functions of alginates.

Encystment and alkylresorcinol production by Azotobacter vinelandii strains impaired in poly-beta-hydroxybutyrate synthesis

The lipids poly-beta-hydroxybutyrate (PHB) and alkylresorcinols are the major metabolic products of Azotobacter vinelandii cysts. Cysts are formed in less than 0.01% of late stationary phase cells grown on sucrose. Culturing vegetative cells in n-butanol or beta-hydroxybutyrate induces encystment. After induction of encystment, PHB rapidly accumulates in large granules. Then, the cells begin the synthesis of alkylresorcinols that replace the phospholipids in the membranes and are components of the exine, the outer layer of the cyst envelope. Vegetative cells do not synthesize alkylresorcinols. We report here the effect of mutations in the phbBAC operon, coding for the enzymes of the PHB biosynthetic pathway, on the synthesis of alkylresorcinols and cyst formation. The phb mutations did not impair the capacity to form mature cysts. However, the cysts formed by these strains posses a thicker exine layer and a higher content of alkylresorcinols than the cysts formed by the wild-type strain. A blockage of PHB synthesis caused by phb mutations resulted in the synthesis of alkylresorcinols and encystment even under non-inducing conditions. We propose that, as a consequence of the blockage in the PHB biosynthetic pathway, the acetyl-CoA and reducing power pools are increased causing the shift to lipid metabolism required for the synthesis of alkylresorcinols and cyst formation.

The global regulators GacA and sigma(S) form part of a cascade that controls alginate production in Azotobacter vinelandii

Transcription of the Azotobacter vinelandii algD gene, which encodes GDP-mannose dehydrogenase (the rate-limiting enzyme of alginate synthesis), starts from three sites: p1, p2, and p3. The sensor kinase GacS, a member of the two-component regulatory system, is required for transcription of algD from its three sites during the stationary phase. Here we show that algD is expressed constitutively throughout the growth cycle from the p2 and p3 sites and that transcription from p1 started at the transition between the exponential growth phase and stationary phase. We constructed A. vinelandii strains that carried mutations in gacA encoding the cognate response regulator of GacS and in rpoS coding for the stationary-phase sigma(S) factor. The gacA mutation impaired alginate production and transcription of algD from its three promoters. Transcription of rpoS was also abolished by the gacA mutation. The rpoS mutation impaired transcription of algD from the p1 promoter and increased it from the p2 sigma(E) promoter. The results of this study provide evidence for the predominant role of GacA in a regulatory cascade controlling alginate production and gene expression during the stationary phase in A. vinelandii.

Azotobacter vinelandii aldehyde dehydrogenase regulated by sigma(54): role in alcohol catabolism and encystment

Encystment in Azotobacter vinelandii is induced by n-butanol or beta-hydroxybutyrate (BHB). We identified a gene, encoding an aldehyde dehydrogenase, that was named aldA. An aldA mutation impaired bacterial growth on n-butanol, ethanol, or hexanol as the sole carbon source. Expression of aldA increased in cells shifted from sucrose to n-butanol and was shown to be dependent on the alternative sigma(54) factor. A mutation in rpoN encoding the sigma(54) factor also impaired growth on alcohols. Encystment on n-butanol, but not on BHB, was impaired in aldA or rpoN mutants, indicating that n-butanol is not an inducer of encystment by itself but must be catabolized in order to induce encystment.

The catalytic activities of the bifunctional Azotobacter vinelandii mannuronan C-5-epimerase and alginate lyase AlgE7 probably originate from the same active site in the enzyme

The Azotobacter vinelandii genome encodes a family of seven secreted Ca(2+)-dependent epimerases (AlgE1--7) catalyzing the polymer level epimerization of beta-D-mannuronic acid (M) to alpha-L-guluronic acid (G) in the commercially important polysaccharide alginate. AlgE1--7 are composed of two types of protein modules, A and R, and the A-modules have previously been found to be sufficient for epimerization. AlgE7 is both an epimerase and an alginase, and here we show that the lyase activity is Ca(2+)-dependent and also responds similarly to the epimerases in the presence of other divalent cations. The AlgE7 lyase degraded M-rich alginates and a relatively G-rich alginate from the brown algae Macrocystis pyrifera most effectively, producing oligomers of 4 (mannuronan) to 7 units. The sequences cleaved were mainly G/MM and/or G/GM. Since G-moieties dominated at the reducing ends even when mannuronan was used as substrate, the AlgE7 epimerase probably stimulates the lyase pathway, indicating a complex interplay between the two activities. A truncated form of AlgE1 (AlgE1-1) was converted to a combined epimerase and lyase by replacing the 5'-798 base pairs in the algE1-1 gene with the corresponding A-module-encoding DNA sequence from algE7. Furthermore, substitution of an aspartic acid residue at position 152 with glycine in AlgE7A eliminated almost all of both the lyase and epimerase activities. Epimerization and lyase activity are believed to be mechanistically related, and the results reported here strongly support this hypothesis by suggesting that the same enzymatic site can catalyze both reactions.

Hexuronyl C5-epimerases in alginate and glycosaminoglycan biosynthesis

The sugar residues in most polysaccharides are incorporated as their corresponding monomers during polymerization. Here we summarize the three known exceptions to this rule, involving the biosynthesis of alginate, and the glycosaminoglycans, heparin/heparan sulfate and dermatan sulfate. Alginate is synthesized by brown seaweeds and certain bacteria, while glycosaminoglycans are produced by most animal species. In all cases one of the incorporated sugar monomers are being C5-epimerized at the polymer level, from D-mannuronic acid to L-guluronic acid in alginate, and from D-glucuronic acid to L-iduronic acid in glycosaminoglycans. Alginate epimerization modulates the mechanical properties of seaweed tissues, whereas in bacteria it seems to serve a wide range of purposes. The conformational flexibility of iduronic acid units in glycosaminoglycans promotes apposition to, and thus functional interactions with a variety of proteins at cell surfaces and in the extracellular matrix. In the bacterium Azotobacter vinelandii the alginates are being epimerized at the cell surface or in the extracellular environment by a family of evolutionary strongly related modular type and Ca(2+)-dependent epimerases (AlgE1-7). Each of these enzymes introduces a specific distribution pattern of guluronic acid residues along the polymer chains, explaining the wide structural variability observed in alginates isolated from nature. Glycosaminoglycans are synthesized in the Golgi system, through a series of reactions that include the C5-epimerization reaction along with extensive sulfation of the polymers. The single, Ca(2+)-independent, epimerase in heparin/heparan sulfate biosynthesis and the Ca(2+)-dependent dermatan sulfate epimerase(s) also generate variable epimerization patterns, depending on other polymer-modification reactions. The alginate and heparin epimerases appear unrelated at the amino acid sequence level, and have probably evolved through independent evolutionary pathways; however, hydrophobic cluster analysis indicates limited similarity. Seaweed alginates are widely used in industry, while heparin is well established in the clinic as an anticoagulant.

Alginate formation in Azotobacter vinelandii UWD during stationary phase and the turnover of poly-beta-hydroxybutyrate

Azotobacter vinelandii UWD is a mutant of strain UW that is defective in the respiratory oxidation of NADH. This mutation causes an overproduction of polyhydroxyalkanoates (PHAs), as polyester synthesis is used as an alternative electron sink. Since PHAs have potential for use as natural, biodegradable plastics, studies of physiology related to their production are of interest. Alginate production by this strain is limited to < 11 microg (mg cell protein)(-1), which permits high efficiency conversion of carbon source into PHA. However, < or = 400 microg (mg cell protein)(-1) was formed when UWD cells were oxygen-limited and in the stationary phase of growth. Alginate formation was fuelled by PHA turnover, which was coincident with the synthesis of alkyl resorcinols, under conditions of exogenous glucose limitation. However, alginate production was a phenotypic and reversible change. Alginate production was stopped by interruption of algD with Tn5lacZ. LacZ activity in UWD was shown to increase in stationary phase, while LacZ activity in a similarly constructed mutant of strain UW did not. Transcription of algD in strain UWD started from a previously identified RpoD promoter and not from the AlgU (RpoE) promoter. This is because strain UWD has a natural insertion element in algU. Differences between strain UW and UWD may reside in the defective respiratory oxidation of NADH, where the NADH surplus in strain UWD may act as a signal of stationary phase. Indeed, a backcross of UW DNA into UWD generated NADH-oxidase-proficient cells that failed to form alginate in stationary phase. Evidence is also presented to show that the RpoD promoter may be recognized by the stationary phase sigma factor (RpoS), which may mediate alginate production in strain UWD.

Role of Azotobacter vinelandii mucA and mucC gene products in alginate production

Azotobacter vinelandii produces the exopolysaccharide alginate, which is essential for its differentiation to desiccation-resistant cysts. In different bacterial species, the alternative sigma factor sigma(E) regulates the expression of functions related to the extracytoplasmic compartments. In A. vinelandii and Pseudomonas aeruginosa, the sigma(E) factor (AlgU) is essential for alginate production. In both bacteria, the activity of this sigma factor is regulated by the product of the mucA, mucB, mucC, and mucD genes. In this work, we studied the transcriptional regulation of the A. vinelandii algU-mucABCD gene cluster, as well as the role of the mucA and mucC gene products in alginate production. Our results show the existence of AlgU autoregulation and show that both MucA and MucC play a negative role in alginate production.

Inactivation of the ampDE operon increases transcription of algD and affects morphology and encystment of Azotobacter vinelandii

Transcription of algD, encoding GDP-mannose dehydrogenase, the key enzyme in the alginate biosynthetic pathway, is highly regulated in Azotobacter vinelandii. We describe here the characterization of a Tn5 insertion mutant (AC28) which shows a higher level of expression of an algD::lacZ fusion. AC28 cells were morphologically abnormal and unable to encyst. The cloning and nucleotide sequencing of the Tn5-disrupted locus in AC28 revealed an operon homologous to the Escherichia coli ampDE operon. Tn5 was located within the ampD gene, encoding a cytosolic N-acetyl-anhydromuramyl-L-alanine amidase that participates in the intracellular recycling of peptidoglycan fragments. The ampE gene encodes a transmembrane protein, but the function of the protein is not known. We constructed strains carrying ampD or ampE mutations and one with an ampDE deletion. The strain with a deletion of the ampDE operon showed a phenotype similar to that of mutant AC28. The present work demonstrates that both alginate production and bacterial encystment are greatly influenced by the bacterial ability to recycle its cell wall.

The GacS sensor kinase regulates alginate and poly-beta-hydroxybutyrate production in Azotobacter vinelandii

Azotobacter vinelandii produces two polymers: the extracellular polysaccharide alginate and the intracellular polyester poly-beta-hydroxybutyrate (PHB). A cosmid clone (pSMU588) from an A. vinelandii gene library diminished alginate production by A. vinelandii mucoid strain ATCC 9046. The nucleotide sequence and predicted amino acid sequence of the locus responsible for the mucoidy suppression revealed 65% identity to Pseudomonas GacS, a transmembrane sensor kinase of the two-component regulators, whose cognate response regulator, GacA, is a global activator regulating several products and virulence factors. Plasmid pMC15, harboring gacS, and a strain carrying a gacS nonpolar mutation were constructed. Either pMC15 or the gacS mutation significantly reduced alginate production and transcription of algD, the gene coding for the key enzyme GDP-mannose dehydrogenase of the alginate biosynthetic pathway. We found that the gacS mutation also reduced PHB accumulation and impaired encystment. Taken together, these data indicate that in A. vinelandii the gacSA global system regulates polymer synthesis.

Transcriptional organization of the Azotobacter vinelandii algGXLVIFA genes: characterization of algF mutants

Azotobacter vinelandii forms desiccation-resistant cysts which contain a high proportion of the exopolysaccharide alginate in their envelope. We have previously shown that the A. vinelandii alginate biosynthetic genes algA and algL are transcribed from a promoter located somewhere upstream of algL. In this study we sequenced the A. vinelandii algX, algL, algV, algI and algF genes located between algG and algA. We carried out primer extension analysis of the algG, algX and algL genes and detected transcription start sites upstream algG but not upstream algX or algL, implying that algG and algX form part of the previously identified algL-A operon. A promoter upstream algA was also detected; however, transcription of algA exclusively from this promoter is not sufficient for the AlgA levels required for alginate production. An algF mutant (AJ34) was constructed by insertion of the Omega-tetracycline cassette in the non-polar orientation. As expected, AJ34 produced unacetylated alginate. Viability of 35day old cysts formed by strain AJ34, but not of those formed by the wild type, was reduced, indicating that acetylation of alginate plays a role in cyst resistance to desiccation.

The A modules of the Azotobacter vinelandii mannuronan-C-5-epimerase AlgE1 are sufficient for both epimerization and binding of Ca2+

The industrially important polysaccharide alginate is composed of the two sugar monomers beta-D-mannuronic acid (M) and its epimer alpha-L-guluronic acid (G). In the bacterium Azotobacter vinelandii, the G residues originate from a polymer-level reaction catalyzed by one periplasmic and at least five secreted mannuronan C-5-epimerases. The secreted enzymes are composed of repeats of two protein modules designated A (385 amino acids) and R (153 amino acids). The modular structure of one of the epimerases, AlgE1, is A1R1R2R3A2R4. This enzyme has two catalytic sites for epimerization, each site introducing a different G distribution pattern, and in this article we report the DNA-level construction of a variety of truncated forms of the enzyme. Analyses of the properties of the corresponding proteins showed that an A module alone is sufficient for epimerization and that A1 catalyzed the formation of contiguous stretches of G residues in the polymer, while A2 introduces single G residues. These differences are predicted to strongly affect the physical and immunological properties of the reaction product. The epimerization reaction is Ca2+ dependent, and direct binding studies showed that both the A and R modules bind this cation. The R modules appeared to reduce the Ca2+ concentration needed for full activity and also stimulated the reaction rate when positioned both N and C terminally.