Identification of Cyanobacteria and Its Potential Toxins in the Joanes I Reservoir, Bahia, Brazil.

The Joanes I Reservoir is responsible for 40% of the drinking water supply of the Metropolitan Region of Salvador, Bahia, Brazil. For water sources such as this, there is concern regarding the proliferation of potentially toxin-producing cyanobacteria, which can cause environmental and public health impacts. To evaluate the presence of cyanobacteria and their cyanotoxins in the water of this reservoir, the cyanobacteria were identified by microscopy; the presence of the genes of the cyanotoxin-producing cyanobacteria was detected by molecular methods (polymerase chain reaction (PCR)/sequencing); and the presence of toxins was determined by liquid chromatography with tandem mass spectrometry (LC-MS/MS). The water samples were collected at four sampling points in the Joanes I Reservoir in a monitoring campaign conducted during the occurrence of phytoplankton blooms, and the water quality parameters were also analysed. Ten cyanobacteria species/genera were identified at the monitoring sites, including five potentially cyanotoxin-producing species, such as Cylindrospermopsis raciborskii, Cylindrospermopsis cf. acuminato-crispa, Aphanocapsa sp., Phormidium sp., and Pseudanabaena sp. A positive result for the presence of the cylindrospermopsin toxin was confirmed at two sampling points by LC-MS/MS, which indicated that the populations are actively producing toxins. The analysis of the PCR products using the HEPF/HEPR primer pair for the detection of the microcystin biosynthesis gene mcyE was positive for the analysed samples. The results of this study point to the worrisome condition of this reservoir, from which water is collected for public supply, and indicate the importance of the joint use of different methods for the analysis of cyanobacteria and their toxins in reservoir monitoring.

Identification of 7-Deoxy-desulfo-argino-cylindrospermopsin, the Missing Piece in Cylindrospermopsin Biosynthesis.

Cylindrospermopsin, a major cyanotoxin, is produced by freshwater cyanobacteria. Its biosynthesis starts from arginine and glycine and involves five polyketide synthases and several tailoring enzymes. We report the identification of 7-deoxy-desulfo-argino-cylindrospermopsin in several cylindrospermopsin-producing cyanobacteria using mass spectrometry experiments. We have purified this new metabolite and established its structure by 1D and 2D NMR spectroscopy using scalar-based 1H-1H, 1H-13C, and 1H-15N as well as 2D 1H-1H ROESY correlation experiments. Using labeled arginines in isotopic incorporation experiments, we have shown that arginine is fully incorporated into 7-deoxy-desulfo-argino-cylindrospermopsin and that the uracil ring of cylindrospermopsin originates from the guanidino moiety of arginine, thus solving a long-standing puzzling question. CyrG and CyrH from the cylindrospermopsin-producing Oscillatoria sp. PCC 6506 were overproduced in Escherichia coli and purified to homogeneity. We showed that CyrG is a zinc-dependent hydrolase, homologous to adenosine deaminases, that transforms 7-deoxy-desulfo-argino-cylindrospermopsin into 7-deoxy-desulfo-cylindrospermopsin and ornithine, with the following kinetic parameters: KM = 0.21 ± 0.05 µM and kcat = 0.19 ± 0.02 min-1. CyrG contained 0.55 mol of zinc per mol of monomer but could be activated by FeII or CoII. CyrH contained almost no metal and showed no such activity even in the presence of excess metal. Using structure-based alignments and secondary structure predictions, we propose that the fifth and last polyketide synthase CyrF in cylindrospermopsin biosynthesis contains an unprecedented C-terminal domain homologous to N-acetyltransferases. We suggest that this domain catalyzes the condensation of the CyrF product with arginine to give 7-deoxy-desulfo-argino-cylindrospermopsin. This would be an unprecedented termination step for a polyketide synthase.

Biosynthesis of Cylindrospermopsin in Cyanobacteria: Characterization of CyrJ the Sulfotransferase.

7-Deoxy-desulfo-cylindrospermopsin was purified at small-scale from the supernatant of a culture of the cyanobacterium Oscillatoria sp. PCC 10702. This metabolite was obtained in a pure form using a three-step chromatographic procedure, and its identity was confirmed by liquid chromatography-tandem mass spectrometry (LC-MS/MS). LC-MS quantification showed that this metabolite was excreted in the culture medium of Oscillatoria sp. PCC 10702. Isotopic incorporation studies using [2-13C,15N]glycine, a cylindrospermopsin precursor, and Oscillatoria sp. PCC 10702 cells showed that glycine was incorporated into 7-deoxy-desulfo-cylindrospermopsin, 7-deoxy-cylindrospermopsin, 7-epi-cylindrospermopsin, and cylindrospermopsin. The isotopic incorporation rate was consistent with the following metabolic flux: 7-deoxy-desulfo-cylindrospermopsin → 7-deoxy-cylindrospermopsin → 7-epi-cylindrospermopsin and cylindrospermopsin. We have cloned the cyrJ gene into an expression vector and overproduced the putative sulfotransferase CyrJ in Escherichia coli. The purified protein CyrJ catalyzed, in vitro, the transfer of a sulfonate group from 3'-phosphoadenosine-5'-phosphosulfate (PAPS) to 7-deoxy-desulfo-cylindrospermopsin to give 7-deoxy-cylindrospermopsin. Kinetic analysis afforded the following apparent constants: KM app. (PAPS) = 0.12 µM, Vmax app. = 20 nM/min, KM app. (7-deoxy-desulfo-cylindrospermopsin) = 0.12 µM, and KI app. (7-deoxy-desulfo-cylindrospermopsin) = 4.1 µM. Preliminary data suggested that CyrJ catalyzed the reaction through a ternary-complex kinetic mechanism. All these data confirmed that CyrJ catalyzed a sulfotransfer during the penultimate step of the biosynthesis of cylindrospermopsin.

Biosynthesis of Anatoxins in Cyanobacteria: Identification of the Carboxy-anatoxins as the Penultimate Biosynthetic Intermediates.

Anatoxin-a, homoanatoxin-a, and dihydroanatoxin-a are potent cyanobacterial neurotoxins. They are biosynthesized in cyanobacteria from proline and acetate by a pathway involving three polyketide synthases. We report the identification of carboxy-anatoxin-a, carboxy-homoanatoxin-a, and carboxy-dihydroanatoxin-a in acidic extracts of Cuspidothrix issatschenkoi CHARLIE-1, Oscillatoria sp. PCC 6506, and Cylindrospermum stagnale PCC 7417, respectively, using liquid chromatography coupled to mass spectrometry. The structure of these carboxy derivatives was confirmed by mass spectrometry and by isotopic incorporation experiments using labeled proline and acetate. Each of these three cyanobacteria only produce one carboxy-anatoxin, suggesting that these metabolites are the product of the hydrolysis by AnaA, the type II thioesterase, of the thioesters bound to AnaG, the last polyketide synthase of the pathway. By measuring the rate of isotopic incorporation of labeled proline into carboxy-homoanatoxin-a and homoanatoxin-a produced by Oscillatoria sp. PCC 6506, we show that carboxy-homoanatoxin-a is the intracellular precursor of homoanatoxin-a, and that homoanatoxin-a is then excreted into the extracellular medium. The transformation of carboxy-homoanatoxin-a into homoanatoxin-a is a very slow two-step process, with accumulation of carboxy-homoanatoxin-a, suggesting that the decarboxylation is spontaneous and not enzymatically catalyzed. However, an unidentified and extracellular catalyst accelerates the decarboxylation when the cell extracts are prepared at neutral pH.

Characterization of CyrI, the hydroxylase involved in the last step of cylindrospermopsin biosynthesis: Binding studies, site-directed mutagenesis and stereoselectivity.

Cylindrospermopsin, a cytotoxin from cyanobacteria, is biosynthesized by a complex pathway, which involves CyrI, an iron and 2-oxoglutarate dependent hydroxylase that transforms 7-deoxy-cylindrospermopsin into cylindrospermopsin and its epimer, 7-epi-cylindrospermopsin, in the last step. The activity of CyrI from Oscillatoria sp. PCC 7926 depends on Fe(II) (Km = 2.1 µM), and 2-oxoglutarate (Km = 3.2 µM), and is strongly inhibited by 7-deoxy-cylindrospermopsin at concentration higher than 1 µM. Using tryptophan fluorescence, we measured the binding to CyrI of Fe(II) (KD = 0.02 µM) and 2-oxoglutarate (KD = 53 µM and KD = 1.1 µM in the absence or presence of 10 µM Fe(II), respectively). The Oscillatoria sp. PCC 6506 CyrI mutants H157A, D159A, H247A, and R257A were all inactive, and impaired in the binding of Fe(II) or 2-oxoglutarate, confirming the identity of the iron ligands and the role of R257 in the binding of 2-oxoglutarate. We constructed several chimeric enzymes using the Oscillatoria sp. PCC 7926 CyrI protein (stereoselective) and that from Oscillatoria sp. PCC 6506 (not stereoselective) to help understanding the structural factors that influence the stereoselectivity of the hydroxylation. Our data suggest that a predicted α-helix in CyrI (positions 87-108) seems to modulate the stereoselectivity of the reaction.

Dihydroanatoxin-a Is Biosynthesized from Proline in Cylindrospermum stagnale PCC 7417: Isotopic Incorporation Experiments and Mass Spectrometry Analysis.

LC-MS and GC-MS analytical conditions have been developed to detect the cis- and trans-epimers (relative configuration of the carbon bearing the acetyl or propionyl group) of dihydroanatoxin-a and dihydrohomoanatoxin-a, in biological samples. These compounds epimerize under acidic conditions, yielding a major species that was tentatively assigned as the cis-epimer. Cylindrospermum stagnale PCC 7417 was definitively shown to produce dihydroanatoxin-a (1.2 mg/g dried cells). Oscillatoria sp. PCC 9107, Oscillatoria sp. PCC 6506, and C. stagnale PCC 7417, which produce anatoxin-a, homoanatoxin-a, and dihydroanatoxin-a, respectively, were cultivated in the presence of isotopically labeled proline, and the toxins were extracted. Interpretation of the GC-MS electron ionization mass spectra of these labeled anatoxins showed that they are all biosynthesized from proline and that the positions of the labels in these molecules are identical. These data and the fact that the ana cluster of genes is conserved in these cyanobacteria suggest that dihydroanatoxin-a is formed by the reduction of either anatoxin-a or its precursor in a specific step involving AnaK, an F420-dependent oxido-reductase whose gene is found in the ana gene cluster in C. stagnale PCC 7417. This is the first report of a cyanobacterium producing dihydroanatoxin-a, suggesting that other producers are present in the environment.

A simple protocol for attenuating the auto-fluorescence of cyanobacteria for optimized fluorescence in situ hybridization (FISH) imaging.

Cyanobacteria contain pigments, which generate auto-fluorescence that interferes with fluorescence in situ hybridization (FISH) imaging of cyanobacteria. We describe simple chemical treatments using CuSO4 or H2O2 that significantly reduce the auto-fluorescence of Microcystis strains. These protocols were successfully applied in FISH experiments using 16S rRNA specific probes and filamentous cyanobacteria.

Irreversible collective migration of cyanobacteria in eutrophic conditions.

In response to natural or anthropocentric pollutions coupled to global climate changes, microorganisms from aquatic environments can suddenly accumulate on water surface. These dense suspensions, known as blooms, are harmful to ecosystems and significantly degrade the quality of water resources. In order to determine the physico-chemical parameters involved in their formation and quantitatively predict their appearance, we successfully reproduced irreversible cyanobacterial blooms in vitro. By combining chemical, biochemical and hydrodynamic evidences, we identify a mechanism, unrelated to the presence of internal gas vesicles, allowing the sudden collective upward migration in test tubes of several cyanobacterial strains (Microcystis aeruginosa PCC 7005, Microcystis aeruginosa PCC 7806 and Synechocystis sp. PCC 6803). The final state consists in a foamy layer of biomass at the air-liquid interface, in which micro-organisms remain alive for weeks, the medium lying below being almost completely depleted of cyanobacteria. These "laboratory blooms" start with the aggregation of cells at high ionic force in cyanobacterial strains that produce anionic extracellular polymeric substances (EPS). Under appropriate conditions of nutrients and light intensity, the high photosynthetic activity within cell clusters leads the dissolved oxygen (DO) to supersaturate and to nucleate into bubbles. Trapped within the EPS, these bubbles grow until their buoyancy pulls the biomass towards the free surface. By investigating a wide range of spatially homogeneous environmental conditions (illumination, salinity, cell and nutrient concentration) we identify species-dependent thresholds and timescales for bloom formation. We conclude on the relevance of such results for cyanobacterial bloom formation in the environment and we propose an efficient method for biomass harvesting in bioreactors.

Searching for a link between the L-BMAA neurotoxin and amyotrophic lateral sclerosis: a study protocol of the French BMAALS programme.

INTRODUCTION: Amyotrophic lateral sclerosis (ALS) is the most common motor neurone disease. It occurs in two forms: (1) familial cases, for which several genes have been identified and (2) sporadic cases, for which various hypotheses have been formulated. Notably, the ß-N-methylamino-L-alanine (L-BMAA) toxin has been postulated to be involved in the occurrence of sporadic ALS. The objective of the French BMAALS programme is to study the putative link between L-BMAA and ALS. METHODS AND ANALYSIS: The programme covers the period from 1 January 2003 to 31 December 2011. Using multiple sources of ascertainment, all the incident ALS cases diagnosed during this period in the area under study (10 counties spread over three French regions) were collected. First, the standardised incidence ratio will be calculated for each municipality under concern. Then, by applying spatial clustering techniques, overincidence and underincidence zones of ALS will be sought. A case-control study, in the subpopulation living in the identified areas, will gather information about patients' occupations, leisure activities and lifestyle habits in order to assess potential risk factors to which they are or have been exposed. Specimens of drinking water, food and biological material (brain tissue) will be examined to assess the presence of L-BMAA in the environment and tissues of ALS cases and controls. ETHICS AND DISSEMINATION: The study has been reviewed and approved by the French ethical committee of the CPP SOOM IV (Comité de Protection des Personnes Sud-Ouest & Outre-Mer IV). The results will be published in peer-reviewed journals and presented at national and international conferences.

Biosynthesis of anatoxin-a and analogues (anatoxins) in cyanobacteria.

Freshwater cyanobacteria produce secondary metabolites that are toxic to humans and animals, the so-called cyanotoxins. Among them, anatoxin-a and homoanatoxin-a are potent neurotoxins that are agonists of the nicotinic acetylcholine receptor. These alkaloids provoke a rapid death if ingested at low doses. Recently, the cluster of genes responsible for the biosynthesis of these toxins, the ana cluster, has been identified in Oscillatoria sp. PCC 6506, and a biosynthetic pathway was proposed. This biosynthesis was reconstituted in vitro using purified enzymes confirming the predicted pathway. One of the enzymes, AnaB a prolyl-acyl carrier protein oxidase, was crystallized and its three dimensional structure solved confirming its reaction mechanism. Three other ana clusters have now been identified and sequenced in other cyanobacteria. These clusters show similarities and some differences suggesting a common evolutionary origin. In particular, the cluster from Cylindrospermum stagnale PCC 7417, possesses an extra gene coding for an F420-dependent oxidoreductase that is likely involved in the biosynthesis of dihydroanatoxin-a. This review summarizes all these new data and discusses them in relation to the production of anatoxins in the environment.

Structure of the prolyl-acyl carrier protein oxidase involved in the biosynthesis of the cyanotoxin anatoxin-a.

Anatoxin-a and homoanatoxin-a are two potent cyanobacterial neurotoxins biosynthesized from L-proline by a short pathway involving polyketide synthases. Proline is first loaded onto AnaD, an acyl carrier protein, and prolyl-AnaD is then oxidized to 1-pyrroline-5-carboxyl-AnaD by a flavoprotein, AnaB. Three polyketide synthases then transform this imine into anatoxin-a or homoanatoxin-a. AnaB was crystallized in its holo form and its three-dimensional structure was determined by X-ray diffraction at 2.8 Šresolution. AnaB is a homotetramer and its fold is very similar to that of the acyl-CoA dehydrogenases (ACADs). The active-site base of AnaB, Glu244, superimposed very well with that of human isovaleryl-CoA dehydrogenase, confirming previous site-directed mutagenesis experiments and mechanistic proposals. The substrate-binding site of AnaB is small and is likely to be fitted for the pyrrolidine ring of proline. However, in contrast to ACADs, which use an electron-transport protein, AnaB uses molecular oxygen as the electron acceptor, as in acyl-CoA oxidases. Calculation of the solvent-accessible surface area around the FAD in AnaB and in several homologues showed that it is significantly larger in AnaB than in its homologues. A protonated histidine near the FAD in AnaB is likely to participate in oxygen activation. Furthermore, an array of water molecules detected in the AnaB structure suggests a possible path for molecular oxygen towards FAD. This is consistent with AnaB being an oxidase rather than a dehydrogenase. The structure of AnaB is the first to be described for a prolyl-ACP oxidase and it will contribute to defining the structural basis responsible for oxygen reactivity in flavoenzymes.

Validation of the analytical procedure for the determination of the neurotoxin ß-N-methylamino-L-alanine in complex environmental samples.

The neurotoxic l-2-amino-3-methylaminopropionic acid (BMAA) was hypothesized to be involved in sporadic cases of amyotrophic lateral sclerosis (ALS). Studies highlighting a possible implication of environmental factors in the incidence of sporadic ALS have become more numerous over recent years. Over the past years, the most widely used method for quantifying BMAA was based on the derivatization of this polar and basic molecule with a fluorescent compound (6-aminoquinolonyl-N-hydroxysuccinimidyl, 6-AQC). This derivatization allows the retention of the conjugate by reversed-phase liquid chromatography and its detection by fluorescence. Nevertheless, recent findings have shown that this method applied to complex samples may cause false positive responses. We therefore developed an analytical procedure for the determination of underivatized BMAA at trace level in complex environmental matrices (river water, cyanobacteria and biofilm) using solid-phase extraction (SPE) based on mixed mode sorbent to concentrate and clean up real samples. Analyzes were performed by hydrophilic interaction chromatography (HILIC) coupled to electrospray ionization and tandem mass spectrometry used in multiple reaction monitoring scan mode. Analytical procedures were validated for the different natural samples using the total error approach. BMAA can be quantified by these reliable and highly selective analytical methods in a range of only a few ng mL(-1) in river water and a few ng mg(-1) dry weight in cyanobacteria and biofilm matrices.

Synthesis, configuration assignment, and simultaneous quantification by liquid chromatography coupled to tandem mass spectrometry, of dihydroanatoxin-a and dihydrohomoanatoxin-a together with the parent toxins, in axenic cyanobacterial strains and in environmental samples.

We have synthesized cis- and trans-dihydroanatoxin-a and cis- and trans-dihydrohomoanatoxin-a using a short synthetic route. The relative configuration of N-tert-butoxycarbonyl-cis-dihydroanatoxin-a was determined by X-ray crystallography, while that of N-tert-butoxycarbonyl-trans-dihydroanatoxin-a was confirmed by epimerization leading to the cis-diastereoisomer. The relative configuration of N-tert-butoxycarbonyl-trans- and cis-dihydrohomoanatoxin-a was inferred from their NMR spectra. Using an optimized LC-MS/MS analytical method and pure standards we have simultaneously determined anatoxin-a, homoanatoxin-a and their dihydroderivatives in axenic strains of cyanobacteria and in environmental samples from the Tarn River, France. However, in these analytical conditions, the cis- and trans-dihydroanatoxin-a and cis- and trans-dihydrohomoanatoxin-a could not be separated. In axenic strains, the dihydroderivatives represented less than 3% of the total toxin content, while in field samples dihydroanatoxin-a represented from 17% to 90% of the total toxin content. Thus, the reduction of anatoxin-a to dihydroanatoxin-a is predominant in the environment. The ratio of anatoxin-a concentration over that of homoanatoxin-a in axenic strains was variable, and among the eight strains studied we found three exclusive anatoxin-a producers and five producers of homoanatoxin-a and anatoxin-a, the latter representing from 0.5% to 2.0% of the total. In the strains studied, we have amplified by PCR, and sequenced the region of anaG coding for the methylation domain proposed to be responsible for the formation of homoanatoxin-a. The sequences showed at least 88% identity and we could not relate the toxin profile of the strains to the sequence of the methylation domain.

Insights into the reaction mechanism of the prolyl-acyl carrier protein oxidase involved in anatoxin-a and homoanatoxin-a biosynthesis.

Anatoxin-a and homoanatoxin-a are two potent cyanobacterial neurotoxins. We recently reported the identification of the gene cluster responsible for the biosynthesis of these toxins as well as the in-vitro reconstitution of the first steps of this biosynthesis. We now report experimental evidence supporting the proposed reaction mechanism of AnaB, a flavoprotein homologous to acyl-CoA dehydrogenase. AnaB catalyzes the two-electron oxidation of prolyl-AnaD, which is proline linked to the acyl carrier protein holo-AnaD, to dehydroprolyl-AnaD using oxygen as the second substrate. AnaB is thus an oxidase. By using liquid chromatography coupled to tandem mass spectrometry (LC-MS/MS), we have identified and characterized dehydroprolyl-AnaD, the AnaB product. We estimated an apparent catalytic constant of 1 min(-1) for AnaB catalysis. We synthesized several deuterium-labeled prolines and enzymatically transformed them into their corresponding prolyl-AnaD. These deuterium-labeled prolyl-AnaDs were oxidized in the presence of AnaB, and the deuterium labeling in the remaining substrate and in the product was determined by LC-MS/MS. The data supported a reaction mechanism starting with a rapid enolization followed by a slow oxidation to give the conjugated imine, which in turn was isomerized to pyrroline-5-carboxyl-AnaD. We also showed that cis- and trans-4-fluoro-L-prolyl-AnaD and 3,4-dehydro-L-prolyl-AnaD were transformed into pyrrole-2-carboxyl-AnaD by AnaB. Thus, the 4-fluoro-analogues experienced a ß-elimination supporting the AnaB-catalyzed aza-allylic isomerization. We identified by sequence alignment the AnaB active site base, Glu244. We produced, purified, and characterized the E244A AnaB mutant, which is inactive, supporting the catalytic role of E244 as a base.

The genome sequence of the cyanobacterium Oscillatoria sp. PCC 6506 reveals several gene clusters responsible for the biosynthesis of toxins and secondary metabolites.

We report a draft sequence of the genome of Oscillatoria sp. PCC 6506, a cyanobacterium that produces anatoxin-a and homoanatoxin-a, two neurotoxins, and cylindrospermopsin, a cytotoxin. Beside the clusters of genes responsible for the biosynthesis of these toxins, we have found other clusters of genes likely involved in the biosynthesis of not-yet-identified secondary metabolites.

Biosynthesis of cylindrospermopsin and 7-epicylindrospermopsin in Oscillatoria sp. strain PCC 6506: identification of the cyr gene cluster and toxin analysis.

Cylindrospermopsin is a cytotoxin produced by Cylindrospermopsis raciborskii and other cyanobacteria that has been implicated in human intoxications. We report here the complete sequence of the gene cluster responsible for the biosynthesis of this toxin in Oscillatoria sp. strain PCC 6506. This cluster of genes was found to be homologous with that of C. raciborskii but with a different gene organization. Using an enzyme-linked immunosorbent assay and an optimized liquid chromatography analytical method coupled to tandem mass spectrometry, we detected 7-epicylindrospermopsin, cylindrospermopsin, and 7-deoxycylindrospermopsin in the culture medium of axenic Oscillatoria PCC 6506 at the following relative concentrations: 68.6%, 30.2%, and 1.2%, respectively. We measured the intracellular and extracellular concentrations, per mg of dried cells of Oscillatoria PCC 6506, of 7-epicylindrospermopsin (0.18 microg/mg and 0.29 microg/mg, respectively) and cylindrospermopsin (0.10 microg/mg and 0.11 microg/mg, respectively). We showed that these two toxins accumulated in the culture medium of Oscillatoria PCC 6506 but that the ratio (2.5 +/- 0.3) was constant with 7-epicylindrospermopsin being the major metabolite. We also determined the concentrations of these toxins in culture media of other Oscillatoria strains, PCC 6407, PCC 6602, PCC 7926, and PCC 10702, and found that, except for PCC 6602, they all produced 7-epicylindrospermopsin and cylindrospermopsin, with the former being the major toxin, except for PCC 7926, which produced very little 7-epicylindrospermopsin. All the cylindrospermopsin producers studied gave a PCR product using specific primers for the amplification of the cyrJ gene from genomic DNA.

First identification of the neurotoxin homoanatoxin-a from mats of Hydrocoleum lyngbyaceum (marine cyanobacterium) possibly linked to giant clam poisoning in New Caledonia.

We report the first identification of homoanatoxin-a from benthic marine cyanobacteria (Hydrocoleum lyngbyaceum) samples collected in Lifou (Loyalty Islands, New Caledonia), where cases of giant clams (Tridacna maxima) intoxications were recorded during a severe ciguatera fish poisoning outbreak. Homoanatoxin-a was also detected in extracts of giant clams harvested in the surroundings of the contaminated area suggesting the possible link between these poisoning events and the occurrence of potentially neurotoxic Hydrocoleum.

In vitro reconstitution of the first steps of anatoxin-a biosynthesis in Oscillatoria PCC 6506: from free L-proline to acyl carrier protein bound dehydroproline.

Anatoxin-a and homoanatoxin-a are two potent cyanobacterial neurotoxins. We recently reported the identification of the gene cluster responsible for the biosynthesis of these toxins in cyanobacteria and proposed a biosynthetic scheme starting from L-proline and involving three polyketide synthases for which the starter would be (S)-1-pyrroline-5-carboxylate bound to an acyl carrier protein, AnaD. We now report the in vitro reconstitution of the first steps of this biosynthesis in Oscillatoria PCC 6506. We identified in PCC 6506 the gene coding for an Sfp-like phosphopantetheinyl transferase and purified the gene product, OsPPT, that catalyzed the transfer of the phosphopantetheinyl arm to the serine 41 of AnaD. The pure adenylation protein AnaC loaded L-proline on holo-AnaD and was specific for L-proline (K(m) = 0.97 mM, k(cat) = 68 min(-1)) among the 20 natural amino acids. Among six close structural analogues of L-proline, including (S)-1-pyrroline-5-carboxylate, we only found 3,4-dehydro-L-proline to be an alternate substrate for AnaC (K(m) = 1.5 mM, k(cat) = 29 min(-1)). The putative prolyl-AnaD dehydrogenase, AnaB, purified to homogeneity as a histidine-tagged protein, showed an absorption spectrum characteristic of FAD-containing proteins. It oxidized prolyl-AnaD to dehydroprolyl-AnaD as shown by tryptic digestion of the protein followed by liquid chromatography coupled to tandem mass spectrometry. Alignment of the amino acid sequence of this dehydrogenase with related enzymes showed that AnaB belongs to the acyl-CoA dehydrogenase superfamily and thus probably catalyzes an alpha-beta-dehydrogenation of the thioester-bound proline followed by an aza-allylic isomerization to yield (S)-pyrroline-5-carboxyl-AnaD, the proposed starter for the subsequent polyketide synthase, AnaE.

Evidence that biosynthesis of the neurotoxic alkaloids anatoxin-a and homoanatoxin-a in the cyanobacterium Oscillatoria PCC 6506 occurs on a modular polyketide synthase initiated by L-proline.

Anatoxin-a and homoanatoxin-a are potent neurotoxins produced by cyanobacteria such as Oscillatoria PCC 6506. Sequencing of the genome of this strain is underway, and we have identified a 29 kb DNA fragment containing a sequence called ks2 that we previously showed to be specific to Oscillatoria cyanobacteria producing anatoxin-a and homoanatoxin-a. Bioinformatic analysis of this 29 kb fragment revealed a cluster of genes, which were annotated. The function assigned to the products of eight contiguous genes, from anaA to anaH, provides a clue to the biosynthesis of anatoxin-a and homoanatoxin-a. Proline is first loaded on an acyl carrier protein and its five-membered cycle oxidized to the pyrroline oxidation state. This activated ring is then successively loaded on three polyketide synthase modules for elongation, reduction, cyclization, and methylation. The final step is the hydrolysis of the thioester with subsequent decarboxylation. GC-MS and NMR analyses of homoanatoxin-a produced by PCC 6506 using labeled precursors confirm that proline is very likely the starter of these polyketide synthases. Using specific PCR amplifications, we have also shown that the anaC, anaE, anaF, and anaG genes are always present in the genome of cyanobacteria producing anatoxin-a and homoanatoxin-a and absent in nonproducing strains. Histidine-tagged AnaC was purified to homogeneity and showed to catalyze the loading of proline on purified histidine-tagged AnaD that had been previously transformed into its holo form using the Bacillus subtilis Sfp phosphopantetheinyl transferase. All of these data provide strong evidence that we have successfully identified the gene cluster responsible for the production of anatoxin-a and homoanatoxin-a in Oscillatoria PCC 6506.