Three novel metabolites from a bloom of the cyanobacterium Microcystis sp

Ecotoxicological assessment of cyclic peptides produced by a Planktothrix rubescens bloom: Impact on aquatic model organisms

Cyanobacterial blooms, a natural phenomenon in freshwater ecosystems, have increased in frequency and severity due to climate change and eutrophication. Some cyanobacteria are able to produce harmful substances called cyanotoxins. These metabolites possess different chemical structures and action mechanisms representing a serious concern for human health and the environment. The most studied cyanotoxins belong to the group of microcystins which are potent hepatotoxins. Anabaenopeptins are another class of cyclic peptides produced by certain species of cyanobacteria, including Planktothrix spp. Despite limited knowledge regarding individual effects of anabaenopeptins on freshwater organisms, reports have identified in vivo toxicity in representatives of freshwater zooplankton by cyanobacterial extracts or mixtures containing anabaenopeptins. This study focused on the isolation and toxicity evaluation of the cyanotoxins produced in the 2022 Planktothrix rubescens bloom in Averno lake, Italy. The three main cyclic peptides have been isolated and identified by nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry (MS) and optical analyses as anabaenopeptins A and B, and oscillamide Y. Ecotoxicological tests on the aquatic model organisms Daphnia magna (crustacean), Raphidocelis subcapitata (algae), and Aliivibrio fischeri (bacterium) revealed that anabaenopeptins A and B do not generate significant toxicity at environmentally relevant concentrations, being also found a stimulatory effect on R. subcapitata in the case of anabaenopeptin A. By contrast, oscillamide Y displayed toxicity. Ecological implications based on ECOSAR predictions align with experimental data. Moreover, long-term exposure bioassays on different green unicellular algae species showed that R. subcapitata was not significantly affected, while Scenedesmus obliquus and Chlorella vulgaris exhibited altered growth patterns. These results, together with the already-known background in literature, highlight the complexity of interactions between organisms and the tested compounds, which may be influenced by species-specific sensitivities, physiological differences, and modes of action, possibly affected by parameters like lipophilicity.

Naturally Occurring Organohalogen Compounds-A Comprehensive Review

The present volume is the third in a trilogy that documents naturally occurring organohalogen compounds, bringing the total number-from fewer than 25 in 1968-to approximately 8000 compounds to date. Nearly all of these natural products contain chlorine or bromine, with a few containing iodine and, fewer still, fluorine. Produced by ubiquitous marine (algae, sponges, corals, bryozoa, nudibranchs, fungi, bacteria) and terrestrial organisms (plants, fungi, bacteria, insects, higher animals) and universal abiotic processes (volcanos, forest fires, geothermal events), organohalogens pervade the global ecosystem. Newly identified extraterrestrial sources are also documented. In addition to chemical structures, biological activity, biohalogenation, biodegradation, natural function, and future outlook are presented.

Recent progression of cyanobacteria and their pharmaceutical utility: an update

Cyanobacteria (blue-green algae) are Gram-negative photosynthetic eubacteria that are found everywhere. This largest group of photosynthetic prokaryotes is rich in structurally novel and biologically active compounds; several of which have been utilized as prospective drugs against cancer and other ailments, as well. Consequently, the integument of nanoparticles-synthetic approaches in cyanobacterial extracts should increase pharmacological activity. Moreover, silver nanoparticles (AgNPs) are small materials with diameters below 100 nm that are classified into different classes based on their forms, sizes, and characteristics. Indeed, the biosynthesized AgNPs are generated with a variety of organisms, algae, plants, bacteria, and a few others, for the medicinal purposes, as the bioactive compounds of curio and some proteins from cyanobacteria have the potentiality in the treatment of a wide range of infectious diseases. The critical focus of this review is on the antimicrobial, antioxidant, and anticancer properties of cyanobacteria. This would be useful in the pharmaceutical industries in the future drug development cascades.Communicated by Ramaswamy H. Sarma.

Anabaenopeptins from Cyanobacteria in Freshwater Bodies of Greece

Cyanobacteria are photosynthetic microorganisms that are able to produce a large number of secondary metabolites. In freshwaters, under favorable conditions, they can rapidly multiply, forming blooms, and can release their toxic/bioactive metabolites in water. Among them, anabaenopeptins (APs) are a less studied class of cyclic bioactive cyanopeptides. The occurrence and structural variety of APs in cyanobacterial blooms and cultured strains from Greek freshwaters were investigated. Cyanobacterial extracts were analyzed with LC-qTRAP MS/MS using information-dependent acquisition in enhanced ion product mode in order to obtain the fragmentation mass spectra of APs. Thirteen APs were detected, and their possible structures were annotated based on the elucidation of fragmentation spectra, including three novel ones. APs were present in the majority of bloom samples (91%) collected from nine Greek lakes during different time periods. A large variety of APs was observed, with up to eight congeners co-occurring in the same sample. AP F (87%), Oscillamide Y (87%) and AP B (65%) were the most frequently detected congeners. Thirty cyanobacterial strain cultures were also analyzed. APs were only detected in one strain (Microcystis ichtyoblabe). The results contribute to a better understanding of APs produced by freshwater cyanobacteria and expand the range of structurally characterized APs.

Bacterial terpenome

Covering: up to mid-2020 Terpenoids, also called isoprenoids, are the largest and most structurally diverse family of natural products. Found in all domains of life, there are over 80 000 known compounds. The majority of characterized terpenoids, which include some of the most well known, pharmaceutically relevant, and commercially valuable natural products, are produced by plants and fungi. Comparatively, terpenoids of bacterial origin are rare. This is counter-intuitive to the fact that recent microbial genomics revealed that almost all bacteria have the biosynthetic potential to create the C5 building blocks necessary for terpenoid biosynthesis. In this review, we catalogue terpenoids produced by bacteria. We collected 1062 natural products, consisting of both primary and secondary metabolites, and classified them into two major families and 55 distinct subfamilies. To highlight the structural and chemical space of bacterial terpenoids, we discuss their structures, biosynthesis, and biological activities. Although the bacterial terpenome is relatively small, it presents a fascinating dichotomy for future research. Similarities between bacterial and non-bacterial terpenoids and their biosynthetic pathways provides alternative model systems for detailed characterization while the abundance of novel skeletons, biosynthetic pathways, and bioactivies presents new opportunities for drug discovery, genome mining, and enzymology.

The use of cyanobacterial metabolites as natural medical and biotechnological tools: review article

Cyanobacteria are photosynthetic, Gram-negative bacteria that are considered one of the most morphologically diverse groups of prokaryotes with a chief role in the global nutrient cycle as they fixed gaseous carbon dioxide and nitrogen to organic materials. Cyanobacteria have significant adaptability to survive in harsh conditions due to they have different metabolic pathways with unique compounds, effective defensive mechanisms, and wide distribution in different habitats. Besides, they are successfully used to face different challenges in several fields, including industry, aquaculture, agriculture, food, dairy products, pollution control, bioenergy, and pharmaceutics. Analysis of 680 publications revealed that nearly 1630 cyanobacterial molecules belong to different families have a wide range of applications in several fields, including cosmetology, agriculture, pharmacology (immunosuppressant, anticancer, antibacterial, antiprotozoal, antifungal, anti-inflammatory, antimalarial, anticoagulant, anti-tuberculosis, antitumor, and antiviral activities) and food industry. In this review, we nearly mentioned 92 examples of cyanobacterial molecules that are considered the most relevant effects related to anti-inflammatory, antioxidant, antimicrobial, antiviral, and anticancer activities as well as their roles that can be used in various biotechnological fields. These cyanobacterial products might be promising candidates for fighting various diseases and can be used in managing viral and microbial infections.Communicated by Ramaswamy H. Sarma.

Eighteen New Aeruginosamide Variants Produced by the Baltic Cyanobacterium Limnoraphis CCNP1324

Cyanobactins are a large family of ribosomally synthesized and post-translationally modified cyanopeptides (RiPPs). Thus far, over a hundred cyanobactins have been detected in different free-living and symbiotic cyanobacteria. The majority of these peptides have a cyclic structure. The occurrence of linear cyanobactins, aeruginosamides and virenamide, has been reported sporadically and in few cyanobacterial taxa. In the current work, the production of cyanobactins by Limnoraphis sp. CCNP1324, isolated from the brackish water Baltic Sea, has been studied for the first time. In the strain, eighteen new aeruginosamide (AEG) variants have been detected. These compounds are characterized by the presence of prenyl and thiazole groups. A common element of AEGs produced by Limnoraphis sp. CCNP1324 is the sequence of the three C-terminal residues containing proline, pyrrolidine and methyl ester of thiazolidyne-4-carboxylic acid (Pro-Pyr-TzlCOOMe) or thiazolidyne-4-carboxylic acid (Pro-Pyr-TzlCOOH). The aeruginosamides with methylhomotyrosine (MeHTyr1) and with the unidentified N-terminal amino acids showed strong cytotoxic activity against human breast cancer cells (T47D).

New microginins from cyanobacteria of Greek freshwaters

Cyanobacteria can form extensive blooms in water with concurrent production and release of a large number of chemically diverse and bioactive metabolites, including hazardous toxins. Significant number of the metabolites belongs to non-ribosomal peptides, with unique residues, unusual structures and great potential for biotechnological application. The biosynthetic pathways of the peptides generate tens of variants, but only part of them has been identified. Microginins are an understudied class of cyanobacterial linear peptides with a characteristic decanoic acid derivative amino acid residue in their structure. In this study, cyanobacterial blooms and isolated strains from Greek lakes were analyzed for the presence of microginins by liquid chromatography coupled to hybrid triple quadrupole/linear ion trap mass spectrometer (LC-qTRAP MS/MS). Microginin structures were elucidated based on the obtained fragmentation spectra. A large number of microginins occurred in blooms of Greek freshwaters and the most frequently detected were Microginin FR1 (70% of samples), Microginin T1 (52%), Microginin 565B (52%), Microginin T2 (43%), and Microginin 565A (43%). Additionally, nine cyanobacterial strains i.e. Nostoc oryzae, Synechococcus sp., Microcystis aeruginosa, Microcystis viridis, and five Microcystis sp., were found to produce microginins. Thirty-six new microginin structures were characterized out of fifty-one totally detected variants. This is the first time that such a diversity of microginins is reported to be present in water bodies. Results clearly demonstrate the great metabolomic potential of cyanobacteria that inhabit Greek freshwaters and significantly expand the knowledge of cyanobacterial secondary metabolites with regards to the class of microginins.

The cyanobacterial oligopeptides microginins induce DNA damage in the human hepatocellular carcinoma (HepG2) cell line

Microginins (MGs) are bioactive metabolites mainly produced by Microcystis spp., (Cyanobacteria) commonly found in eutrophic environments. In this study, the cytotoxic and genotoxic activities of four MG congeners (MG FR3, MG GH787, cyanostatin B, MGL 402) and a well characterized cyanobacterial extract B-14-01 containing these metabolites were evaluated in the human hepatocellular carcinoma (HepG2) cell line. The cytotoxicity was measured with the MTT assay, while genotoxicity was studied with the comet, γH2AX and cytokinesis block (CBMN) micronucleus assays. The viability of cells after 24 h was significantly affected only by the extract, whereas after 72 h a concentration dependent decrease in cell proliferation was observed for the extract and tested microginins, with MGL 402 being the most potent and MG FR3 the least potent congener. The extract and all tested congeners induced DNA strand breaks after 4 and 24 h exposure. The most potent was the extract, which induced concentration and time dependent increase in DNA damage at concentrations ≥0.01 μg mL^-1. Among microginins the most potent was MGL 402 (increase in DNA strand breaks at ≥ 0.01 μg mL^-1) and MG FR3 was the least potent (increase in DNA strand breaks at ≥ 1 μg mL^-1). However, no induction of DNA double strand breaks was observed after 24 and 72-h exposure to the cyanobacterial extract or MGs. Induction of genomic instability was observed in cells exposed to MG GH787, cyanostatin B and the extract B-14-01. This study is the first to provide the evidence that microginins exert genotoxic activity.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Natural Products from Cyanobacteria: Focus on Beneficial Activities

Cyanobacteria are photosynthetic microorganisms that colonize diverse environments worldwide, ranging from ocean to freshwaters, soils, and extreme environments. Their adaptation capacities and the diversity of natural products that they synthesize, support cyanobacterial success in colonization of their respective ecological niches. Although cyanobacteria are well-known for their toxin production and their relative deleterious consequences, they also produce a large variety of molecules that exhibit beneficial properties with high potential in various fields (e.g., a synthetic analog of dolastatin 10 is used against Hodgkin's lymphoma). The present review focuses on the beneficial activities of cyanobacterial molecules described so far. Based on an analysis of 670 papers, it appears that more than 90 genera of cyanobacteria have been observed to produce compounds with potentially beneficial activities in which most of them belong to the orders Oscillatoriales, Nostocales, Chroococcales, and Synechococcales. The rest of the cyanobacterial orders (i.e., Pleurocapsales, Chroococcidiopsales, and Gloeobacterales) remain poorly explored in terms of their molecular diversity and relative bioactivity. The diverse cyanobacterial metabolites possessing beneficial bioactivities belong to 10 different chemical classes (alkaloids, depsipeptides, lipopeptides, macrolides/lactones, peptides, terpenes, polysaccharides, lipids, polyketides, and others) that exhibit 14 major kinds of bioactivity. However, no direct relationship between the chemical class and the respective bioactivity of these molecules has been demonstrated. We further selected and specifically described 47 molecule families according to their respective bioactivities and their potential uses in pharmacology, cosmetology, agriculture, or other specific fields of interest. With this up-to-date review, we attempt to present new perspectives for the rational discovery of novel cyanobacterial metabolites with beneficial bioactivity.

Cyanobacterial bioactive metabolites-A review of their chemistry and biology

Cyanobacterial blooms occur when algal densities exceed baseline population concentrations. Cyanobacteria can produce a large number of secondary metabolites. Odorous metabolites affect the smell and flavor of aquatic animals, whereas bioactive metabolites cause a range of lethal and sub-lethal effects in plants, invertebrates, and vertebrates, including humans. Herein, the bioactivity, chemistry, origin, and biosynthesis of these cyanobacterial secondary metabolites were reviewed. With recent revision of cyanobacterial taxonomy by Anagnostidis and Komárek as part of the Süβwasserflora von Mitteleuropa volumes 19(1-3), names of many cyanobacteria that produce bioactive compounds have changed, thereby confusing readers. The original and new nomenclature are included in this review to clarify the origins of cyanobacterial bioactive compounds. Due to structural similarity, the 157 known bioactive classes produced by cyanobacteria have been condensed to 55 classes. This review will provide a basis for more formal procedures to adopt a logical naming system. This review is needed for efficient management of water resources to understand, identify, and manage cyanobacterial harmful algal bloom impacts.

Unique Biosynthetic Pathway in Bloom-Forming Cyanobacterial Genus Microcystis Jointly Assembles Cytotoxic Aeruginoguanidines and Microguanidines

The cyanobacterial genus Microcystis is known to produce an elaborate array of structurally unique and biologically active natural products, including hazardous cyanotoxins. Cytotoxic aeruginoguanidines represent a yet unexplored family of peptides featuring a trisubstituted benzene unit and farnesylated arginine derivatives. In this study, we aimed at assigning these compounds to a biosynthetic gene cluster by utilizing biosynthetic attributes deduced from public genomes of Microcystis and the sporadic distribution of the metabolite in axenic strains of the Pasteur Culture Collection of Cyanobacteria. By integrating genome mining with untargeted metabolomics using liquid chromatography with mass spectrometry, we linked aeruginoguanidine (AGD) to a nonribosomal peptide synthetase gene cluster and coassigned a significantly smaller product to this pathway, microguanidine (MGD), previously only reported from two Microcystis blooms. Further, a new intermediate class of compounds named microguanidine amides was uncovered, thereby further enlarging this compound family. The comparison of structurally divergent AGDs and MGDs reveals an outstanding versatility of this biosynthetic pathway and provides insights into the assembly of the two compound subfamilies. Strikingly, aeruginoguanidines and microguanidines were found to be as widespread as the hepatotoxic microcystins, but the occurrence of both toxin families appeared to be mutually exclusive.

Microginins from a Microcystis sp. Bloom Material Collected from the Kishon Reservoir, Israel

During blooms, cyanobacteria produce diverse modified peptides. Among these are the microginins, which inhibit zinc-containing metalloproteases. Ten microginins, microginins KR767 (1), KR801(2), KR835 (3), KR785 (4), KR604 (5), KR638 (6), KR781 (7), KR815 (8), FR3 (9), and FR4 (10), were isolated from the extract of a bloom material of Microcystis sp. (IL-405) collected from the Kishon Reservoir, Israel in the fall of 2009. The structures of the pure compounds were elucidated using 1D and 2D NMR techniques and high-resolution mass spectrometry. The absolute configuration of the chiral centers of the amino acids were determined by Marfey’s and advance Marfey’s methods and by comparison of ¹H and ¹³C NMR chemical shifts of the Ahda derivatives with those of known microginins. These microginins differ in sequence and absolute configuration of the chiral centers of the Ahda moieties and by N-methylation of the Ahda amine group and extent of chlorination of the Ahda terminal methyl group. The compounds were evaluated for inhibition of the zinc metalloprotease, aminopeptidase M, and exhibited low- to sub-nanomolar half maximal inhibitory concentration (IC₅₀) values.

New prenylated aeruginosin, microphycin, anabaenopeptin and micropeptin analogues from a Microcystis bloom material collected in Kibbutz Kfar Blum, Israel

Thirteen new and eighteen known natural products were isolated from a bloom material of an assembly of various Microcystis spp. collected in November, 2008, from a commercial fishpond near Kibbutz Kfar Blum, the Jordan Valley, Israel. The new natural products included the prenylated aeruginosin KB676 (1), microphycin KB921 (2), anabaenopeptins KB906 (3) and KB899 (4) and micropeptins KB928 (5), KB956 (6), KB970A (7), KB970B (8), KB984 (9), KB970C (10), KB1048 (11), KB992 (12) and KB1046 (13). Their structures were elucidated primarily by interpretation of their 1D and 2D nuclear magnetic resonance spectra and high-resolution mass spectrometry. Marfey's and chiral-phase high performance liquid chromatography methods were used to determine the absolute configurations of their chiral centers. Aeruginosin KB676 (1) contains the rare (2S,3aS,6S,7aS)-Choi and is the first prenylated aeruginosin derivative described in the literature. Compounds 1 and 5-11 inhibited trypsin with sub-μM IC50s, while Compounds 11-13 inhibited chymotrypsin with sub-μM IC50s. The structures and biological activities of the new natural products and our procedures of dereplication are described.

Protease inhibitors from Microcystis aeruginosa bloom material collected from the Dalton Reservoir, Israel

Nine new metabolites, aeruginosins DA495A (1), DA511 (2), DA642A (3), DA642B (4), DA688 (5), DA722 (6), and DA495B (7), microguanidine DA368 (8), and anabaenopeptin DA850 (9), were isolated along with the known micropeptins MZ924, MZ939A, and MZ1019, cyanopeptolins S and SS, microcin SF608, and aeruginazoles DA1497, DA1304, and DA1274 from bloom material of the cyanobacterium Microcystis aeruginosa collected from the Dalton reservoir, Israel, in October 2007. Their structures were elucidated by a combination of various spectroscopic techniques, primarily NMR and MS, while the absolute configurations of the asymmetric centers were determined by Marfey's and chiral-phase HPLC methods. Two of the new aeruginosins, DA511 (1) and DA495A (2), contain a new Choi isomer, (2S,3aS,6S,7aS)-Choi. The structure elucidation and biological activities of the new metabolites are described.

Metabolites from Microcystis aeruginosa bloom material collected at a water reservoir near Kibbutz Hafetz Haim, Israel

An aqueous MeOH extract of Microcystis aeruginosa (IL-399) afforded three new protease inhibitors, micropeptin HH978 (1), micropeptin HH960 (2), and micropeptin HH992 (3), as well as the known aeruginosin GH553 and microguanidine AL772. The structures of the compounds were elucidated using 1D and 2D NMR techniques, as well as high-resolution mass spectrometry. The absolute configurations of 1-3 were determined using Marfey's method for amino acid and chiral-phase HPLC for hydroxy acids. The inhibitory activity of the compounds was determined for the serine proteases trypsin, thrombin, elastase, and chymotrypsin. The structure elucidation and biological activities of the new natural products are discussed.

SAR analysis and bioactive potentials of freshwater and terrestrial cyanobacterial compounds: a review

Freshwater and terrestrial cyanobacteria resemble the marine forms in producing divergent chemicals such as linear, cyclic and azole containing peptides, alkaloids, cyclophanes, terpenes, lactones, etc. These metabolites have wider biomedical potentials in targeting proteases, cancers, parasites, pathogens and other cyanobacteria and algae (allelopathy). Among the various families of non-marine cyanobacterial peptides reported, many of them are acting as serine protease inhibitors. While the micropeptin family has a preference for chymotrypsin inhibition rather than other serine proteases, the aeruginosin family targets trypsin and thrombin. In addition, cyanobacterial compounds such as scytonemide A, lyngbyazothrins C and D and cylindrocyclophanes were found to inhibit 20S proteosome. Apart from proteases, metabolites blocking the other targets of cancer pathways may exhibit cytotoxic effect. Colon and rectum, breast, lung and prostate are the worst affecting cancers in humans and are deduced to be inhibited by both peptidic and non-peptidic compounds. Moreover, the growth of infections causing parasites such as Plasmodium, Leishmania and Trypanosoma are well controlled by peptides: aerucyclamides A-D, tychonamides and alkaloids: nostocarboline and calothrixins. Likewise, varieties of cyanobacterial compounds tend to inhibit serious infectious disease causing bacterial, fungal and viral agents. Interestingly, portoamides, spiroidesin, nostocyclamide and kasumigamide are the allelopathic peptides determined to suppress the growth of toxic cyanobacteria and nuisance algae. Thus cyanobacterial compounds have a broad bioactive spectrum; the analysis of SAR studies will not only assist to find out the mode of action but also reveal bioactive key components. Thereby, developing the drugs bearing these bioactive skeletons to treat various illnesses is wide open.

Metabolites of Microcystis aeruginosa bloom material from Lake Kinneret, Israel

Six new metabolites, micropeptin KT1042, microguanidine KT636, aeruginosins KT608A, KT608B, and KT650, and pseudoaeruginosin KT554, were isolated along with the known micropeptins SF909 and HM978, cyanopeptolin S, anabaenopeptin F, and the two isomers of planktocyclin-S-oxide from a bloom material collected from Lake Kinneret, Israel, in March 2005. The structure elucidation and biological activity of the six new natural products isolated from this bloom material and the related aeruginosin GH553 are described.

Linking chemistry and genetics in the growing cyanobactin natural products family

Ribosomal peptide natural products are ubiquitous, yet relatively few tools exist to predict structures and clone new pathways. Cyanobactin ribosomal peptides are found in ~30% of all cyanobacteria, but the connection between gene sequence and structure was not defined, limiting the rapid identification of new compounds and pathways. Here, we report discovery of four orphan cyanobactin gene clusters by genome mining and an additional pathway by targeted cloning, which represented a tyrosine O-prenylating biosynthetic pathway. Genome mining enabled discovery of five cyanobactins, including peptide natural products from Spirulina supplements. A phylogenetic model defined four cyanobactin genotypes, which explain the synthesis of multiple cyanobactin structural classes and help direct pathway cloning and structure prediction efforts. These strategies were applied to DNA isolated from a mixed cyanobacterial bloom containing cyanobactins.

Cyanobactins-ribosomal cyclic peptides produced by cyanobacteria

Cyanobactins are small cyclic peptides that are produced by a diverse selection of cyanobacteria living in symbioses as well as terrestrial, marine, or freshwater environments. They include compounds with antimalarial, antitumor, and multidrug reversing activities and potential as pharmaceutical leads. Cyanobactins are produced through the proteolytic cleavage and cyclization of precursor peptides coupled with further posttranslational modifications such as heterocyclization, oxidation, or prenylation of amino acids. Cyanobactin gene clusters encode two proteases which cleave and cyclisize the precursor peptide as well as proteins participating in posttranslational modifications. The bioinformatic mining of cyanobacterial genomes has led to the discovery of novel cyanobactins. Heterologous expression of these gene clusters provided insights into the role of the genes participating in the biosynthesis of cyanobactins and facilitated the rational design of novel peptides. Enzymes participating in the biosynthesis of cyanobactins may prove useful as catalysts for producing novel cyclic peptides in the future. The recent discovery of the cyanobactin biosynthetic pathway in cyanobacteria extends our knowledge of their potential as producers of interesting metabolites.

Highly diverse cyanobactins in strains of the genus Anabaena

Cyanobactins are small, cyclic peptides recently found in cyanobacteria. They are formed through proteolytic cleavage and posttranslational modification of short precursor proteins and exhibit antitumor, cytotoxic, or multi-drug-reversing activities. Using genome project data, bioinformatics, stable isotope labeling, and mass spectrometry, we discovered novel cyclic peptides, anacyclamides, in 27 Anabaena strains. The lengths of the anacylamides varied greatly, from 7 to 20 amino acids. Pronounced sequence variation was also detected, and only one amino acid, proline, was present in all anacyclamides. The anacyclamides identified included unmodified proteinogenic or prenylated amino acids. We identified an 11-kb gene cluster in the genome of Anabaena sp. 90, and heterologous expression in Escherichia coli confirmed that this cluster was responsible for anacyclamide production. The discovery of anacyclamides greatly increases the structural diversity of cyanobactins.

Biosynthesis of the proteasome inhibitor syringolin A: the ureido group joining two amino acids originates from bicarbonate

BACKGROUND

Syringolin A, an important virulence factor in the interaction of the phytopathogenic bacterium Pseudomonas syringae pv. syringae B728a with its host plant Phaseolus vulgaris (bean), was recently shown to irreversibly inhibit eukaryotic proteasomes by a novel mechanism. Syringolin A is synthesized by a mixed non-ribosomal peptide synthetase/polyketide synthetase and consists of a tripeptide part including a twelve-membered ring with an N-terminal valine that is joined to a second valine via a very unusual ureido group. Analysis of sequence and architecture of the syringolin A synthetase gene cluster with the five open reading frames sylA-sylE allowed to formulate a biosynthesis model that explained all structural features of the tripeptide part of syringolin A but left the biosynthesis of the unusual ureido group unaccounted for.

RESULTS

We have cloned a 22 kb genomic fragment containing the sylA-sylE gene cluster but no other complete gene into the broad host range cosmid pLAFR3. Transfer of the recombinant cosmid into Pseudomonas putida and P. syringae pv. syringae SM was sufficient to direct the biosynthesis of bona fide syringolin A in these heterologous organisms whose genomes do not contain homologous genes. NMR analysis of syringolin A isolated from cultures grown in the presence of NaH(13)CO(3) revealed preferential (13)C-labeling at the ureido carbonyl position.

CONCLUSION

The results show that no additional syringolin A-specific genes were needed for the biosynthesis of the enigmatic ureido group joining two amino acids. They reveal the source of the ureido carbonyl group to be bicarbonate/carbon dioxide, which we hypothesize is incorporated by carbamylation of valine mediated by the sylC gene product(s). A similar mechanism may also play a role in the biosynthesis of other ureido-group-containing NRPS products known largely from cyanobacteria.