mazEF is a toxin-antitoxin module located on many bacterial chromosomes, including those of pathogens. Here, we report that Escherichia coli mazEF-mediated cell death is a population phenomenon requiring a quorum-sensing molecule that we call the extracellular death factor (EDF). Structural analysis revealed that EDF is a linear pentapeptide, Asn-Asn-Trp-Asn-Asn. Each of the five amino acids of EDF is important for its activity.

Aphanizomenon ovalisporum (Forti) was identijied and isolated j o m Lake Kinneret upon its first appearance as a dominant bloom in late 1994. This cyanobactm'al species, not previously known to be toxic, was evaluated by a commonly used mouse bioassay and was demonstrated to induce toxic symptoms that were distinguishabb from the typical symptoms of the neurotoxins previously repmted in Aphanizomenon flos-aquae (L.) Ralfs. Mice died 5-24 h after crude extracts were injected intrapm'toneally, and the LDj0 value was estimated as 465 mg dry wt biomass.kg-mouse. A toxicity-guided ji-actionation of the active extract indicated that the potent substance is polar an nature. The structure of the active compound was determined by its mass spectrometry and NMR data. The compound was found to be the sulfate-guanidinium zm'tterion, cylindrosperm@in, previously isolated @om the cyanobacterium Cylindrospermopsis raciborskii (Woloszynska) and recently also reported in Umezakia natans (Watanabe). This is the first time that Aphanizomenon ovalisporum has been reported to contain a toxic compound.Lake Kinneret serves as the main freshwater reservoir in Israel, providing about 30% of the national water requirements, besides being a recreational site and supporting a commercial fishery. The importance of maintaining high water quality led to an intensive and routine monitoring program of the

Microcystins constitute a serious threat to the quality of drinking water worldwide. These protein phosphatase inhibitors are formed by various cyanobacterial species, including Microcystis sp. Microcystins are produced by a complex microcystin synthetase, composed of peptide synthetases and polyketide synthases, encoded by the mcyA-J gene cluster. Recent phylogenetic analysis suggested that the microcystin synthetase predated the metazoan lineage, thus dismissing the possibility that microcystins emerged as a means of defence against grazing, and their original biological role is not clear. We show that lysis of Microcystis cells, either mechanically or because of various stress conditions, induced massive accumulation of McyB and enhanced the production of microcystins in the remaining Microcystis cells. A rise in McyB content was also observed following exposure to microcystin or the protease inhibitors micropeptin and microginin, also produced by Microcystis. The extent of the stimulation by cell extract was strongly affected by the age of the treated Microcystis culture. Older cultures, or those recently diluted from stock cultures, hardly responded to the components in the cell extract. We propose that lysis of a fraction of the Microcystis population is sensed by the rest of the cells because of the release of non-ribosomal peptides. The remaining cells respond by raising their ability to produce microcystins thereby enhancing their fitness in their ecological niche, because of their toxicity.

The EtOAc extract of the whole culture medium of Vibrio parahaemolyticus, which inhabits the toxic mucus of the box fish Ostracion cubicus, afforded a new indole-derived natural product, vibrindole A [1], along with some known cyclic dipeptides and indoles. The structure of 1 was determined by analysis of its physicochemical characteristics.

A toxic minor metabolite, 7-epicylindrospermopsin (1), was isolated from a culture of the cyanobacterium Aphanizomenon ovalisporum isolated from Lake Kinneret in Israel. Homonuclear and inverse-heteronuclear 2D NMR techniques, as well as HRMS and comparison of the NMR data with model compounds, enabled the structure determination of the new compound. Four polymethoxy-1-alkenes, 3-6, were isolated from the lipophilic extract of the cyanobacterium as well.

Biofilms are consortia of bacteria that are held together by an extracellular matrix. Cyanobacterial biofilms, which are highly ubiquitous and inhabit diverse niches, are often associated with biological fouling and cause severe economic loss. Information on the molecular mechanisms underlying biofilm formation in cyanobacteria is scarce. We identified a mutant of the cyanobacterium Synechococcus elongatus, which unlike the wild type, developed biofilms. This biofilm-forming phenotype is caused by inactivation of homologues of type II secretion /type IV pilus assembly systems and is associated with impairment of protein secretion. The conditioned medium from a wild-type culture represses biofilm formation by the secretion-mutants. This suggested that the planktonic nature of the wild-type strain is a result of a self-suppression mechanism, which depends on the deposition of a factor to the extracellular milieu. We also identified two genes that are essential for biofilm formation. Transcript levels of these genes are elevated in the mutant compared with the wild type, and are initially decreased in mutant cells cultured in conditioned medium of wild-type cells. The particular niche conditions will determine whether the inhibitor will accumulate to effective levels and thus the described mechanism allows switching to a sessile mode of existence.

Phytoalexin scoparone (6,7-dimethoxycoumarin) generally was not detected in noninoculated lemon fruit (Citrus limon [L.] Burm., cv Eureka) but accumulated in fruit after inoculation with Penicillium digitatum Sacc. A much greater increase in the amount of scoparone was found in fruit exhibiting an incompatible response to Penicillium after heat treatment at 360C for 3 days. Heat treatment prevented development of decay in the inoculated fruit. The concentration of the compound after inoculation continued to increase during and after the heat treatment period, reaching 178 micrograms per gram fresh weight of the flavedo 6 days after the heat treatment. Changes in scoparone concentration in fruit were closely correlated with the changes in the antifungal activity of the fruit extract. A low concentration of the phytoalexin was detected in fruit injured mechanically. Scoparone also accumulated in the fruit following ultraviolet illumination; the concentration of the compound was dose-dependent. Median effective dose values of the inhibition of germ tube elongation and spore germination of P. digitatum were 29 and 46 micrograms per milliliter, respectively. Our findings suggest that the rapid increase in scoparone concentration plays an important role in the increased resistance of heat-treated lemon fruit to infection by P. digitatum.

Bioassay-guided fractionation of the 7:3 MeOH/water extract of a cultured cyanobacterium strain identified as Fischerella sp. yielded nine isonitrile-containing alkaloids. Three of the compounds, ambiguine H isonitrile (1), ambiguine I isonitrile (2), and ambiguine J isonitrile (3), are new, while the other six, 12-epi-hapalindole H, ambiguine A isonitrile, ambiguine B isonitrile, ambiguine D isonitrile, ambiguine E isonitrile, and ambiguine F isonitrile, have been previously isolated from Fischerella ambigua. The structures of the compounds were determined by 1D and 2D NMR techniques and mass spectrometric data. Ambiguine H isonirile and ambiguine I isonirile possess antibacterial and antimycotic activity.