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.

Summary: 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.

A new natural derivative of the sulfated guanidinium zwitterionic toxin cylindrospermopsin, 7-epi-cylindrospermopsin, was recently isolated from the cyanobacterium Aphanizomenon ovalisporum (Forti). The toxicity of the molecule (LD50 ip 5 d), estimated by mouse bioassay, was 200 µg/kg mouse, a value similar to that of cylindrospermopsin. Treatment of cylindrospermopsin with chlorine solution or chlorine-related oxidants produced two new derivatives. The chemical structure of these products was elucidated by nuclear magnetic resonance (NMR) and mass spectrometry (MS) techniques and toxicity was determined. In the first derivative, the vinylic proton at position 5 of the pyrimidine ring was substituted by chlorine to yield 5-chlorocylindrospermopsin. The other product is a truncated one, where C-6 of the pyrimidine ring was oxidized to a carboxylic acid. A trivial name, cylindrospermic acid, was given to this compound. Both products showed no toxic effects even at doses 50 times higher than the LD50 of cylindrospermopsin (10 mg/kg mouse ip). Based on these results, the pyrimidine ring is postulated as the molecule component essential for the toxicity of cylindrospermopsin.