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Funding This work was supported by grants from the Academy of Finland (259505, D.P.F.), Helsinki University Research grant (490085, D.P.F.) ESCMID grant (4720572, D.P.F.), the Industrial Biotechnology Innovation Centre (IBioIC) studentship (L. D.), the Jane and Aatos Erkko Foundation (K.S.), the BBSRC FoF grant (no BB/M013669/1, W. E. H.), IBCatalyst grant (no. BB/M028526/1, W. E. H.), the Sarcoma UK grant (W. E. H.) and the SULSA Leaders and SULSA PECRE awards (W. E. H.). W. E. H. acknowledges the fund from the ERC grant no. 339367. ACKNOWLEDGEMENTS D.P.F. and K.S. are grateful to Lyudmila Saari, Department of Microbiology, University of Helsinki, for her valuable help in handling the Anabaena sp. UHCC-0232 culture. W. E. H. thanks the Aberdeen Proteomics Facility and the Marine Biodiscovery Centre Mass Spectrometry Facility for extensive MS analysis. W. E. H. is grateful to Mr. Russell Gray (Marine Biodiscovery Centre, University of Aberdeen) for the NMR analysis of our samples.

Macrocyclic peptides have promising therapeutic potential but the scaling up of their chemical synthesis is challenging. The cyanobactin macrocyclase PatGmac is an efficient tool for production but is limited to substrates containing 6–11 amino acids and at least one thiazoline or proline. Here we report a new cyanobactin macrocyclase that can cyclize longer peptide substrates and those not containing proline/thiazoline and thus allows exploring a wider chemical diversity.

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.

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.

Cyanobactins are small cyclic peptides produced by cyanobacteria. Here we demonstrate the widespread but sporadic occurrence of the cyanobactin biosynthetic pathway. We detected a cyanobactin biosynthetic gene in 48 of the 132 strains included in this study. Our results suggest that cyanobactin biosynthetic genes have a complex evolutionary history in cyanobacteria punctuated by a series of ancient horizontal gene transfer events.