Schizotrins B-E from the Cultured Cyanobacterium CCIBt3280 - Unique Biosynthetic Signatures and Evidence for an Antibacterial Target

We report the discovery and chemical structures of schizotrins B-E (1-4) isolated from cultures of Phormidium sp. CCIBt3280. These peptides were identified by spectroscopic and spectrometric analyses. Their absolute configuration was established by Marfey’s and advanced NMR analysis, computational modeling, and Phormidium sp. CCIBt3280 genome analysis. Conserved A- and B-type ketoreductase motifs revealed a stereochemical control of αketoreduction in polyketide natural products. Phylogenomic analyses placed Phormidium sp. CCIBt3280 within the Floridanema family-level clade, indicating that its current assignment to Phormidium should be reassessed. Genome mining uncovered numerous previously unknown schizotrin-like biosynthetic gene clusters in publicly available genomes. The antibacterial activity of peptides 1-4 were also investigated. Schizotrin B inhibited the growth of Bacillus anthracis Sterne at 1 µM. Whole-genome sequencing of spontaneous resistant mutants of B. anthracis revealed a loss-of-function mutation in psd (phosphatidylserine decarboxylase), implicating membrane phospholipid homeostasis in schizotrin B activity. Our results expanded the chemical diversity of the schizotrin family, provided new insights into the biosynthesis of αcarbonyl-containing amino acids, supported the placement of the schizotrin-producing lineage within a potentially novel cyanobacterial genus, and provided insights into the potential mechanism of schizotrins antibacterial resistance.