A Wagner–Meerwein‐Like Rearrangement Generates a Vinyl Group in the Biosynthesis of the Polychlorinated Lipopeptides Fischerazoles

The discovery of structurally unusual natural products is a major inspiration in the search for novel enzymatic transformations. In this study, we employ stable isotope‐labeled precursor feeding and metabolomics to reveal highly unusual cyanobacterial lipopeptides—fischerazoles A‐C. These metabolites contain a polychlorinated fatty acyl‐derived moiety with a vinyl branch and an N ‐methylated carboxamide‐thiazole terminus. Most strikingly, stable‐isotope‐labeled precursor supplementation showed that during fischerazole biosynthesis, a linear, hexadecanoic acid‐derived intermediate is processed to yield the ultimately branched alkyl moiety. The branched carbon becomes part of the vinyl group together with an S ‐adenosyl methionine (SAM)‐derived carbon. A candidate polyketide synthase/non‐ribosomal peptide synthetase (PKS/NRPS) fischerazoles biosynthetic gene cluster— fsh —was identified in the genome of the producing strain, Fischerella sp. PCC 9431. In vitro assays experimentally connected fsh genes to fischerazole biosynthesis and revealed that the SAM‐dependent methyltransferase FshF catalyzes a Wagner–Meerwein‐like rearrangement to generate the vinyl group from an ACP‐tethered unsaturated fatty acid. The discovery of FshF, which is related to the well‐characterized cyclopropane fatty acid synthases (CFASs), brings to light a new biocatalytic strategy for alkyl chain functionalization.