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.