Animal-encoded nonribosomal pathway to bursatellin analogs

The bursatellin-oxazinin family is a series of tyrosine-derived, nitrile-containing marine natural products from gastro-pod and bivalve molluscs. Although the first analogs were identified and associated with toxicity forty years ago, their biosynthetic origins were unknown. During an investigation of published mollusc genomes and transcriptomes, we serendipitously identified a putative bursatellin biosynthetic gene cluster (referred hereafter as the bur-ox pathway). Through biochemical characterization of some bur-ox genes, we provide evidence suggesting that bursatellin-type metabolites are produced by molluscs themselves rather than by their microbial symbionts. We show that the reductive domain from a monomodular nonribosomal peptide synthetase (NRPS) protein FmtATR performs a four-electron reduction to produce tyrosinols from tyrosine derivatives. Moreover, an aminocarboxypro-pyltransferase enzyme, ACT, uses S -adenosylmethionine (SAM) to transform tyrosinols into their phenolic homoserine ethers, which in bursatellin is further modified to the nitrile. Widespread occurrence of bur-ox in molluscs suggests a common biosynthetic origin for bursatellins and oxazinins as well as an important but currently unidentified physiological role for this metabolite family in molluscs inhabiting diverse ecological niches. Further, the presence of bur-ox pathway homologs in many culinary bivalves such as mussels and geoducks suggests that possible impacts on human consumers should be investigated. As one of the few NRPS pathways of animal origin to be characterized, bur-ox sheds light on underappreciated chemical and biochemical diversity in animals.