Metagenome Mining Reveals Polytheonamides as Posttranslationally Modified Ribosomal Peptides

Made and Modified The polytheonamides are 48-residue toxins derived from marine sponges that include 18 D-amino acids, as well as many other unusual amino acid modifications. Given the complexity, one might guess that these peptides are the product of nonribosomal, peptide synthetase (NRPS). However, Freeman et al. (p. 387, published online 13 September now show that polytheonamides are produced by a bacterial symbiont using a ribosomal pathway. Six candidate enzymes for the 48 posttranslational modifications were identified and three were functionally validated. Such ribosomal systems could be useful in bioengineering. Large toxins that comprise many modified and d-amino acids are ribosomally synthesized and then derivatized. It is held as a paradigm that ribosomally synthesized peptides and proteins contain only l-amino acids. We demonstrate a ribosomal origin of the marine sponge–derived polytheonamides, exceptionally potent, giant natural-product toxins. Isolation of the biosynthetic genes from the sponge metagenome revealed a bacterial gene architecture. Only six candidate enzymes were identified for 48 posttranslational modifications, including 18 epimerizations and 17 methylations of nonactivated carbon centers. Three enzymes were functionally validated, which showed that a radical S-adenosylmethionine enzyme is responsible for the unidirectional epimerization of multiple and different amino acids. Collectively, these complex alterations create toxins that function as unimolecular minimalistic ion channels with near-femtomolar activity. This study broadens the biosynthetic scope of ribosomal systems and creates new opportunities for peptide and protein bioengineering.

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