Nat. Chem. 10.1038/nchem.2666 (2016)

Credit: NATURE CHEMISTRY

Polytheonamides are ribosomally synthesized and post-translationally modified peptide (RiPP) natural products made by a bacterial symbiont in a marine sponge. Among other modifications, the peptide features alternating L- and D-configured amino acids, likely causing it to adopt a β-helical structure. Using heterologous expression in Escherichia coli and in vitro reconstitution with select precursor peptide variants, Freeman et al. characterized the set of enzymes that comprise the polytheonamide biosynthetic pathway and found that only seven enzymes are sufficient to install dozens of post-translational modifications. A single epimerase iteratively introduces all 18 d-amino acids in a C- to N-terminal direction, and then an N-methyltransferase installs methyl groups on d-asparagine residues. The biosynthetic pathway also involves β-hydroxylation of valine and asparagine residues by an oxygenase, modification of the N-terminal residue of the core peptide by a dehydratase and removal of the leader peptide by a protease. To reconstitute the 17 methylations that occur at unactivated carbon centers, the authors also used an unconventional rhizobial heterologous host for expression of the two cobalamin-dependent C-methyltransferases, each of which act on distinct halves of the core peptide. This detailed understanding of polytheonamide biosynthesis adds new options to the collective toolbox of RiPP biosynthetic enzymes, which could be used to engineer new potentially bioactive compounds.