Catalytic Use of a Leader Peptide in the Biosynthesis of 3-Thiaglutamate

Small molecule natural products are key modulators of many types of intra- and interspecies communication. The availability of genome sequences allows the discovery of pathways to previously unknown natural products. We describe here a pathway in which a ribosomally synthesized small peptide serves as a catalytic scaffold on which a small-molecule anti-metabolite is biosynthesized in Pseudomonas syringae. First, a cysteine residue is transferred from Cys-tRNA to the C-terminus of the peptide, a reaction that replaces ribosomal protein synthesis. Then, a translocation of the cysteine thiol from the {beta}-carbon to the -carbon is catalyzed by an oxidase that removes the {beta}-carbon as formate. The resulting thiol is carboxymethylated and proteolysis releases 3-thiaglutamate, in the process regenerating the peptide scaffold. This pathway features three previously unknown biochemical processes.