Parallel evolution of plant alkaloid biosynthesis from bacterial-like decarboxylases

Alkaloids are nitrogen-containing natural products derived from amino acids. The basic amino acids lysine and ornithine are precursors to a wide range of alkaloids including the bioactive compounds nicotine, hyoscyamine and securinine. Feeding experiments have shown that the amino acids can be incorporated into alkaloids in a symmetric or nonsymmetric manner. The symmetric pathway is catalysed by two enzymes, a decarboxylase and oxidase, forming a cyclic iminium which acts as the electrophile in the scaffold forming step. Here, we describe the ornithine/lysine/arginine decarboxylase-oxidases (OLADOs), PLP-dependent enzymes responsible for the nonsymmetric pathway, catalysing the single step decarboxylative oxidative deamination of lysine, ornithine or arginine. These enzymes are group III ornithine/lysine/arginine decarboxylases (OLADs), an enzyme class previously exclusively associated with prokaryotes. We reveal OLADs to be widespread in plants and show that OLADOs have repeatedly emerged through parallel evolution from OLADs, via similar active site substitutions. This investigation introduces a new class of eukaryotic decarboxylases, and describes enzymes involved in multiple alkaloid biosynthesis pathways. It furthermore demonstrates how the principle of parallel evolution at a genomic and enzymatic level can be leveraged for gene discovery across multiple lineages.