Domoic acid biosynthesis in the red alga <i>Chondria armata</i> suggests a complex evolutionary history for toxin production.

Abstract

Domoic acid (DA), the causative agent of amnesic shellfish poisoning, is produced by select organisms within two distantly related algal clades: planktonic diatoms and red macroalgae. The biosynthetic pathway to isodomoic acid A was recently solved in the harmful algal bloom-forming diatom <i>Pseudonitzschia multiseries</i>, establishing the genetic basis for the global production of this potent neurotoxin. Herein, we sequenced the 507-Mb genome of <i>Chondria armata</i>, the red macroalgal seaweed from which DA was first isolated in the 1950s, identifying several copies of the red algal DA (<i>rad</i>) biosynthetic gene cluster. The <i>rad</i> genes are organized similarly to the diatom DA biosynthesis cluster in terms of gene synteny, including a cytochrome P450 (CYP450) enzyme critical to DA production that is notably absent in red algae that produce the simpler kainoid neurochemical, kainic acid. The biochemical characterization of the <i>N</i>-prenyltransferase (RadA) and kainoid synthase (RadC) enzymes support a slightly altered DA biosynthetic model in <i>C. armata</i> via the congener isodomoic acid B, with RadC behaving more like the homologous diatom enzyme despite higher amino acid similarity to red algal kainic acid synthesis enzymes. A phylogenetic analysis of the <i>rad</i> genes suggests unique origins for the red macroalgal and diatom genes in their respective hosts, with native eukaryotic CYP450 neofunctionalization combining with the horizontal gene transfer of <i>N-</i>prenyltransferases and kainoid synthases to establish DA production within the algal lineages.