Swainsonine Biosynthesis Genes in Diverse Symbiotic and Pathogenic Fungi

Abstract

Swainsonine-a cytotoxic fungal alkaloid and a potential cancer therapy drug-is produced by the insect pathogen and plant symbiont <i>Metarhizium robertsii</i>, the clover pathogen <i>Slafractonia leguminicola</i>, locoweed symbionts belonging to <i>Alternaria</i> sect. <i>Undifilum</i>, and a recently discovered morning glory symbiont belonging to order Chaetothyriales. Genome sequence analyses revealed that these fungi share orthologous gene clusters, designated "<i>SWN</i>," which included a multifunctional <i>swnK</i> gene comprising predicted adenylylation and acyltransferase domains with their associated thiolation domains, a β-ketoacyl synthase domain, and two reductase domains. The role of <i>swnK</i> was demonstrated by inactivating it in <i>M. robertsii</i> through homologous gene replacement to give a ∆<i>swnK</i> mutant that produced no detectable swainsonine, then complementing the mutant with the wild-type gene to restore swainsonine biosynthesis. Other <i>SWN</i> cluster genes were predicted to encode two putative hydroxylases and two reductases, as expected to complete biosynthesis of swainsonine from the predicted SwnK product. <i>SWN</i> gene clusters were identified in six out of seven sequenced genomes of <i>Metarhzium</i> species, and in all 15 sequenced genomes of Arthrodermataceae, a family of fungi that cause athlete's foot and ringworm diseases in humans and other mammals. Representative isolates of all of these species were cultured, and all <i>Metarhizium</i> spp. with <i>SWN</i> clusters, as well as all but one of the Arthrodermataceae, produced swainsonine. These results suggest a new biosynthetic hypothesis for this alkaloid, extending the known taxonomic breadth of swainsonine producers to at least four orders of Ascomycota, and suggest that swainsonine has roles in mutualistic symbioses and diseases of plants and animals.