Discovery and Biosynthesis of Cytotoxic Myroverols: A Phenalenone Scaffold Constructed by a Fungal Highly Reducing Polyketide Synthase and a Cytochrome P450

Through genome mining and heterologous expression, a fungal highly reducing polyketide synthase with a fused <i>C</i>-terminal reductase domain (HRPKS-R) encoding gene cluster, the <i>myr</i> cluster, was demonstrated to be responsible for the production of myroverols with antitumor activity. The biosynthesis of myroverols includes the generation of a decalin intermediate by an HRPKS-R enzyme and its partner enoyl reductase, and this intermediate can be converted into two different types of products depending on whether it is reduced to an alcohol. Without reduction, this intermediate can be further oxidized and cyclized to a phenalenone derivative by the cytochrome P450 MyrE. With reduction catalyzed by a short chain dehydrogenase/reductase MyrC, this intermediate can be further converted into a carboxylated decalin derivative by the same P450 MyrE. Our study unraveled a novel biosynthetic route to construct phenalenone derivatives and expand the chemical space of natural products derived from fungal HRPKS-R biosynthetic pathways.