Biosynthesis of a Tricyclo[6.2.2.02,7]dodecane System by a Berberine Bridge Enzyme‐Like Aldolase

The aldol reaction is one of the most fundamental stereocontrolled carbon–carbon bond‐forming reactions and is mainly catalyzed by aldolases in nature. Despite the fact that the aldol reaction has been widely proposed to be involved in fungal secondary metabolite biosynthesis, a dedicated aldolase that catalyzes stereoselective aldol reactions has only rarely been reported in fungi. Herein, we activated a cryptic polyketide biosynthetic gene cluster that was upregulated in the fungal wheat pathogen Parastagonospora nodorum during plant infection; this resulted in the production of the phytotoxic stemphyloxin II (1). Through heterologous reconstruction of the biosynthetic pathway and in vitro assay by using cell‐free lysate from Aspergillus nidulans, we demonstrated that a berberine bridge enzyme (BBE)‐like protein SthB catalyzes an intramolecular aldol reaction to establish the bridged tricyclo[6.2.2.02,7]dodecane skeleton in the post‐assembly tailoring step. The characterization of SthB as an aldolase enriches the catalytic toolbox of classic reactions and the functional diversities of the BBE superfamily of enzymes. Activation of a polyketide biosynthetic gene cluster sth in a fungal plant pathogen Parastagonospora nodorum resulted in the production of a phytotoxin stemphyloxin II. Heterologous pathway reconstruction and in vitro biochemical characterization demonstrated a BBE‐like enzyme SthB catalyzes an intramolecular aldol reaction to synthesize the tricyclo[6.2.2.02,7]dodecane scaffold (see scheme).