Genome Mining-Guided Discovery of the Glycosylated Griseorhodin Congener Ruskamycin

Aromatic polyketides from Actinobacteria are a class of natural products with potential for clinical application due to their antibiotic and cytotoxic activities. Among the most complex members are the closely related rubromycins and griseorhodins, which are derived from a pentangular backbone that undergoes extensive oxidative tailoring to afford a characteristic bisbenzannulated spiroketal pharmacophore. Recent insights into the characteristic genetics and enzymology underlying the biosynthesis of these polyketides now enable the bioinformatics-driven search for further congeners. Here, we analyzed 154 available and manually selected rubromycin/griseorhodin biosynthetic gene clusters (BGCs) with the help of the tool BisCEET, which allowed the rapid identification and visualization of "variant-specific tailoring genes" (VSTGs), encoding putative tailoring enzymes required only for individual congeners. This approach revealed a striking BGC variant in the genome of the Actinomycete Actinacidiphila soli, predicted to enable the biosynthesis of a glycosylated griseorhodin variant. Following the successful cultivation of the strain, the production of a previously uncharacterized griseorhodin, named ruskamycin, was confirmed that featured a distinctive digitoxose-substituted epoxy-spiroketal pharmacophore and exhibited antibacterial activity against Gram-positive strains.