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Cobalamin- or B12-dependent radical S-adenosylmethionine (SAM) enzymes acting during carbapenem antibiotic biosynthesis carry out radical-mediated methyl transfers that underlie the therapeutic usefulness of these essential medicines. Here we present x-ray crystal structures of TokK, which are representative of this functional class, containing its two metallocofactors and determined in the presence and absence of carbapenam substrate. The structures give the first visualization of a cobalamin-dependent radical SAM methylase that employs the radical mechanism shared by a vast majority of these enzymes. The structures provide insight into the stereochemistry of initial C6 methylation and suggests that substrate positioning governs the rate of each methylation event.One Sentence SummaryStructural insight into a cobalamin-dependent radical SAM methylase that performs three sequential radical-mediated methylations to install the C6 side chain of a carbapenem antibiotic.

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Species in the genus Cercospora cause economically devastating diseases in sugar beet, maize, rice, soy bean and other major food crops. Here we sequenced the genome of the sugar beet pathogen C. beticola and found it encodes 63 putative secondary metabolite gene clusters, including the cercosporin toxin biosynthesis (CTB) cluster. We show that the CTB gene cluster has experienced multiple duplications and horizontal transfers across a spectrum of plant pathogenic fungi, including the wide host range Colletotrichum genus as well as the rice pathogen Magnaporthe oryzae. Although cercosporin biosynthesis has been thought to-date to rely on an eight gene CTB cluster, our phylogenomic analysis revealed gene collinearity adjacent to the established cluster in all CTB cluster-harboring species. We demonstrate that the CTB cluster is larger than previously recognized and includes cercosporin facilitator protein (CFP) previously shown to be involved with cercosporin auto-resistance, and four additional genes required for cercosporin biosynthesis including the final pathway enzymes that install the unusual cercosporin methylenedioxy bridge. Finally, we demonstrate production of cercosporin by Colletotrichum fioriniae, the first known cercosporin producer within this agriculturally important genus. Thus, our results provide new insight into the intricate evolution and biology of a toxin critical to agriculture and broaden the production of cercosporin to another fungal genus containing many plant pathogens of important crops worldwide.Significance StatementSpecies in the fungal genus Cercospora cause diseases in many important crops worldwide. Their success as pathogens is largely due to the secretion of cercosporin during infection. We report that the cercosporin toxin biosynthesis (CTB) cluster is ancient and was horizontally transferred to diverse fungal pathogens on an unprecedented scale. Since these analyses revealed genes adjacent to the established CTB cluster, we evaluated their role in C. beticola to show that four are necessary for cercosporin biosynthesis. Finally, we confirmed that the apple pathogen Colletotrichum fioriniae produces cercosporin, the first case outside the family Mycosphaerellaceae. Other Colletotrichum plant pathogens also harbor the CTB cluster, which points to a wider concern that this toxin may play in virulence and human health.

ble mode of biosynthesis of corrins (as 3), for although it has been known for almost 20 years that vitamin B12 shares the "early" part of heme, chlorophyll and tetrapyrrole biosynthesis in that it is built up via the succinate-glycine/ -aminolevulinate sequence,10 the point at which the "cobalt" route divides from the "iron" and "magnesium" pathways was unknown at the outset of our investigation. From