LAUSR

Xantholipin (1), a polycyclic xanthone antibiotic, exhibited strong antibacterial activities and showed potent cytotoxicity. The biosynthetic gene cluster of 1 has been identified in our previous work and the construction of xanthone nucleus has been well demonstrated. However, limited information of the halogenation involved in 1 biosynthesis is known. In this study, based on the genetic manipulation and biochemical assay, we characterized XanH as an indispensable FAD-dependent halogenase (FDH) for the biosynthesis of 1 XanH was found to be a bifunctional protein capable of flavin reduction and chlorination, and exclusively used the reduced nicotinamide adenine dinucleotide (NADH). However, the reduced flavin could not be fully and effectively utilized, and the presence of an extra flavin reductase (FDR) and chemical reducing agent could promote the halogenation. XanH accepted its natural free-standing substrate with angular fused polycyclic aromatic systems. Meanwhile, it exhibited moderate halogenation activity and possessed high substrate specificity. The requirement of extra FDR for higher halogenation activity is tedious for future engineering development. To facilitate efforts in engineering construction of XanH derivative proteins, we constructed the self-sufficient FDR-XanH fusion proteins. The fusion protein E1 with comparable activities could be used as a good alternative for future protein engineering. Taken together, these findings reported here not only improved the understanding of polycyclic xanthones biosynthesis but also expand the substrate scope of FDH and pave the way for future engineering construction of biocatalysts for new active substance synthesis.IMPORTANCE Halogenation is important in medicinal chemistry and plays essential role in the biosynthesis of active secondary metabolites. Halogenases have evolved to catalyze reactions with high efficiency and selectivity, and engineering efforts have been made to engage the selective reactivity in natural product biosynthesis. The enzymatic halogenations are environmentally friendly green approach with high regio- and stereoselectivity, which make it a potential complement to organic synthesis. FDHs is one of the most extensively elucidated class of halogenases, however the inventory awaits to be expanded for biotechnology applications and for the generation of halogenated natural product analogues. Here, XanH was found to reduce flavin and halogenated the freely diffusing natural substrate with angular fused hexacyclic scaffold, which were different from the exclusively studied FDHs. Moreover, the FDR-XanH fusion protein E1 with comparable reactivity serves as a successful example of genetic fusions and set important stage for future protein engineering development.