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Chartreusin is a potent antiproliferative agent that contains a unique aromatic pentacyclic bislactone carbon scaffold. The biosynthesis of type II polyketide aglycone has been extensively investigated and shown to proceed through a tetracyclic anthracycline intermediate. The last remaining unknown steps are the conversion of auramycinone to resomycin C. Here we have discovered three enzymes that play crucial roles in two mechanistically distinct dehydration reactions. We show that ChaX is an NAD(P)H-dependent auramycinone quinone reductase that allows the cyclase-like ChaU to catalyze the formation of 9,10-dehydroauramycinone via a carbanion intermediate. In contrast, the cyclase-like ChaJ, homologous to ChaU, is responsible for subsequent 7,8-dehydration via a canonical carbocation intermediate, yielding resomycin C. The results were confirmed via assembly of the biosynthetic pathway for production of resomycin C in Streptomyces coelicolor M1152ΔmatAB. The work expands the catalytic repertoire of the SnoaL protein family, which has previously been associated with anthracycline fourth-ring cyclization and two-component 1-hydroxylation. 

Abstract In₂O₃, an n‐type semiconducting transparent transition metal oxide, possesses a surface electron accumulation layer (SEAL) resulting from downward surface band bending due to the presence of ubiquitous oxygen vacancies. Upon annealing In₂O₃in ultrahigh vacuum or in the presence of oxygen, the SEAL can be enhanced or depleted, as governed by the resulting density of oxygen vacancies at the surface. In this work, an alternative route to tune the SEAL by adsorption of strong molecular electron donors (specifically here ruthenium pentamethylcyclopentadienyl mesitylene dimer, [RuCp*mes]₂) and acceptors (here 2,2′‐(1,3,4,5,7,8‐hexafluoro‐2,6‐naphthalene‐diylidene)bis‐propanedinitrile, F₆TCNNQ) is demonstrated. Starting from an electron‐depleted In₂O₃surface after annealing in oxygen, the deposition of [RuCp*mes]₂restores the accumulation layer as a result of electron transfer from the donor molecules to In₂O₃, as evidenced by the observation of (partially) filled conduction sub‐bands near the Fermi level via angle‐resolved photoemission spectroscopy, indicating the formation of a 2D electron gas due to the SEAL. In contrast, when F₆TCNNQ is deposited on a surface annealed without oxygen, the electron accumulation layer vanishes and an upward band bending is generated at the In₂O₃surface due to electron depletion by the acceptor molecules. Hence, further opportunities to expand the application of In₂O₃in electronic devices are revealed.