Abstract
Ophiobolins are a group of sesterterpenoids with a 5-8-5 tricyclic skeleton. They exhibit a significant cytotoxicity and present potential medicinal prospects. However, the biosynthesis and transport mechanisms of these valuable compounds have not been fully resolved. Herein, based on a transcriptome analysis, gene inactivation, heterologous expression and feeding experiments, we fully explain the biosynthesis pathway of ophiobolin K in Aspergillus ustus 094102, especially proved to be an unclustered oxidase OblCAu that catalyzes dehydrogenation at the site of C16 and C17 of both ophiobolin F and ophiobolin C. We also find that the intermediate ophiobolin C and final product ophiobolin K could be transported into a space between the cell wall and membrane by OblDAu to avoid the inhibiting of cell growth, which is proved by a fluorescence observation of the subcellular localization and cytotoxicity tests. This study completely resolves the biosynthesis mechanism of ophiobolins in strain A. ustus 094102. At the same time, it is revealed that the burden of strain growth caused by the excessive accumulation and toxicity of secondary metabolites is closely related to compartmentalized biosynthesis.
Highlights
Nine new sesterterpenoids synthesized by a silent gene cluster in Aspergillus ustus 094102 were identified by gene mining, among which aspergilol A and B showed cytotoxic activities against MCF-7 [5]
In the obl of A. ustus 094102, both oblAAu and oblBAu showed a higher identity of 82.28% and 79.38% with genes in oblAs compared to genes in oblAc or oblBm by amino acid sequence alignments (Table S1), which showed their similar functions to genes in oblAs
To confirm the genes involved in ophiobolin biosynthesis in A. ustus 094102, we performed RNA-Seq and quantitative real-time analyses at ophiobolin-producing and non-ophiobolin-producing conditions
Summary
Terpenoids, with complicated and diverse structures, are the largest category of natural products. More than 175,000 kinds of terpenoid-like and terpenoidderived compounds have been collected in TeroKit (http://terokit.qmclab.com/, accessed on 1 November 2021) [1]. Sesterterpenoids are a class of rare terpenoids containing five isoprenyl units and are mainly isolated from metazoan, plants and fungi [1]. The amount of sesterterpenoids only accounts for less than 1.5% of the total terpenoids, many of these compounds show broad-spectrum biological activities and promising application prospects [1]. The 14/18-membered sesterterpenoids synthesized by terpene synthase LcTPS2 from Leucosceptrum canum exhibit significant immunosuppressive activity [2]. Asperterpenols A and B isolated from Aspergillus sp. Nine new sesterterpenoids synthesized by a silent gene cluster in Aspergillus ustus 094102 were identified by gene mining, among which aspergilol A and B showed cytotoxic activities against MCF-7 [5]
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