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Marine Cyclopeptide Biosynthesis: Heterologous Characterization
Update time: 2021-09-03
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Recently, Gong-Li Tang’s group from Shanghai Institution of Organic Chemistry, University of Chinese Academy of Science, reported the biosynthetic pathway of mechercharmycin A (MCM-A), a marine natural product belonging to a family of polyazole cyclopeptides with remarkable bioactivities and unique structures. This article was published on Cell Chemical Biology (Heterologous characterization of mechercharmycin A biosynthesis reveals alternative insights into posttranslational modifications for RiPPs, Cell Chem. Biol. https://doi.org/10.1016/j.chembiol.2021.08.005).
Through bioinformatics analysis and structural analysis of MCM-A, the authors presumed that the compound is a natural product of ribosomally synthesized and post-translationally modified peptides (RiPPs). With this knowledge, the authors sequenced the genome of MCM producer, Thermoactinomyces sp. YM3-251, and searched the genome data with the predicted core sequence (FIVSSSCS) and identified a candidate biosynthetic gene cluster BGC (mcm) containing the possible precursor gene mcmA and a dehydratase gene mcmL.
Given the fact that the original producing strain Thermoactinomyces sp. YM3-251 is difficult to genetically manipulate, to investigate the biosynthetic pathway, the presumptive BGC (mcmA–mcmL) were heterologously expressed in Bacillus subtilis 168. After successfully activating the BGC by adding a strong promoter pLaps, the authors successfully detected the target product MCM-A in the fermentation products. Next, based on this heterologous expression system, two MCM-A analogs 17 and 18 with comparable antitumor activity are generated by engineering the biosynthetic pathway. 
Due to the degradation of the precursor peptides in the heterologous expression host, each knockout mutant strain (inactivation of gene mcmA-mcmL) did not provide more information about intermediates or the modification of precursor peptide, the authors carried out the combinatorial co-production of the precursor peptide with different modifying enzymes in Escherichia coli, which lead to the identification of a different timing of modifications, showing that a tRNAGlu-dependent highly regioselective dehydration is the first modification step, followed by polyazole formation through heterocyclization and dehydrogenation in an N- to C-terminal direction.
In summary, the authors identified the BGC of MCM by heterologous expression, gene inactivation, and co-expression. This work clarified the early steps of the biosynthetic pathway, which provides an alternative insight into the timing of post-translational modifications for RiPPs. These results set the stage for not only evaluating novel biochemical reactions and enzymatic mechanism but also generating more active polyazole-containing cyclopeptides by combinatorial biosynthesis and genome-mining approaches.
Figure (A) The gene organization of the mcm; (B) The early steps of the biosynthetic pathway of MCM-A; (C) The structures and biological activity of MCM-A and its analogs 17 and 18.(Image by TANG Gongli)
Tang Gongli Ph.D.Professor
Shanghai Institute of Organic Chemistry, Chinese Academy of Sciences
Ling Ling Road 345 Shanghai 200032 China
Tel: 021-54925113
Email: gltang@mail.sioc.ac.cn
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Ling Ling Road 345 Shanghai 200032 China Email: sioc@mail.sioc.ac.cn