Aurantinins (ARTs) are antibacterial polyketides featuring a unique 6/7/8/5-fused tetracyclic ring system and a triene side chain with a carboxyl terminus. Here we identify the art gene cluster and dissect ART’s C-methyl incorporation patterns to study its biosynthesis. During this process, an apparently redundant methyltransferase Art28 was characterized as a malonyl-acyl carrier protein O-methyltransferase, which represents an unusual on-line methyl esterification initiation strategy for polyketide biosynthesis. The methyl ester bond introduced by Art28 is kept until the last step of ART biosynthesis, in which it is hydrolyzed by Art9 to convert inactive ART 9B to active ART B.
Fig: Structures and the biosynthetic gene cluster of ARTs: a Structures of ART A-D. The isotope labeling pattern of the ART B aglycone is shown as two separate polyketide chains. The C-1 carbon (marked with a light blue star) was previously proposed to originate from the C-1 of acetate; in this study, we propose it originates from the free carboxyl end of the malonyl starter unit. b Genetic organization of the art gene cluster. cHPLC profiles of B. subtilis fmb60 wild-type and mutant strains. B. subtilisΔart28/art28 and Δart28/Bc-bioC are two complementation strains of theart28 in-frame deletion mutant B. subtilis Δart28.
The cryptic reactions catalyzed by Art28 and Art9 represent a protecting group biosynthetic logic to render the ART carboxyl terminus inert to unwanted side reactions and to protect producing organisms from toxic ART intermediates. Further analyses revealed a wide distribution of this initiation strategy for polyketide biosynthesis in various bacteria.
Li, P., Chen, M., Tang, W. et al. Initiating polyketide biosynthesis by on-line methyl esterification. Nat Commun 12, 4499 (2021). https://doi.org/10.1038/s41467-021-24846-7