Ammosamide C (6) and its congeners A (7) and B (8) are marine natural products that contain a highly decorated pyrroloquinoline core. Ammosamides A and B were first isolated in 2003 from Streptomyces sp. CNR698 collected from sea bottom sediments in the Bahamas Islands. These compounds were shown to be highly cytotoxic against diverse cancer cell lines and to target myosin. In 2016, the ammosamide biosynthetic gene cluster (BGC) was identified. Through bioinformatic analysis, it was shown that this cluster did not contain the typical NRPS PKS architecture, instead elements from ribosomally synthesized and posttranslationally modified peptide (RiPP) gene clusters were present. Later work revealed that the starting point of the biosynthetic pathway was a truncated precursor peptide (1) as shown in the scheme below. Next, a peptide amino acyl-tRNA ligase (PEARL), AmmB₂, was shown to add a C-terminal tryptophan (W) that could be transformed into the pyrroloquinoline core. Then, a tailoring enzyme AmmC₁ trihydroxylated the indole of the C-terminal W and PEARL AmmB₃ gave an imine adduct with glycine appended to the 7^th position of the W indole moiety (2). During this presentation, I will showcase the activity of the next acting enzymes from the ammosamide BGC elucidated through in vivo and in vitro studies. Briefly, three enzymes from the cluster including an oxidoreductase and two PEARL enzymes further modified the C-terminal W to produce 3 and then proteolytic cleavage and chlorination produced a chlorinated ammosamic acid intermediate (5) en route to ammosamide C (6.)

Wilfred Van Der Donk

HHMI at the University of Illinois

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