Unexpected transformations during pyrroloiminoquinone biosynthesis

Pyrroloiminoquinone containing natural products have long been known for their biological activities. They are derived from tryptophan, but their biosynthetic pathways have remained elusive. Studies on the biosynthetic gene cluster (BGC) that produces the ammosamides revealed that the first step is attachment of Trp to the C-terminus of a scaffold peptide in an ATP and tRNA dependent manner catalyzed by a PEptide Amino-acyl tRNA ligase (PEARL). The indole of the Trp is then oxidized to a hydroxyquinone. We previously proposed a chemically plausible and streamlined pathway for converting this intermediate to the ammosamides using additional enzymes encoded in the BGC. In this study, we report the activity of four additional enzymes that show that the proposed pathway is incorrect and that Nature’s route towards pyrroloiminoquinones is much more complicated. We demonstrate that, surprisingly, the amino groups in pyrroloiminoquinones are derived from three different sources, glycine, asparagine, and leucine, all introduced in a tRNA dependent manner. We also show that an FAD-dependent putative glycine oxidase is required for the process that incorporates the nitrogens from glycine and leucine, and that a quinone reductase is required for the incorporation of the asparagine. Additionally, we provide the first insights into the evolutionary origin of the PEARLs as well as related enzymes such as the glutamyl-tRNA dependent dehydratases involved in the biosynthesis of lanthipeptides and thiopeptides. These enzymes appear to all have descended from the ATP-GRASP protein family.