Metagenomic Identification of Brominated Indole Biosynthetic Machinery from Cyanobacteria

Halogenated indole natural products have been isolated from a variety of organisms, including plants, marine algae, marine invertebrates, and bacteria. Aquatic cyanobacteria, in particular, are rich producers of brominated indoles, but their cognate biosynthetic enzymes have only been successfully linked in a limited number of natural products, such as the eagle-killing toxin aetokthonotoxin (AETX). The biosynthetic pathway for AETX involves five enzymes, two of which were previously undescribed due to incomplete annotations as hypothetical proteins. Our recent elucidation of AETX biosynthesis established functions of the two previously unknown proteins as enzymes responsible for tryptophan halogenation (AetF) and nitrile synthesis (AetD). Given their sequence novelty, we queried metagenomic data sets for these two enzymes and identified two new cyanobacterial haloindole biosynthetic gene clusters (BGCs) from marine sediment in Moorea, French Polynesia, and soil-derived samples in Maunawili Falls, Hawaii. We characterized the recovered BGCs by biochemically validating a new AetF homologue that exclusively halogenates free indole, rather than tryptophan as observed in AETX biosynthesis, and a new AetD homologue that harbors distinct substrate preferences, expanding the scope of nitrile biosynthesis. Additional characterization of core and accessory enzymes within these AETX-like BGCs highlights the breadth and diversity of haloindole biosynthetic machinery in cyanobacteria.