Indigoidine synthetases: Versatile biocatalysts in nature and the laboratory

Indigoidine synthetases are unique single-module non-ribosomal peptide synthetases (NRPSs) that can act autonomously to create a pigmented product, the vivid blue 3,3'-bipyridyl compound indigoidine. Recent techno-economic and life cycle analyses demonstrate that the microbial production of indigoidine is approaching commercial feasibility, and we compare different advances in metabolic engineering and fermentation technology that have achieved fermentation titres of up to 49.3 g/L. We also explore the catalytic cycle of indigoidine synthetases and examine how the core 5-amino-3H-pyridine-2,6-dione intermediate they produce can also be used to generate a variety of other natural products. However, it is the ability of indigoidine to act as a robust chromogenic reporter that has spurred the greatest interest for biotechnology applications, and we discuss how this has underpinned development of environmental biosensors, effective cell-free expression systems, and high-throughput drug discovery assays. Collectively, these advances position indigoidine synthetases as pivotal tools for both commercial and research applications.