indigo

Indigo is an insoluble blue pigment produced through the oxidation of indole catalyzed by enzymes such as flavin-containing monooxygenases (FMOs) or naphthalene dioxygenases, followed by spontaneous dimerization [15, 16]. In the FMO-mediated biosynthetic pathway, indole is first oxidized to indole oxide, which is subsequently converted into indoxyl (3-hydroxyindole). The non-enzymatic dimerization of two indoxyl molecules then yields indigo [12, 15]. Traditionally, indigo has been extracted from plants belonging to the genera Indigofera, Isatis, Polygonum, and Lonchocarpus [16].

Indigo is a major target for the sustainable production of dyes using microbial cell factories as an environmentally friendly alternative to conventional chemical synthesis [15, 16]. Metabolic engineering strategies have focused on increasing intracellular indole availability by deleting the trpR repressor, relieving feedback inhibition of aroG and trpE, and enhancing phosphoenolpyruvate (PEP) and erythrose-4-phosphate (E4P) supply through pykA and pykF deletion together with tktA overexpression [12]. In addition, cysteine supplementation has been used to modulate the regioselectivity of FMO, as elevated cysteine levels favor hydroxylation at the C-2 position of indole, thereby promoting indirubin formation instead of indigo [12, 16].