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. 2017 Jun 15;8:15804. doi: 10.1038/ncomms15804

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Copyright © 2017, The Author(s)

Open Access This article is licensed under a Creative Commons Attribution 4.0 International License, which permits use, sharing, adaptation, distribution and reproduction in any medium or format, as long as you give appropriate credit to the original author(s) and the source, provide a link to the Creative Commons license, and indicate if changes were made. The images or other third party material in this article are included in the article’s Creative Commons license, unless indicated otherwise in a credit line to the material. If material is not included in the article’s Creative Commons license and your intended use is not permitted by statutory regulation or exceeds the permitted use, you will need to obtain permission directly from the copyright holder. To view a copy of this license, visit http://creativecommons.org/licenses/by/4.0/

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(a) Blue pigment production of xiaF-expressing E. coli (E. coli/pXU520) plated on agar supplemented with tryptophan. (b) Dimethylformamide extract of E. coli/pXU520, which turns red after several days of incubation; reduction of indigo yields indigo white and reveals the red indirubin that is no longer overlaid by the dark blue indigo. (c) HPLC-HRMS monitoring (extracted ion chromatograms) of extracts from cultures of E. coli/pXU520. E. coli carrying the empty vector (E. coli/pET-28b(+)) was used as a negative control. The profiles showing the reference compounds indigo (11) and indirubin (12) are composed of different measurements of pure compounds (colour coded). (d) Hydroxylation of indole (10) catalysed by XiaF yields indoxyl (13), which is spontaneously oxidized to yield indigo (11). Alternatively, isatin (15) is formed from indoxyl as a side reaction, which can condense with indoxyl to yield indirubin (12), the structural isomer of 11.