Tyrian purple has a range of striking purple to red, color-fast and resistance to fading, and also has a promising application in dye-sensitized solar cells, functional polymers, and conductive materials ( Głowacki et al., 2012 Głowacki et al., 2013 Guo et al., 2015 Kim et al., 2018 Schnepel et al., 2021). Tyrian purple, known as royal purple, and mainly composed of 6, 6′-dibromoindigo, is an ancient dye extracted from the murex shellfish ( Ngangbam et al., 2015 Lee et al., 2021). This is the first study to show the existence of an indole biodegradation pathway in Providencia rettgeri, and the indole biodegradation gene cluster can contribute to the selective production of Tyrian purple. Interestingly, the monooxygenase GS-C co-expressed with its corresponding reductase GS-D in the cluster has better activity for the biosynthesis of Tyrian purple compared with the previously reported monooxygenase from Methylophaga aminisulfidivorans (MaFMO) or Streptomyces cattleya cytochrome P450 enzyme (CYP102G4).
To further explore the underlying mechanism of the selectivity, we explored the intermediates in this indole biodegradation pathway using liquid chromatography electrospray ionization quadrupole time-of-flight mass spectrometry (LC-ESI-QTOF-MS/MS), which indicated that the indole biodegradation pathway in Providencia rettgeri is the catechol pathway. The heterologous expression of the indole degradation gene cluster in Escherichia coli BL21 (DE3) and in vitro enzymatic reaction demonstrated that the indole biodegradation gene cluster may contribute to selectively biosynthesizing Tyrian purple. An indole degradation gene cluster for indole metabolism was identified from this GS-2 strain. This GS-2 strain was then identified as Providencia rettgeri based on bacterial genome sequencing analysis. In this study, we found Tyrian purple can be selectively produced by a bacterial strain GS-2 when fed with 6-bromotryptophan in the presence of tryptophan. Tyrian purple, mainly composed of 6, 6′-dibromoindigo, is a precious dye extracted from sea snails.
1School of Life Sciences, Inner Mongolia University, Hohhot, China.
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