In the days of the Roman Empire, Tyrian purple dye cost more than gold. The rare and royal pigment had to be painstakingly extracted from Murex brandaris sea snails—as many as 10,000 of the mollusks perished to make 1 g of dye. Tyrian purple’s principal chemical component is 6,6'-dibromoindigo, which can’t be synthesized on industrial scale because it requires harsh reagents like bromine or hydrobromic acid, and it’s tough to get the bromines in the right spots on the indigo molecule. Now, by using biochemistry’s finely tuned machinery, a team led by Seoul National University’s Byung-Gee Kim has coaxed Escherichia coli into producing 6,6'-dibromoindigo (Nat. Chem. Biol. 2020, DOI: 10.1038/s41589-020-00684-4). The researchers engineered three enzymes, including a halogenase, into the bacteria. These enzymes were able to transform tryptophan into 6,6'-dibromoindigo in three steps using oxygen, sodium bromide, and a coenzyme. Pellets made by compacting the purple-producing E. coli via centrifuge can impart the characteristic hue to fabrics, including wool (shown). By using different halogenase enzymes, the researchers were able to make other substituted indigo molecules, including 5,5'-dibromoindigo and 6,6'-dichloroindigo, which have distinct colors and could also be used as dyes. These bacterial dyes have fewer toxic by-products than chemical dyes, the researchers say.
CORRECTION: This article was updated on Nov. 9, 2020, to correct the structure label. It should be 6,6'-dibromoindigo, not 6,6'-dibromoindingo.