Coumarins are a major group of plant secondary metabolites that serves as defense compounds against pathogens. Although coumarins can be obtained from diverse plant sources, the use of microorganisms to synthesize them could be an alternative way to supply building blocks for the synthesis of diverse coumarin derivatives. Constructs harboring two genes, F6′H (encoding feruloyl CoA 6′ hydroxylase) and 4CL (encoding 4-coumarate CoA:ligase), were manipulated to increase the productivity of coumarins. Escherichia coli expressing the two genes was cultured in medium supplemented with hydroxycinnamic acids (HCs) including p-coumaric acid, caffeic acid, and ferulic acid, resulting in the synthesis of the corresponding coumarins, umbelliferone, esculetin, and scopoletin. Cell concentration and initial substrate feeding concentration were optimized. In addition, umbelliferone, and esculetin were synthesized from glucose by using a ybgC deletion mutant and co-expressing tyrosine ammonia lyase and other genes involved in the tyrosine biosynthesis pathway. To produce coumarin derivatives (umbelliferone, scopoletin, and esculetin) in E. coli, several constructs containing F6′H and 4CL were made, and their ability to synthesize coumarin derivatives was tested. The solubility of F6′H was critical for the final yield. After optimization, 82.9 mg/L of umbelliferone, 79.5 mg/L of scopoletin, and 52.3 mg/L of esculetin were biosynthesized from the corresponding HCs, respectively in E. coli. Umbelliferone and esculetin were also synthesized from glucose using engineered E. coli strains. The final yields of umbelliferone and esculetin were 66.1 and 61.4 mg/L, respectively.

Microbial glycosyltransferases can convert many small lipophilic compounds such as phenolics, terpenoids, cyanohydrins and alkaloids into glycons usin

Most flavonoids exist as sugar conjugates. Naturally occurring flavonoid sugar conjugates include glucose, galactose, glucuronide, rhamnose, xylose, and arabinose. These flavonoid glycosides have diverse physiological activities, depending on the type of sugar attached. To synthesize an unnatural flavonoid glycoside,Actinobacillus actinomycetemcomitansgenetll(encoding dTDP-6-deoxy-l-lyxo-4-hexulose reductase, which converts the endogenous nucleotide sugar dTDP-4-dehydro-6-deoxy-l-mannose to dTDP-6-deoxytalose) was introduced intoEscherichia coli. In addition, nucleotide-sugar dependent glycosyltransferases (UGTs) were screened to find a UGT that could use dTDP-6-deoxytalose. Supplementation of this engineered strain ofE. coliwith quercetin resulted in the production of quercetin-3-O-(6-deoxytalose). To increase the production of quercetin 3-O-(6-deoxytalose) by increasing the supplement of dTDP-6-deoxytalose inE. coli, we engineered nucleotide biosynthetic genes ofE. coli, such asgalU(UTP-glucose 1-phosphate uridyltransferase),rffA(dTDP-4-oxo-6-deoxy-d-glucose transaminase), and/orrfbD(dTDP-4-dehydrorahmnose reductase). The engineeredE. colistrain produced approximately 98 mg of quercetin 3-O-(6-deoxytalose)/liter, which is 7-fold more than that produced by the wild-type strain, and the by-products, quercetin 3-O-glucose and quercetin 3-O-rhamnose, were also significantly reduced.