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Arbutin is a plant-derived hydroquinone glycoside that has been used in the healthcare and cosmetic industry due to its anti-oxidant, anti-bacterial, and anti-inflammatory properties and its ability to lighten the skin. Inhibition of the enzyme tyrosinase within the pigment producing skin cells, the melanocytes, blocks formation of melanin and thus leads to lightening of the skin. Currently, arbutin is produced through plant-based extraction or chemical synthesis which are expensive, complex processes that do not produce a high product yield. Metabolic engineering of microbes is becoming a powerful method for the large-scale production of valuable compounds. Previously, the biosynthetic production of arbutin within E. coli engineered with plasmid-base expression of biosynthetic enzymes was minimal. However, researchers in a recent study published in Microbial Cell Factories sought to create an economically feasible candidate for industrial arbutin production by utilizing a plasmid-free biosynthetic system in bacteria.
For this study, the researchers chose to utilize a mutant strain of the bacterium, Pseudomonas chlororaphis, which is typically used to produce phenazine, an antibiotic to root rot in agricultural crops. This bacterium has been well characterized and typically has a faster growth rate when glycerol is used as a renewable carbon source. Glycerol is becoming an emerging feedstock for biosynthesis of compounds since it is an inevitable by-product. In order to validate industrial scale production of arbutin, the researchers constructed a plasmid-free biosynthetic pathway in the mutant strain Pseudomonas chlororaphis P3. This allowed the steady production of arbutin from a sustainable carbon source without being plasmid and inducer dependent, which has significant economic and environmental benefits. The biosynthetic pathway included genes for precursor generation and enzymes for arbutin production. These genes were inserted in the phenazine synthesis gene cluster under the native bacterial promoter using chromosomal integration, which promoted stability and reduced the metabolic burden of plasmid expression within the bacteria.
Interestingly, following several modifications to the biosynthetic pathway, this plasmid-free metabolic engineered bacterial strain had a significantly higher production of arbutin than the previous plasmid-based biosynthetic pathway expressed in E. coli. The ability to achieve a high yield of arbutin using this plasmid-free engineered bacterial strain suggests that large-scale microbial biosynthesis of arbutin utilizing this system is feasible. In addition, the use of this plasmid-free system can be extended to the large-scale production of other plant-derived and natural products using engineered environmental microbes, providing an economical way to increase the amount of these valuable compounds.