One of the largest components of plant cells, lignin, could be converted into useful chemical building blocks, but scientists currently lack efficient methods to unlock this potential. Lignin is composed of polyphenylpropanoid units that, when broken down, afford chemicals that can be turned into plastics, nylon, or resins. So far, lignin has escaped digestion by bacterial culture because its aromatic byproducts, i.e. vanillin, inhibit bacterial growth. In addition, it is hard for bacteria to take up these aromatic compounds and much of the substrate remains in the surrounding culture.
Seema Singh and colleagues, Sandia National Laboratories, CA, USA, have tackled these issues by engineering E. coli that can both take up vanillin and use it to produce catechol without suffering from toxic effects. The team alleviated most of the toxicity associated with this molecule by introducing a biosynthetic pathway that breaks down vanillin. In addition, they put the pathway under the control of a promoter that is induced by vanillin, which means the pathway is self-regulated. Such a promoter element also eliminates the need for the more expensive inducer, isopropyl-β-D-1-thiogalactopyranoside (IPTG), which would be too costly for the industrial-scale break down of lignin.
In a second modification, the researchers added CouP (a transporter known to import aromatic substrates into bacteria) so the E. coli could more efficiently process vanillin. Together, these modifications significantly increased the production of catechol from lignin aromatics, which represents an important prototype for the industrial-scale use of lignins. Further engineering of the promoter and the biosynthetic pathway could increase the production of catechol or allow other important building blocks to be made.
- Toward engineering E. coli with an autoregulatory system for lignin valorization,
Weihua Wu, Fang Liu, Seema Singh,
Proc. Natl. Acad. Sci. USA 2018, 115, 2970–2975.