Metabolic Engineering of the Actinomycete Amycolatopsis sp. Strain ATCC 39116 towards Enhanced Production of Natural Vanillin

ABSTRACT

The Gram-positive bacterium Amycolatopsis sp. ATCC 39116 is used for the fermentative production of natural vanillin from ferulic acid on an industrial scale. The strain is known for its outstanding tolerance to this toxic product. In order to improve the productivity of the fermentation process, the strain's metabolism was engineered for higher final concentrations and molar yields. Degradation of vanillin could be decreased by more than 90% through deletion of the vdh gene, which codes for the central vanillin catabolism enzyme, vanillin dehydrogenase. This mutation resulted in improvement of the final concentration of vanillin by more than 2.2 g/liter, with a molar yield of 80.9%. Further improvement was achieved with constitutive expression of the vanillin anabolism genes ech and fcs, coding for the enzymes feruloyl-coenzyme A (CoA) synthetase ( fcs) and enoyl-CoA hydratase/aldolase ( ech). The transcription of both genes was shown to be induced by ferulic acid, which explains the unwanted adaptation phase in the fermentation process before vanillin was efficiently produced by the wild-type cells. Through the constitutive and enhanced expression of the two genes, the adaptation phase was eliminated and a final vanillin concentration of 19.3 g/liter, with a molar yield of 94.9%, was obtained. Moreover, an even higher final vanillin concentration of 22.3 g/liter was achieved, at the expense of a lower molar yield, by using an improved feeding strategy. This is the highest reported vanillin concentration reached in microbial fermentation processes without extraction of the product. Furthermore, the vanillin was produced almost without by-products, with a molar yield that nearly approached the theoretical maximum.

IMPORTANCE Much effort has been put into optimization of the biotechnological production of natural vanillin. The demand for this compound is growing due to increased consumer concerns regarding chemically produced food additives. Since this compound is toxic to most organisms, it has proven quite difficult to reach high concentrations and molar yields. This study shows that improvements in the final vanillin concentrations and molar yields can be made through a combination of modification of the fermentation parameters and molecular strain engineering, without the need for methods such as continuous extraction from the fermentation broth. Using this approach, we were able to reach a final vanillin concentration of 22.3 g/liter, which is the highest vanillin concentration reported to date that was generated with Amycolatopsis sp. ATCC 39116 without additional extraction of the toxic product.