Enhancing metabolic efficiency via novel constitutive promoters to produce protocatechuic acid in Escherichia coli.

The antioxidant molecule protocatechuic acid (PCA) can also serve as a precursor for polymer building blocks. PCA can be produced in Escherichia coli overexpressing 3-dehydroshikimate dehydratase (DSD), an enzyme that catalyses the transformation of 3-dehydroshikimate to PCA. Nevertheless, optimizing the expression rate of recombinant enzymes is a key factor in metabolic engineering when producing biobased chemicals. In this study, a degenerate synthetic promoter approach was investigated to improve further the production of PCA. By limited screening of a randomized promoter library made using pSEVA221 plasmid in E. coli, three novel synthetic constitutive promoters were selected that increased the PCA yield from glucose by 10-21% compared to the inducible T7-promoter. RT-qPCR analysis showed that the DSD gene, regulated by the synthetic promoters, had high expression during the exponential phase, albeit the gene expression level dropped 250-fold during stationary phase. Besides the increased product yield, the synthetic promoters avoided the need for a costly inducer for gene expression. Screening of the entire promoter library is likely to provide more positive hits. The study also shows that E. coli transformed with the DSD gene on either pSEVA221 or pCDFDuet plasmids exhibit background PCA levels (~ 0.04 g/L) in the absence of a transcriptional regulatory element. KEY POINTS: • Degenerate synthetic promoters are remarkable tools to produce protocatechuic acid. • The constitutive synthetic promoters did not affect the growth rate of the bacterial host. • The use of constitutive synthetic promoters avoids the need for the costly inducer.