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The biosynthesis of carboxylic acids including fatty acids from biomass is central in envisaged biorefinery concepts. The productivities are often, however, low due to product toxicity that hamper whole-cell biocatalyst performance. Here, we have investigated factors that influence the tolerance of  Escherichia coli  to medium chain carboxylic acid (i.e., n-heptanoic acid)-induced stress. The metabolic and genomic responses of  E .  coli  BL21(DE3) and MG1655 grown in the presence of n-heptanoic acid indicated that the GadA/B-based glutamic acid-dependent acid resistance (GDAR) system might be critical for cellular tolerance. The GDAR system, which is responsible for scavenging intracellular protons by catalyzing decarboxylation of glutamic acid, was inactive in  E .  coli  BL21(DE3). Activation of the GDAR system in this strain by overexpressing the  rcsB  and  dsrA  genes, of which the gene products are involved in the activation of GadE and RpoS, respectively, resulted in acid tolerance not only to HCl but also to n-heptanoic acid. Furthermore, activation of the GDAR system allowed the recombinant  E .  coli  BL21(DE3) expressing the alcohol dehydrogenase of  Micrococcus luteus  and the Baeyer-Villiger monooxygenase of  Pseudomonas putida  to reach 60% greater product concentration in the biotransformation of ricinoleic acid (i.e., 12-hydroxyoctadec-9-enoic acid ( 1 )) into n-heptanoic acid ( 5 ) and 11-hydroxyundec-9-enoic acid ( 4 ). This study may contribute to engineering  E .  coli -based biocatalysts for the production of carboxylic acids from renewable biomass.

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Activation of the Glutamic Acid-Dependent Acid Resistance System in Escherichia coli BL21(DE3) Leads to Increase of the Fatty Acid Biotransformation Activity