title: Evolution for exogenous octanoic acid tolerance improves carboxylic acid production and membrane integrity.
abstract: Carboxylic acids are an attractive biorenewable chemical, but as with many biorenewables, their toxicity to microbial biocatalysts limits their fermentative production. While it is generally accepted that membrane damage is the main mechanism of fatty acid toxicity, previous metabolic engineering efforts that increased membrane integrity did not enable increased carboxylic acid production. Here we used an evolutionary approach to improve tolerance to exogenous octanoic acid, with the goal of learning design strategies from this evolved strain. This evolution of an Escherichia coli MG1655 derivative at neutral pH in minimal media produced a strain with increased tolerance not only to octanoic acid, but also to hexanoic acid, decanoic acid, n-butanol and isobutanol. This evolved strain also produced carboxylic acids at a 5-fold higher titer than its parent strain when expressing the Anaerococcus tetradius thioesterase. While it has been previously suggested that intracellular acidification may contribute to carboxylic acid toxicity, we saw no evidence that the evolved strain has increased resistance to this acidification. Characterization of the evolved strain membrane showed that it had significantly altered membrane polarization (fluidity), integrity (leakage) and composition relative to its parent. The changes in membrane composition included a significant increase in average lipid length in a variety of growth conditions, including 30°C, 42°C, carboxylic acid challenge and ethanol challenge. The evolved strain has a more dynamic membrane composition, showing both a larger number of significant changes and larger fold changes in the relative abundance of membrane lipids. These results highlight the importance of the cell membrane in increasing microbial tolerance and production of biorenewable fuels and chemicals.
authors.affiliation: Department of Chemical and Biological Engineering, Iowa State University, Ames, IA 50011, USA.
authors.forename: Liam A
authors.initials: LA
authors.lastname: Royce
authors.affiliation: Department of Chemical and Biological Engineering, Iowa State University, Ames, IA 50011, USA.
authors.forename: Jong Moon
authors.initials: JM
authors.lastname: Yoon
authors.affiliation: Department of Chemical and Biological Engineering, Iowa State University, Ames, IA 50011, USA.
authors.forename: Yingxi
authors.initials: Y
authors.lastname: Chen
authors.affiliation: Honors Program, Iowa State University, Ames, IA 50011, USA.
authors.forename: Emily
authors.initials: E
authors.lastname: Rickenbach
authors.affiliation: Department of Chemical and Biological Engineering, Iowa State University, Ames, IA 50011, USA.
authors.forename: Jacqueline V
authors.initials: JV
authors.lastname: Shanks
authors.affiliation: Department of Chemical and Biological Engineering, Iowa State University, Ames, IA 50011, USA. Electronic address: ljarboe@iastate.edu.
authors.forename: Laura R
authors.initials: LR
authors.lastname: Jarboe
mesh_terms: D001426:Bacterial Proteins / Q000096:biosynthesis / Q000235:genetics; D002210:Caprylates / Q000494:pharmacology*; D019020:Directed Molecular Evolution; D024881:Drug Resistance, Bacterial / Q000187:drug effects / Q000235:genetics; D004926:Escherichia coli / Q000235:genetics / Q000378:metabolism; D000068536:Bacillota / Q000235:genetics* / Q000378:metabolism; D006863:Hydrogen-Ion Concentration; D013869:Thiolester Hydrolases / Q000096:biosynthesis / Q000235:genetics
chemical_list: D001426:Bacterial Proteins; D002210:Caprylates; D013869:Thiolester Hydrolases; C031492:octanoic acid
keywords: Fatty acids; Intracellular acidification; Membrane composition; Membrane fluidity; Membrane integrity; Membrane leakage