Investigating the regulation of long-chain fatty acid metabolism by the Cpx envelope stress response in Escherichia coli

Abstract

Long-chain fatty acids (LCFAs) represent a tremendous energy source for bacteria including many pathogens; however, their utilization confers stress in bacteria. Using Escherichia coli as a model, previous work from our lab showed that the oxidation of a large number of reduced cofactors generated during LCFA metabolism increases electron flow towards ubiquinone, a lipid-soluble electron carrier in the electron transport chain (ETC). Because ubiquinone also re-oxidizes the disulfide bond (DSB)-forming machinery that performs oxidative protein folding in the envelope, the outermost multi-layered compartment critical for cellular growth and viability, increased electron flow during LCFA metabolism hampers the essential process of DSB formation, thereby compromising envelope redox balance. Notably, E. coli induces the CpxAR two-component system to counteract stress. The upregulation of envelope-localized chaperones and proteases is a well-recognized remedial mechanism by which Cpx restores cellular integrity. However, my work has identified Cpx as a global regulator of LCFA metabolism that uses a preventive measure to maintain envelope homeostasis in LCFA-grown cells; it facilitates DSB formation by downregulating LCFA metabolism and increasing the oxidizing power of ETC. Interestingly, contrary to its conventional mode of imparting regulation via CpxR working mainly as a transcriptional regulator, during LCFA metabolism, Cpx uses its non-coding arm to counteract envelope redox stress. The Cpx-regulated small RNA (sRNA) CpxQ i) represses fad genes involved in LCFA transport and -oxidation, ii) downregulates components of the glyoxylate shunt, gluconeogenesis, and ETC, and iii) stabilizes another sRNA OmrA, and both these sRNAs increase ubiquinone content. My work in E. coli revealing the interconnection between LCFA metabolism, redox stress, and envelope stress response provides the rationale for investigating similar networks in other LCFA utilizing bacteria.