Ubiquinone Accumulation Improves Osmotic-Stress Tolerance in Escherichia coli

Microbial production organisms in bioprocesses are often challenged by medium conditions that unfavorably change over time. High product concentrations achieved by modern production strains – often a prerequisite for economic feasibility – may hamper growth and productivity of the biocatalyst, thus reducing overall process efficiency. Besides product-specific inhibitory effects, general issues caused by product accumulation are solvent stress frequently encountered in biofuel production and osmotic stress imposed by up to molar concentrations of produced bulk chemicals. A detailed understanding of the microbial response mechanisms to such general stresses can guide metabolic engineering strategies by identifying cellular targets, thereby leading to more effective production organisms.
                A principal mechanism microbes use to cope with osmotic stress is to adjust their intracellular osmolality by accumulating compatible solutes. To gain detailed insights into this process, we analyzed the global metabolic response of the bacterium Escherichia coli to salt-induced hyperosmotic stress using nontargeted high-resolution mass spectrometry (ref. 1), and unexpectedly found the respiratory electron carrier, antioxidant and isoprenoid lipid ubiquinone-8 (Q8) as the by far most accumulating metabolite. Using various in vivo and in vitro experiments, we demonstrated that Q8 is required for acute and sustained osmotic-stress tolerance and that the main mechanism of Q8-mediated osmoprotection is the stabilization of the cytoplasmic membrane. Thus, we find that besides regulating intracellular osmolality, E. coli enhances the stability of its cytoplasmic membrane to withstand long-term osmotic stress (ref. 2).
                To investigate how widespread osmotic-stress induced ubiquinone accumulation is, we are currently analyzing the metabolic responses of diverse prokaryotic and eukaryotic microbes, including several established production hosts. Furthermore, we are extending our research to a range of additional stresses including temperature and solvent stress to understand whether ubiquinone accumulation contributes to general stress tolerance. At the conference, we will report our findings and provide a perspective on how they might contribute to future metabolic engineering approaches.

REFERENCES

Ref. 1:   Fuhrer T., Heer D., Begemann B. & Zamboni N. High-throughput, accurate mass metabolome     profiling of cellular extracts by flow injection-time-of-flight mass spectrometry. Analytical Chemistry 83, 7074-7080 (2011).

Ref. 2:   Sévin D. C. & Sauer U. Ubiquinone accumulation improves osmotic-stress tolerance in   Escherichia coli. Nature Chemical Biology, published online ahead of print (2014).