title: The respiratory chain provides salt stress tolerance by maintaining a low NADH/NAD+ ratio in Zymomonas mobilis.
abstract: The respiratory chain of ethanol-producing Zymomonas mobilis shows an unusual physiological property in that it is not involved in energy conservation, even though this organism has a complete electron transport system. We reported previously that respiratory-deficient mutants (RDMs) of Z. mobilis exhibit higher growth rates and enhanced ethanol productivity under aerobic and high-temperature conditions. Here, we demonstrated that the salt tolerance of RDM strains was drastically decreased compared with the wild-type strain. We found that the NADH/NAD+ ratio was maintained at low levels in both the wild-type and the RDM strains under non-stress conditions. However, the ratio substantially increased in the RDM strains in response to salt stress. Complementation of the deficient respiratory-chain genes in the RDM strains resulted in a decrease in the NADH/NAD+ ratio and an increase in the growth rate. In contrast, expression of malate dehydrogenase, activity of which increases the supply of NADH, in the RDM strains led to an increased NADH/NAD+ ratio and resulted in poor growth. Taken together, these results suggest that the respiratory chain of Z. mobilis functions to maintain a low NADH/NAD+ ratio when the cells are exposed to environmental stresses, such as salinity.
authors.affiliation: 1​Department of Food and Fermentation Science, Faculty of Food and Nutrition, Beppu University, Beppu, Oita 874-8501, Japan 2​Food Science and Nutrition, Graduate School of Food Science and Nutrition, Beppu University, Beppu, Oita 874-8501, Japan.
authors.forename: Takeshi
authors.initials: T
authors.lastname: Hayashi
authors.affiliation: 2​Food Science and Nutrition, Graduate School of Food Science and Nutrition, Beppu University, Beppu, Oita 874-8501, Japan.
authors.forename: Tsuyoshi
authors.initials: T
authors.lastname: Kato
authors.affiliation: 1​Department of Food and Fermentation Science, Faculty of Food and Nutrition, Beppu University, Beppu, Oita 874-8501, Japan.
authors.forename: Satoshi
authors.initials: S
authors.lastname: Watakabe
authors.affiliation: 1​Department of Food and Fermentation Science, Faculty of Food and Nutrition, Beppu University, Beppu, Oita 874-8501, Japan.
authors.forename: Wonjoon
authors.initials: W
authors.lastname: Song
authors.affiliation: 1​Department of Food and Fermentation Science, Faculty of Food and Nutrition, Beppu University, Beppu, Oita 874-8501, Japan.
authors.forename: Shizuho
authors.initials: S
authors.lastname: Aikawa
authors.affiliation: 1​Department of Food and Fermentation Science, Faculty of Food and Nutrition, Beppu University, Beppu, Oita 874-8501, Japan.
authors.forename: Kensuke
authors.initials: K
authors.lastname: Furukawa
mesh_terms: D004579:Electron Transport; D000431:Ethanol / Q000378:metabolism; D009243:NAD / Q000378:metabolism*; D055049:Salt Tolerance; D012965:Sodium Chloride / Q000378:metabolism*; D017355:Zymomonas / Q000235:genetics / Q000254:growth & development / Q000378:metabolism*
chemical_list: D009243:NAD; D000431:Ethanol; D012965:Sodium Chloride