Respiration is essential for aerobic growth ofZymomonas mobilisZM4

Abstract

Zymomonas mobilisis an alpha-proteobacterium that is a promising platform for industrial scale production of biofuels or valuable products due to its efficient ethanol fermentation and low biomass generation.Z. mobilishas also intriguing physiology, sometimes difficult to explain by the rules and strategies commonly observed in other bacteria. One of the most mysterious features ofZ. mobilisis its growth in oxic conditions.Z. mobilisis an aerotolerant bacterium that encodes a complete respiratory electron transport chain but the benefit of respiration for growth in oxic conditions has never been confirmed, despite decades of research. Quite the opposite, growth and ethanol production of WTZ. mobilisis poor in oxic conditions indicating that it does not benefit from oxidative phosphorylation. Additionally, in previous studies, aerobic growth improved significantly when respiratory genes were disrupted (ndh) or acquired point mutations (cydA, cydB)even if respiration was significantly reduced by these changes. Here, we obtained clean deletions of respiratory genesndhandcydAB, individually and in combination, and showed, for the first time, that deletion ofcydABcompletely inhibited O2respiration and dramatically reduced growth in oxic conditions. Both respiration and aerobic growth were restored by expressing a heterologous, water-forming NADH oxidase,noxE. This result shows that the main role of the electron transport chain inZ. mobilisis reducing the toxicity of molecular oxygen, helping to explain why it is beneficial forZ. mobilisto use electron transport chain complexes that contribute little to oxidative phosphorylation.ImportanceA key to producing next generation biofuels is to engineer microbes that efficiently convert non-food materials into drop-in fuels and to engineer microbes effectively we must understand their metabolism thoroughly.Zymomonas mobilisis a bacterium that is a promising candidate biofuel producer but its metabolism remains poorly understood, especially its metabolism when exposed to oxygen. AlthoughZ. mobilisrespires with oxygen, its aerobic growth is poor and disruption of genes related to respiration counterintuitively improves aerobic growth. This unusual result has sparked decades of research and debate regarding the function of respiration inZ. mobilis. Here, we used a new set of mutants to determine that respiration is essential for aerobic growth and likely protects the cells from oxidative damage caused by molecular oxygen. These results indicate that respiration has a non-canonical function inZ. mobilisand expand our understanding of the role of respiration in metabolism and oxidative stress.

Oxidative Phosphorylation for Aerobic Survival, but Not for Growth: The Peculiar ‘Make-Accumulate-Consume’ Strategy in Zymomonas mobilis

“…This should result in a more oxidized steady state of respiratory electron carriers, and hence, in a lower rate of respiratory ROS generation. Such reasoning seems to be in line with the recent findings of Felczak and TerAvest [48]. They reported that, while the decrease in the respiration rate due to the deletion of ndh in the strain ZM4 stimulated aerobic growth, a similar decrease in respiration attained in result of the deletion of cytochrome bd had an opposite effect.…”

The Genetics of Aerotolerant Growth in a Naturally Reduced Genome Alphaproteobacterium

“…4A, Table S9). This discrepancy could be explained by the possibly redundant function of atp with the Z. mobilis aerobic respiratory chain, which has an elusive function but may contribute to proton gradient generation and/or detoxification of oxygen (43,87).…”

“…1B); these genes largely encode horizontally-transferred elements (e.g., CRISPR-associated immune systems) and hypothetical proteins (Table S8). Genes carried by other Alphaproteobacteria but not by Z. mobilis are largely related to oxidative phosphorylation (e.g., type I NADH dehydrogenase, cytochrome c oxidase) and some TCA cycle components that are known to be absent in Z. mobilis (Table S8) (41)(42)(43)(44)(45).…”