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

Understanding the energy metabolism and its regulation is one of the clues to metabolic engineering of stress-resistant lignocellulose-converting microbial strains, also including the promising ethanologen Zymomonas mobilis. Z. mobilis is an obligately fermentative, facultatively anaerobic bacterium, carrying an active respiratory chain with low energy-coupling efficiency. Its respiration does not supply energy to aerobically growing cultures on sugary media, yet oxidative phosphorylation has been demonstrated in non-growing cells with ethanol. Here, we show, for the first time, that in respiring, non-growing Z. mobilis cells receiving regular small amounts of ethanol, oxidative phosphorylation significantly contributes to the maintenance of their viability. No improvement of viability is seen in the NADH dehydrogenase (ndh)-deficient respiratory mutant, which is unable to oxidize ethanol. The ethanol effect is also hampered by the protonophoric uncoupler CCCP, or the inhibitor of ATP synthase, DCCD. At higher concentrations (6% v/v), ethanol causes stress that slows down culture growth. By monitoring the activity of several respiratory gene promoters under ethanol stress with the green fluorescent protein reporter system, we demonstrate downregulation of these promoters, in particular the ndh promoter. We speculate that the decrease in respiratory chain activity in response to stress conditions mitigates the production of reactive oxygen species.