Achromobacter denitrificans Strain YD35 Pyruvate Dehydrogenase Controls NADH Production To Allow Tolerance to Extremely High Nitrite Levels

We identified the extremely nitrite-tolerant bacteriumAchromobacter denitrificans YD35 that can grow in complex medium containing 100 mM nitrite (NO2 − ) under aerobic conditions. Nitrite induced global proteomic changes and upregulated tricarboxylate (TCA) cycle enzymes as well as antioxidant proteins in YD35. Transposon mutagenesis generated NO2 − -hypersensitive mutants of YD35 that had mutations at genes for aconitate hydratase and α-ketoglutarate dehydrogenase in the TCA cycle and a pyruvate dehydrogenase (Pdh) E1 component, indicating the importance of TCA cycle metabolism to NO2 − tolerance. A mutant in which thepdh gene cluster was disrupted (Δpdh mutant) could not grow in the presence of 100 mM NO2 − . Nitrite decreased the cellular NADH/NAD+ ratio and the cellular ATP level. These defects were more severe in the Δpdh mutant, indicating that Pdh contributes to upregulating cellular NADH and ATP and NO2 − -tolerant growth. Exogenous acetate, which generates acetyl coenzyme A and then is metabolized by the TCA cycle, compensated for these defects caused by disruption of thepdh gene cluster and those caused by NO2 − . These findings demonstrate a link between NO2 − tolerance and pyruvate/acetate metabolism through the TCA cycle. The TCA cycle mechanism in YD35 enhances NADH production, and we consider that this contributes to a novel NO2 − -tolerating mechanism in this strain.