The FNR-type regulator GoxR of the obligatory aerobic acetic acid bacterium Gluconobacter oxydans affects expression of genes involved in respiration and redox metabolism

Gene expression in the obligately aerobic acetic acid bacterium Gluconobacter oxydans responds to oxygen limitation, but the regulators involved are unknown. In this study, we analyzed a transcriptional regulator named GoxR (GOX0974), which is the only member of the FNR family in this species. Evidence was obtained that GoxR contains an iron-sulfur cluster, suggesting that GoxR functions as an oxygen sensor similar to FNR. The direct target genes of GoxR were determined by combining several approaches including a transcriptome comparison of a ΔgoxR mutant with the wild type and detection of in vivo GoxR binding sites by ChAP-Seq. Prominent targets were the cioAB genes encoding a cytochrome bd oxidase with low O2 affinity, which were repressed by GoxR, and the pnt operon, which was activated by GoxR. The pnt operon encodes a transhydrogenase (pntA1A2B), an NADH-dependent oxidoreductase (GOX0313), and another oxidoreductase (GOX0314). Evidence was obtained for GoxR being active despite a high dissolved oxygen concentration in the medium. We suggest a model in which the very high respiration rates of G. oxydans due to the periplasmic oxidations cause an oxygen-limited cytoplasm and insufficient reoxidation of NAD(P)H in the respiratory chain, leading to an inhibited cytoplasmic carbohydrate degradation. GoxR-triggered induction of the pnt operon enhances fast interconversion of NADPH and NADH by the transhydrogenase and NADH reoxidation by the GOX0313 oxidoreductase via reduction of acetaldehyde formed by pyruvate decarboxylase to ethanol. In fact, small amounts of ethanol were formed by G. oxydans under oxygen-restricted conditions in a GoxR-dependent manner.IMPORTANCE Gluconobacter oxydans serves as cell factory for oxidative biotransformations based on membrane-bound dehydrogenases and as model organism for elucidating the metabolism of acetic acid bacteria. Surprisingly, to our knowledge none of the more than 100 transcriptional regulators encoded in the genome of G. oxydans has been studied experimentally up to now. In this work, we analyzed the function of a regulator named GoxR, which belongs to the FNR family. Members of this family serve as oxygen sensors by means of an oxygen-sensitive [4Fe-4S] cluster and typically regulate genes important for growth under anoxic conditions by anaerobic respiration or fermentation. Because G. oxydans has an obligatory aerobic respiratory mode of energy metabolism, it was tempting to elucidate the target genes regulated by GoxR. Our results show that GoxR affects the expression of genes that support the interconversion of NADPH and NADH and NADH reoxidation by reduction of acetaldehyde to ethanol.