ALTERNATIVE RESPIRATORY PATHWAYS OF ESCHERICHIA-COLI - ENERGETICS ANDTRANSCRIPTIONAL REGULATION IN RESPONSE TO ELECTRON-ACCEPTORS

The electron-transport chains of Escherichia coli are composed of many different dehydrogenases and terminal reductases (or oxidases) which are linked by quinones (ubiquinone, menaquinone and demethylmenaquinon e). Quinol:cytochrome c oxido-reductase ('bc(1) complex') is not prese nt. For various electron accepters (O-2, nitrate) and donors (formate, H-2, NADH, glycerol-3-P) isoenzymes are present. The enzymes show gre at variability in membrane topology and energy conservation. Energy is conserved by conformational proton pumps, or by arrangement of substr ate sites on opposite sides of the membrane resulting in charge separa tion. Depending on the enzymes and isoenzymes used, the H+/e(-) ratios are between 0 and 4 H+/e(-) for the overall chain. The expression of the terminal reductases is regulated by electron accepters. O-2 is the preferred electron acceptor and represses the terminal reductases of anaerobic respiration. In anaerobic respiration, nitrate represses oth er terminal reductases, such as fumarate or DMSO reductases. Energy co nservation is maximal with O-2 and lowest with fumarate. By this regul ation pathways with high ATP or growth yields are favoured. The expres sion of the dehydrogenases is regulated by the electron accepters, too . In aerobic growth, non-coupling dehydrogenases are expressed and use d preferentially, whereas in fumarate or DMSO respiration coupling deh ydrogenases are essential. Coupling and non-coupling isoenzymes are ex pressed correspondingly. Thus the rationale for expression of the dehy drogenases is not maximal energy yield, but could be maximal flux or g rowth rates. Nitrate regulation is effected by two-component signal tr ansfer systems with membraneous nitrate/nitrite sensors (NarX, NarQ) a nd cytoplasmic response regulators (NarL, NarP) which communicate by p rotein phosphorylation. O-2 regulates by a two-component regulatory sy stem consisting of a membraneous sensor (ArcB) and a response regulato r (ArcA). ArcA is the major regulator of aerobic metabolism and repres ses the genes of aerobic metabolism under anaerobic conditions. FNR is a cytoplasmic O-2 responsive regulator with a sensory and a regulator y DNA-binding domain. FNR is the regulator of genes required for anaer obic respiration and related pathways. The binding sites of NarL, NarP , ArcA and FNR are characterized for various promoters. Most of the ge nes are regulated by more than one of the regulators, which can act in any combination and in a positive or negative mode. By this the hiera rchical expression of the genes in response to the electron acceptors is achieved. FNR is located in the cytoplasm and contains a 4Fe4S clus ter in the sensory domain. The regulatory concentrations of O-2 are 1- 5 mbar. Under these conditions O-2 diffuses to the cytoplasm and is ab le to react directly with FNR without involvement of other specific en zymes or protein mediators. By oxidation of the FeS cluster, FNR is co nverted to the inactive state in a reversible process. Reductive activ ation could be achieved by cellular reductants in the absence of O-2. In addition, O-2 may cause destruction and loss of the FeS cluster, It is not known whether this process is required for regulation of FNR f unction. (C) 1997 Elsevier Science B.V.