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Chapter 13 : Signal Transduction in the Arc System for Control of Operons Encoding Aerobic Respiratory Enzymes
Aerobic respiration is the most efficient way to extract energy from carbon and energy sources. It has long been known that aerobically grown Escherichia coli contain elevated levels of many enzymes associated with aerobic metabolism. Examples of the enzymes include members of the tricarboxylic acid cycle and the cytochrome o complex, the major terminal oxidase. Because the synthesis of so many enzymes apparently depends on a single variable, O2 tension, there is reason to suspect the existence of a global control mechanism. Supposing further that such a regulation would likely be transcriptional, the authors decided to search for pleiotropic mutants that highly express operons of aerobic function under anaerobic conditions. To this end, they constructed a chromosomal merodiploid bearing both sdh + (encoding the succinate dehydrogenase complex) and a (sdh-lacZ) operon fusion using a Δlac strain and picked a red papilla from each colony on MacConkey lactose agar after anaerobic incubation for 5 days. ArcB variants deprived of the receiver module were significantly diminished in ArcA-phosphorylating activity. As a membrane sensor, a special feature of ArcB is its possession of a receiver module, but a surprising feature is the cytosolic origin of at least some of the signals. It is tempting to suggest that other sensors with a receiver module can also detect changes within the cell, including those in yeast and plants. These sophisticated sensors might have arisen from a simpler two-component system by some mechanism involving gene duplication.
Redox pathways. The numbers designate the reactions catalyzed by the enzymes: (1) L-lactate dehydrogenase (flavoprotein); (2) D-amino acid dehydrogenase (flavoprotein); (3) acylcoenzyme A dehydrogenase (flavoprotein); (4) 3-hydroxyacyl-coenzyme A dehydrogenase (NAD+-linked); (5) D-lactate:NAD+ oxidoreductase; (6) D-lactate dehydrogenase (flavoprotein); (7) formate dehydrogenase (the FDHN enzyme); (8) pyruvate dehydrogenase; (9) ethanol:NAD+ oxidoreductase; (10) citrate synthase; (11) aconitase; (12) isocitrate dehydrogenase; (13) 2-oxoglutarate dehydrogenase; (14) succinate dehydrogenase; (15) fumarate reductase; (16) fumarase A (Bell et al., 1989); (17) malate dehydrogenase; (18) isocitrate lyase; and (19) ubiquinol-1 oxidase. Reactions catalyzed by enzymes under the arc control are represented by thick arrows. Reactions catalyzed by enzymes that may or may not be under the arc control are represented by thin uninterrupted arrows. Reactions catalyzed by enzymes not under the arc control are represented by interrupted arrows. 2H boxed by solid lines represents reducing equivalents yielded by the reaction, and 2H boxed by dotted lines represents reducing equivalents consumed by the reaction. Modified from Fig. 1 of Iuchi and Lin(1988).
Primary structures of ArcB, ′ArcB, and ArcA. Dark boxes indicate putative transmembrane portions. Amino acid sequence between the transmembrane portions is supposed to be in the periplasmic compartment. Hatched boxes indicate the conserved transmitter module. Open boxes indicate the receiver module. Numbers over the diagrams indicate the positions of the highly conserved histidyl (H) and aspartyl (D) residues (Iuchi and Lin, 1992b).
Effect of ArcB and its variants on Φ(sdh-lac) expression. Cells were grown either aerobically (solid bar) or anaerobically (hatched bar), pls and chr indicate ArcB encoded by plasmid-borne and chromosome-borne genes, respectively. The + or — sign indicates the presence or absence of wild-type or variant ArcB. Symbols for variant proteins: H292Q (ArcBHis-292-Gln), FN517 (ArcB possessing 517 amino acid residues from the N-terminal end), FN516 (ArcB possessing 516 amino acid residues from the N-terminal end),D533A (ArcBAsp-533-Ala), and D576A (ArcBAsp-575-Ala)H292 (Iuchi and Lin, 1992a).
Model for the signaling process by ArcB under aerobic and anaerobic conditions. H, D, P, and m, respectively, indicate conserved His-292, conserved Asp-576, phosphoryl group, and cellular metabolites such as D-lactate and NADH. Arrows indicate reaction steps. The signs + and - indicate a positive and negative effect, respectively. ArcB could be a dimer or oligomer, and the phosphoryl group could be transferred between the subunits (Iuchi, 1993).