ABSTRACT_x000D_ Facultative anaerobes, such as Escherichia coli, possess a genetic regulatory network to capitalize on the best mode of energy extraction under different environments. The most efficient mode is aerobic respiration. Expression of many genes in this process is controlled by the ArcA/B two-component regulatory system. This system comprises the ArcB protein, a complex membrane-associated sensor kinase having kinase, phospotransfer and kinase activity, and the ArcA protein, a typical response regulator controlling more than 300 operons. Under reducing conditions, ArcB autophosphorylates by using ATP through an intramolecular reaction, a process enhanced by certain anaerobic metabolites, such as D-lactate and acetate. The phosphoryl group is then transferred to ArcA via an ArcBHis292-P→ArcBAsp576-P→ArcBHis717-P→ArcAAsp54-P phosphorelay, and activates it as a transcriptional regulator. When oxygen becomes available, the oxidized forms of the quinone electron carriers act as ArcB-specific signals that silence the kinase activity of ArcB. The molecular mechanism for kinase silencing involves the oxidation of two cytosol-located redox-active cysteine residues (Cys-180 and Cys-241) that participate in intermolecular disulfide bond formation. Furthermore, dephosphosphorylation of ArcA-P, a reaction needed to curtail its regulatory activity proceeds via an ArcAAsp54-P→ArcBHis717-P→ArcBAsp576-P→Pi reverse pathway. We propose to further characterize the relationships of structure and function in the phosphorelays for signal transmition and signal decay, to probe the biological significance of the H. influenzae ArcB protein, which lacks almost the entire linker region, and finally to probe for antibacterial agents that inhibit signal transduction systems. Accomplishment of the goals should not only clarify the mechanisms of Arc signal transduction, but could also improve our understanding of the integration of genetic circuits for bacterial adaptation to environmental changes, including those associated with host invasion during pathogenesis. Gaining more precise information on two-component systems, abundant in bacteria but not found in mammals, could also facilitate rational screening for novel drugs to treat infections._x000D_ _x000D_ _x000D_

_x000D_ The Arc two-component system is a complex signal transduction system that plays a key role in regulating energy metabolism at the level of transcription in bacteria (42), including the pathogens Vibrio cholerae, Salmonella typhimurium, Yersinia pestis, and Hemophilus influenzae (4, 13, 43). In E. coli, the complexity of this system is manifested by multiple inputs and successive phosphoryl group transfers in signal transmission and decay. More than 300 operons in the genome are known to be under Arc control. Unravelling the molecular mechanisms of this signal transduction system should cast light on how various metabolically connected pathways are coordinated in their adaptive responses to changes in respiratory conditions. Two-component signal transduction systems, dependent on histidine and aspartyl residues as phosphoryl group donors and acceptors, play extensive roles in sensing environmental conditions (21). Although a few homologs have been found in fungi and higher plants (2), they are not known to exist in mammals. These systems are therefore considered potential targets for new families of drugs in infectious disease control, and better understanding of their structure and function should facilitate the design of specific inhibitors, and could also facilitate rational screening for novel drugs to treat infections. _x000D_ _x000D_