Escherichia coli and related bacteria employ TorT/TorS/TorR signal transduction system to sense and respond to the presence of the small molecule trimethylamine oxide (TMAO) by controlling expression of the proteins that convert TMAO to trimethylamine. When oxygen is absent, this respiratory process enables E. coli to extract more energy from the surroundings than would be possible by fermentation alone. In contrast with many other anaerobic respiratory systems, TMAO respiration is also active when oxygen is present. However, under aerobic growth conditions, the output of the TorT/TorS/TorR system is highly heterogeneous across the population of cells, whereas anaerobically growing cells are much more homogeneous. As part of the long-term goal of understanding the interplay between the diverse signal transduction systems that E. coli employs to adapt to diverse environmental conditions, this proposal will explore the mechanistic basis for the oxygen-dependent regulation of cell-to-cell variability in Tor signaling as well as the physiological significance of this behavior. The first aim will test the hypothesis that the variability results from stochasticity in relative levels of the TMAO-binding protein TorT and the sensor kinase TorS, the two proteins that mediate the initial steps of sensing TMAO, and will determine the mechanism of oxygen regulation. The second aim will test the hypothesis that aerobic variability enables a subpopulation to more readily adapt to a rapid decrease in oxygen levels. TMAO respiration may be important for E. coli and other pathogens to thrive in the context of intestinal inflammation, colonization of the urinary tract infections, and possibly other types of infections. Thus, a better understanding of the Tor signaling system may enable new therapies for combatting pathogens and modulating gut flora.