Indisputably, the redox state of the ubiquinone pool and/or the concentration of ubiquinone per se (11, 12, 26) plays an important role in transducing environmental signals and may be a key factor in the regulatory network that governs catabolism. The results presented here show that the ArcB regulation system is not a straightforward linear system and also show that there is a nonlinear response system with regard to oxygen availability. Cold Spring Harbor Laboratory Press, Cold Spring Harbor, NY. 31.↵ Potter, L., H. Angove, D. Richardson, and J. Cole. 2001. Nitrate reduction in the periplasm of gram-negative bacteria.