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By its direct contact with outer membrane receptor BtuB, the cytoplasmic membrane transducer TonB delivers energy that mediates cyanocobalamin uptake in Escherichia coli. This activity has been generally proposed to be the role of TonB in cyanocobalamin uptake. We now report the discovery and characterization of interactions between TonB and periplasmic binding protein BtuF. Phage display experiments predicted interaction between TonB and BtuF, identifying potential binding residues on each protein. Dynamic light scattering experiments measured a complex of 55 kDa, consistent with a TonB-BtuF heterodimer. The hydrodynamic radius of the complex was unchanged in the presence of cyanocobalamin. Surface plasmon resonance measured TonB-BtuF interaction kinetics that were independent of cyanocobalamin and that deviated from a simple binding model. Binding isotherms from intrinsic fluorescence suggested a multifaceted interaction that was independent of cyanocobalamin. In addition, the presence of TonB did not abrogate subsequent binding of cyanocobalamin by BtuF. Taken together, these data support a previously proposed model wherein TonB serves as a scaffold to optimally position BtuF for initial binding of cyanocobalamin and for its subsequent release. These results substantiate a diverse role for TonB with its multiple protein-protein interactions in bacterial nutrient uptake systems.

This paper presents a summary of the general behavior of cylinder clusters in axial flow and especially of the fluidelastic instabilities which occur at high flow velocities. Experiments were conducted in a water tunnel with three- and four-cylinder clusters, and the behavior was monitored either optically or by instrumenting one of the cylinders with strain gauges. With increasing flow, the amplitude of small random vibrations of the cylinders increased; simultaneously, the natural frequencies, as a group, decreased, which is in good agreement with theory. The cylinders eventually lost stability by buckling (divergence), and at higher flow by flutter. Agreement between theoretical and experimental critical flow velocities for these fluidelastic instabilities has been found to be good.

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