Physical Properties of the Escherichia coli Transcription Termination Factor Rho. 1. Association States and Geometry of the Rho Hexamer

Abstract

To function as a DNA-RNA helicase in rho-dependent transcript termination, six genetically identical subunits of the Escherichia coli transcription termination protein rho must first assemble into a hexameric complex. To help determine the quaternary structure of this complex, we have studied the association equilibria of the rho protomers. Sedimentation equilibrium, sedimentation velocity, diffusion, X-ray scattering, and neutron-scattering data have been combined to create a “phase diagram” of the association states of this protein as a function of protein concentration and ionic environment. The results show that rho exists predominantly as a hexamer under approximately hysiological conditions and that this hexamer is in equilibrium with both lower and higher states of association that may also have physiologicial relevance. Small-angle X-ray scattering measurements and theoretical calculations indicate that the rho hexamer has a radius of gyration of 50 ± 3 Å. The radius of gyration measured by small-angle neutron scattering in ^2H_2O is 47 ± 3 Å. These scattering studies also support earlier models of rho as a planar hexagon which have been developed on the basis of electron microscopy. In the following paper in this issue [Geiselmann, J., Seifried, S. E., Yager, T. D., Liang, C., & von Hippel, P. H. (1992)], these results are combined with information on symmetry, subunit interactions, and packing geometry to obtain a model of the quaternary structure of the functional rho hexamer.