Physical properties of the Escherichia coli transcription termination factor rho. 1. Association states and geometry of the rho hexamer.
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 physiological conditions and that this hexamer is in equilibrium with both lower and higher states of association that may also have physiological relevance. Small-angle X-ray scattering measurements and theoretical calculations indicate that the rho hexamer has a radius of gyration of 50 +/- 3 A. The radius of gyration measured by small-angle neutron scattering in 2H2O is 47 +/- 3 A. 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.
Geiselmann
J
J
Institute of Molecular Biology, University of Oregon, Eugene 97403.
Yager
T D
TD
Gill
S C
SC
Calmettes
P
P
von Hippel
P H
PH
D004247:DNA / Q000378:metabolism; D004058:Diffusion; D004926:Escherichia coli / Q000737:chemistry*; D008024:Ligands; D008433:Mathematics; D008958:Models, Molecular; D008970:Molecular Weight; D009097:Multienzyme Complexes / Q000378:metabolism; D009841:Oligonucleotides / Q000378:metabolism; D011485:Protein Binding; D011487:Protein Conformation; D012313:RNA / Q000378:metabolism; D012234:Rho Factor / Q000737:chemistry* / Q000235:genetics / Q000302:isolation & purification; D012492:Salts / Q000378:metabolism; D012542:Scattering, Radiation; D013329:Structure-Activity Relationship; D014158:Transcription, Genetic; D014461:Ultracentrifugation; D014961:X-Ray Diffraction
D008024:Ligands; D009097:Multienzyme Complexes; D009841:Oligonucleotides; D012234:Rho Factor; D012492:Salts; D012313:RNA; D004247:DNA