Nucleic Acids Research, 1988, Vol. 16, No. 17 8633-8644
© 1988
Articles |
An elongated model of the Xenopus laevis transcription factor IIIA-5S ribosomal RNA complex derived from neutron scattering and hydrodynamic measurements
Institut Laue-Langevin 156X, 38042 Grenoble Cedex 1EMBL Outstation, c/o ILL 156X, 38042 Grenoble Cedex, France 2 EMBL, Postfach 102209, Meyerhofstrasse 1, D-6900 Heidelberg, FRG
*To whom correspondence should be addressed
Received July 8, 1988. Accepted August 5, 1988.
The precise molecular composition of the Xenopus laevis TFIIIA-5S ribosomal RNA complex (7S particle) has been established from small angle neutron and dynamic light scattering. The molecular weight of the particle was found to be 95700±l0000 and 86700±9000 daltons from these two methods respectively. The observed match point of 54.4% D2O obtained from contrast variation experiments indicates a 1:1 molar ratio. It is concluded that only a single molecule of TFIIIA, a zinc-finger protein, and of 5S RNA are present in this complex. At high neutron scattering contrast a radius of gyration of 42.3±2 Å was found for the 7S particle. In addition a diffusion coefficient of 4.4.1011 [m2S–1] and a sedimentation coefficient of 6.2S were determined. The hydrodynamic radius obtained for the 7S particle is 48±5 A. A simple elongated cylindrical model with dimensions of 140 Å length and 59 Å diameter is compatible with the neutron results. A globular model can be excluded by the shallow nature of the neutron scattering curves. It is proposed that the observed difference of 15 Å in length between the 7S particle and isolated 5S RNA most likely indicates that part(s) of the protein protrudes from the end(s) of the RNA molecule. There is no biochemical evidence for any gross alteration in 5S RNA conformation upon binding to TFIIIA [1].
+Present address: Department of Microbiology and Molecular Genetics, Harvard Medical School and Howard Hughes Medical Institute, Harvard University, 7 Divinity Avenue, Cambridge, MA 02138