The symmetries of the protein and nucleic acid in turnip yellow mosaic virus: X-ray diffraction studies

Abstract

Turnip yellow mosiac virus (TYMV) is an approximately spherical plant virus having a (hydrated) spherically-averaged diameter of 280 A and containing about 60% protein and 40% RNA (Markham, 1951). The protein is in the form of a spherical shell with the RNA contained inside. Associated with the virus is the “top component” which consists of empty protein shells containing no RNA. Previous X-ray diffraction studies (Klug, Finch & Franklin, 1957) of crystals of TYMV have shown that the virus particles have the symmetry of the tetrahedral point group (23), with a very strong tendency towards icosahedral (532) symmetry. It was suggested that the icosahedral symmetry was due to the protein component. We have now extended these studies on the virus crystals to higher angles of diffraction. Our previous observations are confirmed and it is shown that some part of the virus particle has 532 symmetry developed to a resolution of at least 4 A. We have now also obtained photographs of a single crystal of the top component and these confirm that the protein shell has 532 symmetry. It is therefore made up of 60 asymmetric (structurally equivalent) units so that the number of chemical units of any one kind must be a multiple of 60. Recent electron microscope observations presented in the accompanying papers (Huxley & Zubay, 1960; Nixon & Gibbs, 1960), show that the virus is composed of 32 protuberances arranged so that 20 fall at the vertices of a pentagonal dodecahedron and 12 at those of an icosahedron. The relation between the X-ray and the electron microscope results and the available chemical evidence is discussed. All the present data can be reconciled if the protein shell is made up of 180 (or, less likely, 120) chemical subunits arranged regularly in 20 groups of 6 (or 3) at the dodecahedral vertices and in 12 groups of 5 at the icosahedral vertices, but other, only statistically regular, models cannot be ruled out. Differences are observed between the X-ray diffraction patterns of crystals of the virus and of the top component. These must be due directly or indirectly to the RNA, and show the RNA to possess, at least in part, some kind of regular structure. The differences observed are consistent with a configuration having 23 symmetry, which, if correct, would imply that the RNA is made up of 12 geometrical subunits. This could still be reconciled with a single chemical chain. The problem of packing such a chain into a spherical particle with cubic symmetry is discussed. However, the apparent 23 symmetry of the RNA may arise as a statistical average over the many virus particles in a crystal, the RNA in a single particle having lesser symmetry. The various possibilities are discussed.