Diffraction studies of tulip virus X particles

1 Carnegie Laboratory of Physics, The University, Dundee, DD1 4HN and 2 Scottish Crop Research Institute, Invergowrie, Dundee, DD2 5DA, U.K. X-ray diffraction from oriented tulip virus X (TVX) particles and optical diffraction from electron micrographs show that the virus particles have a helical structure with a pitch of about 3.25 nm and a true repeat in five turns of the helix. Both X-ray and optical diffraction indicate that the number of subunits in the repeat period is 5q + 4, where q is an integer. The X-ray diffraction patterns suggest that 9 q 6, and the optical diffraction patterns suggest that q is probably 7 or 8. The X-ray diffraction patterns indicate that there is a marked feature in the virus particle at a radial position of about 3.3 nm, which is probably the RNA. The equatorial diffraction further suggests that the virus has an axial hole of about 1.5 nm radius. Keywords: tulip virus X, potexvirus, optical diffraction, X-ray diffraction Received 23 April 1981; accepted 5 June 1981.