Fine structure of maize rough dwarf virus inner core in cryosections of maize tissue.

Information on the fine structure of viruses is generally obtained by negative staining of particles in crude or purified extracts or, more recently, in cryo-vitrified suspensions. This information however concerns the external protein coat, and the inner core can be seen more or less clearly only in ultrathin sections. In negative staining, the particles of maize rough dwarf virus (MRDV), a plant reovirus, have a diameter of about 70 nm and show a double protein shell. The inner core, containing double stranded RNA, appears both in negative staining and resin sections as an electron-opaque sphere of 35-40 nm of diameter. To gain a better knowledge of inner core morphology we examined virus particles in plant cryosections stained with neutral uranyl acetate and coated with methyl cellulose. With this technique, MRDV particles clearly showed their outer and inner shells, and the inner core was resolved in a series of repeating geometrical forms. Amon these, some were easily identified as having 2, 3 or 5-fold symmetry. Others were less clearly identified, appearing as superposed or tilted images of the same forms. From these images it was possible to obtain a tentative three-dimensional reconstruction of MRDV inner core. This is likely to be a pentadodecahedron, whose twelve pentagonal faces might match the base plates of the B spikes, one for each of the twelve vertices of the icosahedral inner shell (Francki et al., 1985).