Protein-nucleic acid interactions in tobacco mosaic virus

Tobacco mosaic virus (TMV), a positive-strand RNA virus, provides a unique perspective on nucleic acid structure, in that it has been for some years the only macromolecular assembly in which the structure of a ribonucleic acid interacting with a protein has been visualized in molecular detail, and one of the very few showing protein-nucleic acid interactions at all. Crystallographers have determined the structures of a number of spherical RNA viruses (for references see Liljas, 1986; Stubbs, 1989); but in those structures so far determined, the nucleic acid is not seen in the electron density maps, because it does not conform to the icosahedral symmetry of the coat proteins. TMV, by contrast, is a helical virus, and its RNA is well ordered, with the same symmetry as the protein. It is the type member of the tobamovirus group, rod-shaped viruses 3000 A long and 180 A in diameter, with a central hole of diameter 40 A. Approximately 2130 identical protein subunits of molecular weight 17500 form a right-handed helix of pitch 23A with 49 subunits in 3 turns. A single strand of RNA follows the basic helix between the protein subunits at a radius of 40 A, with three nucleotides bound to each protein subunit. An overall view of part of the viral rod is shown in Figure 5.1.