Six so called spherical viruses (four plant and two animal) are shown to exhibit magnetically induced birefringence in solu-tion. They must therefore be magnetically and optically anisotropic. This is attributed to static structural anisotropy of the interiors as neither natural shape nor field-induced deformations are likely causes. Thus at least part of these virus cores have a symmetry differing from that of their cap-sids. An estimate of the average orientation of the RNA bases is given for the plant viruses: turnip yellow mosaic, bromegrass mosaic, tomato bushy stunt and turnip crinkle. The packing geometry of the nucleic acid/protein cores of adenovirus and probably influenza virus are anisotropic but to an extent that cannot be quantified. Key words: adenovirus/influenza virus/magnetic birefrin-gence/plant virus/virus internal structure Introduction Magnetically induced birefringence has previously been us-ed to investigate the solution behaviour and structure of two flamentous bacteriophages Pfl and fd (Maret et al., 1979; Torbet and Maret, 1981). Consequently, highly oriented fibres, which gave improved X-ray diffraction patterns, were formed by partial drying in a magnetic field (Torbet and Maret, 1979). This technique was subsequently developed and exploited for detailed structural studies (Nave et al., 1979, 1981; Banner et al., 1981). This paper is an extension of that work to the study of the spherical viruses listed in Table I. The four plant viruses (Table I) each contain single-stranded RNA encapsidated in a T = 3, 180 protein subunit icosahedron. Electron microscopy has revealed little about the RNA structure in small plant viruses. This is also true of X-ray crystallography which, however, has detailed the struc-ture of much of the protein of three viruses (Harrison et al., 1978; Abad-Zapatero et al., 1980; Unge et al., 1980). Most of the information which is summarized below on the disposi-tion of protein and nucleic acid has been obtained from neutron scattering (reviewed by Jacrot, 1981). In turnip yellow mosaic virus (TYMV) the protein forms a very dense shell and is hardly penetrated by the RNA which is distributed throughout the interior (Jacrot et al., 1977). The protein of bromegrass mosaic virus (BMV) capsid is not very densely packed and is interpenetrated to a limited extent by RNA which is localized into a narrow sheet (Jacrot et al., 1977; Chauvin et al., 1978a). In tomato bushy stunt virus (TBSV) the protein subunit has two domains, one is closely packed on the surface, the other extends inwards into the RNA shell and there is a central cavity which is smaller than that in BMV (Chauvin et al., 1978b; Harrison et al., 1978). Both the N-terminal part of the protein subunits and the RNA are believed to be spatially disordered because they do not give rise to clear features in high resolution electron density maps (Harrison et al., 1978). No neutron scattering results have been published on the structure of turnip crinkle virus (TCV) but it is expected to be very like TBSV (Munowitz et al., 1980). Thus apart from the latter two these plant viruses have significant differences in their protein-RNA interactions and distribution. Measurements are also reported on two spherical animal viruses, adenovirus and influenza virus. The former has an icosahedron (T = 25) capsid which is composed of three ma-jor structural proteins. The core contains double-stranded DNA in association with four species of protein. The nucleo-protein may be in the form of folded rods (Nermut, 1979) with a structure akin to chromatin (Corden et al., 1976; Mirza and Weber, 1982). The outer envelope of influenza virus, recently shown to be spherical in solution (Mellema et al., 1981), is a lipid bilayer from which project two types of glycoprotein. It has a segmented RNA genome and the eight segments are believed to interact with the protein in the core to form a regular double-helical structure (Compans et al., 1972). In general, because it has proved to be a difficult prob-lem to approach experimentally, little is known about the packing geometry of the cores in most spherical viruses. Here it is demonstrated that the internal symmetry of spherical viruses can differ from that of their exteriors.
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