3.2 THE STRUCTURE OF FILAMENTOUS VIRUSES AND NUCLEOPROTEINS

 

One of the simplest ways of symmetrically arranging nonsymmetrical components is to place them round the circumference of a circle to form discs (Fig. 3.2). This gives a two-dimensional structure. If a large number of discs is stacked on top of one another, the result is a “stacked-disc” structure. Thus a symmetrical three-dimensional structure can be generated from a nonsymmetrical component such as protein and still leave room for nucleic acid. Examination of published electron micrographs of viruses reveals that some of them have a tubular structure. One such virus is the tobamovirus, tobacco mosaic virus (TMV) (Fig. 3.1). However, closer examination reveals that the TMV subunits are not arranged cylindrically, i.e. in rings, but helically. There is an obvious explanation for this. A helical nucleic acid could not be equivalently bonded in a stacked-disc structure. However, by arranging the subunits helically, the maximum number of bonds can still be formed and each subunit equivalently bonded, except, of course, for those at either end. All filamentous viruses so far examined are helical rather than cylindrical and the insertion of the nucleic acid may be the factor governing this arrangement. A helical arrangement offers considerable stability because of the subunits. This is greater than would be found with a cylinder which has no linking along the long axis (see Section 11.3). Many nucleoprotein structures inside enveloped viruses (Fig. 3.24) are constructed in the same way

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