The anatomy of tobacco mosaic virus

This chapter explores those aspects of tobacco mosaic virus (TMV) that help in interpreting the structure of the particle. The internal anatomy of the particle has two aspects: how are the protein and nucleic acid disposed relative to one another, and from what sort of subunits is each made. There are likewise two main techniques by which these questions are tackled: X-ray analysis of oriented preparations and the examination of the fragments resulting after various types of physical and chemical fragmentation. X-ray analysis does not give unequivocal evidence about the position of the nucleic acid in TMV, but the observations are compatible with its running axially up the middle of the rod. Studies on the products of partial disintegration of TMV lead to the same conclusion. X-ray evidence suggests protein in TMV is arranged in a helix. It is suggested that there are several types of protein in TMV and that they differ in the avidity with which they associate with nucleic acid after the particle has disintegrated. There is a hypothesis that TMV more than a simple electrostatic union is involved. There is no evidence for a covalent link, though this is possible; hydrogen bonding is also possible. Alternatively TMV may be a clathrate compound with the nucleic acid and possibly some of the protein held centrally by purely mechanical features so that it can only escape when, through denaturation or other action, the protein tube begins to disperse