Virus Structure: General Principles

Introduction IN this article we shall discuss some general ideas about the structure of viruses. This is a hazardous undertaking. We know of no principles so compelling that we can be certain that they must be true; or, more correctly, those that must be true-the rules for inter-atomic distances, for instance-do not lead directly to any interesting conclusions. However, there are certain ideas suggested by experience in related fields (such as the study of protein crystals) which we might well expect to apply to viruses, or at any rate to small viruses. Moreover we can make some use of that powerful but dangerous weapon, the pri?ciple of simplicity. Our ideas fall into two groups. There is good evidence in the case of three plant viruses, and indirect evidence for certain animal viruses, that the protein component of a virus is made up of sub-units. Our first set of ideas concerns the question: why does a virus have protein sub-units? We have not previously published this argument. Our second deals with the problem: if there are sub-units, how are they arranged? This we have recently put forward elsewhere, SO that we shall only deal with it briefly. This paper should therefore be read in conjunction with our previous one (Crick and Watson, 1956). We shall restrict our discussion in the first place to those small viruses which contain only protein and ribonucleic acid (RNA): that is, the majority of known plant viruses, and