Chapter 14 Structure and Assembly of Phage Particles

Abstract

The simplest virus particles, such as those of phage M12 (Fig. 1), are composed of a single molecule of nucleic acid enclosed in a capsid made from protein subunits of a single kind. In such viruses, the form of the capsid is determined mainly or entirely by the properties of the protein subunit. By contrast many viruses, including the well-known phages T2, T4, and λ, have several structural parts each composed of its own type of subunit. For these viruses, particle-formation (or assembly) is a complex process involving components not all of which appear in the finished particle.

The assembly of λ particles starts near the middle of the latent period and continues until terminated by lysis of the cell. The time required for making a virus particle is only a few minutes. Thus the period of assembly is a steady state situation where various precursor molecules are continually being synthesized and used. Virus assembly is not regulated through mechanisms controlling the time of synthesis of components but by the specificity of their interactions. In a sequential process new specificities are likely to arise through conformational changes induced by the interactions themselves, as proposed for allosteric regulation of enzyme action (Monod et al., 1965).

The formation of phage particles provides a model not only for assembly of organelles but also for regulation of protein specificity in general. Although only a few examples have been clearly demonstrated, it is likely that regulation through induced conformational changes may play as important a...