DNA viruses: cooperativity and regulation through conformational changes as features of phage assembly

The assembly of bacteriophages provides experimental model systems for the study of regulation at the level of gene products. We discuss the hypothesis of regulation through sequentially induced conformational changes by which precursor-assemblies become ready at a specific stage of maturation to interact with an additional gene product or nucleic acids. Phage mutants provide excellent experimental model systems for studying, for example, the role and fate of the core in the prehead assembly. The conservative maturation of the prehead to the final, stable head consists of several steps whose complexity reflect that of the virus. It includes packaging of DNA. The surface lattice of maturing preheads apparently undergoes several steps characterized by different conformational states as suggested by in vitro studies on a morphological variant of the prehead, the polyhead of phage T4 (Steven, Couture, Aebi & Showe 1976; Laemmli, Amos & Klug 1976). Addition of a partly purified, enriched proteolytic fraction--which is gene 21-dependent--to empty purified polyheads leads to different conformational states. These seem to go in a direction approaching the structure of the surface of finished capsids as studied by Aebi et al. on gene 24 related (Bijlenga, Aebi & Kellenberger 1976) and other genetically defined giant-variants of T4 phage (Doermann, Eiserling & Boehner 1973). We show some experiments which suggest that high cooperativity is responsible for the stabilization of capsids. The activation energy necessary for dissociation of capsids is very high, 247 kJ for T4 capsids, and 10% smaller for T2. Once the energy barrier has been overcome, the capsids are first structurally modified before they undergo partial and finally complete dissociation.