Morphogenesis of the Isometric Phages

Abstract

Virus assembly involves a wide spectrum of protein-protein and protein-nucleic acid interactions (Casjens and King 1975). Although information about morphogenetic pathways in other systems has accumulated, studies of isometric single-stranded (SS) DNA phage assembly have started only recently and references are scanty. Most information about morphogenesis in this class of phages comes from studies of ϕX174. We speculate that, because of their kinship, other phages in this class, such as S13 or the G phages, assemble virions in a similar way. Two basic strategies to elucidate the process of ϕX assembly are those that have been used in other virus systems:
1. Identification of precursors for mature phage.

a. The precursors exist transiently in infected cells. They are usually pulse-labeled with radioactive material. When a pulse-labeled culture is chased with unlabeled material, the radioactivity existing in the precursors should appear in mature phage.

b. Cells infected with conditionally lethal mutants accumulate precursors under restrictive conditions. When the infected cells are brought to permissive conditions, the accumulated precursors should be converted to mature phage.

2. Use of in vitro systems. Several laboratories have developed in vitro systems that synthesize viral SS DNA and infectious phage particles. Biochemical analyses of these systems provide further insight into phage morphogenesis.

Phage structures and infectious particles have been reconstituted from their separate precursors in several viral systems. Such reconstitution experiments have been useful in elucidating the sequence of events in assembly processes in other viral systems. However, such experiments have not been used extensively in...