Membrane-containing enterobacterial phage PRD1 was isolated from sewage more than 40 years ago. At that time none would have expected the impact that unravelling its biology would have on modern virology and on the way we understand virus assembly, evolution and classification today. PRD1 structural analyses have provided a framework for understanding some aspects of virus evolution-introducing the concept of "viral lineages"-where the three-dimensional structures of virus capsids represent the fingerprint for evolutionary relationship which cannot be traced from the sequence data. In this review we summarise those findings that have led to the notion of viral lineages and the multidisciplinary efforts made in elucidating PRD1 life cycle. These studies have rendered PRD1 a model system not only for the family Tectiviridae to which it belongs, but more generally to complex DNA viruses enclosing a membrane vesicle beneath the capsid shell.
Keywords: 3D structure; Adenovirus; Assembly; Bacteriophage; DNA ejection; PRD1; Packaging; Tectiviridae; Uncoating; Viral lineage.