Tailed Bacteriophages: The Order Caudovirales

This chapter discusses the common properties of tailed phages and potential criteria for their classification as an order and situating tailed phages with respect to other viruses. Tailed bacteriophages have a common origin and constitute an order with three families, named Caudovirales. Their structured tail is unique. Tailed phages share a series of high-level taxonomic properties and show many facultative features that are unique or rare in viruses—for example, tail appendages and unusual bases. They share with other viruses, especially herpesviruses, elements of morphogenesis and lifestyle that are attributed to convergent evolution. Tailed phages present three types of lysogeny, exemplified by phages λ, Mu, and P1. Lysogeny appears as a secondary property acquired by horizontal gene transfer. Amino acid sequence alignments (notably of DNA polymerases, integrases, and peptidoglycan hydrolases) indicate frequent events of horizontal gene transfer in tailed phages. Common capsid and tail proteins have not been detected. Present-day tailed phages appear as chimeras, but their monophyletic origin is still inscribed in their morphology, genome structure, and replication strategy. It may also be evident in the three-dimensional structure of capsid and tail proteins. It is unlikely to be found in amino acid sequences because constitutive proteins must be so old that relationships were obliterated and most or all replication-, lysogeny-, and lysis-related proteins appear to have been borrowed.