Contractile tail machines of bacteriophages.

Bacteriophages with contractile tails epitomize the concepts of "virus" and "phage" for many because the tails of these phages undergo a large conformational change - resembling the action of a syringe - upon the attachment to the host cell. The contractile tails belong to the recently recognized class of "contractile systems," which includes phage tails, their close relatives R-type pyocins, the bacterial type VI secretion system, and the virulence cassette of Photorhabdus. Their function is to deliver large proteins and/or DNA into the cytoplasm of a bacterial or eukaryotic cell. The structure of the core components of all contractile tail-like systems is conserved, but the corresponding genes have diverged to such a degree that the common ancestry can no longer be easily detected at the level of amino acid sequence. At present, it is unclear, whether the contractile systems originated in bacteria or in phages. This chapter describes the structure and function of phage contractile tails and compares them with other phage tails and with other known contractile systems.