Function and horizontal transfer of the small terminase subunit of the tailed bacteriophage Sf6 DNA packaging nanomotor

Bacteriophage Sf6 DNA packaging series initiate at many locations across a 2 kbp region. Our in vivo studies show that Sf6 small terminase subunit (TerS) protein recognizes a specific packaging (pac) site near the center of this region, that this site lies within the portion of the Sf6 gene that encodes the DNA-binding domain of TerS protein, that this domain of the TerS protein is responsible for the imprecision in Sf6 packaging initiation, and that the DNA-binding domain of TerS must be covalently attached to the domain that interacts with the rest of the packaging motor. The TerS DNA-binding domain is self-contained in that it apparently does not interact closely with the rest of the motor and it binds to a recognition site that lies within the DNA that encodes the domain. This arrangement has allowed the horizontal exchange of terS genes among phages to be very successful.

► We identify the bacteriophage Sf6 DNA packaging recognition (pac) site.
► We show that the N-terminal domain of Sf6 TerS protein recognizes the pac site.
► The C-terminal TerS domain binds to the rest of the packaging apparatus.
► The “linker” between the TerS N- and C-terminal domains can be very flexible.
► We show that the P22-like TerS genes have undergone extensive horizontal exchange.