Structural characterization of the terminases from some siphoviridae

Terminases are responsible for the packaging of DNA into the procapsids of dsDNA bacteriophages. They are multimeric enzymes formed from two basic subunits. The structure of such multimers varies in different phages.

SPP1 and SF6 are two phages that infect Bacillus subtilis. They belong to the siphoviridae family, characterised by having a long, non-contractile tail. The terminase of these phages is composed of a small subunit (Gp1) and a large subunit (Gp2).

Gp1 is known to form homo-oligomers of 10 polypeptides. It has been shown to bind DNA, with a high preference for the so-called pac sequence of the phage genome. Recognition of the pac site is the initiating event in the packaging of the phage DNA into the procapsid.

Gp2 has ATPase and endonuclease activities. As an endonuclease, it cuts the concatemeric DNA of the phage within the pac site bound to Gp1. The ternary DNA-Gp1-Gp2 complex would then interact with the phage procapsid, probably through the portal protein Gp6, after what the DNA would be translocated into the procapsid. The ATPase activity of Gp2 would provide a source of energy to drive the packaging.

My aim, at Dr. A.A. Antson lab, was to structurally characterize the two subunits of the SPP1 terminase to further understand the mechanism by which the concatemeric DNA of this phage is recognized and handled to the portal vertex.

After leaving from York, this project was successfully taken up by Stefano Benini.