Assembly of the head of bacteriophage P22: x-ray diffraction from heads, proheads and related structures.

Abstract We have used electron microscopy and small-angle X-ray diffraction to study the three principal structures found in the head assembly pathway of Salmonella phage P22. These structures are, in order of their appearance in the pathway: proheads, unstable filled heads (which lose their DNA and become empty heads upon isolation), and phage. In addition, we can convert proheads to an empty head-like form (the empty prohead) in vitro by treating them with 0.8% sodium dodecyl sulfate at room temperature. We have shown that proheads are composed of a shell of coat protein with a radius of 256 A, containing within it a thick shell or a solid ball (outer radius 215 A) of a second protein, the scaffolding protein, which does not appear in phage. The three other structures studied are all about 10% larger than proheads, having outer shells with radii of about 285 A. Empty heads and empty proheads appear identical by small-angle X-ray diffraction to a resolution of 25 A, both being shells about 40 A thick. Phage appear to be made up of a protein shell identical to that in empty heads and empty proheads, within which is packed the DNA. Some details of the DNA packing are also revealed by the diffraction pattern of phage. The inter-helix distance is about 28 A, and the hydration is about 1.5 g of water per g of DNA. Certain aspects of the pattern suggest that the DNA interacts in a specific mariner with the coat protein subunits on the inside edge of the protein shell. Thus, the prohead-to-head transformation involves, in addition to the loss of an internal scaffold and its replacement by DNA, a structural transition in the outer shell. Diffraction from features of the surface organization in these structures indicates that the clustering of the coat protein does not change radically during the expansion. The fact that the expansion occurs in vitro during the formation of empty proheads shows that it is due to the bonding properties of the coat protein alone, although it could be triggered in vivo by DNA -protein interactions. The significance of the structural transition is discussed in terms of its possible role in the control of head assembly and DNA packaging.