Structural studies of bacteriophage lambda heads and proheads by small angle X-ray diffraction.

We have examined a series of lambda proheads and mature structures by small angle X-ray diffraction. This technique yields spherically averaged density distributions and some information about surface organization of particles in solution.We find that gpE ² of proheads and heads forms shells with one of two radii; A^?, B^?, groE^?, and Nu3^? proheads have shells of radius 246 Å, while mature heads, urea-treated A^? proheads and C^? proheads have a radius of 300 Å. The expansion of proheads to mature heads is accompanied by a corresponding decrease in the thickness of the shell. groE^? proheads contain a core. This core is lost spontaneously from the structure and is only observed if the structures are fixed with glutaraldehyde prior to examination by X-ray diffraction or electron microscopy.C^? proheads expand to mature head size spontaneously. A preparation of C^? proheads which was fixed with glutaraldehyde at an early stage of the purification had the smaller, prohead radius. Unfixed particles from this preparation expanded to the mature head size after further purification and standing in the cold for several days. This result suggests that gpC may be involved in regulating head expansion.The radii of the protein shells of mature heads are identical for a series of phages that contain between 78% and 105% of the wild-type complement of DNA, and this radius is the same as that of proheads expanded in the absence of DNA. These results with phage lambda indicate that assembly of a double shell structure composed of coat and scaffolding protein, followed by expansion to a larger shell containing only coat protein is a general feature of the morphogenesis of dsDNA phages.