To determine the capsid structure of bacteriophage T7, we have investigated polycapsids, tubular capsid-related structures isolated from lysates of the T7 mutant am16. Biochemical analysis shows polycapsids to be composed of gp10, the major structural protein of the wild-type capsid. The conformational state of gp10 in polycapsids is indistinguishable from that in the mature virus capsid by the criteria of surface charge, buoyant density, and insensitivity to proteolysis by trypsin. Optical diffraction of electron micrographs of negatively stained polycapsids reveals a hexagonal surface lattice of periodicity 12.6 ± 0.2 nm and is used to analyze the distribution of cylindrical foldings of this lattice into polycapsids (polymorphic variation). These foldings are found to be related to that of the capsid proper through the intrinsic curvature of gp10, each folding having a set of lattice lines whose radius of curvature is close to 29 nm. The fine structure of this surface lattice has been elucidated by digital image processing of electron micrographs. The capsomer is shown unequivocally to be a hexamer of characteristic morphology. By collating these results with earlier observations, we conclude that the structure of the normal T7 capsid is an orthodox icosahedron of triangulation class T = 7, composed of 60 hexamers and 12 pentamers.
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