Reconstruction of the three-dimensional structure of simian virus 40 and visualization of the chromatin core

The three-dimensional structure of the capsid and the nucleohistone core of simian virus 40 (SV40) has been reconstructed by image analysis of electron micrographs of frozen hydrated samples. The 72 prominent capsomere units that comprise the T = 7d icosahedral surface lattice of the capsid are clearly resolved. Both the pentavalent and hexavalent capsomeres appear with pentameric substructure, indicating that bonding specificity in the shell is not quasi-equivalent. There is a remarkable similarity between the structure of the SV40 virion capsid and the structure reported for the polyoma empty capsid. This result establishes that (i) the unexpected pentameric substructure of the hexavalent capsomeres is also present in virions and (ii) the arrangement of the 72 pentamers in the capsid lattice may be a characteristic feature of the entire papova family of viruses. The center of the SV40 reconstruction reveals electron density corresponding to the nucleohistone core. This density is smeared, suggesting that the minichromosome is not organized with icosahedral symmetry matching the capsid symmetry. The visualization of the virion chromatin provides a basis for invoking new models for the higher order structure of the encapsidated minichromosome.