Helical Nucleocapsid Structure of the Oncogenic Ribonucleic Acid Viruses (Oncornaviruses)

Negative staining of virions and isolated nucleoids from avian myeloblastosis virus, murine leukemia virus, murine mammary tumor virus, and feline leukemia virus reveals common internal structures. The majority of virions that are penetrated by phosphotungstate show spherical nucleoids with no apparent symmetry. In a small percentage of virions, two distinctive structures are found: (i) single strands (3 to 5 nm in diameter) which are presumed to be the nucleoprotein and are found randomly oriented throughout the viral interior and (ii) helical structures (7 to 9 nm in diameter) which contain these nucleoprotein strands and are observed at the periphery of the nucleoid. The finding of helical nucleocapsid segments at the periphery of the nucleoid, as well as the hollow spherical structure observed in thin section of budding virions, has led to the hypothesis that the nucleocapsid of the freshly budded oncornavirus is supercoiled as a hollow sphere. This symmetry, however, is considered transient, as the internal structure of the extracellular virus undergoes a conformational rearrangement; thus, due to structural instability, the nucleocapsid uncoils and the nucleoprotein strands fill the interior of the virion. The extracellular virion is therefore considered degenerate in respect to symmetry, explaining the difficulty in detecting a helical nucleocapsid.