A temperature-sensitive mutant of vaccinia virus defective in an early stage of morphogenesis

The isolation and characterization of a temperature-sensitive mutant of vaccinia virus is described. It is shown that at the nonpermissive temperature immature vaccinia virus particles accumulated in the cytoplasm of infected cells. These particles displayed an empty apperance due to their failure to incorporate the dense viroplasmic matrix found in the normal intermediates of vaccinia morphogenesis. The synthesis of functional viral DNA and viral-specific polypeptides occurred in an apparently normal fashion at the nonpermissive temperature. However the major vaccinia virus polypeptides (4A and 4B) whose appearance is associated with the assembly process were not formed by cleavage from their high-molecular-weight precursors. It is suggested that this defect is a consequence of an earlier one in the assembly of viral particles. Leakiness of the blockade led to the formation of morphologically aberrant virions possessing reduced infectiousness and a slightly altered polypeptide composition. It is concluded that the temperature-sensitive function in this mutant is normally involved in an early stage of vaccinia morphogenesis.