Vaccinia virus reexamined: Development and release

Vaccinia virus was examined in thin sections by electron microscopy at intervals after release of infected cells from hydroxyurea, which permits the synthesis of early proteins but blocks DNA replication. There was sufficient synchrony in the ensuing process of development to recognize early stages. The foci of assembly lack polyribosomes and endoplasmic reticulum suggesting that the late proteins, at least, are synthesized elsewhere in the cytoplasm and transported to the maturation sites. The proposal by previous investigators that the surface spicules account for the contour and rigidity of the trilaminar membrane enclosing immature forms of the virus is consistent with the present study. The nucleoprotein appears to be inserted into the immature virus and to condense into the nucleoid just before closure of the envelope. The nucleoid is believed to move from its eccentric position to the center of the particle and there to become transformed into the biconcave disc characteristic of the mature, infectious virion. Previous reports were confirmed that virions are packaged by membranes of the Golgi apparatus and transported to the cell surface where they are released by exocytosis.