Cytoplasmic origin of an intranuclear DNA virus (duck plague virus).

Electron microscopic and autoradiographic studies have revealed that the assembly and maturation of duck plague virus (DPV, an intranuclear DNA virus) occur both in the nucleus and in the cytoplasm. A dense cytoplasmic matrix (tentatively called viroplast) is the basic organelle in which the assembly of nucleocapsids occurs. At the early stage of virion maturation, dense particles, approximately 40 nm in size, appear and become assembled nucleocapsid structures later on. As capsids become mature, they obtain a diameter of 95-105 nm. The viroplast matrix gradually decreases in size, and a loose structure appears inside as the nucleocapsids increase in number. When viroplast has almost disappeared, nucleocapsids become abundant, regularly arranged around the cytoplasmic vacuoles. They bud into these vacuoles, thereby acquiring their envelopes. These nucleocapsids differ from those found in the nucleus both in morphology and in distribution. Electron microscopic autoradiography has shown that there is a huge amount of DNA in viroplast. DNA for the assembly of the nucleocapsids in the cytoplasm is derived from the very viroplast.