During the course of virus replication, the different
stages of virus self-assembly were detected
in the cytoplasm. A slightly electron-lucent viromatrix
was detected in a virus-infected cell (Fig.
2). The cell nucleus changed shape and displaced
around edge of the cell indicative of a central
cytoplasmic viromatrix. The viromatrix contained
large amounts of virus particles in different stages
of assembly including empty capsids, capsids with
partial core and the fully matured nucleocapsids
located at assembly sites in paracrystalline arrays
or scattered as individuals. In some ultrathin sections,
the nucleo-protein core-like materials, tubular
membrane-like structure, empty capsids and
partially formed capsids by sealing tubular mem-enzyme digestion was carried out. After digestion
with protease K at 37 °C for 10 min, the external
envelope of virus particles was partially or totally
removed, and the inner double loose electron
lucent membranes were observed. However, subunits
of the viral capsid were still not able to be
visualised (Fig. 6B). The grouper iridovirus particles
were treated with 1% Triton X-100 or 1%
SDS in an attempt to reveal the fine structure of
the virus capsid. When the virus particles were
treated with 1% Triton X-100 for 2 min, the
external envelope of the viron was completely
removed, and the viral capsid structure was
clearly appeared as a loose double-layered membrane
forming a ring around the electron-dense
core. A regular periodic array of subunits was
visible on the surface of the virons. The subunit
was about 5 nm in an average diameter and
seemed to be connected to the underlying layer
through thin rods (Fig. 6C). When the virus particles
were treated with 1% Triton X-100 for 5 min,brane were observed (Fig. 3A). More interestingly,
the matured ‘full’ nucleocapsids were probably
formed by inserting striped electron-dense
material into ‘empty’ capsids. The striped material
could be inserted into one or two empty capsids at
the same time (Fig. 3B). In the late phase of virus
infection, the nucleocapsids were matured, formed
a typical icosahedral shape and aggregated in
pseudocrystalline array at the assembly sites (Fig.
4A). Some viral nucleocapsids were distributed
around cytoplasm membrane and partially coated
with the membrane (Fig. 4B). In the last stage of
infection, the virus was enveloped and released by
budding through the plasma membrane (Fig. 5A
and B). In some cases, the budding virons
entered neighbouring cells simultaneously by endocytosis
to start the next round infection (Fig. 5C and D).