Biology Reference
In-Depth Information
The opposite technical problem is the much larger appearance of a structure
with dimensions below the wavelength of light due to the dispersion of light
from a high-intensity source. In this instance, a structure can appear much larger
than it really is. One such type of structure is ND10 (nuclear domain 10), most
often identified by immunofluorescence using antibody to the promyelocytic
leukemia protein (PML; Fig. 1b, c, e). ND10 figure prominently in the early
stages after DNA virus infections and appear to function like nuclear depots
(Ishov and Maul 1996; Maul 1998). Most ND10 appear to be substantially larger
than 300 nm when examined by fluorescent microscopy but smaller than 300 nm
when examined by electron microscopy (Maul et al. 1995). This disparity is
important to consider when interpreting images that show physical association
between viral genomes and ND10. In human fibroblasts examined at 3 h postin-
fection (p.i.) (Fig. 1b), HCMV genomes appear throughout the nucleus as
diffraction-spot-sized signals. Only a few of these signals localize beside ND10;
none localize in these domains. Localization of genomes next to ND10 could be
due to random events, particularly when one excludes the large volume of the
nucleolus and the apparent location of various DNA viruses in only the inter-
chromosomal space, that is, not within the chromosomal territories but the space
occupied where ND10 are positioned (Bell et al. 2001).
Shortly after infection, IE transcripts appear to emanate only from a few ND10
(Fig. 1c and e) (Ishov et al. 1997). The conclusion drawn from these images was
that most of the viral genomes that had reached the nucleus were incompetent to
transcribe and only those that reached ND10 found a congenial space where
transcription was possible. However, most of the major ND10-associated proteins,
such as PML and Sp100, are upregulated by interferon (Maul 1998). More ominous,
all of these proteins, including the PML-interacting Daxx, are transcriptional
repressors involved in the formation of heterochromatin (Seeler et al. 1998; Xu
et al. 2001; Ishov et al. 2004). ND10, therefore, appear more like sites for nuclear
defense. Also, CMV and other DNA viruses possess genes whose products can
eliminate or disperse these nuclear domains, as first identified for herpes simplex
virus (Maul et al. 1993). For CMV, the dispersing protein is IE1, which first
accumulates at ND10 for some time before ND10 are dispersed (Ishov et al. 1997;
Ahn et al. 1998). Surprisingly, the highest concentration of the major immediate
early viral transactivator and major immediate early promoter (MIEP) repressor,
IE2, is at the site of the highest concentration of viral IE transcripts (Fig. 1c).
These transcripts seem to pass from the IE2-covered site into the splicing-factor-
containing domain (Fig. 1e). The number of transcripts is much larger than
expected, suggesting either a very high transcription rate from a single transcription
unit or, as we suspect presently, an inhibition of splicing and thus accumulation
of unspliced transcripts. The general arrangement of ND10, the region of accumu-
lated transcripts, and the IE2-containing site were described as the immediate
transcript environment which resolves approximately 6 h p.i (Ishov et al. 1997).
At first glance, these observations indicate that the virus dispersed a site inhibi-
tory to its replicative success. The immediate early transcript environment also
includes the UL112/113 gene products (Fig. 1d). The UL112/113 gene product is
