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J. Virol., Sep 1997, 6850-6862, Vol 71, No. 9
R Jordan and PA Schaffer
ICP0 is a nuclear phosphoprotein involved in the activation of herpes
simplex virus type 1 (HSV-1) gene expression during lytic infection and
reactivation from viral latency. Although available evidence suggests that
ICP0 acts at the level of transcription, definitive studies specifically
addressing this issue have not been reported. In the present study we
measured the ability of ICP0 to activate gene expression (i) from promoters
representing the major kinetic classes of viral genes in transient
expression assays and (ii) from the same promoters during viral infection
at multiplicities of infection ranging from 0.1 to 5.0 PFU/cell. The levels
of synthesis and steady-state accumulation of mRNA, mRNA stability, and
levels of protein synthesis were compared in cells transfected with a
reporter plasmid in the presence and absence of ICP0 and in cells infected
with wild-type HSV-1 or an ICP0 null mutant, n212. In transient expression
assays and during viral infection at all multiplicities tested, the levels
of steady- state mRNA and protein were significantly lower in the absence
of ICP0, indicating that ICP0 activates gene expression at the level of
mRNA accumulation. In transient expression assays and during infection at
low multiplicities (< 1 PFU/cell) in the presence or absence of ICP0,
marked increases in the levels of viral mRNAs accompanied by proportional
increases in the levels of protein synthesis were observed with increasing
multiplicity. At a high multiplicity (5 PFU/cell) in the presence or
absence of ICP0, mRNA levels did not increase as a function of multiplicity
and changes in the levels of protein were no longer related to changes in
the levels of mRNA. Collectively, these tests indicate that transcription
of viral genes is rate limiting at low multiplicities and that translation
is rate limiting at high multiplicities, independent of ICP0. Consistent
with the lower levels of mRNA detected in the absence of ICP0, the rates of
transcription initiation measured by nuclear run-on assays were uniformly
lower in cells infected with the ICP0 null mutant at all multiplicities
tested, implying that ICP0 enhances transcription at or before initiation
or both. No evidence was found of posttranscriptional effects of ICP0
(i.e., effects on the stability of mRNA, nuclear-cytoplasmic distribution,
polyribosomal mRNA distribution, or rates of protein synthesis). Taken
together, these results suggest that ICP0 activates gene expression prior
to or at the level of initiation of mRNA synthesis in transient expression
assays and during viral infection. Based on these findings; we hypothesize
that the exaggerated multiplicity-dependent growth phenotype characteristic
of ICP0 null mutants reflects the requirement for ICP0 under conditions
where the steady-state level of mRNA is rate limiting, such as during low-
multiplicity infection and reactivation from latency.
Copyright © 1997, American Society for Microbiology
Activation of gene expression by herpes simplex virus type 1 ICP0 occurs at the level of mRNA synthesis
Division of Molecular Genetics, Dana-Farber Cancer Institute, Boston, Massachusetts, USA.
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