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Journal of Virology, June 2005, p. 7273-7278, Vol. 79, No. 11
0022-538X/05/$08.00+0 doi:10.1128/JVI.79.11.7273-7278.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
Genome Replication and Progeny Virion Production of Herpes Simplex Virus Type 1 Mutants with Temperature-Sensitive Lesions in the Origin-Binding Protein
Oliver Schildgen,
Sascha Gräper,
Johannes Blümel,
and
Bertfried Matz*
Institut für Medizinische Mikrobiologie und Immunologie, Universität Bonn, Bonn, Germany
Received 30 November 2004/
Accepted 7 January 2005

ABSTRACT
Genome replication of herpes simplex viruses (HSV) in cultured
cells is thought to be started by the action of the virus-encoded
origin-binding protein (OBP). In experiments using two HSV-1
mutants with temperature-sensitive lesions in the helicase domain
of OBP, we demonstrated that this function is essential during
the first 6 hours of the lytic cycle. Once DNA synthesis has
started, this function is no longer required, suggesting that
origin-driven initiation of viral DNA replication is a single
event rather than a continuous process.

TEXT
One of the virus-encoded factors essential for herpes simplex
virus type 1 (HSV-1) DNA replication (
5,
6,
9,
42) is the UL9
gene product (
13), which due to its affinity for the origins
of HSV-1 DNA replication (
36,
38) is termed origin-binding protein
(OBP). Structural and functional features of OBP have been thoroughly
characterized (see, e.g., references
1,
2,
4,
8,
12,
14,
15,
17-
19,
23,
28,
29,
31,
40, and
41). Nevertheless, there is but
limited information on the temporal requirement for OBP in vivo.
It is still unknown whether OBP acts exclusively as an initiator
of HSV DNA synthesis or whether it is needed also for elongation
and possibly even for the maturation/packaging of progeny genomes.
To address this question, we used two temperature-sensitive
(
ts) HSV-1 mutants (
tsR,
tsS) that had already been characterized
partially (
7,
25,
27,
37). In an earlier study (
3), we identified
the mutations responsible for the
ts phenotypes within the helicase-encoding
portion of the UL9 gene (
tsR, V220 M;
tsS, A90T), and in an
origin-dependent plasmid amplification assay, we found that
the activity of OBP affected by the
ts lesions is in fact indispensable
during the first 8 h of the lytic cycle in cultured cells.
Since it had been argued that plasmid amplification assays might not faithfully reflect the genuine herpesviral genome replication process and definitely are unsuitable for the assessment of potentially late functions of OBP, we scrutinized, refined, and extended our earlier investigations in the following way. Vero cell cultures were inoculated with tsR or tsS and, for control purposes, with tsH (a temperature-sensitive DNA polymerase mutant, tsPOL [10]); with the non-ts revertants tsRrev (M220L; 3) or tsSrev (T90A; 3); or with the parental wild-type (wt) HSV-1 17syn+ (7). Cultures were kept at 33°C (the temperature at which all the viruses inoculated are known to grow) for a defined period of time until being either harvested or transferred to 39°C (the temperature at which only the wild-type virus and the non-ts revertants are able to grow). All cultures at 39°C were harvested simultaneously 30 h postinoculation (p.i.). Total DNA was isolated from all harvested cell cultures, aliquots were applied in duplicate onto nitrocellulose membranes using a filtration manifold (Schleicher & Schüll, Dassel, Germany), and HSV-1-specific DNA was quantitated by dot hybridization (21) with 32P-labeled HSV-1 DNA. Hybridization signals were calculated as genome equivalents using a dilution series of purified HSV-1 virion DNA. As illustrated in Fig. 1, with all non-ts viruses (wt, tsRrev, tsSrev), temperature elevation from 33°C to 39°C had little influence on the final yields of viral DNA. In contrast, DNA synthesis of tsH (tsPOL) was abruptly blocked as soon as the temperature was elevated to 39°C. With the tsOBP mutants, viral DNA synthesis could be inhibited by shifting the cultures to the nonpermissive temperature only within the first 6 hours postinfection. Temperature upshifts at later times had no major effect on the final yields of viral DNA at 30 h p.i. These results indicate that the function of OBP, affected by the mutations in tsR and tsS, is essential for an early phase of genome replication but irrelevant for the gross synthesis of HSV DNA in the advanced stages of the lytic cycle.
Since it could still be possible that the OBP function were
essential for a step in the processing of the newly synthesized
progeny genomes, we quantitated the infectious particles in
the harvested cell cultures described in the legend to Fig.
1 by plaque assays at 33°C. As expected, in the case of
non-
ts viruses, temperature upshift had no major influence on
the production of virions (Fig.
2). The virion synthesis of
tsH, on the other hand, was severely inhibited by temperature
elevation at any stage of the replicative cycle. In the case
of the
tsOBP mutants, virion production was inhibited by temperature
upshift only within the first 6 hours p.i. Considerable amounts
of plaque-forming particles were produced when the cultures
had been upshifted later. An active role of the (temperature-sensitive)
OBP activity in DNA processing thus appears very unlikely.
When the cultures were upshifted between 9 and 24 h postinfection,
the yields of infectious particles in the
tsOBP virus-infected
cultures were marginally (i.e., 0.5 to 1.0 log steps) but reproducibly
lower than the final yields in those cultures held at 33°C
for 30 h (i.e., the respective endpoint yields, represented
by the last triangles/circles in the graphs in Fig.
2). One
possible explanation for this observation seemed to be an inhibitory
effect of inactive OBP molecules on late events in the lytic
cycle. Since inhibition of DNA replication had previously been
observed with genetically engineered mutants of OBP (
22,
24,
32,
39), it appeared reasonable to test the two
tsOBP mutants
for a possible dominant negative phenotype. Cell cultures were
inoculated with wild-type HSV-1 at multiplicities of infection
(MOI) of 1.0 or 0.1 and coinfected at an MOI of 5 either with
one of a group of viruses consisting of
tsR,
tsS,
tsRrev,
tsSrev,
tsA (a putative glycoprotein B mutant),
tsH, and
tsO (a putative
helicase mutant) or with wild-type HSV-1, giving total multiplicities
of either 6 or 5.1 PFU, respectively, per cell. The infected
cultures were incubated at 39°C and harvested after 24 h,
and the number of particles able to form plaques at 39°C
was determined (Fig.
3A and B). As a control, single infections
were carried out at both 33°C and 39°C with all the
viruses involved in the experiment, and the yields of infectious
particles were quantitated by plaque assays at 33°C (Fig.
3C and D). It is obvious that in coinfections of wild-type viruses
with
tsOBP virus mutants, the number of infectious wild-type
particles was drastically reduced, whereas other temperature-sensitive
mutants exhibited no significant inhibitory effects. Thus, under
nonpermissive conditions, the temperature-sensitive UL9 gene
products of
tsR and
tsS seemed to impede the action of unmutated
OBP molecules specified by the wild-type virus. One could speculate
that at 39°C the mutated variants of OBP, although unable
to initiate DNA replication, might still be able to bind (in
competition with the wt molecules) to the origins of replication
and/or to associate with other essential DNA replication factors.
In spite of the genotypic difference between
tsR and
tsS, their
phenotypes in vivo appeared to be very similar in all experiments.
In search for a possible phenotypic correlate for the genotypic
difference, we analyzed the kinetics of DNA synthesis and virion
production in temperature downshift experiments. Multiple cell
cultures were infected with
tsR or
tsS and incubated at 39°C.
After 8 h, some cultures were shifted to 33°C and harvested
at the time points shown in Fig.
4. The other cultures were
further kept at 39°C and harvested at 24 h p.i. From all
the harvested cultures, total DNA was isolated. HSV-specific
DNA was quantitated by dot hybridization. In the case of
tsS,
genome replication started almost immediately upon temperature
downshift, whereas with
tsR a measurable increase in viral DNA
could be detected only as late as 12 h after the downshift (Fig.
4A). The harvested samples were also assayed for plaque-forming
particles at 33°C (Fig.
4B). As could be anticipated from
the kinetics of genome replication, the synthesis of virions
started early with
tsS, whereas the onset of virus growth was
considerably delayed in the case of
tsR. The present data suggest
that the structural alteration caused by the mutation V220M
in the
tsR-OBP, in contrast to the A90T mutation in the
tsS-OBP,
may be irreversible. For this reason, the initiation of viral
DNA synthesis probably has to await de novo synthesis of new
OBP that is correctly folded at the permissive temperature (33°C).
Conclusion.
The objective of this study was, firstly, to phenotypically
characterize virus mutants which in our opinion are highly useful
tools for future studies of HSV DNA replication and, secondly,
to investigate the temporal requirement for the UL9 gene product
in lytic HSV infection.
These findings furnish experimental evidence for the commonly accepted opinion that in an initial phase of productive HSV infection, viral DNA is synthesized in an origin-dependent manner under the control of OBP, and that in a later stage, DNA synthesis switches to an origin-independent replication mode, finally giving rise to progeny genomic DNA molecules packaged into capsids (reviewed in references 5 and 34). It is noteworthy that the late phase appears to start relatively early, i.e., by 6 hours postinfection (Fig. 1). When the time required for the expression of the beta genes and the subsequent assembly of the replication complexes is subtracted, theoretically just a quite short period remains for but a few genome copies to be synthesized. Consequently, the huge mass of viral DNA accumulated after the switch (shift to nonpermissive temperature) must have arisen by some kind of origin-independent replication. A rolling-circle (sigma-like) mechanism in a strict sense (i.e., without any kind of reinitiation on the nascent DNA molecules) appears too slow for that rapid burst of progeny genome production. Nor does it explain the occurrence of four isomeric forms of genomic DNA molecules, in any case. It is true that the HSV DNA replication machinery is in fact capable of acting exactly in a sigma-like fashion in generating large concatemeric head-to-tail multimers starting from relatively small monomeric DNA seed units, as observed in defective interfering particles (16) or even for heterologous (i.e., polyomaviral) DNA sequences (11, 26), and yet it is very unlikely that the majority of HSV genome copies are multiplied in this way, as is particularly evidenced by the apparent absence or paucity of circular HSV DNA molecules in productive infections (20). Thus, one must postulate some kinds of reinitiation processes occurring concomitantly with the elongation of herpesviral DNA chains without the involvement of the origins and/or the origin-binding proteins. The observation of branched DNA molecules in HSV-infected cells (35) and the demonstration of HSV-encoded factors being able to induce recombination-dependent DNA replication (30, 33) could explain in a most elegant way the very early switch from origin-dependent to origin-independent DNA replication in our experiments.

ACKNOWLEDGMENTS
We are grateful to H. S. Marsden and J. H. Subak-Sharpe for
providing the HSV mutants.
The work was financially supported by Deutsche Forschungsgemeinschaft (grant Ma846/3-1) and by the University of Bonn (grant BONFOR 153/07).

FOOTNOTES
* Corresponding author. Mailing address: Institut für Medizinische Mikrobiologie und Immunologie, Universität Bonn, Sigmund-Freud-Strasse 25, D-53105 Bonn, Germany. Phone: (49)-228-287-5881. Fax: (49)-228-287-4433. E-mail:
bertfried.matz{at}ukb.uni-bonn.de.

Present address: Aventis Pharma, Industriepark Hoechst, Gebäude G650, D-65926 Frankfurt a.M., Germany. 
Present address: Paul-Ehrlich-Institut, Paul-Ehrlich-Strasse 51-59, D-63225 Langen, Germany. 

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Journal of Virology, June 2005, p. 7273-7278, Vol. 79, No. 11
0022-538X/05/$08.00+0 doi:10.1128/JVI.79.11.7273-7278.2005
Copyright © 2005, American Society for Microbiology. All Rights Reserved.
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