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J Virol, May 1998, p. 3635-3645, Vol. 72, No. 5
Dana-Farber Cancer Institute and Department
of Microbiology and Molecular Genetics, Harvard Medical School,
Boston, Massachusetts 02115, and Department of Microbiology,
University of Pennsylvania School of Medicine, Philadelphia,
Pennsylvania 19104
Received 6 November 1997/Accepted 15 January 1998
The herpes simplex virus type 1 (HSV-1) origin of DNA replication,
oriS, contains three binding sites for the viral origin binding protein
(OBP) flanked by transcriptional regulatory elements of the
immediate-early genes encoding ICP4 and ICP22/47. To assess the role of
flanking sequences in oriS function, plasmids containing oriS and
either wild-type or mutant flanking sequences were tested in transient
DNA replication assays. Although the ICP4 and ICP22/47 regulatory
regions were shown to enhance oriS function, most individual elements in these regions, including the VP16-responsive TAATGARAT elements, were found to be dispensable for oriS function. In contrast, two oriS core-adjacent regulatory (Oscar) elements, OscarL and OscarR,
at the base of the oriS palindrome were shown to enhance oriS function
significantly and additively. Specifically, mutational disruption
of either element reduced oriS-dependent DNA replication by 60 to 70%,
and disruption of both elements reduced replication by 90%. The
properties of protein-DNA complexes formed in gel mobility shift assays
using uninfected and HSV-1-infected Vero cell nuclear extracts
demonstrated that both OscarL and OscarR are binding sites for cellular
proteins. Whereas OscarR does not correspond to the consensus binding
site of any known transcription factor, OscarL contains a consensus
binding site for the transcription factor Sp1. Gel mobility shift and
supershift experiments using antibodies directed against members of the
Sp1 family of transcription factors demonstrated the presence of Sp1
and Sp3, but not Sp2 or Sp4, in the protein-DNA complexes formed at
OscarL. The abilities of OscarL and OscarR to bind their respective
cellular proteins correlated directly with the efficiency of
oriS-dependent DNA replication. Cooperative interactions between
the Oscar-binding factors and proteins binding to adjacent OBP binding
sites were not observed. Notably, Oscar element mutations that impaired
oriS-dependent DNA replication had no detectable effect on either basal
or induced levels of transcription from the ICP4 and ICP22/47
promoters, as determined by RNase protection assays. The Oscar elements
thus appear to provide binding sites for cellular proteins that
facilitate oriS-dependent DNA replication but have no effect on
transcription of oriS-flanking genes.
0022-538X/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
Cellular Transcription Factors Enhance Herpes
Simplex Virus Type 1 oriS-Dependent DNA Replication
*
Corresponding author. Mailing address: Department of
Microbiology, University of Pennsylvania School of Medicine, 225A
Johnson Pavilion, 3610 Hamilton Walk, Philadelphia, PA 19104-6076. Phone: (215) 573-9863. Fax: (215) 573-5344. E-mail:
pschfr{at}mail.med.upenn.edu.
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