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Molecular and Cellular Biology, August 2001, p. 4938-4948, Vol. 21, No. 15
Department of Cancer Genetics, Roswell Park
Cancer Institute, Buffalo, New York 14263
Received 11 October 2000/Returned for modification 4 December
2000/Accepted 9 May 2001
Chromosomal replicators in budding yeast contain an autonomously
replicating sequence (ARS) that functions in a plasmid, but certain
ARSs are silent as replication origins in their natural chromosomal
context. In chromosome III, the HML ARS cluster
(ARS302-ARS303-ARS320) and ARS301 flank the
transcriptionally silent mating-type locus HML, and all of
these ARSs are silent as replication origins. ARS301 and
ARS302 function in transcriptional silencing mediated by
the origin recognition complex (ORC) and a heterochromatin structure,
while the functions of ARS303 and ARS320 are
not known. In this work, we discovered replication fork pause sites at
the HML ARS cluster and ARS301 by analyzing DNA
replication intermediates from the chromosome via two-dimensional gel
electrophoresis. The replication fork pause at the HML ARS
cluster was independent of cis- and
trans-acting mutations that abrogate transcriptional silencing at HML. Deletion of the HML ARS
cluster led to loss of the pause site. Insertion of a single,
heterologous ARS (ARS305) in place of the HML
ARS cluster reconstituted the pause site, as did multiple copies of DNA
elements (A and B1) that bind ORC. The orc2-1 mutation,
known to alter replication timing at origins, did not detectably affect
the pause but activated the silent origin at the HML ARS
cluster in a minority of cells. Delaying the time of fork arrival at
HML led to the elimination of the pause sites at the
HML ARS cluster and at the copy of ARS305
inserted in place of the cluster. Loss of the pause sites was
accompanied by activation of the silent origins in the majority of
cells. Thus, replication fork movement near HML pauses at a
silent origin which is competent for replication initiation but kept
silent through Orc2p, a component of the replication initiator.
Possible functions for replication fork pause sites in checkpoints,
S-phase regulation, mating-type switching, and transcriptionally silent
heterochromatin are discussed.
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.15.4938-4948.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
DNA Replication Forks Pause at Silent Origins
near the HML Locus in Budding Yeast

*
Corresponding author. Mailing address: Department of
Cancer Genetics, Roswell Park Cancer Institute, Buffalo, NY 14263. Phone: (716) 845-4462. Fax: (716) 845-5906. E-mail:
David.Kowalski{at}RoswellPark.org.
Present address: Department of Structural Biology, Hauptman
Woodward Medical Research Institute, Buffalo, NY 14203.
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