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Molecular and Cellular Biology, September 1999, p. 6098-6109, Vol. 19, No. 9
Department of Cancer Genetics, Roswell Park
Cancer Institute, Buffalo, New York 14263
Received 10 March 1999/Returned for modification 5 May
1999/Accepted 1 June 1999
In the budding yeast, Saccharomyces cerevisiae,
replicators can function outside the chromosome as autonomously
replicating sequence (ARS) elements; however, within chromosome III,
certain ARSs near the transcriptionally silent HML locus
show no replication origin activity. Two of these ARSs comprise the
transcriptional silencers E (ARS301) and I
(ARS302). Another, ARS303, resides between
HML and the CHA1 gene, and its function is not
known. Here we further localized and characterized ARS303
and in the process discovered a new ARS, ARS320. Both
ARS303 and ARS320 are competent as chromosomal
replication origins since origin activity was seen when they were
inserted at a different position in chromosome III. However, at their
native locations, where the two ARSs are in a cluster with
ARS302, the I silencer, no replication origin activity was
detected regardless of yeast mating type, special growth conditions
that induce the transcriptionally repressed CHA1 gene,
trans-acting mutations that abrogate transcriptional silencing at HML (sir3, orc5), or
cis-acting mutations that delete the E and I silencers
containing ARS elements. These results suggest that, for the
HML ARS cluster (ARS303, ARS320,
and ARS302), inactivity of origins is independent of local
transcriptional silencing, even though origins and silencers share key
cis- and trans-acting components. Surprisingly,
deletion of active replication origins located 25 kb (ORI305) and 59 kb
(ORI306) away led to detection of replication origin function at the
HML ARS cluster, as well as at ARS301, the E
silencer. Thus, replication origin silencing at HML ARSs is
mediated by active replication origins residing at long distances from
HML in the chromosome. The distal active origins are known
to fire early in S phase, and we propose that their inactivation delays
replication fork arrival at HML, providing additional time
for HML ARSs to fire as origins.
0270-7306/99/$04.00+0
Copyright © 1999, American Society for Microbiology. All rights reserved.
Activation of Silent Replication Origins at Autonomously
Replicating Sequence Elements near the HML Locus in
Budding Yeast
and
*
Corresponding author. Mailing address: Department of
Cancer Genetics, Roswell Park Cancer Institute, Elm & Carlton St.,
Buffalo, NY 14263. Phone: (716) 845-4462. Fax: (716) 845-5906. E-mail: kowalski{at}sc3101.med.buffalo.edu.
Present address: Department of Environmental Health Sciences,
Tulane University, New Orleans, LA 70112.
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