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Molecular and Cellular Biology, August 2001, p. 5541-5553, Vol. 21, No. 16
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.16.5541-5553.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
Yeast RNA Polymerase I Enhancer Is Dispensable for
Transcription of the Chromosomal rRNA Gene and Cell Growth, and
Its Apparent Transcription Enhancement from Ectopic Promoters
Requires Fob1 Protein
Hobert
Wai,1,
Katsuki
Johzuka,1
Loan
Vu,1
Kristilyn
Eliason,1
Takehiko
Kobayashi,2
Takashi
Horiuchi,2 and
Masayasu
Nomura1,*
Department of Biological Chemistry,
University of California
Irvine, Irvine, California
92697-1700,1 and National Institute
for Basic Biology, Okazaki 444-8585, Japan2
Received 26 March 2001/Returned for modification 8 May
2001/Accepted 21 May 2001
At the end of the 35S rRNA gene within ribosomal DNA
(rDNA) repeats in Saccharomyces cerevisiae lies an
enhancer that has been shown to greatly stimulate rDNA transcription in
ectopic reporter systems. We found, however, that the enhancer is not necessary for normal levels of rRNA synthesis from chromosomal rDNA or
for cell growth. Yeast strains which have the entire enhancer from rDNA
deleted did not show any defects in growth or rRNA synthesis. We found
that the stimulatory activity of the enhancer for ectopic reporters is
not observed in cells with disrupted nucleolar structures, suggesting
that reporter genes are in general poorly accessible to RNA polymerase
I (Pol I) machinery in the nucleolus and that the enhancer improves
accessibility. We also found that a fob1 mutation
abolishes transcription from the enhancer-dependent rDNA promoter
integrated at the HIS4 locus without any effect on
transcription from chromosomal rDNA. FOB1 is required
for recombination hot spot (HOT1) activity, which also
requires the enhancer region, and for recombination within rDNA
repeats. We suggest that Fob1 protein stimulates interactions between
rDNA repeats through the enhancer region, thus helping ectopic rDNA
promoters to recruit the Pol I machinery normally present in the nucleolus.
*
Corresponding author. Mailing address: Department of
Biological Chemistry, 240D Medical Sciences I, University of
California
Irvine, Irvine, CA 92697-1700. Phone: (949) 824-4564. Fax:
(949) 824-3201. E-mail: mnomura{at}uci.edu.

Present address: Beckman Coulter, Inc., Bioresearch Division,
Fullerton, CA 92834-3100.
Molecular and Cellular Biology, August 2001, p. 5541-5553, Vol. 21, No. 16
0270-7306/01/$04.00+0 DOI: 10.1128/MCB.21.16.5541-5553.2001
Copyright © 2001, American Society for Microbiology. All rights reserved.
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