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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,dagger 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.

dagger 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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