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

Abstract

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.

ACKNOWLEDGMENTS

The first two authors, Hobert Wai and Katsuki Johzuka, contributed equally to this work.

We thank Jonathan R. Warner for providing plasmids and for helpful discussions; C. Greer, S. M. Arfin, and S. Jinks-Robertson for critical reading of the manuscript; and M. Oakes and S. VanAmburg for help in preparation of the manuscript.

This work was supported in part by Public Health Service grant GM35949 from the National Institute of Health (to M.N.) and by grants from the Ministry of Education, Culture, Sports, Science and Technology of Japan (to T.H. and T.K.) and the Ministry of Health, Labour and Welfare, Japan (to T.K.).