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Mol Cell Biol, May 1998, p. 3021-3033, Vol. 18, No. 5
Departments of Microbiology and Immunology
and of Biochemistry and Biophysics, University of California at San
Francisco, San Francisco, California 94143-0414
Received 16 October 1997/Returned for modification 14 November
1997/Accepted 11 February 1998
In the ciliated protozoan Tetrahymena thermophila the
ribosomal DNA (rDNA) minichromosome replicates partially under cell cycle control and is also subject to a copy number control mechanism. The relationship between rDNA replication and rRNA gene transcription was investigated by the analysis of replication, transcription, and
DNA-protein interactions in a mutant rDNA, the rmm3 rDNA. The rmm3 (for rDNA maturation or maintenance mutant 3) rDNA
contains a single-base deletion in the rRNA promoter region, in a
phylogenetically conserved sequence element that is repeated in the
replication origin region of the rDNA minichromosome. The multicopy
rmm3 rDNA minichromosome has a maintenance defect in the
presence of a competing rDNA allele in heterozygous cells. No
difference in the level of rRNA transcription was found between
wild-type and rmm3 strains. However, rmm3 rDNA
replicating intermediates exhibited an enhanced pause in the region of
the replication origin, roughly 750 bp upstream from the
rmm3 mutation. In footprinting of isolated nuclei, the
rmm3 rDNA lacked the wild-type dimethyl sulfate (DMS)
footprint in the promoter region adjacent to the base change. In
addition, a DMS footprint in the origin region was lost in the
rmm3 rDNA minichromosome. This is the first reported
correlation in this system between an rDNA minichromosome maintenance
defect and an altered footprint in the origin region. Our results
suggest that a promoter region mutation can affect replication without
detectably affecting transcription. We propose a model in which
interactions between promoter and origin region complexes facilitate
replication and maintenance of the Tetrahymena rDNA
minichromosome.
0270-7306/98/$04.00+0
Copyright © 1998, American Society for Microbiology. All rights reserved.
A Promoter Region Mutation Affecting Replication of
the Tetrahymena Ribosomal DNA Minichromosome
*
Corresponding author. Mailing address: Department of
Microbiology and Immunology, University of California at San Francisco, Room 5447, Box 0414, 513 Parnassus, San Francisco, CA 94143-0414. Phone: (415) 476-4912. Fax: (415) 476-8201. E-mail:
porter{at}itsa.uscf.edu.
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