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Nucleic Acids Research, 1995, Vol. 23, No. 9 1561-1569
© 1995

MOLECULAR BIOLOGY

Tandem repeats of the 5' non-transcribed spacer of Tetrahymena rDNA function as high copy number autonomous replicons in the macronucleus but do not prevent rRNA gene dosage regulation

Wei-Jun Pan and Elizabeth H. Blackburn*

Department of Microbiology and Immunology, University of Califomia-San Francisco San Francisco, CA 94143-0414, USA

* To whom correspondence should be addressed

Received December 15, 1994. Revised March 16, 1995. Accepted March 16, 1995.

The rRNA genes in the somatic macronucleus of Tetrahymena thermophlla are normally on 21 kb linear palindromic molecules (rDNA). We examined the effect on rRNA gene dosage of transforming T.thermophlla macronuclel with plasmld constructs containing a pair of tandemly repeated rDNA replication origin regions unlinked to the rRNA gene. A significant proportion of the plasmld sequences were maintained as high copy circular molecules, eventually consisting solely of tandem arrays of origin regions. As reported previously for cells transformed by a construct in which the same tandem rDNA origins were linked to the rRNA gene [Yu,G.-L. and Blackburn,E.H. (1990) Mol. Cell. Blol., 10,2070–2080], origin sequences recombined to form linear molecules bearing several tandem repeats of the origin region, as well as rRNA genes. The total number of rDNA origin sequences eventually exceeded rRNA gene copies by {small tilde}20- to 40-fold and the number of circular replicons carrying only rDNA origin sequences exceeded rRNA gene copies by 2- to 3-fold. However, the rRNA gene dosage was unchanged. Hence, simply monitoring the total number of rDNA origin regions Is not sufficient to regulate rRNA gene copy number.


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