TANDEM REPEATS OF THE 5' NONTRANSCRIBED SPACER OF TETRAHYMENA RDNA FUNCTION AS HIGH COPY NUMBER AUTONOMOUS REPLICONS IN THE MACRONUCLEUS BUT DO NOT PREVENT RIBOSOMAL-RNA GENE DOSAGE REGULATION

The rRNA genes in the somatic macronucleus of Tetrahymena thermophila are normally on 21 kb linear palindromic molecules (rDNA), We examined the effect on rRNA gene dosage of transforming T.thermophila macronuc lei with plasmid constructs containing a pair of tandemly repeated rDN A replication origin regions unlinked to the rRNA gene, A significant proportion of the plasmid sequences were maintained as high copy circu lar 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. Biol,, 10, 2070-2080], ori gin sequences recombined to form linear molecules bearing several tand em repeats of the origin region, as well as rRNA genes. The total numb er of rDNA origin sequences eventually exceeded rRNA gene copies by si milar to-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.