Tetrahymena thermophila mutants defective in the developmentally programmed maturation and maintenance of the rDNA minichromosome.

The abundant rDNA minichromosome of Tetrahymena thermophila is generated by a series of developmentally controlled processing steps, termed rDNA maturation, during the formation of the new macronucleus in conjugating cells. rDNA maturation involves excision of a region encoding the single copy rRNA gene (rDNA) from its germline location, rearrangement of the rDNA into a palindromic minichromosome, de novo telomere addition, and amplification to approximately 10(4) copies. The rDNA is maintained at this high level in vegetatively growing cells. Using a previously developed genetic scheme for studying rDNA maturation and maintenance, we report the isolation of a new class of mutants defective for rDNA maturation. Several new rDNA maintenance mutants were also obtained. The maturation mutant, rmm10, is severely defective for the production of both monomeric and palindromic rDNA in the developing macronucleus. The mm10 mutation is recessive-lethal and cis-acting. None of the previously identified DNA sequence elements that control rDNA maturation or maintenance is mutated in rmm10. Therefore, additional cis-acting sequence elements must be required for rDNA maturation. Based on our current understanding of rDNA maturation processes, we suggest that the rmm10 mutation affects rDNA excision rather than subsequent rDNA amplification/replication.