TETRAHYMENA-THERMOPHILA MUTANTS DEFECTIVE IN THE DEVELOPMENTALLY PROGRAMMED MATURATION AND MAINTENANCE OF THE RDNA MINICHROMOSOME

The abundant rDNA minichromosome of Tetrahymena thermophila generated by a series of developmentally controlled processing steps, termed rDN A maturation, during the formation of the new macronucleus in conjugat ing 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 addit ion, and amplification to approximately 10(4) copies. The rDNA is main tained at this high level in vegetatively growing cells. Using a previ ously developed genetic scheme for studying rDNA maturation and mainte nance, we report the isolation of a new class of mutants defective for rDNA maturation. Several new rDNA maintenance mutants were also obtai ned. The maturation mutant, rmm10, is severely defective for the produ ction of both monomeric and palindromic rDNA in the developing macronu cleus. The rmm10 mutation is recessive-lethal and cis-acting. None of the previously identified DNA sequence elements that control rDNA matu ration or maintenance is mutated in rmm10. Therefore, additional cis a cting sequence elements must be required for rDNA maturation. Based on our current understanding of rDNA maturation processes, we suggest th at the rmm10 mutation affects rDNA excision rather than subsequent rDN A amplification/replication.