Elimination of specific DNA sequences from the somatic nucleus of the ciliate Tetrahymena.

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Abstract
Tetrahymena micronuclear DNA fragments were cloned in the plasmid pBR322. One clone, pTt 2512, was found to contain the C-C-C-C-A-A hexanucleotide repeat which is also present in the macronuclear r[ribosomal]DNA. Further restriction enzyme digestion and hybridization studies suggest that the clone also contains sequences that are not present in the somatic macronucleus. The flanking sequences of the C4A2 repeats in this clone were separated into 4 restriction fragments, 1 from one side and 3 from the other. These fragments were used as probes for Southern hybridization to study the organizations of similar sequences in the macronucleus and micronucleus. All 4 fragments hybridized to many fragments of restriction enzyme digested micronuclear DNA. None of these hybridizations were detected in the macronucleus. These families of repetitive DNA are completely eliminated from the macronucleus. The 4 different sequences may be linked at other locations of the genome. Using nullisomic strains of Tetrahymena, at least 1 of these seuqences is found in more than 1 chromosome. Studies of various normal and star strains of Tetrahymena suggest that these sequences are stable in the normal micronucleus but are altered drastically in the defective micronuclei of the star strains. Eliminated DNA of similar nature was also found in at least five other randomly selected clones of micronuclear DNA and may be present widely in the genome.