Specific DNA rearrangements in synchronously developing nuclei of Tetrahymena.

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Abstract
Specific rearrangement of internal chromosomal regions occurs during development of the somatic macronucleus in Tetrahymena thermophila and results in elimination of germ-line (micronuclear) DNA sequences. The timing and mechanism of genome rearrangement within one particular 9.3-kilobase region, which contains 3 distinct eleminated sequences, were investigated. Portions of this cloned region were used as probes in Southern hybridization experiments to analyze DNA from developing macronuclei (anlagen). All 3 deletions were found to occur predominantly within a 2-h time period in which the nuclear DNA contents increased from 4C to 8C (1C represents the amount of DNA present in a haploid genome). The 3 deletion events can occur independently because intermediate forms, having sustained 1 or 2 deletions, were detected. One of the deletions occurs in 2 alternative ways, resulting in 2 equally abundant products of different size. Because reciprocal products expected from unequal sister chromatid exchange were not detected, an intramolecular DNA splicing mechanism is suggested.