Abstract
Specific rearrangement of internal chromosomal regions occurs during development of the somatic macronucleus in Tetrahymena thermophila and results in elimination of germ-like (micronuclear) DNA sequences. The timing and mechanism of genome rearrangement within one particular 9.3-kilobase region, which contains three distinct eliminated sequences, were investigated. Portions of this cloned region were used as probes in Southern hybridization experiments to analyze DNA from developing macronuclei (anlagen). All three deletions were found to occur predominantly within a 2-hr 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 three deletion events can occur independently because intermediate forms, having sustained one or two deletions, were detected. One of the deletions occurs in two alternative ways, resulting in two 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.