Specific DNA rearrangements in synchronously developing nuclei of Tetrahymena.

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.