Rearrangement of repeated DNA sequences during development of macronucleus in Tetrahymena thermophila

Three clones of non-repetitive sequences and six clones containing repetitive sequences were obtained from micronuclear DNA of Tetrahymena thermophila. All the non-repetitive and three repetitive sequences had the sane organization in micro-and macronuclear DNAs as revealed by blot hybridization. On the other hand, the remaining three clones with repetitive sequences had apparently different organization in the two nuclear DNAs. All these repetitive sequences showed a smear on the blot in addition to a number of discrete bands when micronuclear DNA was digested with EcoR I. In macronuclear DNAs, these sequences invariably became one or two bands and the smear disappeared. We conclude that, when a macronucleus develops from a micronucleus, the non-repetitive sequences amplify by more than 20 times with relatively few rearrangement, whereas some selected portions of repeated and/or repeat-contiguous sequences are amplified with rather extensive reorganization. INTRODDCTION The ciliated protozoan Tetrahymena thermophila possesses a transcriptionally inactive, diploid, germinal micronucleus and a transcriptionally active, polyploid, somatic macronucleus. During the course of sexual conjugation, the macronucleus is destroyed and resorbed, while the micronucleus undergoes raeiosis. After subsequent reciprocal exchange and fusion of the two haploid genetic nuclei, a new macronucleus develops through a process of amplification of raicronuclear DNA to ^45-ploid (3,12). The multiple copies of rRNA genes in the macronucleus are generated by amplification of the apparently single gene in the micronucleus (13). There is evidence that rRNA gene amplification may be associated with chromosome breakage and DNA elimination (15)