Sequence organization within and flanking clusters of 5S ribosomal RNA genes in Tetrahymena

Macro- and micronuclei of Tetrahymena thermophila each contain approximately 30 clusters of 5S genes per haploid genome. Structural changes in DNA sequences associated with some of these clusters occur during the development of the transcriptionally active macronucleus from the transcriptionally inert micronucleus. Exonuclease digestion indicates that DNA fragmentation is not responsible for these changes, making it likely that sequence rearrangements occur near some 5S genes during macronuclear development. These rearrangements appear to be random in location with respect to the 5S genes and do not alter either the tandem repeat organization of the genes, the average number (five) or the range in number (one to about 16) of genes per cluster. The 5S gene clusters are not closely linked and are not flanked by common repeating elements large enough to cross-hybridize. Sequence analysis of one tandem repeat suggests that Tetrahymena 5S genes have intragenic promoters. These observations indicate that features other than DNA rearrangements or DNA sequence per se are responsible for the transcriptional activation of 5S genes during macronuclear development.