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

Abstract

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.

Rearrangement of the 5S ribosomal RNA gene clusters during the development and replication of the macronucleus in Tetrahymena thermophila

“…Using similar amounts of DNA in the gels, the total number of 5s gene copies in the newly formed MAC was found to be one-third greater than in the MIC [17]. A similar number of gene copies was found in both MIC and MAC by Pederson et a1 [23].…”

“…In a study comparing MIC and MAC DNA, approximately the same total number as well as about the same number of MIC and MAC-limited fragments with 5s gene clusters was observed using either EcoRI or Hind III to cut the DNA [23]. Hind 111, like EcoRI, cuts at sites external to the 5s gene cluster, but Hind 111, which cuts at AAGCTT, is insensitive to methylation, since the in vivo methylation site contains the bases 5'-NAT [38].…”

“…Plasmid pDP5, obtained from D . Pederson (University of Rochester), contains a 280-bp BamHI fragment of a MIC 5s rRNA gene plus AT-rich spacer cloned into the BamHI site of pBR322 [23]. DNA from this plasmid was used to transform E coli strain HB101.…”

Formation of stable chromatin structures on the histone H4 gene during differentiation in Tetrahymena thermophila.

“…Like most or all genes in T. thermophila, it appears to be transcriptionally silent in the germinal micronucleus and active in the macronucleus. Frequent DNA sequence rearrangements (chromosome fragmentation, interstitial deletions) accompany gene activation during macronuclear development in T. thermophila (1,2,6,10,16,22,29,30). Although the role of these rearrangements, if any, in the expression of macronuclear genes has yet to be demonstrated (3,22), they nonetheless offer the possibility that ciliates have unique mechanisms of gene activation.…”

“…Frequent DNA sequence rearrangements (chromosome fragmentation, interstitial deletions) accompany gene activation during macronuclear development in T. thermophila (1,2,6,10,16,22,29,30). Although the role of these rearrangements, if any, in the expression of macronuclear genes has yet to be demonstrated (3,22), they nonetheless offer the possibility that ciliates have unique mechanisms of gene activation. This possibility is heightened by reports that ciliates show peculiarities in the genetic code (11,14,15,23), suggesting that they are evolutionarily ancient.…”