Macronuclear persistence of sequences normally eliminated during development in Tetrahymena thermophila

Abstract

During conjugation in the ciliated protozoan, Tetrahymena thermophilu , a somatic MACronucleus develops from the germinal MICronucleus. Ten to 20 percent of the MIC genome is eliminated during this process. Three repetitive families have been identified which have different levels of repetition in the MIC and are eliminated to different degrees in the MAC. Some members of two of these families persist in the MAC. In this study, we have looked at these persistent sequences in the MAC of cell lines from a variety of sources including several inbed strains, two sets of caryonides, caryonidal subclones, and vegetatively aged cell clones. The results suggest that the sequences that remain in the MAC have a genetic predisposition to persist. However, epigenetic variations occur as the MAC develops so that only some of the persistent sequences are actually observed in a particular MAC. Polymorphisms may be generated if alternative processing of a single MIC segment occurs. These polymorphisms can later be resolved by phenotypic assortment during vegetative growth. These facultatively persistent sequences appear to differ from sequences previously described in this organism.

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

“…It is in this context, then, that a loss or gain of a fragment with a 5s gene cluster would be stable. Similar observations have been made for other DNA sequences, ones normally eliminated from the MIC that persist in the MAC [35].…”

“…At the molecular level, what kinds of processes could lead to the alterations in the 5s gene clusters that we are seeing in the MAC? Processes identified as occurring in the MAC during its development following conjugation are the following: fragmentation of the chromosomes [6,36]; addition of C4Az repeats to the ends of the MAC fragments [6]; interstitial deletions [ 10,111 ; elimination and/or underreplication of MIC sequences [7][8][9]35,36]; methylation of adenines [37,38,39]; gene amplification [5]; and changes in chromosomal proteins [12]. The 5s gene alterations are seen as a change in the size of EcoRI fragments containing the tandem arrays of 5s genes.…”

Methylation site within a facultatively persistent sequence in the macronucleus of Tetrahymena thermophila.

“…In some cell lines the sequence is eliminated, and in other cell lines it is retained in the macronucleus (10). In this report, we investigated a double-stranded methylation site which is found within this persistent sequence.…”

Alternative Processing of Sequences During Macronuclear Development in Tetrahymena thermophila1

“…in approximately 300 copies in the MIC genome and is almost completely eliminated from the MAC genome (36,37). This family was identified by its homology to DNA subclones C, B, and A1 from a clone of MIC DNA, pT1008 (see the top two lines of Fig.…”

“…We have recently studied three eliminated families, two of which have sequences that persist in the MAC (37). These facultatively persistent sequences may persist because of weak or less accessible signals for processing at sites of elimination (36). In the present study, we have isolated a clone of MAC DNA from a cell line that contains a persistent member of a repetitive family that is eliminated from the MAC DNA of other cell lines, as well as sequences normally retained in the MAC.…”