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Programmed DNA Elimination
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The micronuclear genome of T. thermophila contains
many sequences that are not found in the mature macronuclear genome. These
segments are eliminated from the developing macronucleus following division of the zygotic nucleus.
Though some of the elimination events are imprecise, the set of
sequences removed during this process is deterministic. Each sequence can be classified into one of two groups:
Breakage Elimination Sequences (BES) and Internal Eiminated Sequences (IES). The
remaining DNA fragments, which are joined together to form the macronuclear
chromosomes, are called Macronuclear Destined Sequences (MDS) remaining DNA.
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| Internal Eliminated Sequences (IES) are found
between Macronuclear Destined Sequences (MDS) located on the same
macronuclear chromosome. It has been estimated that 6000 of these
segments exist in T. thermophila. All known IES in T. thermophila are
longer than 500 bp, and are not found in the coding regions of
genes. One type of IES, the Tlr element, clearly had its origin
as a transposable element. The mechanism of IES removal has recently been linked to the
production of small (~28 nt) RNAs during macronuclear development.
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| Breakage Elimination Sequences are found adjacent to
Chromosome Breakage Sequences (CBS), 15 nucleotide sequences
(consensus sequence: 5'-AAAGAGGTTGGTTTA-3') found between
the presumptive macronuclear chromosomes in the zygotic nucleus.
CBS direct the action of an as yet
unidentified nuclease, which cleaves the zygotic genome into
distinct macronuclear chromosomes. The ends of the
freed chromosomes are capped with telomeres; however, a comparison
of micronuclear and macronuclear sequences show that up to 30 nucleotides
at the extremities are lost before the telomeres are added.
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